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Lithos 101 (2008

Magma–carbonate interaction: An experimental study onultrapotassic rocks from Alban Hills (Central Italy)

Carmela Freda a,⁎, Mario Gaeta b, Valeria Misiti a, Silvio Mollo c,Daniela Dolfi c, Piergiorgio Scarlato a

a Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Rome, Italyb Dipartimento di Scienze della Terra, Sapienza Università di Roma, P. le Aldo Moro 5, 00185 Rome, Italy

c Dipartimento di Scienze Geologiche, Università degli Studi Roma Tre, Largo San Leonardo Murialdo 1, 00146 Rome, Italy

Received 5 February 2007; accepted 6 August 2007Available online 16 August 2007

Abstract

TheAlbanHills ultrapotassic volcanic district is one of themain districts emplaced duringQuaternary time along the Tyrrhenianmarginof Italy. Alban Hills lava flows and scoria clasts are made up essentially of clinopyroxenes and leucites and their chemical composition ismostly K-foiditic. Differentiated products (MgOb3 wt.%) are characterised by low SiO2 concentration (b50 wt.%) and geochemicalfeatures indicate that this unique differentiation trend is driven by crystal fractionation plus carbonate crust interaction. Notably, the AlbanHills Volcanic District was emplaced into thick limestone units. With the aim of constraining the magmatic differentiation, we performedexperiments on theAlbanHills parental composition (plagioclase-free phono-tephrite) under anhydrous, hydrous, and hydrous-carbonatedconditions. Experimentswere carried out at 1 atm, 0.5GPa and 1GPa, temperatures ranging from 1050 to 1300 °C, andH2O andCaCO3 inthe starting material up to 2 and 7 wt.%, respectively. The experiments performed at 0.5 GPa are the most representative of the Alban Hillsplumbing system. Clinopyroxene and leucite are the main phases occurring under all the investigated conditions and the liquidus phases.Nevertheless, our experimental results demonstrate that the occurrence of CaCO3 in the starting material strongly affects phase relations.Experiments performed under hydrous conditions crystallize magnetite and phlogopite at relatively high temperature. This earlycrystallization drives the glass composition towards a silica enrichment, resulting in a differentiation trend moving from phono-tephritic(AlbanHills parental composition) to phonolitic compositions. This is in contrast withmicro-textural evidence showing late crystallizationof magnetite and phlogopite in the natural products and with the composition of the juvenile products. On the contrary, in the CaCO3-bearing experiments (i.e., simulatingmagma–carbonate interaction) themagnetite and phlogopite stability fields are strongly reduced.As aconsequence, the melt differentiation is mainly controlled by the cotectic crystallization of clinopyroxene and leucite, resulting in adifferentiation trendmoving towardsK-foiditic compositions. These experimental results are in agreement withmicro-textural features andchemical compositions of Alban Hills natural products and with the magmatic differentiation model inferred by geochemical data.Magma–carbonate interaction is not a rare process and its occurrence has been demonstrated for different plumbing systems.However, theuniqueness of the Alban Hills liquid line of descent suggests that the efficacy of the carbonate contamination process is controlled bydifferent factors, the dynamics of the plumbing system being one of the most important.© 2007 Elsevier B.V. All rights reserved.

Keywords: Alban Hills; Ultrapotassic rocks; Experimental petrology; Magma–carbonate interaction

⁎ Corresponding author. Tel.: +39 06 51860502; fax: +39 06 51860507.E-mail addresses: freda@ingv.it (C. Freda), mario.gaeta@uniroma1.it (M. Gaeta), misiti@ingv.it (V. Misiti), smollo@uniroma3.it (S. Mollo),

dolfi@uniroma3.it (D. Dolfi), scarlato@ingv.it (P. Scarlato).

0024-4937/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.lithos.2007.08.008

398 C. Freda et al. / Lithos 101 (2008) 397–415

1. Introduction

The Alban Hills is one of the main volcanic districtsof the ultrapotassic Roman Province (Washington,1906; Fornaseri et al., 1963; Peccerillo, 2001). TheAlban Hills District developed along the Tyrrhenianmargin of Italy during Quaternary time in an area that iscurrently densely urbanised and very close to the city ofRome (only 10 km southeastern).

Primary magmas of the Roman Province volcanicdistricts were generated from a metasomatized uppermantle source (Peccerillo and Lustrino, 2005 andreference therein). The most primitive (possibly prima-ry) ultrapotassic rocks sampled in the Roman Provinceare K-basalts outcropping at Monti Vulsini VolcanicDistrict (Kamenetsky et al., 1995). The liquid line ofdescent of Roman Province ultrapotassic magmasgenerally starts from this parental composition andevolves towards phonolitic composition (Perini et al.,2004). Melzer and Foley (2000) constrained this trendexperimentally and demonstrated the important roleplayed by clinopyroxene crystallization during theliquid differentiation; as a consequence the differentiat-ed melts are characterised by low CaO concentration.

Differently, the Alban Hills liquid line of descentevolves towards K-foiditic composition and the differ-entiated melts show low SiO2 (down to 40 wt.%) andhigh CaO (up to 9 wt.%) contents (Trigila et al. 1995;Freda et al., 2006; Gaeta et al., 2006). This differenti-ation process takes place in a plumbing systemcharacterised by high values of fO2 (from NNO toHM; Gaeta et al., 2000) and at pressure of at least0.3 GPa (as inferred by H2O+CO2 solubility datacombined with volatile concentrations in melt inclu-sions; Palladino et al., 2001; Marra et al., 2003; Misitiet al., 2006; Freda et al., 2006).

Platz et al. (2004) explained the occurrence of SiO2-depleted, CaO-enriched differentiated nephelinites atNyiragongo by means of a fractionation processdominated by leucite crystallization. This fractionationmodel, however, is not applicable to the Alban Hillsmagmatic system because it does not explain theformation of the high-K2O differentiated compositionsoccurring in the district. Veksler et al. (1998) demon-strated experimentally that equilibrium crystallization ofclinopyroxene, leucite, and phlogopite drives a leucite–kalsilite normative liquid (saturated in forsterite) to-wards SiO2-depleted, CaO-enriched residual composi-tions. Neither is this model strictly applicable to AlbanHills magmatic differentiation, because the AlbanHills primitive compositions are orthoclase norma-tive; moreover the Alban Hills natural products do not

show any micro-textural evidence of early phlogopitefractionation.

Recent papers (Dallai et al., 2004; Gaeta et al., 2006;Freda et al., 2006) suggest that the unique Alban Hillsliquid line of descent can be explained by invoking theinteraction between magmas and carbonate countryrocks. In particular, Gaeta et al. (2006), through massbalance calculations, demonstrated that the Alban Hillsdifferentiation trend is driven by crystal fractionation ofclinopyroxene and leucite (plus magnetite and apatite asaccessory phases) and by calcite assimilation (up toabout 7 wt.%). It is worth stressing that Alban Hillsmagma chambers were emplaced into carbonate units(Funiciello and Parotto, 1978) whose top was localisedat about 2 km depth and thickness was estimated up toabout 5 km (Bianchi et al., 2006). It is also worth stres-sing that evidences of the interaction between magmasand carbonate country rocks are present in the AlbanHills volcanic deposits at both macroscopic (limestoneenclosed in scoria clasts) and microscopic level (calcitecrystals enclosed in quenched melt in scoria clasts).Although magma–carbonate interaction is not a rareprocess, and its occurrence was suggested and demon-strated for other magmatic systems (cf., Wenzel et al.,2002; Barnes et al., 2005 and references therein), theAlban Hills liquid line of descent is quite unique.

Geochemical data constrains the processes acting atAlban Hills, but give no indications on the mechanismleading to the formation of SiO2-poor, CaO- and K2O-enriched differentiated melts starting from an ultra-potassic parental composition. Changes in the phaserelations, in turn related to changes in the CaO and CO2

activities in the magmas during carbonate-magmainteraction, can represent the mechanism explainingthe Alban Hills liquid line of descent. Notwithstandingthe peculiarity of the Alban Hills ultrapotassic magmas,very few papers have been devoted to experimentallyconstrain their phase relations (Thompson, 1977; Trigilaet al., 1995; Freda et al., 1997).

In this study we performed experiments with the aimof modelling the effect of pressure, temperature, H2Oand CaCO3 content on the phase relations of an AlbanHills primitive composition and on the composition ofresidual melts. This will allow us to constrainexperimentally the liquid line of descent of the AlbanHills magmas.

