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Lithos 72 (2004) 209–229

Derivation of potassic (shoshonitic) magmas by

decompression melting of phlogopite+pargasite lherzolite

R.V. Conceicaoa,b,*, D.H. Greenb

a Laboratorio de Geologia Isotopica, Instituto de Geociencias, Universidade Federal do Rio Grande do Sul, Campus do Vale,

Av. Bento Goncalves, 9500. C.P. 15.001 CEP: 91.501-970, Porto Alegre, RS, BrazilbResearch School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia

Received 27 September 2002; accepted 29 September 2003

Abstract

A model metasomatized lherzolite composition contains phlogopite and pargasite, together with olivine, orthopyroxene,

clinopyroxene and spinel or garnet as subsolidus phases to 3 GPa. Previous works established that at z 1.5 GPa, phlogopite is

stable above the dehydration solidus, determined by the melting behaviour of pargasite and coexisting phases. At 2.8 GPa, melts

with residual phlogopite + garnet lherzolite mineralogy at 1195 jC and with garnet lherzolite mineralogy at 1250 jC are both

olivine nephelinite with K/Na (atomic) = 0.51 and K/Na = 0.65, respectively. Recent work shows that melting along the

dehydration (fluid-absent) solidus of the phlogopite + pargasite lherzolite at pressures < 1.5 GPa is very different with the

presence of phlogopite, decreasing the solidus below that of pargasite lherzolite. At 1.0 GPa, both phlogopite and pargasite

disappear at temperatures at or slightly above the solidus. The compositions of two melts at 1.0 GPa, 1075 jC (with different

water contents), in equilibrium with residual spinel lherzolite mineralogy are silica-saturated trachyandesite (f 5% melt

fraction, f 3% H2O) to silica-oversaturated basaltic andesite (f 8% melt fraction, 4.5% H2O). Both compositions may be

classified as ‘shoshonites’ on the basis of normative compositions, silica-saturation, and K/Na ratio. Decompression melting of

metasomatized lithospheric lherzolite with minor phlogopite and pargasite may produce primary ‘shoshonitic’ magmas by

dehydration melting at f 1 GPa, 1050–1150 jC. Such magmas may be parental to Proterozoic batholithic syenites occurring

in Brazil.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Mantle; Metasomatized lithosphere; Syenite; Experimental petrology; High-pressure; Shoshonites

1. Introduction

The experimental determination of the solidus of

the upper mantle and of near-solidus melt composi-

0024-4937/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.lithos.2003.09.003

* Corresponding author. Isotopica, Instituto de Geociencias,

Universidade Federal do Rio Grande do Sul, Av. Bento Goncalves,

9500, Porto Alegre 91501-970, Brazil.

E-mail address: rommulo.conceicao@ufrgs.br

(R.V. Conceicao).

tions has been a major research theme with emphases

both on anhydrous compositions and on the large

effects of small amounts of water or of C–H–O

fluids. Whether approached from the experimental

petrology direction or from the direction of minor

element, trace-element or isotope geochemistry, it has

been argued that mantle compositions are variable

and no single-mantle composition can provide the

geochemical diversity observed in primitive magmas

by variations in pressure, temperature or volatile

R.V. Conceic�ao, D.H. Green / Lithos 72 (2004) 209–229210

content (C–H–O system) alone. At one extreme are

the ultrapotassic magmas (K/NaH1; Foley et al.,

1987) of mantle derivation. Experimental study of

the leucite–olivine–quartz system under anhydrous

conditions (Conceicao and Green, 2000) showed that

highly potassic, silica-saturated and silica-oversatu-

rated liquids were the partial melt products of potassic

harzburgite at pressures up to 2.0 GPa and temper-

atures around 1200 jC. This contrasts with the sodic

and calcic systems in which near-solidus melts coex-

isting with olivine (Mg90) and enstatite are silica-

undersaturated (nepheline normative) at z 1 GPa

(sodic system) or silica-saturated (hypersthene + oli-

vine normative) and olivine-rich at z 1 GPa (calcic

system; Walter and Presnall, 1994).

In the presence of hydrogen [(OH)�] in the

earth’s mantle, the mineral pargasite is a host for

potassium in lherzolite compositions with significant

Na, Al, Ti, K, Ca contents. For small ‘water’

contents, the lherzolite solidus is given by the

dehydration melting reactions of pargasite from

f 0.2 to 3.0 GPa (Green, 1973). Depending on the

pargasite composition and bulk composition, the

‘dehydration-solidus’ at 1.5 GPa varies from

f 1050 to 1150 jC (Wallace and Green, 1991). In

the most detailed study to date, Niida and Green

(1999) mapped the composition and modal abun-

dance of pargasite in a fertile peridotite composition

(MORB pyrolite) from 0.5 to 3.0 GPa and from 950

jC to the solidus (1050–1075 jC). This pargasite

was essentially K2O-free, but Green (1973) and

Wallace and Green (1991) showed higher tempera-

ture solidi and pargasite stability for the K- and Ti-

bearing pargasite stabilised in the ‘enriched’ mantle

lherzolite composition known as ‘Hawaiian pyrolite’.

Mengel and Green (1989) used a lherzolite compo-

sition enriched in phlogopite, with K/Na = 0.8 (atom-

ic), and in which phlogopite coexisted with pargasite

in subsolidus conditions from 1.5 to 3.0 GPa. The

composition was presented as a model ‘metasomat-

ized mantle’ composition based on the addition of

phlogopite to a refractory spinel lherzolite composi-

tion. Mengel and Green (1989) showed that, from

1.5 to 3 GPa, the pargasite stability in the ‘meta-

somatized mantle’ composition was closely compa-

rable with pargasite stability in the ‘Hawaiian

pyrolite’ composition but that phlogopite stability

extended above the solidus at z 1.5 GPa. They

obtained compositions for melts coexisting with olivi-

ne + orthopyroxene + clinopyroxene + garnet +phlogo-

pite at 2.8 GPa, 1195 jC and with the four-phase

garnet lherzolite assemblage at 2.8 GPa, 1250 jCabove phlogopite breakdown. These liquids are oliv-

ine-rich nephelinite to basanite compositions with K/

Na = 0.51 at 1195 jC (phlogopite stable) and K/

Na = 0.65 at 1250 jC after phlogopite breakdown.

The present study was undertaken to link the

work of Mengel and Green (1989) with the 1–2

GPa work on the anhydrous olivine–leucite–quartz

system, in which liquids in the potassic system were

much more silica-rich than in the sodic system. In

particular, one aim was to explore the possibility that

potassic syenitic complexes and potassic volcanic

rocks with high K/Na ratio and moderate to high

SiO2 contents (Foley et al., 1987) could be direct

partial melting products from ‘metasomatized lherzo-

lite’ or ‘metasomatized harzburgite’ mantle contain-

ing minor phlogopite, with low-pressure magma

segregation. Our interest in mantle derivation of

potassic syenites also arises from observations on

large posttectonic syenite complexes of Early and

Late Peroterozoic ages in Brazil (Conceic� ao et al.,

2000). Recent studies of Late Proterozoic (594 Ma)

potassic syenites intruded in south Brazil (Pla Cid et

al., 2003) demonstrated microinclusions of potassi-

um-rich diopside (K2O around 1.5 wt.%), potassic

pargasite and pyrope-rich garnet included in diop-

sides. Textural and chemical evidence suggest that

the crystallization of these minerals is related to a

lamprophyric magma mingling with the host potassic

syenite at mantle conditions. This suggestion implies

that both magmas were generated at high pressure

and, noting their distinctive K-enriched nature, both

may be derived from metasomatized mantle. The

geochemical characteristics (major and trace ele-

ments; isotopic ratios) of these Late Proterozoic

syenites are similar to other syenites (Conceicao et

al., 2000) occurring in north-western Brazil with

Early Proterozoic ages (around 2.1 Ga), suggesting

that the generation of these syenites has occurred

repeatedly through the evolution of the Earth. Both

syenite complexes are posttectonic without signifi-

cant deformation textures and are emplaced in ter-

rains with previous histories of subduction and

collision events. A specific purpose in this study is

to investigate possible ways of forming potassic

R.V. Conceic�ao, D.H. Green / Lithos 72 (2004) 209–229 211

syenite magmas from metasomatized upper mantle

lherzolite.

The study is divided in two parts. Firstly, we

present the solidus temperatures and stability fields

of phlogopite- and pargasite-bearing lherzolite under

water-saturated (subsolidus) and water-undersaturated

conditions from 0.5 to 1.5 GPa. Secondly, we derive

melt compositions close to the phlogopite-out bound-

ary at 1 GPa.

2. Experimental methods

2.1. Starting material

The peridotite composition used (‘NHD perido-

tite’—Mengel and Green, 1989) is that of a depleted

peridotite (NHD—Table 1) calculated as the average of

36 lherzolite xenoliths from basanitic eruptive centres

of the Northern Hessian Depression (Wedepohl, 1985),

northwest Germany (Table 1). The mineralogy,

expressed as the spinel lherzolite assemblage, is 73%

olivine, 18% orthopyroxene, 7% clinopyroxene and

1% chromian spinel. Veinlets enriched in phlogopite

occur within this xenolith population and represent a

mantle-metasomatism event. The metasomatized peri-

dotite model composition was derived by adding 1.5%

phlogopite to the depleted spinel lherzolite composi-

tion and the newmetasomatized lherzolite composition

was named ‘NHD peridotite’ (Table 1) following

Mengel and Green (1989).

Consistent with earlier studies, 60% of olivine

(Fo89) was subtracted from the NHD peridotite com-

position in order to diminish the dominance of this

mineral, to enhance the proportion of liquid to crystals

in partially molten samples and to facilitate the

analyses of minor phases and melt pools. Olivine

remained in all experiments.

The startingmaterial was prepared from amixture of

correct proportions of analytical grade carbonates (Ca,

K and Na), and oxides (Mg, Si, Al, Ti, Mn, Ni, Cr),

ground under acetone in an agate mortar and fired at

900 jC for 5 h to release CO2. After firing, it was

ground in acetone again, pelletised and sintered at 1000

jC overnight (f 16–20 h). An appropriate amount of

synthetic fayalite was then added to the sintered mix,

and the mixture was ground again under acetone, stored

in glass vials and kept dry in an oven at 110 jC.

For experiments under water-saturated conditions,

1% water (0.14 mg) was added by a microsyringe to

14 mg of the starting mixture and run at different

conditions of pressure and temperature. Ag50Pd50 or

Ag75Pd25 capsules were used, depending on the

condition of the experiment, in order to minimise Fe

loss (Table 2). During sealing by welding, the tem-

perature of the capsule was maintained cold to avoid

water evaporation. The capsule was weighed before

and after the sealing, and before and after the runs to

ensure that there was no leak. Bulk compositions were

checked by broad-area (50–100 Am2) analysis of the

polished surface of the product (Table 1). As noted

below, 0.35% to 0.4% H2O is held in amphibole and

phlogopite at subsolidus conditions, giving approxi-

mately 0.6% H2O in excess at the solidus.

