ORIGINAL PAPER

In situ U–Pb rutile dating by LA-ICP-MS: 208Pb correctionand prospects for geological applications

Thomas Zack • Daniel F. Stockli • George L. Luvizotto •

Matthias G. Barth • Elena Belousova •

Melissa R. Wolfe • Richard W. Hinton

Received: 14 November 2009 / Accepted: 6 January 2011

� Springer-Verlag 2011

Abstract Rutile is a common accessory mineral that

occurs in a wide spectrum of metamorphic rocks, such as in

blueschists, eclogites, and granulites and as one of the most

stable detrital heavy minerals in sedimentary rocks. The

advent of rutile trace element thermometry has generated

increased interest in a better understanding of rutile forma-

tion. This study documents important analytical advances in

in situ LA-ICP-MS U/Pb geochronology of rutile: (1) Matrix

matching, necessary for robust in situ dating is fulfilled by

calibrating and testing several rutile standards (R10, R19,

WH-1), including the presentation of new TIMS ages for the

rutile standard R19 (489.5 ± 0.9 Ma; errors always stated as

2 s). (2) Initial common lead correction is routinely applied

via 208Pb, which is possible due to extremely low Th/U ratios

(usually \0.003) in most rutiles. Employing a 213 nm

Nd:YAG laser coupled to a quadrupole ICP-MS and using

R10 as a primary standard, rutile U/Pb concordia ages for the

two other rutile standards (493 ± 10 Ma for R19;

2640 ± 50 Ma for WH-1) and four rutile-bearing meta-

morphic rocks (181 ± 4 Ma for Ivrea metapelitic granulite;

339 ± 7 Ma for Saidenbach coesite eclogite; 386 ± 8 Ma

for Fjortoft UHP metapelite; 606 ± 12 Ma for Andrelandia

metepelitic granulite) always agree within 2% with the

reported TIMS ages and other dating studies from the same

localities. The power of in situ U/Pb rutile dating is illus-

trated by comparing ages of detrital rutile and zircon from a

recent sediment from the Christie Domain of the Gawler

Craton, Australia. While the U/Pb age spectrum from zircons

show several pronounced peaks that are correlated with

magmatic episodes, rutile U/Pb ages are marked by only one

pronounced peak (at ca 1,675 Ma) interpreted to represent

cooling ages of this part of the craton. Rutile thermometry of

the same detrital grains indicates former granulite-facies

conditions. The methods outlined in this paper should find

wide application in studies that require age information of

single spots, e.g., provenance studies, single-crystal zoning

and texturally controlled dating.

Keywords Rutile � Geochronology � LA-ICP-MS �SIMS � TIMS � Provenance studies � Granulites � Eclogites

Introduction

Rutile is a common accessory mineral that is found in a

wide range of rock types on Earth and beyond (e.g., on the

Moon; Marvin 1971). On Earth, all major rock types

Communicated by J. Hoefs.

Electronic supplementary material The online version of thisarticle (doi:10.1007/s00410-011-0609-4) contains supplementarymaterial, which is available to authorized users.

T. Zack (&) � M. G. Barth

Institut fur Geowissenschaften, Universitat Mainz,

Becherweg 21, 55128 Mainz, Germany

e-mail: zack@uni-mainz.de

D. F. Stockli � M. R. Wolfe

Department of Geology, University of Kansas,

1475 Jayhawk Boulevard, Lawrence, KS 66045, USA

G. L. Luvizotto

Max-Planck-Institut fur Chemie, Becherweg 27,

55128 Mainz, Germany

E. Belousova

GEMOC ARC National Key Centre,

Department of Earth and Planetary Sciences,

Macquarie University, Sydney, NSW 2109, Australia

R. W. Hinton

Grant Institute, The King’s Buildings,

West Mains Road, Edinburgh EH9 3JW, Scotland, UK

123

Contrib Mineral Petrol

DOI 10.1007/s00410-011-0609-4

exhibit rutile-bearing samples. However, rutile forms pre-

dominantly in metamorphic rocks. In general, rutile is more

common in metapelitic than in metamorphic rocks (see the

Ivrea example; Zingg 1980; Luvizotto and Zack 2009) and

is more common in higher-pressure samples compared to

their lower-pressure counterparts (see discussion in Zack

et al. 2002). Rutile in igneous rocks is restricted to rare

rock types (alkaline intrusions, kimberlites, MARID’s; see

Force 1980; Zack et al. 2002, 2004b). Clastic sediments

are important rutile-bearing rocks since rutile is one of

the most stable minerals during weathering, transport,

and diagenesis (Hubert 1962; Force 1980; Morton and

Hallsworth 1994).

Despite its common occurrence, rutile has so far

received less attention as a geochronometer than zircon and

monazite. One major reason is the low U concentrations in

many rutiles (as low as \0.01 ppm), which resulted in

numerous unsuccessful and hence discouraging U/Pb dat-

ing attempts. To overcome this obstacle, we have chosen

the straightforward practical approach of a fast preselection

where grains are analysed first for their U content and in a

second analytical session only rutile grains with sufficient

U are dated (in this study[5 ppm; see below). As a general

rule, rutile in metapelites and the majority of detrital rutiles

fulfill this requirement, while some hydrothermal rutiles

and only in exceptional cases rutile from eclogites are

suitable dating candidates.