2. Experimental procedures andanalytical techniques

All the experiments performed in this study used asstarting material a sample of the Mt. Mellone lava flow

399C. Freda et al. / Lithos 101 (2008) 397–415

(AH-7a sample, Marra et al., 2003), the most primitiverock outcropping at Alban Hills (Gaeta et al., 2006).This lava flow, outcropping in the northern sector of theAlban Hills Volcanic District, is a dark grey, fine grainedrock with olivine (b1 vol.%), clinopyroxene (5 vol.%),and leucite (2 vol.%) phenocrysts dispersed in agroundmass of olivine, clinopyroxene, leucite, magnetite,nepheline, and amphibole. The Mt. Mellone lava flow isa plagioclase-free, phono-tephrite (SiO2 = 47.96,TiO2=0.94, Al2O3=14.72, MnO=0.14, MgO=6.38,FeO=7.77, CaO= 11.32, BaO= 0.15, SrO= 0.22,Na2O=1.67, K2O=6.97, P2O5=0.50, F=0.22 in wt.%;S=50 ppm; Mg#=63) classified as primitive ultrapotas-sic rock (Foley et al., 1987).

We performed 6 sets of experiments by using startingmaterials containing different combinations of H2O andCaCO3: i) natural AH-7a powder containing 0.7 wt.%of H2O (measured by means of Karl Fisher titrationat Institut fur Mineralogie Universitat Hannover);ii) natural AH-7a powder (containing 0.7 wt.% ofH2O) doped with about 1 wt.% of water, added bymicro-syringe to each run; iii) glassy AH-7a powder,nominally anhydrous, obtained by melting the naturalsample in a vertical rapid-quench furnace at 1400 °C for1/2 h; iv) glassy AH-7a powder, nominally anhydrous,doped with either 3 or 5 wt.% CaCO3 powder (MassaCarrara pure marble); v) natural AH-7a powder (contain-ing 0.7 wt.% of H2O) doped with 3wt.%CaCO3 powder;vi) natural AH-7a powder (containing 0.7 wt.% of H2O)doped with 1, 3, 5, and 7 wt.% CaCO3 powder in order toobtain four different batches of powder to be run at fourdifferent temperatures; the starting material doped withthe lowest CaCO3 content was run at the highesttemperature while the starting material doped with thehighest CaCO3 content was run at the lowest tempera-ture. The amount of CaCO3 in each batch and thecorresponding experimental temperature were chosen bycombining data from mass balance (Gaeta et al., 2006)and MELTS (Ghiorso and Sack, 1995) calculations. Forexample, at 1050 °C and 0.5 GPa under hydrous con-dition (1 wt.% of H2O), MELTS calculations produce aresidual melt coexisting with clinopyroxene, leucite,magnetite, and apatite and having aMgO content of 1 wt.% and a CaO content of 4.5 wt.%. Alban Hills naturalproducts with a similar MgO concentration, have a CaOconcentration of about 9wt.%:mass balance calculationsindicate that this concentration can be obtained byadding about 7 wt.% of CaCO3 to the starting material.

Previous hydrous experiments performed in exter-nally heated pressure vessels on ultrapotassic composi-tions (Trigila et al., 1995; Freda et al., 1997) similar tothe AH-7a were sometimes difficult to analyse because

of quench crystals. These experiments demonstratedthat in order to obtain reliable experimental products byusing low-viscosity ultrapotassic compositions as start-ing material (above all if containing water), the use ofrapid-quench experimental apparata is mandatory. Forthis reason, we performed experiments by using a 1 atmDeltech DT-31 vertical rapid-quench furnace (samplesquenched by dropping them into a water bath) and apiston cylinder apparatus (rate quench∼2000 °C/min).

Experiments were carried out at temperatures rangingfrom 1050 to 1300 °C and pressures of 1 atm, 0.5 GPa,and 1.0 GPa.

Atmospheric pressure experiments were performed atDipartimento di Scienze Geologiche (Università di RomaTre). The internal thermocouple (Pt87Rh13–Pt, type R) islocated at about 5 mm from the sample in the central partof the furnace whose hot spot is about 4.5 cm long. Thisthermocouple was calibrated against the melting point ofgold and is estimated to be accurate within ±3 °C.Samples, loaded in Pt capsules (3 mm outer diameter),were heated up to 1400 °C at 6 °C/min, held at thattemperature for 20 min, and dropped to the experimentaltemperature at a cooling rate of 600 °C/min. Runs wereperformed at air fugacity.

High pressure experiments were carried out at HP-HTLaboratory of Experimental Volcanology and Geophys-ics (Istituto Nazionale di Geofisica e Vulcanologia,Roma). The piston cylinder apparatus we used iscalibrated to perform experiments in the range of 0.5–1.5 GPa, this prevented us from performing experimentsat pressure lower than 0.5GPa.As stated above, however,solubility data combined with volatile concentrations inmelt inclusions, indicate that a pressure value of 0.5 GPacan be considered representative for the Alban Hillsmagma chamber. High pressure experiments used Pt(runs above 1200 °C) and Au75Pd25 capsules (3.0 mmouter diameter). Capsules were welded at one end, filledwith the starting material (previously stored at 110 °C inorder to remove humidity), and then welded at the otherend. Capsules were positioned either into a 19.1 mmNaCl–crushable alumina–pyrophillite–pyrex assembly(sample with free-water added, Freda et al., 2001) orNaCl–crushable alumina–pyrex assembly. These kind ofassemblies produce an fO2 during the experiments ofNNO+2 (Kushiro, 1990; Kawamoto and Hirose, 1994)which is in the range of fO2 values estimated for AlbanHills plumbing system (Gaeta et al., 2000). Runs werefirst pressurised to 0.5 or 1.0 GPa (nominal pressurewithin 25 MPa of the actual pressure), then heated at arate of 200 °C/min up to superliquidus temperature, heldat these conditions for 15 min, and finally dropped to theexperimental temperature at 100 °C/min. In order to check

Table 1Run conditions and phases occurring in experimental products

Run# P (GPa) T (°C) Time (h) Phases

Anhydrous starting material060506 1×10−4 1300 43 G150506 1×10−4 1250 43 Lc+G120406 1×10−4 1200 24 Lc+G170506 1×10−4 1150 43 Lc+Cpx+Mgt+G267/40 0.5 1300 0.5 Lc+Cpx+G179/7 0.5 1250 1.5 Lc+Cpx+G298/33 1.0 1300 7 Lc+Cpx+G

Hydrous starting material (∼1 wt.% H2O)232/22 0.5 1300 5 G284/47 0.5 1275 7 G294/49 0.5 1200 7 Cpx+Lc+G175/9 0.5 1150 7 Cpx+Lc+Mgt+G166/5 0.5 1100 7 Cpx+Lc+Mgt+G227/18 0.5 1050 7 Cpx+Lc+Mgt+

Ap+Phl+G157/1 1.0 1300 8 G158/2 1.0 1200 7 G265/34 1.0 1200 5 G191/10 1.0 1150 7 Cpx+Lc+Mgt+G165/4 1.0 1100 8 Cpx+Lc+Mgt+

Phl+G

Hydrous starting material (∼2 wt.% H2O)232/24 0.5 1300 5 G212a 0.5 1250 1 G225/21 0.5 1200 7 Cpx+G224/20 0.5 1100 7 Cpx+Phl+Mgt+

Ap+Lc+G227/19 0.5 1050 7 Cpx+Phl+

Mgt+Ap+Lc+G208 1.0 1150 0.1 Cpx+G233/25 1.0 1100 7 Cpx+Phl+Mgt+G

Anhydrous-carbonated starting material (various wt.% CaCO3)310106a (3 wt.%) a 1×10−4 1250 7 Lc+G010206a (3 wt.%) a 1×10−4 1200 6 Lc+G060306a (3 wt.%) a 1×10−4 1150 6 Lc+Cpx+G310106b (5 wt.%) a 1×10−4 1250 7 Lc+G010206b (5 wt.%) a 1×10−4 1200 6 Lc+G060306b (5 wt.%) a 1×10−4 1150 6 Lc+Cpx+G

Hydrous-carbonated starting material (∼1 wt.% H2O+various wt.%CaCO3)

243/32 (1 wt.%) a 0.5 1200 4 Cpx+Lc+G223/15 (5 wt.%) a 0.5 1100 7 Cpx+Lc+Mgt+G247/28 (7 wt.%) a 0.5 1050 7 Cpx+Lc+Mgt+

Ap+G243/33 (3 wt.%) a 0.5 1200 4 Cpx+G244/31 (3 wt.%) a 0.5 1150 7 Cpx+Lc+G223/14 (3 wt.%) a 0.5 1100 7 Cpx+Lc+Mgt+G247/29 (3 wt.%) a 0.5 1050 7 Cpx+Lc+Mgt+

Ap+Phl+G233/23 (3 wt.%) a 1.0 1100 7 Cpx+Lc+Mgt+G

G = glass; Lc = leucite; Cpx = clinopyroxene; Mgt = magnetite;Ap = apatite; Phl = phlogopite.a Amount of CaCO3 added to the starting material.

400 C. Freda et al. / Lithos 101 (2008) 397–415

for crystallization and/or quench crystals during thiscooling step, some experiments were quenched immedi-ately after reaching the set point. The experimental pro-ducts, observed by optical microscope, were crystal-free.During the run, temperature was controlled by a factorycalibrated W95Re5–W74Re26 thermocouple and heldwithin 3 °Cof the experimental temperature; gradient alongthe capsules was less than 10 °C (Hudon et al., 1994).