For experiments under water-undersaturated con-

ditions, 80–100 mg of the starting mix was presatu-

rated with 2% water in large-capacity run, which

allowed the use of larger amount of sample (see

below), at 0.5 GPa and 950 jC for 72 h. This approach

was used to overcome unavoidable errors in pipetting

very small amounts of water. The water was added in

sufficient amount to crystallise phlogopite and amphi-

bole, and a gold capsule was used to avoid Fe loss. The

capsule was weighed before and after sealing, and

before and after the run to ensure that there had been

no loss of water. After the run, the capsule was pierced

and dried at 110 jC. The run product was then ground

under acetone and dried at 300 jC overnight to release

the excess of water. Scanning electron microscope

(SEM), microprobe analysis, optical examination and

X-ray diffraction confirmed the mineralogy particular-

ly the presence of both pargasite and phlogopite. The

amount of water remaining in the large-capacity run

product is 0.35–0.4 wt.% H2O, based on proportions

of pargasite and phlogopite in the charge and is

exclusively fixed in amphibole and phlogopite struc-

tures. In addition to amphibole and phlogopite, fine-

grained olivine, diopside, enstatite and spinel were

also found. The bulk composition was checked by

analytical scanning electron microscope, and the result

was compared with the (NHD peridotite � 60% oliv-

ine) composition based in the bulk analysis of exper-

iment 17 (Table 1). From this presaturated material, 5

to 7 mg of (amphibole + phlogopite)-bearing lherzolite

were used in subsequent runs under various conditions

of pressure and temperature (Table 3).

Table 1

Starting material composition and some peridotites used in previous experiments

Chemical composition of the starting material

NHD Phlogopite NHD

peridotite

Olivine S. mat. H2O-

saturatedaH2O-

undersaturatedb

Average S.D. Average S.D.

SiO2 43.40 40.52 43.36 40.80 47.19 48.22 1.02 48.82 0.38

TiO2 0.08 1.06 0.09 0.24 0.27 0.10 0.34 0.05

Al2O3 2.00 17.92 2.24 5.60 5.99 0.70 6.14 0.50

Cr2O3 0.42 1.17 0.43 1.08 0.99 0.09 1.04 0.09

FeO 8.60 5.42 8.55 9.79 6.70 6.52 0.45 5.74 0.52

MnO 0.13 0.03 0.13 0.32 0.02 0.07 0.00 0.00

MgO 43.10 23.35 42.80 48.94 33.60 32.13 2.43 32.11 0.15

CaO 1.80 0.09 1.77 4.44 4.76 0.94 4.96 0.86

Na2O 0.13 0.95 0.14 0.36 0.48 0.08 0.40 0.09

K2O 0.03 9.28 0.17 0.42 0.55 0.14 0.45 0.24

NiO 0.30 0.21 0.30 0.75 0.08 0.16 0.00 0.00

Total 99.99 100.00 99.99 99.53 100.68 100.00 2.32 100.00 1.18

Mg# 89.93 88.48 89.92 89.91 89.94 89.78 90.89

K2O/Na2O 0.23 9.77 1.19 1.19 1.15 1.12

Some sample of peridotites for reference

NHD

metasomatized

mantle (1)

MM-3 (2) MORB

pyrolite (3)

KLB-1 (4) MARID (5) St. Paul’s (6) PLZ (7)

SiO2 43.36 45.50 45.20 44.48 45.45 42.22 47.04

TiO2 0.09 0.11 0.70 0.16 2.67 0.30 0.30

Al2O3 2.24 3.98 3.50 3.59 6.34 4.42 4.23

Cr2O3 0.43 0.68 0.40 0.31 0.19 0.50 0.72

FeO 8.55 7.18 8.10 8.10 6.52 7.02 6.72

MnO 0.13 0.13 0.10 0.12 0.07 0.13 0.12

MgO 42.80 38.30 37.50 39.22 21.58 34.61 35.40

CaO 1.77 3.57 3.10 3.44 5.67 3.92 4.32

Na2O 0.14 0.31 0.60 0.30 0.92 0.43 0.20

K2O 0.17 0.13 0.02 6.91 0.11 0.61

NiO 0.30 0.20 0.08 0.27 0.11

Total 100.68 99.76 100.68 99.74 96.40 93.93 99.77

Mg# 89.94 90.48 89.94 89.62 85.51 89.78 90.38

Na2O+K2O 0.31 0.31 0.73 0.32 7.83 0.54 0.81

K2O/Na2O 1.19 0.22 0.07 7.51 0.26 3.05

NHD-depleted peridotite: average of 28 spinel peridotite xenoliths; phlogopite: average composition of phlogopites from metasomatized

lherzolite xenoliths from the same area; NHD peridotite: NHD+ 1.5% phlogopite; olivine: stoichiometrically calculated; S. Mat.: NHD

peridotite� 60% olivine (see text). The starting material for experiments in water-saturated and water-understaturated conditions are expressed

by the averages of the bulk analyses of the runs EXP-31 and EXP-32 for H2O-saturated condition,and EXP-17 and large capacity run for H2O-

undersaturated condition. S.D.: standard deviation (1 sigma), FeO: Fe total. The references for the peridotites for comparison are in parenthesis:

1—Mengel and Green, 1989; 2—Baker et al., 1995; 3—Green, 1973; 4—Hirose and Kawamoto, 1995; 5—Sweeney et al., 1993; 6—Nehru and

Wyllie, 1975; 7—Thibault et al., 1992.a Check of the correct composition based on bulk analysis of experiments 31 and 32.b Check of the correct composition based on bulk analysis of experiments 17 and large-capacity run.

R.V. Conceic�ao, D.H. Green / Lithos 72 (2004) 209–229212

R.V. Conceic�ao, D.H. Green / Lithos 72 (2004) 209–229 213

2.2. Experiments

Experiments were run at the Research School of

Earth Sciences, The Australian National University in a

solid-media, piston-cylinder apparatus similar to that

described by Boyd and England (1960). Piston diam-

eter is 1.25 cm (0.5 in). The pressure cell consists of a

talc outer sleeve, a graphite heater, and two inner

spacers of MgO. The 2-mm-diameter capsule is placed

between these inner parts and encapsulated in a MgO

sleeve. MgO powder was packed around the welded

capsule. A 0.5 mm Al2O3 disk separates the capsule

from the thermocouple tip. Temperatures were con-

trolled automatically to F 1 jC and measured with

Pt94Rh6–Pt70Rh30 Type B thermocouples encapsulated

in Al2O3 sleeves. A pressure correction of � 10% was

applied for this configuration (talc sleeve). In a typical

experiment, pressure was initially applied to the sample

cell to approach the target value at room temperature

and was allowed to relax as the sample was heated to

the final temperature. Final adjustments were made

through ‘‘piston-in’’ adjustment. Pressure is considered

accurate to F 0.1 GPa, and no correction was made for

the effect of pressure on the thermocouple. Ag75Pd25and Ag50Pd50 capsules were used depending on the

conditions of each experiment.

At the end of each experiment, the entire experi-

mental charge was mounted and sectioned longitudi-

nally before polishing.

2.3. Analytical procedures

Experimental run products were analysed at the

Electron Microscope Unit (EMU), in The Australian

National University, with a JEOL 6400 Scanning

Electron Microscope (SEM) fitted with an Oxford

Instruments light element energy-dispersive analysis

system, using an accelerating voltage of 15 kV, spec-

imen current of 10 nA, and spot size of ca. 1 Am.

Studies of counting times, especially for the best K and

Na analysis, defined the use of 120 s for each analysis.

Dead time was around 25–30%. The melt phase was

analysed using an area scan, usually of 10–30 Am2

area to ensure that there was no K or Na loss. Stand-

ards were natural sanidine (K, Al), olivine (Mg),

enstatite (Fe), anorthite (Ca), chromite (Cr), ilmenite

(Ti) and silica (Si), and pure Nickel (Ni) and Manga-

nese (Mn). Back-scattered and secondary electron

images aided phase identifications and selection of

analytical area or point. Drift in beam current of the

SEM gave analysis totals from 97% to 102%. But

because analyses for all elements are simultaneous by

the energy-dispersive method, totals were normalized

to 100% and the quality of the analysis was assessed

by adherence to stoichiometry for mineral phases. The

analyses reported in the tables are, in most cases,

means of five or more analysis.

3. Experimental results

The phase assemblages of experimental runs at 0.5,

1.0 and 1.5 GPa on NHD peridotite under water-

saturated and water-undersaturated conditions are

summarised in Fig. 1, Table 2 and Fig. 2, Table 3,

respectively. The data from Mengel and Green (1989)

for higher pressure conditions are also shown in these

figures.

3.1. Quality of the runs and iron loss control

Solidus temperature was estimated by textural

characteristics in the SEM images, phase composi-

tions, and the presence of glass and of quench rims on

primary phases. Reversal experiments were also made

at 0.5, 1.0 and 1.5 GPa, increasing temperature 50 jCabove the estimated solidus temperature for 1 h, and

then dropping to the run temperature (Table 2: EXP-

15#, -06# and -02#; Table 3: EXP-20# and -07#).

Iron loss was checked by bulk composition analysis

and was significant in some above-solidus experiments

(notably in experiments in Ag50Pd50 capsules). For

these runs, there was an increase of Mg# [100 MgO/

(MgO+FeO) molar proportion] of bulk composition

and olivine compared to the starting material. At

equilibrium and temperatures of 1000–1200 jC,Mg# decreases in the order Phlog>Cpx>Opx>Ol>Liq,

and this is observed in experiments without Fe loss.

Where Fe loss is significant, phases adjust to this at

rates controlled by intracrystalline diffusion. Our

observations (Figs. 3 and 4) suggest that Fe X Mg

diffusion rates decrease in the order Ol>Opx>Cpx.

Minerals were analysed in most experiments.