Mössbauer spectroscopy was performed at Diparti-mento di Scienze della Terra, Università Sapienza diRoma. Samples were prepared by pressing together thepowdered run product glass and an acrylic resin toobtain self-supporting absorbers with Fe thickness in therange 1–2 mg/cm2. Spectra were collected at roomtemperature using a conventional spectrometer systemoperating in constant acceleration mode with a 57Cosource of nominal 50 mCi in Rh matrix. Spectral data inthe velocity range from −4 to +4 mm/s were recordedby a 512 multichannel analyser. After velocity calibra-tion against a spectrum of high-purity α-iron foil (25 μmthick), the raw data were folded to 256 channels. Thespectra were fitted, assuming Lorentzian peak shapes,using the fitting program Recoil 1.04. The reduced χ2

was used as a parameter to evaluate statistical best fit,and uncertainties were calculated using the covariancematrix. Errors are estimated at about ±0.02 mm/s forisomer shift, quadrupole splitting, and no more than±3% for doublet areas.

Analyses of experimental products were performedat the CNR-Istituto di Geologia Ambientale e Geoin-gegneria (Rome, Italy) with a Cameca SX50 electronmicroprobe equipped with five wavelength-dispersivespectrometers using 15 kV accelerating voltage, 15 nAbeam current, 10 μm beam diameter, and 20 s countingtime. The following standards were used: wollastonite(Si and Ca), corundum (Al), diopside (Mg), andradite(Fe), rutile (Ti), orthoclase (K), jadeite (Na), barite(Ba, S), celestine (Sr), F-phlogopite (F), and metals(Cr and Mn). Ti and Ba contents were corrected for theoverlap of the Ti Kα and Ba Kα peaks.

Image analyses and chemical maps of experimentalproducts were performed at the HP-HT Laboratory ofExperimental Volcanology and Geophysics (INGV,Roma) with a Jeol FE-SEM 6500F equipped with aEnergy Dispersion microanalysis System.

3. Results

Run conditions and phases occurring in experimentalproducts are listed in Table 1. Representative electronmicroprobe analyses of experimental glasses and min-erals are reported in Tables 2, 3 and 4, respectively.

Table 2Electron microprobe chemical analyses of residual glasses and percentage of phases

Anhydrous starting material

Run# 060506 150506 120406 170506 267/40 179/7 298/33

P (GPa) 1×10−4 1×10−4 1×10−4 1×10−4 0.5 0.5 1

T (°C) 1300 1250 1200 1150 1300 1250 1300

SD(11) a SD(3) SD(3) SD(4) SD(4) SD(5) SD(4)SiO2 48.41 0.22 48.67 0.30 48.27 0.71 49.15 0.39 49.05 0.27 47.51 0.63 50.24 0.46TiO2 0.81 0.03 0.85 0.03 0.92 0.08 1.02 0.05 0.84 0.03 1.01 0.15 0.95 0.11Al2O3 14.40 0.15 14.66 0.15 13.60 0.44 15.31 0.12 14.62 0.04 14.92 0.16 15.98 1.07Cr2O3 0.01 0.02 0.02 0.02 0.02 0.02 0.01 0.01 0.02 0.02 0.00 0.00 0.01 0.01FeO 7.71 0.11 7.68 0.08 8.17 0.28 7.59 0.18 7.49 0.15 8.55 0.42 7.34 0.17MnO 0.15 0.04 0.17 0.03 0.17 0.07 0.25 0.05 0.15 0.03 0.17 0.04 0.02 0.01MgO 6.51 0.05 6.47 0.14 6.84 0.27 5.75 0.09 6.26 0.08 5.73 0.21 4.95 1.02CaO 11.38 0.25 11.66 0.29 12.37 0.46 10.41 0.14 11.30 0.15 10.86 0.35 9.97 1.56SrO 0.04 0.03 0.05 0.05 0.07 0.08 0.24 0.07 0.03 0.04 0.11 0.07 0.01 0.01BaO 0.13 0.03 0.16 0.03 0.18 0.04 0.24 0.06 0.12 0.07 0.17 0.06 0.03 0.01Na2O 1.59 0.06 1.58 0.02 1.78 0.08 2.64 0.08 1.69 0.20 2.18 0.08 2.32 0.42K2O 6.84 0.11 6.54 0.07 5.85 0.29 5.36 0.19 6.83 0.55 6.37 0.13 7.34 0.56P2O5 0.25 0.07 0.28 0.08 0.56 0.06 0.51 0.01 0.35 0.03 0.68 0.06 0.06 0.06F 0.04 0.05 0.06 0.02 0.15 0.12 0.43 0.04 0.21 0.14 0.32 0.08 0.01 0.01Total 98.27 98.85 98.95 98.91 98.96 98.58 99.23

Phases (wt.%)∑ 0.81 0.15 0.24 0.67 0.31 0.47Melt 100.00 97.00 90.50 46.00 97.50 73.0(62) b 80.00Cpx – – – 28.00 1.25 15.0(16) 16.00Lc – 3.00 9.50 24.50 1.25 12.0(22) 4.00Mgt – – – 1.50 – – –Ap – – – – – – –Phl – – – – – – –

(continued on next page)

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Hydrous starting material (∼1 wt.% H2O)

Run# 232/22 284/47 294/49 175/9 166/5 227/18 157/1 158/2 191/10 165/4

P (GPa) 0.5 0.5 0.5 0.5 0.5 0.5 1 1 1 1

T (°C) 1300 1275 1200 1150 1100 1050 1300 1200 1150 1100

SD(3) a SD(6) SD(5) SD(7) SD(9) SD(5) SD(3) SD(3) SD(7)SiO2 48.58 0.26 48.66 47.94 1.24 49.83 0.38 50.22 0.25 51.47 0.81 48.29 0.23 48.41 0.26 48.29 0.22 50.70 0.65TiO2 0.85 0.03 0.86 1.00 0.17 0.91 0.03 0.82 0.05 0.56 0.05 0.90 0.04 0.83 0.04 0.87 0.09 0.71 0.09Al2O3 14.07 0.03 14.57 14.68 0.36 17.91 0.11 19.11 0.29 19.43 0.40 14.62 0.12 14.64 0.31 15.32 0.13 18.30 0.68Cr2O3 0.01 0.01 0.03 0.01 0.01 0.01 0.01 0.00 0.01 0.01 0.02 0.04 0.03 0.04 0.03 0.02 0.02 0.00 0.01FeO 7.54 0.05 7.14 8.13 0.92 5.82 0.18 4.83 0.10 5.54 0.61 7.34 0.10 7.13 0.33 7.71 0.06 5.33 0.54MnO 0.17 0.01 0.20 0.15 0.05 0.17 0.03 0.21 0.03 0.18 0.05 0.16 0.03 0.17 0.05 0.14 0.03 0.22 0.05MgO 6.27 0.13 6.21 5.71 0.64 3.08 0.06 2.04 0.10 1.51 0.26 6.19 0.07 6.30 0.17 5.73 0.05 1.60 0.20CaO 11.33 0.17 11.29 10.00 0.73 6.12 0.13 4.86 0.11 4.18 0.47 11.17 0.18 11.41 0.28 10.30 0.13 4.31 0.48SrO 0.01 0.02 0.07 0.10 0.14 0.19 0.04 0.27 0.06 0.30 0.07 0.00 0.00 0.01 0.01 0.07 0.01 0.01 0.08BaO 0.20 0.02 0.17 0.22 0.05 0.32 0.07 0.32 0.06 0.25 0.05 0.20 0.03 0.19 0.03 0.22 0.03 0.32 0.06Na2O 1.63 0.03 1.61 2.08 0.37 3.17 0.11 3.67 0.06 4.02 0.19 1.50 0.01 1.62 0.03 1.80 0.08 3.57 0.09K2O 7.29 0.13 7.04 6.31 0.46 8.66 0.22 9.38 0.09 9.08 0.23 7.21 0.06 7.22 0.15 7.41 0.04 9.86 0.36P2O5 0.55 0.09 0.36 0.68 0.32 0.96 0.11 1.10 0.11 0.46 0.09 0.51 0.07 0.53 0.03 0.52 0.05 1.11 0.25F 0.32 0.06 0.31 0.20 0.17 0.68 0.13 0.55 0.12 0.46 0.11 0.23 0.05 0.25 0.03 0.33 0.07 0.59 0.11Total 98.82 98.52 97.21 97.83 97.38 97.45 98.36 98.75 98.73 96.63

Phases (wt.%)∑ 0.45 0.12 0.24 0.16 0.29 0.07Melt 100 100 70 (85) b 42 37 33 100 100 73 37Cpx – – 18 38.8 42.5 43.1 – – 17.2 43Lc – – 12 17.2 18.2 19 – – 9.2 16.1Mgt – – – 2 2.3 2 – – 0.6 2.1Ap – – – – – 0.3 – – – –Phl – – – – – 2.6 – – – – 1.8

Table 2 (continued )

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Hydrous starting material (∼2 wt.% H2O)

Run# 232/24 212a 225/21 224/20 227/19 208 233/25

P (GPa) 0.5 0.5 0.5 0.5 0.5 1 1

T (°C) 1300 1250 1200 1100 1050 1150 1100

SD(5) a SD(4) SD(4) SD(4) SD(6) SD(3) SD(4)SiO2 47.68 0.64 48.36 0.17 47.90 0.15 50.36 0.21 51.01 0.52 47.06 0.09 50.18 0.15TiO2 0.84 0.04 0.88 0.03 0.85 0.03 0.62 0.06 0.61 0.05 0.89 0.02 0.70 0.04Al2O3 13.85 0.21 14.33 0.04 15.08 0.60 18.13 0.07 18.85 0.26 15.08 0.92 17.71 0.29Cr2O3 0.01 0.02 0.01 0.01 0.01 0.01 0.00 0.00 0.01 0.02 0.01 0.01 0.00 0.00FeO 7.51 0.17 7.80 0.29 7.33 0.07 6.54 0.34 5.52 0.54 7.65 0.13 5.45 0.18MnO 0.18 0.03 0.15 0.01 0.14 0.02 0.15 0.03 0.13 0.04 0.16 0.03 0.17 0.04MgO 5.98 0.04 6.03 0.06 5.35 0.33 1.63 0.32 1.54 0.09 5.34 0.27 1.98 0.16CaO 11.26 0.11 10.81 0.14 9.98 0.56 5.68 0.25 4.64 0.18 10.38 0.66 5.66 0.20SrO 0.00 0.00 0.05 0.03 0.03 0.02 0.08 0.07 0.10 0.05 0.01 0.02 0.09 0.02BaO 0.15 0.04 0.14 0.04 0.17 0.03 0.15 0.08 0.15 0.03 0.16 0.03 0.20 0.06Na2O 1.51 0.07 1.56 0.09 1.52 0.06 2.38 0.09 2.63 0.10 1.65 0.04 2.46 0.05K2O 7.15 0.11 7.11 0.08 7.51 0.26 10.34 0.34 10.91 0.17 7.65 0.52 10.92 0.36P2O5 0.46 0.02 0.51 0.01 0.55 0.09 0.61 0.08 0.35 0.05 0.43 0.14 0.71 0.06F 0.33 0.08 0.11 0.09 0.32 0.10 0.26 0.07 0.18 0.11 0.34 0.09 0.33 0.11Total 96.91 97.85 96.74 96.93 96.63 96.81 96.56

Phases (wt.%)∑ 0.41 0.22 0.25 0.27 0.05Melt 100 100 91.3(96) b 59 (56) b 41 96 (96) b 61Cpx – – 8.7 (4) 30 (35) 42.3 4 (4) 33.2Lc – – – 1.5 (0) 11.8 – –Mgt – – – 0.8 (1) 1.8 – 1.1Ap – – – 1.2 (0) 0.5 – –Phl – – – 7.5 (2) 2.6 – 4.7

(continued on next page)

Table 2 (continued )

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Anhydrous-carbonated starting material (various wt.% CaCO3)

Run# 310106a 010206a 060306a 310106b 010206b 060306b

P (GPa) 1×10−4 1×10−4 1×10−4 1×10−4 1×10−4 1×10−4

T (°C) 1250 1200 1150 1250 1200 1150

CaCO3c 3 3 3 5 5 5

SD(4) a SD(5) SD(5) SD(5) SD(3) SD(5)SiO2 47.39 0.07 46.07 0.48 47.80 0.40 47.06 0.15 46.06 0.32 46.50 0.49TiO2 0.94 0.03 0.98 0.05 0.97 0.06 0.92 0.05 1.04 0.07 1.03 0.08Al2O3 13.60 0.15 13.08 0.11 15.25 0.52 13.47 0.08 12.77 0.02 14.86 0.15Cr2O3 0.01 0.02 0.04 0.03 0.01 0.01 0.02 0.02 0.02 0.02 0.01 0.01FeO 8.30 0.14 8.57 0.13 7.04 0.29 7.95 0.11 8.47 0.23 7.30 0.12MnO 0.18 0.04 0.23 0.01 0.32 0.04 0.16 0.01 0.22 0.01 0.33 0.03MgO 6.74 0.14 7.03 0.11 4.91 0.21 6.54 0.04 6.99 0.15 4.71 0.10CaO 13.20 0.20 15.16 0.62 10.80 0.54 14.73 0.14 16.26 0.34 12.77 0.33SrO 0.01 0.03 0.05 0.05 0.36 0.08 0.03 0.03 0.05 0.04 0.43 0.08BaO 0.16 0.05 0.14 0.03 0.38 0.08 0.16 0.03 0.23 0.05 0.32 0.05Na2O 1.73 0.05 1.69 0.12 3.73 0.34 1.61 0.04 1.86 0.02 4.01 0.10K2O 6.14 0.11 4.77 0.14 5.81 0.10 5.98 0.09 4.56 0.11 5.46 0.12P2O5 0.48 0.07 0.56 0.05 1.04 0.13 0.51 0.05 0.61 0.12 1.09 0.19F 0.14 0.06 0.28 0.08 0.83 0.15 0.03 0.05 0.27 0.02 0.67 0.10Total 99.02 98.65 99.25 99.17 99.41 99.48

Phases (wt.%)∑ 0.24 0.43 0.25 0.11 0.35 0.18Melt 94.00 85.00 37.00 93.00 85.00 32.00Cpx – – 37.80 – – 41.10Lc 6.00 15.00 25.20 7.00 15.00 26.90Mgt – – – – – –Ap – – – – – –Phl – – – – – –

Table 2 (continued )

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Hydrous-carbonated starting material (∼1 wt.% H2O+various wt.% CaCO3)

Run# 243/33 244/31 223/14 247/29 243/32 223/15 247/28 233/23

P (GPa) 0.5 0.5 0.5 0.5 0.5 0.5 0.5 1

T (°C) 1200 1150 1100 1050 1200 1100 1050 1100

CaCO3c 3 3 3 3 1 5 7 3

SD(7) SD(5) SD(5) SD(9) SD(5) SD(3) SD(6) SD(4)SiO2 47.32 0.28 47.07 0.10 45.65 0.74 50.37 0.94 47.24 1.02 44.99 0.02 44.51 0.26 46.64 0.23TiO2 0.86 0.03 0.92 0.10 0.93 0.06 0.74 0.09 0.84 0.06 0.90 0.04 0.90 0.13 0.94 0.02Al2O3 14.38 0.90 15.13 0.12 17.60 0.38 18.89 0.39 14.97 0.45 16.07 0.22 16.32 0.68 16.37 0.10Cr2O3 0.02 0.02 0.01 0.01 0.01 0.02 0.02 0.03 0.02 0.03 0.02 0.02 0.00 0.00 0.00 0.00FeO 7.28 0.22 7.45 0.11 6.03 0.21 4.50 0.19 7.44 0.28 6.53 0.10 6.01 0.52 7.51 0.48MnO 0.18 0.05 0.18 0.03 0.26 0.03 0.18 0.06 0.16 0.04 0.26 0.02 0.26 0.05 0.25 0.03MgO 5.33 0.71 4.52 0.17 2.11 0.11 1.94 0.22 5.36 0.49 2.42 0.05 2.64 0.36 3.16 0.13CaO 11.15 1.12 9.96 0.56 7.84 0.70 5.08 0.64 11.69 0.98 9.06 0.09 8.87 0.33 8.13 0.13SrO 0.02 0.03 0.04 0.07 0.50 0.09 0.21 0.07 0.04 0.04 0.33 0.07 0.33 0.08 0.34 0.04BaO 0.19 0.06 0.21 0.04 0.48 0.07 0.41 0.06 0.18 0.01 0.41 0.07 0.44 0.05 0.28 0.04Na2O 1.79 0.22 2.13 0.13 4.61 0.30 4.42 0.15 1.82 0.15 4.28 0.17 4.54 0.26 3.30 0.26K2O 7.63 0.68 8.18 0.24 8.20 0.48 9.53 0.34 7.46 0.44 8.71 0.07 9.18 0.09 8.27 0.21P2O5 0.53 0.09 0.81 0.08 0.91 0.27 0.60 0.40 0.58 0.10 1.09 0.03 0.67 0.10 1.01 0.20F 0.32 0.14 0.28 0.04 0.82 0.06 0.72 0.14 0.26 0.11 0.81 0.06 0.81 0.15 0.54 0.20Total 97.00 96.89 95.94 97.61 98.06 95.88 95.48 96.74

Phases (wt.%)∑ 0.09 0.30 0.21 0.18 0.58 0.18 0.13 0.09Melt 90.00 74.30 31(58) b 28.00 80.60 29.00 27.00 42.00Cpx 10.00 22.40 43.8(33) 48.00 15.70 47.80 49.70 38.50Lc – 3.30 22.9(8) 20.00 3.70 22.00 21.20 17.60Mgt – – 2.3(1) 2.00 – 1.20 1.30 1.90Ap – – – 0.40 – – 0.80 –Phl – – – 1.60 – – – –

The latter were estimated by means of mass balance calculations. For mass balance calculations we used the chemical composition of glasses and minerals as reported in this table and in tables 3 and 4,respectively.a Number of analyses.b In brackets: phase amounts as determined from FE-SEM image analyses.c Amount of CaCO3 (wt.%) added to the starting material.