Quench minerals were identified by their shapes and

low Mg# relative to primary phases. In some analysis

of small grains, inclusion of quenching rims degrades

Table 2

Experimental run conditions and phase composition produced in the runs at water-saturated conditions

Sample Capsule Temp. Pres. Time Assembly Phase Composition

(jC) (GPa) (h)SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O NiO Total Mg#

EXP-28 Ag75Pd25 950 0.5 12 Ol + En +Di + Olivine 40.70 0.00 0.98 0.44 10.26 0.00 46.61 0.56 0.07 0.00 0.39 100.00 89.01

Sp + Phl +Amp Enstatite 54.16 0.14 3.62 0.87 5.95 0.12 32.75 2.05 0.29 0.06 0.00 100.00 90.75

Diopside 52.64 0.30 2.84 1.42 2.15 0.00 20.92 19.06 0.48 0.18 0.00 100.00 94.55

Phlogopite 43.68 1.77 13.95 2.03 4.73 0.00 25.21 0.76 0.69 7.19 0.00 100.00 90.48

EXP-15 Ag75Pd25 975 0.5 12 Ol + En +Di + Olivine 41.17 0.00 0.58 0.22 9.53 0.02 47.61 0.26 0.08 0.00 0.53 100.00 89.91

Sp + Phl +Amp +Gl Enstatite 54.53 0.32 4.17 0.88 5.65 0.00 29.82 4.48 0.11 0.04 0.00 100.00 90.52

Diopside 53.87 0.38 3.89 0.67 2.75 0.00 18.63 19.17 0.30 0.17 0.17 100.00 92.42

Phlogopite 43.29 2.37 16.03 1.10 3.65 0.00 24.14 0.00 0.65 8.78 0.00 100.00 92.18

EXP-15# Ag75Pd25 1025 / 0.5 12 Ol + En +Di + Sp + Olivine 41.10 0.00 0.45 0.25 9.45 0.00 47.90 0.20 0.00 0.00 0.65 100.00 90.12

975 Phl +Amp +Gl Enstatite 54.49 0.39 4.20 0.88 5.59 0.00 29.93 4.46 0.05 0.01 0.00 100.00 90.60

Diopside 53.70 0.35 3.98 0.70 2.70 0.00 18.70 19.21 0.35 0.18 0.13 100.00 92.57

Phlogopite 43.30 2.40 16.10 1.10 3.50 0.00 24.30 0.00 0.40 8.90 0.00 100.00 92.59

EXP-35 Ag75Pd25 1000 0.5 48 Ol + En +Di + Olivine 40.73 0.00 0.41 0.34 8.90 0.03 48.83 0.19 0.18 0.00 0.40 100.00 90.72

Sp +Gl Enstatite 55.41 0.16 3.88 0.85 5.56 0.04 32.44 1.44 0.21 0.00 0.00 100.00 91.23

Diopside 52.84 0.46 4.02 1.19 3.01 0.00 18.46 19.73 0.22 0.07 0.00 100.00 91.62

EXP-16 Ag75Pd25 1025 0.5 48 Ol + En +Di + Olivine 40.96 0.00 0.00 0.00 9.03 0.00 49.31 0.12 0.19 0.00 0.39 100.00 90.68

Sp +Gl Enstatite 54.78 0.27 4.34 0.99 5.65 0.22 32.23 1.34 0.17 0.00 0.00 100.00 91.04

Diopside 52.55 0.65 4.27 1.26 2.79 0.00 17.72 20.76 0.00 0.00 0.00 100.00 91.87

EXP-05 Ag75Pd25 975 1.0 48 Ol + En +Di + Olivine 41.01 0.00 0.30 0.00 10.30 0.00 47.73 0.11 0.06 0.00 0.49 100.00 89.20

Sp + Phl +Amp Enstatite 55.27 0.10 4.68 0.00 6.31 0.06 32.15 1.10 0.12 0.00 0.19 100.00 90.08

Diopside 52.85 0.33 4.29 0.00 2.96 0.06 18.03 20.80 0.28 0.42 0.00 100.00 91.57

Phlogopite 41.37 0.99 17.92 1.79 3.75 0.00 23.94 0.00 0.78 9.46 0.00 100.00 91.92

EXP-04 Ag75Pd25 1000 1.0 8 Ol + En +Di + Sp + Olivine 40.88 0.05 1.81 0.62 8.65 0.00 46.98 0.37 0.10 0.00 0.54 100.00 90.64

Phl +Amp Enstatite 54.89 0.11 3.93 0.55 5.54 0.00 33.08 1.41 0.15 0.27 0.07 100.00 91.41

Diopside 54.08 0.27 3.94 0.65 2.59 0.04 20.76 17.17 0.44 0.06 0.00 100.00 93.30

Phlogopite 39.72 1.04 17.74 2.07 4.54 0.00 26.69 0.21 0.60 7.03 0.35 100.00 91.31

EXP-06 Ag75Pd25 1025 1.0 10 Ol + En +Di + Sp + Olivine 41.09 0.00 0.35 0.23 9.52 0.00 48.18 0.25 0.10 0.02 0.28 100.00 90.02

Phl +Glass Enstatite 56.18 0.14 2.85 0.98 5.31 0.00 32.78 1.66 0.10 0.00 0.00 100.00 91.66

Diopside 52.86 0.34 3.51 0.98 2.96 0.05 18.79 19.96 0.31 0.24 0.00 100.00 91.88

Phlogopite 41.26 1.66 18.33 1.42 3.62 0.00 23.49 0.16 0.73 9.33 0.00 100.00 92.05

EXP-06# Ag75Pd25 1075 / 1.0 10 Ol + En +Di + Sp + Olivine 41.03 0.00 0.30 0.30 9.57 0.00 48.30 0.20 0.00 0.00 0.30 100.00 90.08

1025 Phl +Glass Enstatite 56.00 0.18 2.88 0.98 5.36 0.00 32.85 1.60 0.15 0.00 0.00 100.00 91.69

Diopside 52.87 0.15 3.60 0.97 2.97 0.00 19.00 19.90 0.29 0.25 0.00 100.00 92.01

Phlogopite 41.23 1.66 18.30 1.40 3.70 0.00 23.52 0.15 0.69 9.35 0.00 100.00 91.96

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EXP-31 Ag75Pd25 1050 1.0 14 Ol + En +Di + Sp + Olivine 40.83 0.00 0.34 0.18 9.49 0.00 48.16 0.18 0.30 0.00 0.51 100.00 90.04

Phl +Glass Enstatite 54.69 0.19 4.66 0.91 5.81 0.04 32.23 1.29 0.19 0.00 0.00 100.00 90.82

Diopside 51.97 0.31 4.89 0.97 3.20 0.00 18.49 19.28 0.44 0.45 0.00 100.00 91.16

Phlogopite 42.46 1.39 16.88 1.34 4.06 0.00 24.57 0.19 0.87 8.25 0.00 100.00 91.52

EXP-32 Ag75Pd25 1075 1.0 13 Ol + En +Di + Sp + Olivine 40.94 0.00 0.39 0.44 8.63 0.00 49.17 0.17 0.04 0.00 0.22 100.00 91.03

Glass Enstatite 56.07 0.12 2.67 1.00 5.26 0.00 32.99 1.78 0.09 0.00 0.00 100.00 91.79

Diopside 53.68 0.29 2.81 0.94 2.89 0.00 18.86 20.37 0.12 0.03 0.00 100.00 92.07

EXP-03 Ag75Pd25 975 1.5 24 Ol + En +Di + Olivine 40.46 0.00 0.77 0.29 9.89 0.00 47.26 0.47 0.16 0.00 0.70 100.00 89.49

Sp + Phl +Amp Enstatite 55.49 0.13 3.29 0.67 6.14 0.00 32.92 1.23 0.14 0.00 0.00 100.00 90.53

Diopside 52.96 0.29 4.17 0.73 2.88 0.00 17.38 21.25 0.34 0.00 0.00 100.00 91.50

Phlogopite 43.98 0.88 15.22 1.71 4.41 0.00 25.79 0.81 0.60 6.21 0.38 100.00 91.19

EXP-01 Ag75Pd25 1000 1.5 8 Ol + En +Di + Sp + Olivine 40.21 0.00 0.76 0.29 10.95 0.00 46.79 0.31 0.14 0.00 0.54 100.00 88.39

Phl +Amp Enstatite 55.15 0.03 3.74 0.65 5.50 0.00 31.68 2.72 0.21 0.23 0.09 100.00 91.15

Diopside 53.35 0.26 3.98 0.64 2.94 0.00 18.12 20.08 0.54 0.10 0.00 100.00 91.65

Phlogopite 40.86 1.17 17.99 1.67 4.00 0.00 24.69 0.24 0.66 8.26 0.46 100.00 91.67

EXP-02 Ag75Pd25 1025 1.5 10 Ol + En +Di + Sp + Olivine 41.14 0.00 0.21 0.10 10.03 0.00 48.05 0.16 0.06 0.00 0.25 100.00 89.52

Phl +Amp +Gl Enstatite 55.39 0.14 4.05 0.70 6.03 0.06 32.36 1.10 0.10 0.00 0.09 100.00 90.54

Diopside 52.46 0.43 3.97 1.04 2.93 0.00 17.43 21.32 0.41 0.00 0.00 100.00 91.37

Phlogopite 41.30 1.28 16.99 1.46 3.80 0.00 25.34 0.00 0.72 8.67 0.44 100.00 92.24

EXP-02# Ag75Pd25 1075 / 1.5 10 Ol + En +Di + Sp + Olivine 40.71 0.00 0.00 0.12 9.95 0.00 48.90 0.07 0.00 0.00 0.25 100.00 89.84

1025 Phl +Amp +Gl Enstatite 55.34 0.16 4.10 0.72 6.07 0.07 32.40 1.00 0.05 0.00 0.09 100.00 90.57

Diopside 52.49 0.40 3.90 1.00 2.95 0.00 17.52 21.35 0.39 0.00 0.00 100.00 91.45

Phlogopite 41.98 1.30 17.01 1.36 3.77 0.00 25.30 0.00 0.20 8.70 0.38 100.00 92.35

EXP-12 Ag75Pd25 1050 1.5 10 Ol + En +Di + Sp + Olivine 40.60 0.00 0.00 0.00 10.04 0.00 48.59 0.13 0.00 0.00 0.64 100.00 89.62

Phl +Amp +Gl Enstatite 54.68 0.00 5.16 0.87 6.01 0.00 32.11 1.00 0.16 0.00 0.00 100.00 90.49

Diopside 52.59 0.48 4.26 0.84 3.06 0.00 17.68 20.60 0.50 0.00 0.00 100.00 91.15

Phlogopite 40.56 1.34 17.90 2.48 4.11 0.00 23.70 0.36 1.12 8.16 0.26 100.00 91.13

EXP-26 Ag75Pd25 1075 1.5 10 Ol + En +Di + Sp + Olivine 40.59 0.00 0.33 0.24 9.95 0.00 48.30 0.00 0.11 0.00 0.48 100.00 89.64

Phl +Gl Enstatite 53.78 0.28 5.58 1.06 6.22 0.00 31.99 0.92 0.18 0.00 0.00 100.00 90.17

Diopside 51.53 0.52 5.46 1.23 3.13 0.00 17.56 19.89 0.54 0.16 0.00 100.00 90.91

Phlogopite 43.14 1.24 16.06 1.24 4.11 0.00 25.08 0.20 0.79 8.13 0.00 100.00 91.59

EXP-27 Ag75Pd25 1100 1.5 10 Ol + En +Di + Sp + Olivine 39.56 0.00 1.26 1.50 9.01 0.00 48.11 0.15 0.09 0.03 0.28 100.00 90.49

Phl +Gl Enstatite 54.93 0.00 3.20 0.78 5.76 0.00 33.94 1.17 0.23 0.00 0.00 100.00 91.30

Diopside 53.32 0.13 4.37 1.11 3.76 0.00 22.84 14.20 0.28 0.00 0.00 100.00 91.58

Phlogopite 41.79 1.09 18.44 0.90 3.58 0.00 23.91 0.04 0.72 9.53 0.00 100.00 92.24

EXP-33 Ag50Pd50 1125 1.5 12 Ol + En +Di + Olivine 40.91 0.00 0.70 0.83 5.08 0.00 51.95 0.15 0.26 0.00 0.13 100.00 94.80

Sp +Gl Enstatite 55.43 0.08 3.93 1.25 4.28 0.00 33.38 1.41 0.23 0.00 0.00 100.00 93.28

Diopside 52.57 0.31 4.19 1.29 2.96 0.15 18.68 19.36 0.49 0.00 0.00 100.00 91.83

Temp.—temperature; Pres.—pressure; Ol—olivine; En—enstatite; Di—diopside; Sp—spinel; Phl—phlogopite; Amp—amphibole; Gl—glass.