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3.1. Superliquidus experiments

Experimental products obtained at superliquidustemperature were used to estimate the oxygen fugacityand the loss of iron during runs.

From the Mössbauer spectra of glasses obtained at1300 and 1200 °C and 1 GPa (runs 157/1 and 158/2 inTable 1) we determined a Fe2+/Fe3+ average ratio of0.67 with a 5% estimated error. By using the Kress andCarmichael (1988) equationwe estimated a fO2 during theruns close toNNO+2, in agreement with the fO2 expectedby using NaCl–crushable alumina–pyrex assemblies andwith the fO2 estimated for Alban Hills plumbing system.

By comparing the iron concentration in the startingmaterial and in the superliquidus glasses (1200–1300 °C)we estimated an average of iron loss below 4% of its totalamount. The high temperatures (Merril andWyllie, 1973)and the use of Pt capsule (Vetere et al., in press) are themost significant factors affecting iron loss. Since sub-

Table 3Representative electron microprobe analyses of experimental clinopyroxenes

Hydrous starting material (∼1 wt.% H2O)

Run# 294/49 294/49 175/9 175/9 166/5 166/5 227/18 2

P (GPa) 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0

T (°C) 1200 1200 1150 1150 1100 1100 1050 1

L, core S L, core S L, core S L, core S

SiO2 51.22 48.29 51.06 46.33 54.24 46.85 52.39 4TiO2 0.82 0.69 0.76 1.08 0.24 1.16 0.48Cr2O3 0.01 0.00 0.07 0.08 0.09 0.04 0.08Al2O3 4.01 5.93 3.89 8.27 1.35 7.26 2.40FeO 5.55 7.50 5.77 8.24 3.10 8.97 4.86MnO 0.10 0.15 0.08 0.22 0.07 0.14 0.17MgO 14.81 13.30 15.04 12.40 17.26 12.17 16.21 1CaO 24.15 22.96 23.92 22.87 24.43 22.81 23.90 2Na2O 0.11 0.34 0.14 0.31 0.11 0.40 0.07Total 100.77 99.16 100.72 99.81 100.88 99.79 100.56 9

Formula, on the basis of 6 oxygensSi 1.872 1.800 1.865 1.722 1.957 1.747 1.909Ti 0.022 0.019 0.021 0.030 0.007 0.032 0.013Cr 0.000 0.000 0.002 0.002 0.003 0.001 0.002AlIV 0.128 0.200 0.135 0.278 0.043 0.253 0.091AlVI 0.045 0.061 0.033 0.084 0.015 0.066 0.012Fe3+ 0.046 0.124 0.067 0.154 0.020 0.151 0.055Fe2+ 0.123 0.110 0.109 0.102 0.074 0.129 0.093Mn 0.003 0.005 0.002 0.007 0.002 0.004 0.005Mg 0.807 0.739 0.819 0.687 0.929 0.677 0.881Ca 0.946 0.917 0.936 0.911 0.944 0.911 0.933Na 0.008 0.025 0.010 0.022 0.007 0.029 0.005

L = large crystal; S = small crystal.a Amount of CaCO3 (wt.%) added to the starting material.

liquidus experiments were performed at temperatureslower than 1200 °C by using AuPd capsules (Vetere et al.,in press), we assume that the loss of iron possibly oc-curring during subliquidus experiments did not signifi-cantly affect experimental phase relations.

3.2. Subliquidus experiments

Subliquidus experimental products were used toconstrain phase relations and chemical evolution ofcoexisting residual melts.

The standard deviation values (SD in Table 2),calculated on the average of multiple analyses performedon experimental glasses demonstrated their chemicalhomogeneity. Chemical homogeneity was verified evenin the low temperature runs, usually characterised bysmall amount of glass.

Clinopyroxene crystals are ubiquitous in the sub-liquidus products (Table 1). In particular, clinopyroxenes

Hydrous-carbonated starting material (∼1 wt.% H2O+variouswt.% CaCO3)

27/18 243/32 243/33 244/31 223/14 247/29 223/15 247/28

.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

050 1200 1200 1150 1100 1050 1100 1050

1 a 3 a 3 a 3 a 3 a 5 a 7 a

S S S S S S S

4.94 45.41 46.33 46.18 46.29 45.66 46.10 44.331.20 1.07 0.88 0.92 1.04 1.22 1.10 1.060.02 0.14 0.23 0.00 0.07 0.10 0.04 0.019.86 9.39 8.08 9.01 9.87 9.98 9.40 8.538.88 8.24 7.51 7.46 7.34 8.66 7.42 11.220.20 0.07 0.09 0.11 0.14 0.10 0.17 0.141.38 11.67 12.24 12.17 11.14 10.53 11.24 10.172.90 24.06 23.79 23.72 22.89 22.27 23.96 23.420.39 0.26 0.25 0.27 0.23 0.75 0.22 0.459.77 100.31 99.42 99.84 99.01 99.27 100.04 100.33

1.675 1.681 1.727 1.712 1.737 1.711 1.719 1.6760.034 0.030 0.025 0.026 0.029 0.034 0.031 0.0300.001 0.004 0.007 0.000 0.002 0.003 0.001 0.0000.325 0.319 0.273 0.288 0.263 0.289 0.281 0.3240.108 0.091 0.083 0.106 0.173 0.151 0.133 0.0560.178 0.183 0.152 0.150 0.046 0.120 0.101 0.2410.099 0.072 0.083 0.081 0.184 0.151 0.131 0.1130.006 0.002 0.003 0.003 0.004 0.003 0.005 0.0050.633 0.644 0.680 0.673 0.623 0.588 0.625 0.5730.914 0.954 0.950 0.942 0.920 0.894 0.957 0.9490.028 0.019 0.018 0.019 0.017 0.054 0.016 0.033

Table 4Representative electron microprobe analyses of experimental minerals

Hydrous starting material(∼1 wt.% H2O)

Hydrous-carbonated startingmaterial (∼1 wt.% H2O+3 wt.% CaCO3)

Run# 175/9

227/18

227/18

227/18

244/31

247/29

247/29

247/29

P (GPa) 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

T (°C) 1150 1050 1050 1050 1150 1050 1050 1050

Phase Lc Mgt Phl Ap Lc Mgt Phl Ap

SiO2 55.24 0.24 36.37 bdl 55.56 0.12 38.12 bdlTiO2 0.05 4.38 2.59 0.01 0.02 1.46 2.25 0.05Al2O3 22.48 7.90 14.33 bdl 21.63 6.02 13.15 bdlCr2O3 0.03 0.03 0.04 bdl 0.06 0.00 0.02 bdlFeO 0.53 76.09 13.34 0.64 0.81 76.76 12.34 0.51MnO 0.05 0.60 0.14 0.00 0.01 0.96 0.14 0.07MgO 0.06 3.40 16.78 0.31 0.00 7.37 17.36 0.24CaO 0.10 0.33 0.10 51.84 0.00 0.13 0.28 48.54SrO bdl bdl 0.00 1.47 bdl bdl 0.00 1.89BaO 0.11 bdl 1.37 bdl 0.07 bdl 1.18 bdlNa2O 0.45 bdl 0.36 0.18 0.30 bdl 0.34 0.19K2O 20.05 bdl 9.05 0.60 20.42 bdl 8.73 0.68P2O5 0.05 bdl 0.00 41.31 0.02 bdl 0.01 41.72F 0.03 bdl 2.09 3.45 0.00 bdl 1.88 3.30Total 99.23 92.96 96.55 99.82 98.91 92.82 95.81 97.18

Formula a

Si 2.018 0.009 2.745 – 2.036 0.004 2.854 –Ti 0.000 0.120 0.147 0.000 0.000 0.039 0.127 0.000Al 0.968 0.339 1.275 – 0.934 0.252 1.160 –Cr 0.000 0.001 0.000 – 0.000 0.000 0.000 –Fe 0.016 2.316 0.949 0.092 0.025 2.281 0.933 0.078Mn 0.000 0.018 0.009 0.000 0.000 0.029 0.009 0.011Mg 0.000 0.184 1.888 0.079 0.000 0.390 1.937 0.064Ca 0.004 0.013 0.008 9.584 0.000 0.005 0.023 9.532Sr – – 0.000 0.147 – – 0.000 0.201Ba 0.002 – 0.040 – 0.001 – 0.035 –Na 0.032 – 0.053 0.031 0.021 – 0.050 0.033K 0.934 – 0.871 0.066 0.955 – 0.834 0.079P 0.000 – 0.000 6.035 0.000 – 0.000 6.475F 0.000 – 0.498 1.884 0.000 – 0.446 1.915

Lc = leucite, Mgt = magnetite, Phl = phlogopite, Ap = apatite; bdl =below detection limit.a Formula calculated on the basis of: 6 oxygens for Lc; 3 cations for

Mgt, 22 oxygens for Phl; 10 cations (=∑cations−P) for Ap.