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Table 3

Experimental run conditions and phase composition produced in the runs at water-undersaturated conditions

Sample Capsule Temp. Pres. Time Assembly Phase Composition

(jC) (GPa) (h)SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O NiO Total Mg#

EXP-38 Ag75Pd25 1025 0.5 30 Ol +En +Di + Olivine 40.71 0.00 0.24 0.29 8.26 0.00 49.46 0.28 0.19 0.00 0.56 100.00 91.43

Sp + Phl +Amp Enstatite 55.05 0.00 3.94 0.91 6.49 0.00 32.07 1.30 0.23 0.00 0.00 100.00 89.80

Diopside 52.32 0.38 4.45 1.13 3.31 0.00 17.51 20.62 0.28 0.00 0.00 100.00 90.42

Phlogopite 36.94 1.85 18.21 4.12 4.84 0.00 25.72 0.00 0.72 7.31 0.30 100.00 90.46

EXP-44 Ag75Pd25 1050 0.5 12 Ol +En +Di + Sp Olivine 41.07 0.00 0.34 0.27 8.09 0.00 49.57 0.25 0.18 0.00 0.22 100.00 91.61

Enstatite 55.18 0.00 3.64 1.05 5.98 0.06 32.66 1.25 0.17 0.00 0.00 100.00 90.68

Diopside 52.63 0.46 3.53 1.15 3.02 0.00 18.32 20.71 0.19 0.00 0.00 100.00 91.52

EXP-22 Ag50Pd50 1075 0.5 11 Ol +En +Di + Sp Olivine 41.62 0.00 0.00 0.00 7.87 0.00 50.18 0.15 0.18 0.00 0.00 100.00 91.91

Enstatite 55.56 0.00 3.98 0.90 5.20 0.00 33.14 1.21 0.00 0.00 0.00 100.00 91.90

Diopside 53.36 0.45 2.49 0.90 2.55 0.00 18.63 21.40 0.21 0.00 0.00 100.00 92.86

EXP-37 Ag75Pd25 1025 1.0 14 Ol +En +Di + Olivine 41.12 0.00 0.29 0.00 9.70 0.00 48.53 0.17 0.18 0.00 0.00 100.00 89.91

Sp + Phl +Amp Enstatite 54.84 0.00 4.24 1.06 6.13 0.00 32.56 1.18 0.00 0.00 0.00 100.00 90.45

Diopside 52.35 0.59 3.80 0.77 3.12 0.00 19.83 19.03 0.41 0.11 0.00 100.00 91.90

Phlogopite 36.84 1.84 18.30 4.17 4.84 0.00 25.50 0.00 0.80 7.31 0.39 100.00 90.37

EXP-29 Ag75Pd25 1050 1.0 14 Ol +En +Di + Olivine 41.16 0.00 0.26 0.09 9.42 0.00 48.72 0.19 0.17 0.00 0.00 100.00 90.21

Sp + Phl +Gl Enstatite 54.04 0.15 4.29 0.92 5.96 0.06 33.13 1.34 0.11 0.00 0.00 100.00 90.82

Diopside 51.98 0.58 4.64 1.13 2.72 0.00 17.35 21.19 0.40 0.00 0.00 100.00 91.91

Phlogopite 40.37 1.97 17.04 2.42 4.07 0.00 24.96 0.12 0.82 7.97 0.27 100.00 91.68

EXP-29# Ag75Pd25 1100 / 1.0 14 Ol +En +Di + Olivine 41.02 0.00 0.20 0.11 9.37 0.00 48.89 0.23 0.18 0.00 0.00 100.00 90.38

1050 Sp + Phl +Gl Enstatite 54.00 0.20 4.25 0.90 5.93 0.04 33.28 1.30 0.10 0.00 0.00 100.00 90.99

Diopside 51.93 0.55 4.70 1.10 2.75 0.00 17.40 21.17 0.35 0.00 0.05 100.00 91.93

Phlogopite 40.50 1.95 17.00 2.45 4.05 0.00 24.99 0.07 0.70 7.99 0.30 100.00 91.74

EXP-17 Ag75Pd25 1075 1.0 15 Ol +En +Di + Olivine 41.72 0.00 0.45 0.13 8.82 0.00 48.27 0.27 0.00 0.00 0.33 100.00 90.70

Sp +Gl Enstatite 55.37 0.07 3.79 0.86 6.03 0.00 32.59 1.22 0.02 0.00 0.06 100.00 90.60

Diopside 53.63 0.24 2.34 0.68 3.53 0.00 18.61 20.65 0.31 0.00 0.00 100.00 90.44

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EXP-18 Ag50Pd50 1100 1.0 10 Ol + En +Di + Olivine 41.83 0.00 0.15 0.15 4.18 0.07 53.46 0.16 0.00 0.00 0.00 100.00 95.80

Sp +Gl Enstatite 56.03 1.18 3.52 0.75 5.05 0.00 33.46 0.00 0.00 0.00 0.00 100.00 92.19

Diopside 53.20 0.26 3.17 0.97 2.13 0.00 18.46 21.51 0.29 0.00 0.00 100.00 93.92

EXP-21 Ag50Pd50 1125 1.0 11 Ol + En +Di + Olivine 41.82 0.00 0.12 0.27 3.07 0.00 54.15 0.17 0.23 0.00 0.18 100.00 96.92

Sp +Gl Enstatite 56.47 0.12 3.11 0.91 3.79 0.00 33.66 1.57 0.30 0.06 0.00 100.00 94.05

Diopside 52.98 0.34 3.66 1.18 2.26 0.00 18.43 20.60 0.54 0.00 0.00 100.00 93.55

EXP-20 Ag50Pd50 1150 1.0 11 Ol + En +Di + Olivine 42.66 0.00 0.00 0.00 1.87 0.00 55.21 0.16 0.10 0.00 0.00 100.00 98.13

Sp +Gl Enstatite 56.96 0.11 2.63 0.84 2.94 0.00 34.82 1.45 0.25 0.00 0.00 100.00 95.47

Diopside 52.77 0.52 4.04 1.16 2.21 0.00 17.85 21.16 0.31 0.00 0.00 100.00 93.50

EXP-08 Ag75Pd25 1100 1.5 8 Ol + En +Di + Sp + Olivine 40.86 0.00 0.60 0.00 8.55 0.00 49.28 0.10 0.12 0.00 0.49 100.00 91.13

Phl +Amp Enstatite 50.27 0.00 3.21 0.00 6.86 0.00 38.50 0.96 0.20 0.00 0.00 100.00 90.91

Diopside 51.99 0.54 5.41 2.01 2.96 0.08 18.17 19.33 0.46 0.05 0.00 100.00 91.61

Phlogopite 42.45 1.35 17.63 1.19 3.15 0.00 24.77 0.00 0.59 8.64 0.23 100.00 93.33

EXP-09 Ag50Pd50 1125 1.5 10 Ol + En +Di + Sp + Olivine 41.64 0.00 0.55 0.00 9.25 0.00 47.36 0.20 0.26 0.00 0.74 100.00 90.12

Phl +Amp Enstatite 55.38 0.00 4.06 0.78 5.08 0.00 33.55 1.16 0.00 0.00 0.00 100.00 92.17

Diopside 52.43 0.41 4.80 1.29 3.05 0.29 17.87 19.12 0.75 0.00 0.00 100.00 91.26

Phlogopite 42.38 1.31 16.47 1.17 3.47 0.00 26.60 0.00 0.45 7.85 0.31 100.00 93.18

EXP-07 Ag50Pd50 1150 1.5 23 Ol + En +Di + Olivine 44.77 0.02 0.67 0.29 1.64 0.03 52.23 0.34 0.01 0.00 0.00 100.00 98.29

Sp +Gl Enstatite 56.32 0.11 3.59 1.14 2.47 0.02 34.93 1.37 0.05 0.00 0.00 100.00 96.22

Diopside 52.90 0.40 4.73 1.45 1.01 0.00 19.34 19.82 0.36 0.00 0.00 100.00 97.19

EXP-07# Ag50Pd50 1200 / 1.5 23 Ol + En +Di + Olivine 44.70 0.02 0.70 0.30 1.60 0.00 52.40 0.28 0.00 0.00 0.00 100.00 98.33

1150 Sp +Gl Enstatite 56.30 0.13 3.61 1.28 2.30 0.00 34.98 1.40 0.00 0.00 0.00 100.00 96.48

Diopside 52.77 0.45 4.80 1.46 1.05 0.00 19.50 19.77 0.20 0.00 0.00 100.00 97.10

EXP-10 Ag50Pd50 1175 1.5 9 Ol + En +Di + Olivine 41.83 0.00 0.63 0.00 6.27 0.00 51.14 0.00 0.13 0.00 0.00 100.00 93.56

Sp +Gl Enstatite 54.84 0.08 4.11 0.78 5.90 0.00 32.72 1.27 0.18 0.00 0.12 100.00 90.82

Diopside 52.99 0.32 3.87 0.87 2.96 0.00 18.45 20.15 0.39 0.00 0.00 100.00 91.74

Abbreviations are the same as in Table 2.

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Fig. 1. Experimental determination of the water-saturated solidus and melting interval of ‘metasomatized NHD peridotite � 60% olivine’, and

above-solidus stability of pargasite and phlogopite. Data of below and including 1.5 GPa are from this study. Data of above 1.5 GPa are from

Mengel and Green (1989).

R.V. Conceic�ao, D.H. Green / Lithos 72 (2004) 209–229218

the stoichiometry and lowers the Mg# of the phase.

Under the experimental conditions of our experi-

ments, equilibrated assemblages have Mg#CpxcMg#Phlog>Mg#Opx>Mg#Ol unless Fe loss perturbs this

relationship so that Mg#Ol>Mg#Opx or Mg#Cpx.

Quench amphibole and phlogopite have low Mg#.

Quench rims on clinopyroxene also have high Al2O3

and low Mg#.

3.2. Solidus and the stability of amphibole and

phlogopite under water-saturated conditions

Above the solidus, melt appears coexisting with

olivine, orthopyroxene, clinopyroxene, spinel, parga-

site and phlogopite in water-saturated condition

experiments. At 0.5 GPa, the solidus temperature is

between 950 and 975 jC, and both pargasite and

phlogopite are eliminated between 975 and 1000 jC.However, at 1 and 1.5 GPa, runs at 1000 jC do not

appear to contain melt, and the solidus is inferred to

lie between 1000 and 1025 jC. Mengel and Green

(1989) inferred that the water-saturated solidus lies

between 975 and 1000 jC at both 1.5 and 2.5 GPa.

However, amphibole, together with phlogopite,

remains in both 1000 jC runs, and there is no obvious

shift in Mg# indicating quench phases in the 2.5 GPa,

1000 jC data of Mengel and Green (1989—Fig. 7.2,

Table 7.2). We have therefore drawn the water-satu-

rated solidus at 1000 jC from 1.5 to 2.5 GPa. The

nature of melting reactions changes along the solidus

with increasing pressure. Attention is drawn to the

different slopes of the phlogopite-out and pargasite-

Fig. 2. Experimental determination of the solidus for water-undersaturated melting of ‘metasomatized NHD peridotite� 60% olivine’ and

above-solidus stability of pargasite and phlogopite. Data of below and including 1.5 GPa are from this study. Data of above 1.5 GPa are from

Mengel and Green (1989). (j) at 2.8 GPa indicate the determined liquid compositions. The shaded arrow (2.0 to 1.0 GPa) indicates a diapiric

upwelling path to yield a melt + lherzolite residue at 1.0 GPa, 1075 jC.

R.V. Conceic�ao, D.H. Green / Lithos 72 (2004) 209–229 219

out curve with the implication that at low pressures

(0.5 GPa), near-solidus liquids are K-rich, whereas at

higher pressures, some K2O remains in the solid

phases and near-solidus liquids may have lower K/

Na ratios at high pressure (see also Mengel and Green,

1989).

Mineral compositions are presented in Table 2 and

Fig. 3. Pargasite crystals were very small to obtain

analyses that are free of overlap with the neighbouring

phases. We are therefore unable to show whether the

K/Na (atomic ratio) of pargasite coexisting with

phlogopite + lherzolite mineralogy varies as a function

of pressure from the value of 0.45 found by Mengel

and Green at 2.5–2.8 GPa, 1000–1150 jC. Phlogo-

pite analysis was more successful with analyses

showing >8.5% K2O approaching expected stoichi-

ometry. Natural rock observations and prior experi-

mental work show that, in the absence of significant

Fe+ 3, the Mg# (100 Mg/(Mg + Fe)) of coexisting

phases decreases in the sequence phlogopite>diopsi-

de>enstatite>olivine>garnet>spinel (Cr–Al spinels).

For those phlogopite compositions with >8.5% K2O,

this relationship holds (Figs. 3 and 4). Other analyses,

with lower Mg# and V 8% K2O, appear to have

overlap, probably with quench outgrowth or glass.

Spinel analyses are not reported in Table 2 as

grains were very small to avoid overlap with neigh-

bouring crystals or quench. Clinopyroxene and

Fig. 3. Variation of the Mg# of the phases analysed in water-

saturated (at and below solidus) conditions are plotted against the

temperature of the experiment. Shaded band shows solidus; (z)

indicates pargasite-out, and (#) indicates phlogopite-out condition.

Fig. 4. Variation of the Mg# of the phases analysed in experiments

under water-undersaturated conditions are plotted against the

temperature of the experiment (symbols as in Fig. 3).

R.V. Conceic�ao, D.H. Green / Lithos 72 (2004) 209–229220

orthopyroxene analyses show some broad systematic

changes in composition with pressure and tempera-

ture. However, as the temperature and pressure

ranges are not large enough and mineral grain size

limits the electron microprobe analyses, we cannot

define compositional changes with respect to pres-

sure and temperature. Most assemblages show the

expected Mg/Fe partitioning noted above but there

are notable exceptions. These are attributed to Fe

loss from sample to capsule wall. Fe loss increases

with temperature, and time and different phases

respond to Fe loss at different rates. Mg# of glass

increases more rapidly than other phases, followed

by olivine, orthopyroxene and clinopyroxene. The

effect is very clear on the Ag50Pd50 capsule at 1.5

GPa, 1125 jC, 12 h (experiment 33) while appar-

ently not affecting the runs in Ag75Pd25 at 1100 jC(experiment 27) or lower temperatures.

3.3. Solidus and the stability of amphibole and

phlogopite under water-undersaturated conditions

In the water-undersaturated experiments, the soli-

dus is that of a phlogopite + pargasite lherzolite with-

out a water-rich fluid phase (but see Niida and Green,

R.V. Conceic�ao, D.H. Green / Lithos 72 (2004) 209–229 221

1999 for a closer examination of the ‘dehydration

solidus’ and variation of pargasite composition as a

function of pressure and temperature). At 1.5 GPa, our

experiments agree with those of Mengel and Green

(1989) with pargasite breakdown and solidus close to

1125 jC. Both hydrous phases disappear close to the

solidus. However, at 1.0 and 0.5 GPa, the solidus is

unexpectedly at much lower temperature (1025 jC),and both phlogopite and pargasite appear to dehydrate

in favour of hydrous melt at the solidus (0.5 GPa) or

close to the solidus. At 1.0 GPa, pargasite disappears

before phlogopite. As established by the earlier

studies, the temperature interval in which phlogo-

pite + garnet lherzolite coexists with a liquid increases

with pressure above 1.5 GPa and particularly above

2.5 GPa, where pargasite stability is very strongly

pressure-limited.

Mineral compositions for experiments in water-

undersaturated conditions are presented in Table 3

and Fig. 4. Above the solidus, olivine, enstatite and

diopside are anhedral to subhedral with phlogopite as

poikilitic plates. Iron loss to capsule-wall is noted in

experiments 18, 20 and 21 at 1 GPa (Ag50Pd50) and is

reflected most clearly in the increase of Mg# of olivine

to values above coexisting enstatite, diopside or

phlogopite, i.e., a disequilibrium situation. Our data

does not show the systematic changes in Al2O3

solution in pyroxenes or in mutual solubility of

orthopyroxene/clinopyroxene pairs which should be

apparent in the pressure and temperature field. We

attribute this to fine grain size and overlapping quench

rims on some pyroxenes. Furthermore, the data is not

of sufficient quality to augment the existing data sets

for variations in spinel lherzolite mineralogy as func-

tions of pressure and temperature.

The attempts at characterisation of pargasite

showed TiO2 contents of f 1.5%, of K2O around

1.1% and of Na2O around of 2.1%—suggesting a

K/Na atomic ratio of 0.35 for pargasite coexisting

with phlogopite at 0.5–1.5 GPa, Tz 1000 jC.Further work on this is required before significance

can be attached to the difference from the value of

0.45 obtained by Mengel and Green (1989) as

noted previously.

The absence of pyrope garnet in our experiments is

consistent with the experiments of Mengel and Green

(1989). Its absence at lower temperatures at 1.5 GPa

may be attributed to the presence of pargasite + phlo-

gopite in P, T fields where garnet would appear in

anhydrous compositions. A similar conclusion applies

to the presence of feldspar at < 1 GPa for anhydrous

compositions.

4. Determination of melt composition from

potassium-enriched lherzolite composition near

phlogopite-saturation at 1 GPa

Mengel and Green (1989) showed that the potas-

sium-enriched NHD peridotite yielded an olivine

nephelinite liquid in equilibrium with phlogopite,

garnet, clinopyroxene, orthopyroxene and olivine

(phlogopite-bearing garnet lherzolite) at 2.8 GPa,

1195 jC and a more potassium-rich olivine nephelin-

ite liquid at 2.8 GPa, 1250 jC beyond the phlogopite-

out melting reaction (i.e., garnet lherzolite residue;

open squares in Fig. 5). The K/Na (atomic) ratios of

the liquids were 0.51 (1195 jC) and 0.66 (1250 jC)from a bulk composition (NHD peridotite + basanite)

with K/Na of 0.53, consistent with the retention of

K2O and Na2O in phlogopite and clinopyroxene

(0.85% Na2O) at 1195 jC and the retention of Na2O

only in clinopyroxene (0.7% Na2O) at 1250 jC.Thibault et al. (1992) found a very similar potassic

olivine nephelinite in equilibrium with phlogopite–

garnet lherzolite mineralogy at 3.0 GPa, 1225 jC (star

in Fig. 5). Although, with a very similar degree of

silica undersaturation to the compositions of Mengel

and Green (1989) at 1195 and 1250 jC, this liquid

composition was much more potassic with 5.5% K2O

and a K/Na of 3.4.