Fig. 1. T–CaCO3+H2O phase diagram for AH-7a phono-tephriticcomposition at 0.5 GPa. Numbers reported on the x axis refer to theamount of CaCO3 and H2O in the starting material. The intersectionbetween the Lc in, Phl in, and Mgt in curves (pointed by the arrow)was inferred by MELTS simulations and by the phase abundances inthe 224/20 run (1100 °C, 2 wt.% H2O). CPML = clinopyroxene +phlogopite + magnetite + liquid; L = liquid; Cpx = clinopyroxene; Lc =leucite; Mgt = magnetite; Phl = phlogopite; Ap = apatite.

407C. Freda et al. / Lithos 101 (2008) 397–415

occurring in hydrous runs are sometimes zoned andcharacterised by a wide compositional range (Table 3)while clinopyroxenes from hydrous-carbonated runshave constant chemical composition (Table 3). Never-theless, each run crystallized a clinopyroxene populationshowing chemical composition in equilibrium with re-sidual melts (according to the K

FeMg

D proposed by Nielsenand Drake, 1979 and Shaw et al., 1998). The compo-sition of the experimental clinopyroxenes also agreeswith the theoretical compositions obtained by means of

MELTS calculations under hydrous conditions and at thesame P–T–fO2 values used during the experiments.

The other subliquidus mineral phases are leucite,magnetite, phlogopite, and apatite; the composition ofthese minerals does not vary significantly even whencrystallizing from different bulk compositions (Table 4).

The amounts of glass and minerals obtained in eachrun were estimated by means of mass balance calcula-tions (Table 2). In order to constrain these calculatedamounts, we performed modal analyses by using FE-SEM images and EDS micro-chemical maps of aselection of experimental products and verified theconsistency between the two methods (Table 2).

3.2.1. Phase relationsAnhydrous experiments performed on the AH-7a

composition indicate that leucite is the only liquidusphase at 1 atm while at 0.5 and 1 GPa (temperature closeto 1300 °C) clinopyroxene also crystallizes. It is worthreminding the reader that micro-textural features of theAlbanHills natural products indicate clinopyroxene as theearly crystallizing phase, while leucite appears just after.The experiments carried out at atmospheric pressure haveleucite as the only liquidus phase over a wide temperaturerange (more than 50 °C, Tables 1 and 2) and this isneither compatible with the chemical composition ofthe Alban Hills differentiated products nor with therocks' micro-textural features. Even doping the startingmaterial with variable amounts of CaCO3 (3 and 5 wt.%,Tables 1 and 2), this temperature range remains large,suggesting that low pressures (close to the atmospheric

Fig. 2. Major oxides (except silica) concentration in experimental glasses vs temperature. Data from 0.5 GPa runs only. Diagrams show the differentiationtrend of residual glasses obtained under different experimental conditions: a) 1 wt.% of H2O in the startingmaterial; b) 2 wt.%H2O in the startingmaterial;c) 1 wt.% of H2O and 3wt.% of CaCO3 in the startingmaterial; d) 1 wt.% ofH2O and variable wt.% of CaCO3 in the starting material; the numbers refer tothe amount of CaCO3 added to each run. The starting material compositions are as follows: AH-7a composition on dry basis for a) and b); AH-7acomposition, on dry basis, doped with 3 wt.% CaCO3 for c); AH-7a composition, on dry basis, doped with 1 wt.% CaCO3 for d).

408 C. Freda et al. / Lithos 101 (2008) 397–415

one) and anhydrous conditions are not appropriate for theAlban Hills plumbing system.

For this reason, we describe in detail phase relations(Fig. 1) and melt compositions (Figs. 2 and 3) obtainedunder volatile-bearing conditions at 0.5 GPa only. Theseconditions, among those investigated, are the mostrepresentative of the Alban Hills plumbing system andthus suitable for the purpose of our study.

Under hydrous conditions (1 wt.% of H2O), clino-pyroxene and leucite co-saturate the melt (Fig. 1) and,as expected, we observe a temperature drop of about100 °C with respect to anhydrous runs. The occurrence ofabout 2 wt.% of H2O in the starting material, reduces theleucite field (Zeng and MacKenzie, 1984 and referencestherein) and it favours clinopyroxene as liquidus phase(Fig. 1).

Under hydrous-carbonated conditions (3 wt.% ofCaCO3 and almost 1 wt.% of H2O in the system) clino-pyroxene is the only liquidus phase (Fig. 1). Yang et al.(1972) demonstrated that a CaO increase in the melt en-larges the clinopyroxene solid solution field by favouring

the formation of Al-Diopside molecule (or Ca-Tschermakclinopyroxene). Notably the Al2O3/FeOtot ratio (calculat-ed for clinopyroxene having comparable MgO content) inclinopyroxenes crystallized under hydrous-carbonatedconditions is higher than that in clinopyroxenes crystal-lized under hydrous conditions (Table 3).

Figs. 2 and 3 show the chemical evolution ofexperimental glasses as a function of decreasing exper-imental temperature under different experimental condi-tions. Fig. 2 shows the variation of the major oxides(except silica) in experimental glasses while Fig. 3 showsthe silica variation and the coexisting crystalline phases(see also Table 2).

Oxide trends (except silica, Fig. 2) are comparableunder both hydrous (1 and 2 wt.% H2O in the startingmaterial) and hydrous-carbonated conditions. In particu-lar, Al2O3 and alkalies increase while FeO, MgO, andCaO decrease with decreasing temperature and increasingcrystallization.

On the contrary, silica trends (Fig. 3) show variablebehaviour depending on the experimental conditions.

Fig. 3. SiO2 concentration in experimental glasses vs temperature. Data from 0.5 GPa runs only. Diagrams show the differentiation trend of residualglasses obtained under different experimental conditions: a) 1 wt.% of H2O in the starting material; b) 2 wt.% H2O in the starting material; c) 1 wt.%of H2O and 3 wt.% of CaCO3 in the starting material; d) 1 wt.% of H2O and variable wt.% of CaCO3 in the starting material; the numbers refer to theamount of CaCO3 added to each run. Labels indicate the mineralogical phases occurring at different temperatures under different experimentalconditions (see also Tables 1 and 2). For the starting material compositions, please see Fig. 2.

409C. Freda et al. / Lithos 101 (2008) 397–415

Under hydrous conditions (1 and 2 wt.% of H2O), theSiO2 concentration in the glass increases significantlywith decreasing temperature (Fig. 3a and b). Underhydrous-carbonated condition a similar silica increase isapparent only in the run performed at 1050 °C by usingthe starting material doped with 3 wt.% CaCO3

(1050 °C, Fig. 3c). In all the other hydrous-carbonatedruns, the amount of silica shows a slightly decreasingtrend (Fig. 3c and d).

The difference in silica content observed amongglasses obtained under hydrous and hydrous-carbonatedconditions cannot be simply explained because of silicadilution in the CaCO3-doped samples. As explained indetail below, this difference is clearly due to changes inphase stability for the different bulk compositions inves-tigated (Fig. 1).

Under hydrous conditions (1 wt.% of H2O, Fig. 3a),clinopyroxene and leucite are the first crystallizing phases.Magnetite crystals (ulvospinel content up to 15%) appearat relatively high temperature (N1150 °C, Fig. 4a) alongwith clinopyroxene and leucite, while apatite andphlogopite occur at 1050 °C (Fig. 1). By increasing the

amount of water in the starting material (about 2 wt.%,Fig. 3b), phlogopite crystals appear above 1100 °C (Figs. 1and 5a) alongwith clinopyroxene while leucite, magnetite,and apatite crystallize later (Fig. 1). The crystallization ofphlogopite at relatively high temperature, even in presenceof small amount of water, is probably favoured by the largeamount of K2O in the system.

Under hydrous-carbonated conditions and constantconcentration of CaCO3 in the system at eachtemperature (Fig. 3c), clinopyroxene is the first mineralphase followed by leucite (at around 1150 °C, Fig. 1),magnetite crystallizes later compared to the CaCO3-free experiments (Fig. 4), and phlogopite appears at thelowest temperature (1050 °C). Under these conditionsthe silica trend is decreasing down to 1100 °C while at1050 °C we observe a dramatic silica increase in thecoexisting residual glass (Fig. 3c). In the experimentsperformed by using different batches of natural powderdoped with variable amounts of CaCO3 (1, 3, 5, and7 wt.%, Fig. 3d) and run at a decreasing temperature(i.e., the starting material doped with 1 wt.% CaCO3

was run at 1200 °C and that doped with 7 wt.% of

Fig. 4. Comparison between experimental products obtained underhydrous and under hydrous-carbonated conditions. The experimentalparagenesis demonstrates that the occurrence of CaCO3 in the systemprevents early magnetite crystallization. a) FE-SEM image acquiredby using backscattered electrons (120×) of Run 175/9: 1 wt.% of H2O inthe starting material, T=1150 °C, P=0.5 GPa, 7 h; b) FE-SEMimage acquired by using backscattered electrons (120×) of Run 244/31:1wt.%ofH2Oplus 3wt.%ofCaCO3 in the startingmaterial,T=1150 °C,P=0.5 GPa, 7 h.