In contrast to these alkali-rich and calcium-rich

olivine nephelinite liquids, the liquid composition at

the vapour-saturated solidus of phlogopite + olivi-

ne + enstatite in the K2O–MgO–Al2O3–SiO2–H2O

system at 2.8 GPa is olivine + hypersthene normative

at a temperature of 1160 jC (inverted solid triangle in

Fig. 5; Foley et al., 1986a). With decreasing water

content, the piercing point with olivine + enstatite + a

potassic phase moves to the silica-undersaturated but

low-olivine field (Lc62 Qz32 Ol6) at 1460 jC (open

inverted triangle in Fig. 5). Small amounts of water

and, in particular, the appearance of a hydrous potas-

sic phase stable to the peridotite solidus have a large

effect on both the temperature and composition of

melt. In the simple system, at a fixed pressure (2.8

Fig. 5. System (Lc + Jd +CaTs)–Ol–Di–Qz (leucite + jadeite +Ca–

Tschermakite–olivine–diopside–quartz) with various liquid com-

positions obtained in experimental studies of liquids in equilibrium

with a peridotitic (lherzolitic or harzburgitic) residue. Large (.) is‘Hawaiian pyrolite � 40% olivine’ composition and large (o) is

melt at 1 GPa, 1100 jC, water-saturated (Green, 1976). The

anhydrous melting curve for Hawaiian pyrolite at 1 GPa (Green et

al., 2001) is shown as a dot–dash line illustrating the shift to more

silica-rich compositions effected by addition of water. Large (n) is

the ‘metasomatized NHD � 60% olivine’ composition and large

(5) are melts derived at 2.8 GPa (Mengel and Green, 1989) under

water-saturated (on the right) and water-understaturated (on the left)

conditions. Large ( w ) are melts obtained from this study derived

from ‘metasomatized NHD � 60% olivine’ at 1 GPa under water-

saturated condition (on the right) and water-undersaturated

condition (on the left; Table 4). Small (5) are compositions with

liquidus ol + opx (on the left), and Ol +Opx +Cpx (on the right) at 1

GPa from Tatsumi (1981, 1982)—see text. Minimum melts at 1 GPa

in Jd + Fo +Qz system [small (o)] and in Lc + Fo +Qz system

[small ( w )] are shown as anhydrous composition. Both are linked

by a dotted line. Minimum melt at 1 GPa in the Jd + Fo +Qz

system+ 10% diopside in small (D) is also shown as (Nimis et al.,

unpublished data). An alkali (Na2O +K2O)-rich melt in equilibrium

with lherzolite at 1.1 GPa obtained by experiments of Draper and

Green (1999) is represented as a small (.). The shift [dotted lines

that join large (q) and (z)] of the minimum melt in the

Lc + Fo +Qz system at 2.8 GPa from anhydrous [large (q)] to

water-saturated compositions [large (z)] is also shown (Gupta and

Green, 1988). The large (B) is the melt composition from Thibault

et al. (1992) in equilibrium with garnet lherzolite residue at 3 GPa

(see text).

R.V. Conceic�ao, D.H. Green / Lithos 72 (2004) 209–229222

GPa), the invariant points Fo–Ens–Phl–Liq–Vapour

(f 1160 jC) and Fo–Ens–San–Liq (1460 jC—anhydrous) are connected by univariant lines (Fo–

Ens – Phl – Liq and Fo –Ens – San –Liq) passing

through (Fo–Ens–San–Phlog–Liq), and along which

water contents in the liquid decrease from approxi-

mately 30 to 0 wt.%.

In the present study, we wished to determine the

composition of the melt from the NHD peridotite

composition at 1 GPa and at a temperature very close

to the phlogopite-out boundary. Our objective was to

evaluate whether silica-oversaturated (quartz-norma-

tive) potassium-rich magmas could be derived direct-

ly from the model metasomatized lherzolite

composition by dehydration melting of phlogopite

lherzolite at 1 GPa.

The experiments described in the previous section

shows that the phlogopite lherzolite contains 0.35–

0.4% H2O in amphibole and phlogopite and has a

solidus at 1 GPa, f 1000 jC (fluid-present) or 1040

jC (fluid-absent, ‘‘dehydration-melting’’). Amphibole

disappears at the solidus, but phlogopite remains

present above the solidus for a small temperature

interval. Amphibole and phlogopite are absent at

1075 jC in both sets of experiments. Because of

quenching problems (Green, 1976), it is not possible

to obtain equilibrium melt compositions directly at 1

GPa, 1075 jC, and thus, a method of estimating melt

composition using small-area scans and mass balance

constraints was used (see Mengel and Green, 1989).

Experiments 17 and 32 were chosen for the deter-

mination of melt composition as both are close to, but

just above, the phlogopite breakdown curve at 1 GPa;

both are water-undersaturated (no free fluid phase)

and appear to have around 15–30% melt. Considering

the inferred water content of f 0.35–0.4% H2O in

experiment 17 and the admixed water content of

f 1% H2O in experiment 32, both melts are inferred

to contain 2–5 wt.% H2O using the estimated %

melting from scanning electron microscope images

of polished mounts. In order to determine the oxide

composition of the melts in experiments 17 and 32,

we followed the same methodology described in

Mengel and Green (1989) which consists of: (a)

plotting the area-scan analyses of quenched glass +

crystals in Mg# vs. oxide diagrams; (b) calculating

regression lines for each Mg# vs. oxide diagrams; (c)

using the composition of olivine in each of the two

R.V. Conceic�ao, D.H. Green / Lithos 72 (2004) 209–229 223

runs and a (KDFe/Mg)Ol/Liq = 0.3 (considered for mantle

olivine in equilibrium with melt) in order to estimate

the Mg# for liquid in experiments 17 and 32 which

resulted in 74.5 and 75.3, respectively; (d) reading

each oxide composition of the melts by crossing the

regression lines for each oxide plotted against the Mg#

calculated in item c.

Both calculated melt compositions have low Na2O

and K2O contents and a K/Na ratio much higher than

the starting material. This is not expected as Na2O

contents of clinopyroxene are small (0.1–0.2%) and

K2O in clinopyroxene, and Na2O in orthopyroxene,

olivine and garnet are negligible. We believe that the

hydrous glasses have lost Na2O under electron-beam

excitation and, accordingly, corrections were made to

the analyses by assuming that K2O was correct and

the melt had K/Na matching the starting material.

These estimated compositions of the quenched melts

in experiments 17 and 32 (Table 4) were then pre-

pared as glasses and used in ‘sandwich experiments’,

equilibrating the estimated melt with a layer of

‘metasomatized NHD peridotite� 60% olivine’.

Sandwich experiments have the goal of equilibrat-

ing the estimated melt composition with the melt and

residual phases in the ‘forward’ (peridotite melting)

experiment. In the ideal case, the sandwich experi-

ment increases the proportion of equilibrium melt in

an experiment and thus enables large-melt pools to

quench to glass without modification by quench

minerals. In the real case, the estimated melt compo-

sition will be imperfect and will dissolve or precipitate

small amounts of the crystalline phases of the perido-

tite layer to reach equilibrium composition. If the

residual phases are of a closely similar composition

in both the forward experiment and the sandwich

experiment, then the analysed liquid in the estimated

melt layer is taken as the equilibrium melt for the P, T

and peridotite bulk composition.

4.1. Results of experiments

The compositions of estimated melt, corrected

melt and residual phases in the forward melting

experiments 32 and 17 are compared with the ana-

lysed melt layer and residual crystals of the peridotite

layer in sandwich experiments 50 and 56, respective-

ly (Table 4). Spinel was present in all experiments

but grains were very small for analysis. The compar-

isons between mineral compositions are very close

with the olivine and pyroxenes of experiments 32 and

50, consistently indicating a higher melt fraction than

those of experiments 17 and 56. In experiments 17

and 56, this is shown in olivine by lower Mg#, in

orthopyroxene by lower Mg#, higher Al2O3 content

and lower Cr/Al, and in clinopyroxene by lower Mg#,

higher Na2O, Al2O3 and TiO2, and lower Cr/Al

(particularly in experiment 56). Mass balance calcu-

lations yield estimates of 20% melt in experiment 32

and 12.5% melt in experiment 17.

Both sets of experiments are at 1 GPa, 1075 jC.and the higher melt fraction in experiments 32 and 50

reflects the higher water content of the starting mate-

rials with the estimated water contents of the melt in

experiment 32 (experiment 50) being f 4.5 wt.% and

that in the melt of experiment 17 (experiment 56)

being f 3 wt.%—both are water-undersaturated at 1

GPa, 1075 jC.Applying these melt and mineral compositions to

the model metasomatized mantle composition (i.e.,

‘‘NHD peridotite + 1.5% phlogopite’’) then the modal

mineralogy calculated for the model metasomatized

peridotite, using the phases of experiment 56, is:

5% meltþ 73% olivineþ 14% orthopyroxene

þ 6% clinopyroxeneþ 2% spinel:

The modal mineralogy calculated for peridotite

melting from experiment 50 is:

8% meltþ 73% olivineþ 13% orthopyroxene

þ 4% clinopyroxeneþ 2% spinel:

The melt compositions (calculated anhydrous)

have SiO2 contents z 57%, Al2O3 contents >18%,

MgO contents < 8%, and CaO contents of 9.3%

(experiment 50) and 6.0% (experiment 56). Their

normative compositions place them as basaltic andes-

ite (experiment 50) to trachy-andesite (experiment

56). However, both experiments 50 and 56 have

K2O>Na2O (wt.%) [K/Na (atomic) = 0.87 and 0.93,

respectively] and are classified as ‘shoshonites’ with-

Table 4

Chemical composition of the best estimated liquid and some phases in the run EXP-32 and EXP-50 under water-saturated conditions, and EXP-

17 and EXP-56 at water-undersaturated conditions

Water-saturated condition Water-undersaturated condition

Composition of the best

estimated liquid

Composition of

the liquid after

sandwich run

Composition of the best

estimated liquid

Composition of

the liquid after

sandwich run

EXP-32 EXP-50a EXP-17 EXP-56a

Measured

valuesbCorrected

liquidc75%NHD+ 25%

corrected liquid

Measured

valuesbCorrected

liquidc50%NHD+ 50%

corrected liquid

Average S.D. Average S.D.

SiO2 61.01 60.68 56.98 0.26 57.31 56.91 58.07 0.28

TiO2 0.60 0.59 0.76 0.08 0.91 0.90 0.93 0.05

Al2O3 20.42 20.31 18.89 0.23 19.22 19.08 18.45 0.27

Cr2O3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

FeO 2.61 2.60 3.81 0.27 3.86 3.83 3.23 0.27

MnO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

MgO 4.86 4.83 6.85 0.85 8.29 8.23 7.25 0.70

CaO 8.14 8.10 9.32 0.67 6.04 6.00 5.96 0.67

Na2O 0.78 1.32 1.40 0.17 1.61 2.30 2.64 0.15

K2O 1.58 1.57 1.99 0.45 2.76 2.74 3.47 0.40

NiO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Total 100.00 100.00 100.00 0.52 100.00 100.00 100.00 0.56

Mg# 76.83 76.83 76.23 1.46 79.30 79.30 80.00 1.46

K2O/Na2O 2.03 1.19 1.42 1.71 1.19 1.31

Olivine

SiO2 40.94 40.68 0.25 41.72 40.90 0.86

TiO2 0.00 0.00 0.00 0.00 0.00 0.00

Al2O3 0.39 0.24 0.26 0.45 0.62 0.26

Cr2O3 0.44 0.60 0.44 0.13 0.32 0.29

FeO 8.63 8.69 0.16 8.82 9.31 0.36

MnO 0.00 0.03 0.06 0.00 0.00 0.00

MgO 49.17 48.74 0.66 48.27 48.00 0.63

CaO 0.17 0.22 0.05 0.27 0.39 0.28

Na2O 0.04 0.24 0.04 0.00 0.06 0.08

K2O 0.00 0.00 0.00 0.00 0.00 0.00

NiO 0.22 0.57 0.08 0.33 0.40 0.20

Total 100.00 100.00 0.56 100.00 100.00 0.77

Mg# 91.03 90.90 0.26 90.70 90.19 0.35

Mg# Liq

(KD = 0.3)d

75.28 74.53

KD (Ol/Liq) 0.32 0.44

Enstatite

SiO2 56.07 55.91 0.35 55.37 54.76 0.84

TiO2 0.12 0.04 0.08 0.07 0.22 0.17

Al2O3 2.67 2.39 0.60 3.79 4.20 1.05

Cr2O3 1.00 1.52 0.79 0.86 0.41 0.41

FeO 5.26 5.41 0.31 6.03 6.72 1.03

MnO 0.00 0.00 0.00 0.00 0.00 0.00

MgO 32.99 32.97 0.18 32.59 32.03 0.48

CaO 1.78 1.65 0.20 0.22 1.48 0.39

Na2O 0.09 0.10 0.08 0.02 0.16 0.08

K2O 0.00 0.00 0.00 0.00 0.02 0.04

R.V. Conceic�ao, D.H. Green / Lithos 72 (2004) 209–229224

Water-saturated condition Water-undersaturated condition

Composition of the best

estimated liquid

Composition of

the liquid after

sandwich run

Composition of the best

estimated liquid

Composition of

the liquid after

sandwich run

EXP-32 EXP-50a EXP-17 EXP-56a

Measured

valuesbCorrected

liquidc75%NHD+ 25%

corrected liquid

Measured

valuesbCorrected

liquidc50%NHD+50%

corrected liquid

Average S.D. Average S.D.

Enstatite

NiO 0.00 0.00 0.00 0.06 0.00 0.00

Total 100.00 100.00 1.32 100.00 100.00 1.54

Mg# 91.79 91.57 0.43 90.60 89.50 1.48

Diopside

SiO2 53.68 53.79 0.76 53.63 51.75 1.52

TiO2 0.29 0.20 0.12 0.24 0.91 0.55

Al2O3 2.81 2.65 0.79 2.34 5.72 1.81

Cr2O3 0.94 1.00 0.15 0.68 0.61 0.53

FeO 2.89 2.74 0.34 3.53 3.24 0.65

MnO 0.00 0.06 0.12 0.00 0.00 0.00

MgO 18.86 18.41 0.69 18.61 17.34 1.61

CaO 20.37 20.85 0.70 20.65 19.93 0.78

Na2O 0.12 0.24 0.06 0.31 0.41 0.08

K2O 0.03 0.06 0.07 0.00 0.08 0.07

NiO 0.00 0.00 0.00 0.00 0.00 0.00

Total 100.00 100.00 0.79 100.00 100.00 1.94

Mg# 92.07 92.29 1.01 90.44 90.53 1.43

Mode

Melt 30.11 29.08 22.97 17.64

Olivine 50.44 51.66 43.39 44.71

Cpx 10.84 8.48 14.69 21.22

Opx 8.62 10.78 18.95 16.43

S.D.—standard deviation error; Res. Sum Sq.—residue sum square. Values are normalized to 100%.a The liquid composition of the run EXP-50 and EXP-56 corresponds to the average of large-area analyses of the layer in the sandwich runs.b The measured value was obtained after correlation between the average of the large-area analyses of oxides and Mg# (see text).c The corrected values correspond to the liquid composition corrected to a 1.19 K2O/Na2O ratio.d Mg# of the liquid calculated for a (KD)Ol/Liq = 0.3.

Table 4 (continued)

R.V. Conceic�ao, D.H. Green / Lithos 72 (2004) 209–229 225

in the broad andesite classification. TiO2 contents are

low, and, in major element terms, both liquids are

well matched to the compositions in shoshonite suites

of island arcs/convergent margins. With olivine,

orthopyroxene, clinopyroxene and chrome-spinel as

residual phases, relative fractionation among incom-

patible elements and high-field strength elements

would principally reflect residual clinopyroxene.

The absence of residual phlogopite, pargasite, garnet

or plagioclase suggests that incompatible element and

high-field strength element ratios of liquids formed in

this way would largely reproduce those of the meta-

somatized source.

5. Comparisons with previous works

Previous experimental studies have sought to es-

tablish the compositions of melts formed by melting

of lherzolite at various pressures in the presence of

water, including both water-saturated and water-un-

dersaturated melts. Emphasis in this study is on melts

R.V. Conceic�ao, D.H. Green / Lithos 72 (2004) 209–229226

hat have significant K2O contents and high K/Na

ratios. Green (1976) explored the problem of melt

determination in lherzolite melting experiments, in

which quench crystallisation modified the melt/glass

compositions. Use of melt estimation and addition of

potential liquidus phases (i.e., similar to the sandwich

technique) established the melt composition in the

Hawaiian pyrolite (i.e., an enriched lherzolite with K/

Na f 0.17) at 1 GPa, 1100 jC under water-saturated

conditions with residual olivine, orthopyroxene, cli-

nopyroxene and spinel. Both the experimental work

and the projection of melt and source compositions

into the basalt tetrahedron (Fig. 5) establish that the

liquid composition (with K/Na f 0.17) was close to

both orthopyroxene-out and clinopyroxene-out bound-

aries. Mass balance calculations show that the melt

formed by 25% melting of the Hawaiian pyrolite

composition plots in the quartz + feldspar + orthopyr-

oxene + clinopyroxene volume and, therefore, is silica-

oversaturated (open circle in Fig. 5).

Tatsumi (1981, 1982) determined the liquidus

phases for natural high magnesian andesite to basalts

from the Setouchi region in Japan. These magmas

have been called ‘sanukitoids’ and are noteworthy in

having high Mg#, and magnesian olivine and ortho-

pyroxene phenocrysts, appropriate to primary melts of

mantle origin. They also have relatively high K/Na

ratios with the two compositions studied (TG1 and

SD261) having K/Na of 0.33 and 0.52, respectively.

Tatsumi (1981, 1982) found that TG1 was saturated

with olivine + orthopyroxene only at 1110 jC, 1.1

GPa with f 8% H2O in the melt and that SD261

was saturated with olivine, orthopyroxene and clino-

pyroxene at 1070 jC, 1.0 GPa with the melt contain-

ing approximately 7% H2O. Both compositions plot in

the quartz + feldspar + orthopyroxene + clinopyroxene

volume of the basalt tetrahedron, with SD261 plotting

appropriately for a melt leaving lherzolite residue, and

TG1 plotting appropriately for olivine + orthopyrox-

ene residue (small squares in Fig. 5).

The melt compositions obtained in hydrous sys-

tems with variable K/Na may be compared with those

in anhydrous systems at similar pressure and in

compositions with variable K/Na. Conceicao and

Green (2000) showed that the isobaric invariant point

(Lc–Ol–Ens–Liq) in the potassic system Lc + Fo +

Qz lay in the quartz-normative field (Lc38 Qz58 Ol4) at

1 GPa, 1265 jC (small open diamond in Fig. 5) in

sharp contrast with the isobaric invariant point (Ab–

Ol–Ens–Liq) in the system Jd + Fo +Qz which lies in

the undersaturated field at 1 GPa, 1220 jC, at Jd59.5Qz34.5 Ol6 (small open circle in Fig. 5). Addition of

diopside to the Jd + Fo +Qz system to produce liquids

along the univariant line olivine–clinopyroxene–

enstatite – sodic plagioclase– liquid yields liquids

which trend across the Fo +Qz+(Jd +CaTs) face at

similar ‘Qz’ values as projected from diopside (small

open triangle in Fig. 5). These liquids have f 10%

normative diopside (Nimis et al. unpublished and

Green et al., 2001; Fig. 5). Experiments by Draper

and Green (1999) obtained an alkali-rich liquid (small

solid circle in Fig. 5) in equilibrium with olivine,

orthopyroxene and clinopyroxene at 1.2 GPa, 1150

jC (run DGA-40 in their work) and approaching

anhydrous conditions, which has K/Na = 0.85. This

melt plots close to the most alkali-rich melts in the

sodic or sodium + calcium system in the projection

from diopside but is significantly lower in normative

diopside and in Ca/Al ratio (Fig. 5). Under anhydrous

conditions at 1 GPa, increasing K/Na to values

approaching 1.0, appears to have little effect on the

degree of silica saturation and alkali (Na +K) enrich-

ment but significantly lowers normative diopside

(decreases Ca/Al).

The effect of addition of water is evident in the

projections of Fig. 5. Melting occurs at lower temper-

atures and liquids are relatively enriched in both MgO

and SiO2 (enstatite component). As in the dry system,

melts with low K/Na, such as that K/Na = 0.18 from

the Hawaiian pyrolite composition, remain at f 15%

normative diopside in the projection from olivine

(open circle in Fig. 5). However, melts with higher

K/Na [SD261 (Tatsumi, 1982) with K/Na = 0.52] have

lower normative diopside, and this continues into the

more potassic liquids obtained in our experiments (K/

Na = 0.86 and 0.91). It appears significant that liquids

in equilibrium with residual olivine, orthopyroxene

and clinopyroxene with K/Na approaching 1 have less

than 5% normative diopside and approach the per-

aluminous chemical boundary (i.e., normative corun-

dum), whether anhydrous (Draper and Green, 1999)

or containing 3–7% H2O.

Our experiments define the melt composition close

to the phlogopite-dehydration lherzolite solidus and

the melts are much more alkali-poor, silica-rich (over-

saturated) and magnesium- and iron-rich than those

R.V. Conceic�ao, D.H. Green / Lithos 72 (2004) 209–229 227

from anhydrous lherzolite with similar K/Na ratio. A

similar result is observed in the K2O–MgO–Al2O3–

SiO2 system at 2.8 GPa, where the invariant point with

olivine + enstatite + sanidine + liquid at 1460 jC is at

Lc62 Qz32 Ol6 (Gupta and Green, 1988; inverted open

triangle in Fig. 5), but the invariant point (in the system

with H2O as an additional component) with olivi-

ne + enstatite + phlogopite + liquid + vapour at 1160

jC is approximately at Lc28 Qz42 Ol30 (olivine + hy-

persthene normativ; inverted solid triangle in Fig. 5).

The addition of CaO, Na2O, TiO2, etc. to yield natural

rock compositions moves the liquids in equilibrium at

2.8 GPa, f 1200–1250 jC with olivine + enstatite +

diopside + garnetF phlogopite (i.e., near ‘phlogopite-

out’ in the melting interval) to strongly silica-under-

saturated olivine-nephelinites to olivine-rich (Mengel

and Green, 1989—basanites—open squares in Fig. 5;

Thibault et al., 1992—star in Fig. 5), including liquids

with K/Na = 0.5, 0.65 and 3.5.