410 C. Freda et al. / Lithos 101 (2008) 397–415

CaCO3 was run at 1050 °C) clinopyroxene and leuciteco-saturate the liquid at highest temperature (1200 °Cand 1 wt.% CaCO3 in the starting material), magnetitecrystallizes later and in scarce quantity while phlogo-pite is totally absent. Under these conditions we ob-serve the lowest silica concentration in the residualmelts (Fig. 3).

Due to the predominance of clinopyroxene and leucitein the experimental products it seems reasonable to con-sider them as responsible for the silica variability ob-served in residual glasses obtained under different

experimental conditions (Fig. 3). Certainly, a differencebetween CaCO3-free and CaCO3-bearing experimentalproducts is the clinopyroxene/leucite ratio. Mass balancecalculations indicate that in hydrous (1 wt.% of H2O inthe system) experiments at 1150 °C, clinopyroxene is twotimes more abundant than leucite and becomes almost 7times more abundant in the CaCO3-bearing run (Table 2,runs 175/9 and 244/31, respectively). Nevertheless,considering the chemistry of clinopyroxene and leucite(Tables 3 and 4), the difference in SiO2 concentrationbetween hydrous and hydrous-carbonated residual meltscannot be simply explained by variations in the modalabundance of these two minerals. As stated above, mag-netite and phlogopite appear at different stages along thedifferentiation path depending on the experimental con-ditions (Figs. 1 and 3). As a consequence, these twominerals are the best candidates to explain the variationsin silica concentration observed in the experimentalglasses. On the other hand, the effect of magnetite andphlogopite crystallization on silica concentration wasalready suggested (cfr., Toplis and Carroll, 1996; Melzerand Foley, 2000).

Notably, the experiments performed at 1.0 GPa undereither hydrous or hydrous-carbonated conditions, al-though not appropriate for the Alban Hills plumbingsystem, confirm the phase relations observed in the0.5 GPa experiments (Tables 1 and 2). In particular, inthe CaCO3-free experiments we observe the enlarge-ment of the phlogopite stability field by increasing theamount of water in the starting material while, by addingCaCO3 to the system the phlogopite field is significantlydepressed (cfr. runs 165/4 and 233/23 in Table 1).

4. Discussion

4.1. Petrological implications for the Alban Hillsplumbing system

The paragenesis obtained in the experiments (leucite,clinopyroxene, magnetite, phlogopite, and apatite)resembles that of the natural Alban Hills juvenileproducts (scoria clasts, lava flows, and cumulates).Nevertheless, it is important to highlight some signif-icant differences between natural and experimentalproducts.

i) Olivine crystals are not stable in our experimentswhile the Mt. Mellone lava flow (our starting material)contains olivine as mm-sized crystals (Fo91) and inthe groundmass (Fo50–70). The absence of olivine inthe experiments suggests that the mm-sized crystals(Fo91) occurring in the natural sample are xenocrysts.The foreign nature of mm-sized AH-7a olivines is

Fig. 6. TAS diagram showing the composition of residual glassesobtained under hydrous conditions. It is apparent that the experimentaldifferentiation trend moves far from the Alban Hills liquid line ofdescent. At these conditions, indeed, melts evolve towards phonoliticcompositions. The Alban Hills natural products field was obtained byaveraging microprobe analyses of interstitial glasses (unpublisheddata; Gaeta et al., 2006; Freda et al., 2006) representative of the wholeAlban Hills eruptive activity. The Vesuvius natural products field wasdrawn by using data from Piochi et al., (2006).

Fig. 5. Comparison between experimental products obtained underhydrous and under hydrous-carbonated conditions. The addition ofCaCO3 to the system results in an enlargement of the clinopyroxenestability filed (Yang et al., 1972) and in a reduction of water solubilitydue to the CO2 dissolution in the melt (Botcharnikov et al., 2005 andreference therein). This, in turn, prevents early crystallization ofphlogopite. a) FE-SEM image acquired by using backscatteredelectrons (200×) of Run 224/20: 2 wt.% of H2O in the starting material,T=1100 °C, P=0.5 GPa, 7 h. Under these conditions, phlogopitecrystallize earlier compared to the other experimental conditions andthe occurrence of large euhedral crystals results in a significant increaseof the silica content in the residual melt (see also Fig. 3b and the text forfurther explanation). b) FE-SEM image acquired by using back-scattered electrons (120×) of Run 223/14: 1 wt.% of H2O and 3 wt.% ofCaCO3 in the starting material, T=1100 °C, P=0.5 GPa, 7 h.

411C. Freda et al. / Lithos 101 (2008) 397–415

c

cRma(

onfirmed by theirXOlFeOX

BulkMgO

XOlMgOX

BulkFeO

¼ 0:16 which is signifi-

antly different from the KFe=MgD value indicated by

oeder and Emslie (1970) as representative of olivine/elt equilibrium (0.27±0.03). These olivine xenocrystsre interpreted as crystallized from a parental magmae.g., K-basaltic in composition, Kamenetsky et al.,

1995) during the early stage of the differentiationprocess leading to the AH-7a phono-tephritic composi-tion (Gaeta et al., 2006). The compositional gap betweenmm-sized olivine xenocrysts (Fo91) and groundmassolivines (Fo50–70) as well as micro-textural features ofthe AH-7a lava flow, strongly indicate that olivinecrystals formed only during the final stage of the lavaflow cooling, after clinopyroxene, leucite, and magne-tite. Although it seems not reasonable to construct aphase diagram in which olivine crystallizes afterclinopyroxene, leucite, and magnetite, this sequence ofcrystallization is confirmed also by MELTS simulationsperformed at atmospheric pressure and fO2 of NNO andNNO+2 by using the AH-7a high-potassium composi-tion. Moreover, our lowest temperature experiments(1050 °C) performed under either hydrous or hydrous-carbonated (3 wt.% of CaCO3) conditions are not incontrast with the occurrence of olivines in the AH-7agroundmass. In these experiments, indeed, we observethe crystallization of phlogopite phase with a Mg/(Mg+Fe) ratio of 0.61–0.70, comparable to that of the olivinemicrocrysts occurring in the AH-7a groundmass (0.51–0.70). This suggests that in the high-potassium AH-7acomposition, olivine crystallization can alternate withphlogopite (after clinopyroxene and leucite) dependingon the occurrence of volatiles in the system.

ii) In the Alban Hills natural products, magnetiteusually crystallizes in the lava flow groundmass whilephenocrysts occur only in differentiated scoria clasts. Asstated above, fO2 value in the Alban Hills plumbingsystem is higher than NNO (Gaeta et al., 2000) thus the

Fig. 7. TAS diagram showing the composition of residual glassesobtained under hydrous-carbonated conditions. The experimentaldifferentiation trend mimics the Alban Hills liquid line of descent.Addition of carbonate to the system, indeed, drives the composition ofresidual melts towards K-foiditic compositions. Numbers close to thesymbols refer to the amount of CaCO3 added to each run. The AlbanHills natural products field was obtained by averaging microprobeanalyses of interstitial glasses (unpublished data; Gaeta et al., 2006;Freda et al., 2006) representative of the whole Alban Hills eruptiveactivity. The Vesuvius natural products field was drawn by using datafrom Piochi et al., (2006).

412 C. Freda et al. / Lithos 101 (2008) 397–415

absence of magnetite as early phase in natural rockscannot be related to unsuitable fO2 values. Moreover,according to MELTS calculations, the AH-7a composi-tion (1 wt.% of water) at fO2≥NNO crystallizesmagnetite at very high temperatures (from 1150 to1250 °C in a pressure range from 0.1 to 0.5 GPa). Asdemonstrated by our experiments, the crystallization ofmagnetite can be strongly depressed, although high fO2

values, by increasing the clinopyroxene stability field(i.e., by adding CaCO3 to the starting material).

iii) In the Alban Hills natural products, phlogopiteusually crystallize in lava flowgroundmass (F-phlogopite,Gaeta et al., 2000; Gaeta and Freda, 2001), as pheno-crysts in differentiated scoria clasts, and as intracu-mulus phase in cumulate rocks. The occurrence of largeexplosive eruptions during the Alban Hills eruptivehistory and data from melt inclusions (Palladino et al.,2001; Marra et al., 2003; Freda et al., 2006) indicate thatthe late crystallization of phlogopite in the Alban Hillsmagmas cannot be related to the scarcity of H2O in theplumbing system. Also in this case we can explain thereduction of the phlogopite stability field, with respectto clinopyroxene, because of the presence of CaCO3

in the system. The addition of CaCO3 into the melt, onone hand enlarges the clinopyroxene stability field(Yang et al., 1972), and on the other hand reduces thewater solubility (Fig. 5b) because of the CO2 dis-solution in the melt (Botcharnikov et al., 2005 andreference therein).