In complex compositions, some earlier studies

have explored the liquidus phase relationships of

natural potassium-rich magmas at high pressures with

H2O, CO2, CH4 and F as volatile components affect-

ing the stability of phlogopite on or near the liquidus

(Edgar et al., 1976; Ryabchikov and Green, 1978;

Foley et al., 1986a,b; Foley, 1989; Sweeney et al.,

1993). These studies have focused on melting at

higher pressures (z 3 GPa) and have collectively

established liquid compositions in equilibrium with

olivine + orthopyroxene + clinopyroxene + phlogopi-

teF fluid which are strongly silica-undersaturated for

CO2-rich (H2O+CO2), reduced C–H–O fluids, and

in fluorine-bearing systems. These studies have pro-

duced models for derivation of olivine leucitite (ugan-

dite) and olivine lamproite magmas as primary melts

from phlogopite-bearing lherzolite and/or phlogopite-

bearing wehrlite with conditions of melting around

2.5–3.5 GPa, temperatures of 1150 to 1300 jC. Boththe direct melting studies on K-enriched lherzolites

and the liquidus studies on K-rich magmas show that

melts derived at pressures f 3 GPa from phlogopite-

bearing lherzolite are olivine-rich, silica-undersaturat-

ed nephelinites to olivine leucitites or lamproites, with

the degree of silica undersaturation increasing with

dissolved carbon (CO3=) and fluorine (F� in the melt).

By contrast, our study shows that at 1 GPa, the

melts formed from fluid-undersaturated melting of

phlogopite lherzolite are silica-oversaturated at melt-

ing temperatures immediately above phlogopite dis-

appearance. Our experiments demonstrate that silica-

oversaturated shoshonitic magmas with K/Na>1 can

be derived from decompression and dehydration melt-

ing of phlogopite + pargasite lherzolite at pressures

around 1 GPa and temperatures of 1075 jC (Fig. 2).

6. Conclusions

Metasomatized lithosphere is characterised by var-

ious melt or fluid imprints superimposed on refractory

or depleted mantle lherzolite to harzburgite. Such

material is sampled within xenolith suites of kimber-

lites, intraplate basalts and, more rarely, by convergent

margin basalts. Typically pargasite, with variable K/

Na ratio, and/or phlogopite are minor phases resulting

from the metasomatic process. Samples of cratonic-

deep lithosphere sampled by kimberlites, potassic

richterite, and samples of the MARID suite (Dawson,

1987) provide evidence for potassic metasomatism

under conditions and in compositions in which parga-

site is not stable.

Our experimental study has explored the lower

pressure P, T field, in which lithospheric thinning or

mantle diapirism is expected to lead to decompression

melting of such metasomatized lithosphere. We have

established that decompression melting of metasom-

atized lithosphere at temperatures of 1050 to 1150 jCwill occur between 1 and 1.5 GPa (Fig. 2). Both

phlogopite and pargasite disappear very close to the

solidus with the result that the melt phase contains the

water from both phases. The combination of water,

K2O and other incompatible elements acting as fluxes

for melting means that the melt fraction formed close

to the solidus is quite large. In our chosen example

(Fig. 2), a metasomatized harzburgite composition

with f 0.14–0.16% H2O, 0.17% K2O and 0.14%

Na2O yields f 5% melt at 1 GPa, 1075 jC.The melt produced by decompression melting

leaves residual lherzolite (olivine + orthopyroxene +

clinopyroxene + spinel) and is a magnesian, silica-

oversaturated shoshonite (K2O>Na2O; wt.%). The

occurence of such magmas in some convergent mar-

gins may be a consequence of decompression melting

of previously metasomatized, phlogopite-bearing lith-

osphere beneath older continental crust. If emplaced in

the middle or upper crust, crystallisation of early

R.V. Conceic�ao, D.H. Green / Lithos 72 (2004) 209–229228

olivine and clinopyroxene joined by plagioclase would

be followed by reaction of olivine with liquid to pre-

cipitate phlogopite/biotite. Magmas of this character

are potential parent magmas for the batholithic potas-

sic syenite complexes of Brazil (Conceic�ao et al.,

2000). If this interpretation is correct, then their

emplacement would be a consequence of crust and

lithospheric thinning in the Early Proterozoic with

upwelling of subcratonic Proterozoic or Archean meta-

somatized lithosphere.

Acknowledgements

CAPES, process number BEX-2486/95-4, spon-

sored R.V. Conc� eicao. We thank members of the

Petrochemistry and Experimental Petrology Group at

RSES and members of the Electron Microscopy Unit,

RSBS (ANU) for fruitful discussions and helpful

assistance.

References

Baker, M.B., Hirschmann, M.M., Ghiorso, M.S., Stolper, E.M.,

1995. Compositions of near-solidus peridotite melts from

experiments and thermodynamic calculations. Nature 375,

308–311.

Boyd, F.R., England, J.L., 1960. Apparatus for phase equilibrium

measurements at pressures up to 50 kb and temperatures up to

1750 jC. J. Geophys. Res. 65, 741–748.Conceicao, R.V., Green, D.H., 2000. Behavior of the cotectic curve

En–Ol in the system leucite–olivine–quartz under dry condi-

tions to 2.0 GPa. Geochem. Geophys. Geosystems 1 (paper No.

200GC000071).

Conceicao, R.V., Nardi, L.V.S., Conceicao, H., 2000. The

Santanapolis syenite: genesis and evolution of Paleoprotero-

zoic shoshonitic syenites in northeastern Brazil. Int. Geol.

Rev. 42, 941–957.

Dawson, J.B., 1987. The MARID suite of xenoliths in kimberlite:

relationship to veined and metasomatised peridotite xenoliths,

465–473. In: Nixon, P.H. (Ed.), Mantle Xenoliths. Wiley-Inter-

science, New York, p. 844.

Draper, D.S., Green, T.H., 1999. P, T phase relations of silicic,

alkaline, aluminous liquids: new results and applications to

mantle melting and metasomatism. Earth Planet. Sci. Lett.

170, 255–268.

Edgar, A.D., Green, D.H., Hibberson, W., 1976. Experimental pet-

rology of a highly potassic magma. J. Petrol. 17, 339–356.

Foley, S.F., 1989. Experimental constraints on phlogopite chemistry

in lamproites: 1. The effect of water activity and oxygen fugac-

ity. Eur. J. Mineral. 1, 411–426.

Foley, S.F., Taylor, W.R., Green, D.H., 1986a. The effect of fluorine

on phase relationships in the system KA1SiO4–Mg2SiO4–SiO2

at 28 kbar and the solution mechanism of fluorine in silicate

melts. Contrib. Mineral. Petrol. 93, 46–55.

Foley, S.F., Taylor, W.R., Green, D.H., 1986b. The role of fluorine

and oxygen fugacity in the genesis of the ultrapotassic rocks.

Contrib. Mineral. Petrol. 94, 183–192.

Foley, S.F., Venturelli, G., Green, D.H., Toscani, L., 1987. The

ultrapotassic rocks: characteristics, classification and con-

straints for petrographic models. Earth Sci. Revs. 24, 81–134.

Green, D.H., 1973. Contrasted melting relations in a pyrolite upper

mantle under mid-oceanic ridge, stable crust and island arc en-

vironments. Tectonophysics 17, 285–297.

Green, D.H., 1976. Experimental testing of ‘equilibrium’ partial

melting of peridotite under water-saturated, high pressure con-

ditions. Can. Mineral. 14, 255–268.

Green, D.H., Falloon, T.J., Eggins, S.M., Yaxley, G.M., 2001. Pri-

mary magmas and mantle temperatures. Eur. J. Mineral. 13,

437–451.

Gupta, A.K., Green, D.H., 1988. The liquidus surface of the system

forsterite –kalsilite –quartz at 28 kb under dry conditions, in

presence of H2O and CO2. Mineral. Petrol. 39, 163–174.

Hirose, K., Kawamoto, T., 1995. Hydrous partial melting of lher-

zolite at 1 GPa: the effect of H on the genesis of basaltic mag-

mas. Earth Planet. Sci. Lett. 133, 463–473.

Mengel, K., Green, D.H., 1989. Stability of amphibole and phlo-

gopite in metasomatized peridotite under water-saturated and

water-undersaturated conditions. Fourth International Kimber-

lite Conference, Perth. Geol. Soc. Aust. Spec. Publ., vol. 14,

pp. 571–581.

Nehru, C.E., Wyllie, P.S., 1975. Compositions of glasses from St.

Paul’s peridotite partially melted at 20 kilobars. J. Geol. 83,

455–471.

Niida, K., Green, D.H., 1999. Stability and chemical composition of

pargasitic amphibole in MORB pyrolite under upper mantle

conditions. Contrib. Mineral. Petrol. 135, 18–40.

Pla Cid, J., Nardi, L.V.S., Stabel, L.Z., Conceicao, R.V., Balzar-

etti, N.M., 2003. High-pressure minerals in mafic microgranu-

lar enclaves: evidences for co-mingling between lamprophyric

and syenitic magmas at mantle conditions. Contrib. Mineral.

Petrol. 145, 444–459.

Ryabchikov, I.E., Green, D.H., 1978. The role of carbon dioxide

in the petrogenesis of highly potassic magmas. Problems of

the Petrology of the Earth’s Crust and Upper Mantle. Trudi,

Inst. Geol. and Geophys. Akad. Nauk., USSR, Siberian Re-

gion, pp. 49–64.

Sweeney, R.J., Thompson, A.B., Ulmer, P., 1993. Phase relations of

a natural MARID composition and implications for MARID

genesis, lithospheric melting and mantle metasomatism. Con-

trib. Mineral. Petrol. 115, 225–251.

Tatsumi, Y., 1981. Melting experiments in a high-magnesian ande-

site. Earth Planet. Sci. Lett. 54, 357–365.

Tatsumi, Y., 1982. Origin of high magnesian andesites in the Se-

touchi volcanic belt, southwest Japan II: melting experiments at

high pressures. Earth Planet. Sci. Lett. 60, 305–317.

Thibault, Y., Edgar, A.D., Lloyd, F.E., 1992. Experimental inves-

tigation of melts from carbonated phlogopite lherzolite: impli-

R.V. Conceic�ao, D.H. Green / Lithos 72 (2004) 209–229 229

cations for metasomatism in the continental lithospheric mantle.

Am. Mineral. 77, 784–794.

Wallace, M.E., Green, D.H., 1991. The effect of bulk rock compo-

sitions on the stability of amphibole in the upper mantle: im-

plications for solidus positions and mantle metasomatism.

Mineral. Petrol. 44, 1–19.

Walter, M.J., Presnall, D.C., 1994. Melting behavior of simplified

lherzolite in the system CaO–MgO–Al2O3–SiO2–Na2O from

7 to 35 kbar. J. Pet. 35, 329–360.

Wedepohl, K.H., 1985. Origin of the tertiary basaltic volcanism in

the northern Hessian depression. Contrib. Mineral. Petrol. 89,

122–143.