As discussed above, the early crystallization ofmagnetite and phlogopite in experiments performedunder hydrous conditions at fO2=NNO+2 drives theglass composition towards a silica enrichment (Fig. 3aand b), resulting in a differentiation trend moving fromphono-tephritic (the AH-7a starting material) to phono-litic compositions (Fig. 6). This is in contrast with theobservation that natural Alban Hills juvenile productsare mostly K-foiditic with silica concentrations in theresidual melts always below 50 wt.% (Gaeta et al.,2006). The composition of residual melts obtainedunder hydrous conditions suggests that this kind ofenvironment, favourable for early crystallization ofphlogopite and magnetite (Figs. 4a and 5a), is notsuitable for the differentiation trend of Alban Hillsnatural products. In the CaCO3-bearing experiments(i.e., simulating a magma–carbonate interaction), in-stead, magnetite and phlogopite stability fields arereduced with respect to clinopyroxene stability field andthe residual melts show a differentiation trend compa-rable to the liquid line of descent of the Alban Hillsrocks (Fig. 7). The reduction of magnetite stability fieldduring the magmatic differentiation in a hydrous-carbonated environment, can result in a concentrationof FeO in the differentiated glasses. This is in agree-ment with the FeO enrichment observed in Alban HillsK-foiditic residual melts (Gaeta et al., 2006; Freda et al.,2006). In turn, high amount of FeO in differentiatedmelts can explain the peculiar textural feature of AlbanHills leucitite lava flows characterised by the suddencrystallization of magnetite in the groundmass. On theother hand, the occurrence of phlogopite crystals asintracumulus phase only, is easily explained if thedifferentiation of Alban Hills magmas occurs in a“Solidification Front” (Marsh, 1995) strongly andcontinuously affected by decarbonation reactions actingat the magma chamber–wall rock interface.

4.2. Factors in the carbonate assimilation

As stated above, we believe that the Alban Hillsplumbing system interacts continuously with the carbon-ate wall rocks. This continuous interaction is guaranteedonly if during each eruptive cycle the plumbing system isfed by discrete arrivals of newmagma batches that remainisolated within carbonate country rocks and do not mixwith old magma batches. Discrete arrivals of magmas inthe Alban Hills shallow crustal reservoir (IntermediateStorage system of Scandone et al., 2007) is supported bythe results of Gaeta et al. (2006) who showed that everyeruptive cycle (lasting 45 kyr, Marra et al., 2004) ischaracterised by a unique 87Sr/86Sr isotopic signature.

413C. Freda et al. / Lithos 101 (2008) 397–415

This signature remains constant within each cycle, sug-gesting no mixing between magma batches of differentage. Moreover, experiments performed simulating car-bonate contamination, demonstrated that in order to havedifferentiated products comparable with the natural AlbanHills products it is necessary to add larger amounts ofCaCO3 to the starting material while decreasing theexperimental temperature (Figs. 3d and 7). This is inagreementwith amodel of continuous interaction betweenmagma and carbonate wall rocks. On the contrary, if car-bonate assimilation is modelled simulating an early inter-action between magmas and limestone when the magmaintrudes the wall rocks followed by a cessation of thisinteraction while the differentiation process occurs (Fig.3c), the most differentiated products (corresponding to thedata point falling in the phonolite field in Fig. 7) showcompositions characterised by high silica concentrationsand are thus not usual in the Alban Hills Volcanic District.

In order to test our model of magma/carbonateinteraction we applied it to Vesuvius Volcano whosemagma chamber was emplaced in carbonate basement(Rittmann, 1933; Barberi and Leoni, 1980) and forwhich the hypothesis of carbonate assimilation to ex-plain the genesis of alkaline rocks (Daly, 1910) wasproposed at the beginning of the last century by Rittmann(1933). Recent geochemical data (Piochi et al., 2006)and preliminary experiments (Iacono Marziano andGaillard, 2006) indicate that limestone assimilationplay an important role also during the magmatic differ-entiation of Vesuvius magmas.

According to the many geochemical studies performedon Vesuvius and Alban Hills, the parental magmas ofthese two plumbing systems are isotopically different,reflecting heterogeneous mantle sources (cf., Peccerilloand Lustrino, 2005 and references therein). Moreover, thechemical composition of Vesuvius products, differentlyfrom Alban Hills, spans from trachybasaltic/tephritic tophonolitic (Figs. 6 and 7). Nevertheless, the primitive Mt.Mellone lava flow (AH-7a starting material) and scoriaclasts from lava fountains of the 1944 Vesuvius eruption(Marianelli et al., 1999) are both phono-tephritic incomposition. This suggests that the “Intermediate Stor-age” systems (Scandone et al., 2007) of the Alban Hillsand of Vesuvius were, at some stage of their history, fedwith a similar primitive composition but then the twosystems differentiated along different paths. In fact, whilethe differentiated products of large Alban Hills eruptionsare mostly K-foiditic (Freda et al., 1997), products of thelarge plinian eruptions from Vesuvius are mostlyphonolitic (see Cioni et al., 1995).

According to our experimental results these twodifferent liquid lines of descent are driven by different

magma–carbonate interaction processes. Alban Hillsplumbing system is characterised by the continuousinteraction between magmas and carbonate during thedifferentiation process (modelled by experiments per-formed by adding higher amount of CaCO3 whiledecreasing the temperature). This continuous interactionis ensured because the “Intermediate Storage” system isnot connectedwith the “Supply System” (Scandone et al.,2007) and is refilled by new batches of magma only afterlong time interval (on the order of 104 years; Marra et al.,2004). In the Vesuvius plumbing system the magma–carbonate interaction is not continuous (in time andspace) because of the occurrence of an “inflating”magmachamber (Fulignati et al., 2004). Vesuvian IntermediateStorage system, indeed, forms and grows due to theperiodic (on the order of 103 years) arrival of K-tephriticmagma batches. This implies the formation of a large,mature magma chamber in which new batches of magmacan differentiate without interacting with the carbonatewall rocks. For example, the 79 AD eruption was relatedto a large (N1 km3), subequant, mature magma chamber(Cioni et al., 1998) and is characterised by phonoliticpumices, phlogopite phenocrysts in magmatic cumulatesand low FeO in residual glasses (Cioni et al., 1995).

The comparison between Alban Hills and Vesuviushighlights that the efficacy of carbonate contaminationduring magmatic differentiation depends on the mode inwhich the plumbing system is fed.

5. Conclusions

Experiments performed on the most primitive AlbanHills composition (plagioclase-free phono-tephrite)under anhydrous, hydrous and hydrous-carbonatedconditions allowed us to constrain experimentally thehypothesis of magma–carbonate interaction at AlbanHills Volcanic District.

Experiments carried out at atmospheric pressureunder anhydrous conditions have leucite as the onlyliquidus phase over a wide temperature range. This is incontrast with the micro-textural features of natural rocksshowing clinopyroxene as early crystallizing phase. Inthe experiments performed under hydrous conditions weobserve a reduction of the leucite stability field withrespect to the clinopyroxene and phlogopite fields.Nevertheless, these experiments produced residualmelts with chemical compositions in contrast with theAlban Hills liquid line of descent. At these conditions,indeed, the evolved melts move towards higher SiO2

contents (i.e., phonolitic composition). Under hydrous-carbonated conditions (i.e., simulating magma–carbon-ate interaction) clinopyroxene is the early crystallizing

414 C. Freda et al. / Lithos 101 (2008) 397–415

phase, leucite occurs as second phase while magnetiteand phlogopite stability fields are significantly reduced.These conditions, by preventing the silica enrichment inresidual melts, are more appropriate to explain theAlban Hills liquid line of descent.

The comparison between Alban Hills and Vesuviusliquid lines of descent indicates that the dynamics of theplumbing system can play a crucial role duringassimilation processes. Our experimental results suggestthat the efficacy of carbonate contamination is possiblyrelated to the way in which parental magmas refill theplumbing system.

Acknowledgments

We are grateful to M. Serracino and J. Taddeucci forhelping us during the EMP and FESEM analyses, res-pectively and to G. Andreozzi for collecting and inter-preting Mössbauer spectra. G. Iacono Marziano andF. Gaillard gave inputs on the Alban Hills vs Vesuviusdiscussion.

Many thanks to D.R. Baker for sharing with us his“experimental skill” and for his advices which improvedthe manuscript.

R. Vieira Conceição and I. Veksler are thanked forrevising the manuscript; S. Foley is also acknowledgedfor his comments and suggestions.

This research was done in the frame of the activitiesof the HP-HT Laboratory of Experimental Volcanologyand Geophysics of INGV and was funded by INGV-DPC Project V3_1 Colli Albani and by Project FIRBMIUR “Development of innovative technologies for theenvironmental protection from natural events”.

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