Motivations for U/Pb rutile dating are multifold: (1) in

contrast to zircon, rutile is assumed to be unstable during

low-grade metamorphic conditions, so that rutile grown

during prograde metamorphic conditions should not con-

tain inherited cores of older metamorphic events (Zack

et al. 2004b, but see Stendal et al. 2006); (2) Zr contents in

rutile are very sensitive to temperature in rutile–zircon–

quartz assemblages (Zack et al. 2004a; Watson et al. 2006;

Tomkins et al. 2007), opening the prospect of combining

age with temperature information in single texturally

controlled minerals; (3) Nb/Cr ratios in rutile can be used

to further evaluate the source rock of single detrital grains

(Triebold et al. 2007), and combined with (2), giving one

rutile crystal several layers of information to be exploited

in provenance studies; (4) Pb diffusion in rutile is fast

compared to monazite and zircon (Cherniak 2000), so that

obtained U/Pb rutile ages have been used in TIMS dating

studies to put constraints on the cooling path (e.g., Mezger

et al. 1989; Moller et al. 2000); and (5) progress in rutile

dating may also spur investigation into rutile Hf isotope

systematics (Ewing et al. 2011), as this mineral has sub-

stantial Hf (we expect [200 ppm Hf in UHT granulites

having up to 8,000 ppm Zr and assuming a typical Zr/Hf

ratio of ca. 35; Jochum et al. 1986; Zack et al. 2004a, b),

but lacks Lu, therefore making it equally amenable as

zircon for obtaining Hf isotope continental crust extraction

ages (e.g., Thirlwall and Walder 1995; Griffin et al. 2000;

Hawkesworth and Kemp 2006; Kemp et al. 2006). Finally,

(6) like other U-bearing phases, ‘‘triple dating’’ (U/Pb,

fission track and U-Th/He; for apatite see Carrapa et al.

2009) on single grains is potentially possible, with the first

successful attempts at (U-Th)/He dating being conducted

on rutile (Stockli et al. 2007).

So far, the majority of rutile U/Pb dating studies have

been performed using thermal ionization mass spectrome-

try (TIMS) on mineral separates (e.g., Mezger et al. 1989;

Davis 1997; Moller et al. 2000; Cox et al. 2002; Schmitz

and Bowring 2003). U/Pb dating studies where single rutile

grains where analysed in situ are confined to Clark et al.

(2000) using SHRIMP, Vry and Baker (2006) using laser

ablation (LA)-MC-ICP-MS, Storey et al. (2007), Allen and

Campbell (2007), Birch et al. (2007) and Zack et al. (2007)

using LA-quadrupole-ICP-MS as well as Kooijman et al.

(2010) using LA-SC-ICP-MS. Where corrections are made

(or at least specified), initial common Pb (Pb not derived by

radioactive decay from Th and U in target phase; either

being incorporated in the phase during crystallization or

added as a contaminant) has been corrected by measuring204Pb. Only Clark et al. (2000), Allen and Campbell

(2007), Zack et al. (2007) and Kooijman et al. (2010) made

use of the observation by Mezger et al. (1989) that rutiles

contain very low Th (hence very low radiogenic 208Pb from232Th decay) and corrected for initial Pb by measuring208Pb in rutiles.

In this study, we have systematically evaluated in situ

dating of rutile by LA-ICP-MS including initial common

Pb correction by the 208Pb method. Here, we made use of

the recently introduced rutile mineral standard R10 as a

primary standard (Luvizotto et al. 2009). For this study, we

selected a variety of different samples where U contents in

rutile are sufficiently high, including granulite-facies

metapelites from Ivrea (IVZ-R25b), Fjørtoft (Z05-724-6)

and Andrelandia (PD-1), one eclogite from Saidenbach

(ERG-R2bb), one modern stream sediment from the

Christie Domain of the Gawler Craton (EGC-4), one

quartzite sample from Windmill Hills (WH-1) as well as a

large hydrothermal rutile from Blumberg (R19). The aim is

to establish and illustrate a routine where rutile in thin

sections can be dated with sufficient precision and accuracy

to exploit the main advantages of laser ablation based in

situ dating: texturally controlled dating with a wide-spread

technique at low cost and high throughput.

Analytical methods

All laser ablation (LA)-ICP-MS analyses for this study

were performed at the Institut fur Geowissenschaften at the

Universitat Mainz utilizing a system consisting of a New

Contrib Mineral Petrol

123

Wave UP213 Nd:YAG 213 nm laser ablation system

coupled to an Agilent 7500 ce quadrupole ICP-MS. The

setup for LA-ICP-MS analysis in general and specifically

U/Pb dating for such a system has been described in Jacob

(2006) and Jackson et al. (2004), respectively. However,

the most important characteristics and modifications are

outlined below.

Samples were either prepared as grain mounts (sepa-

rated grains embedded in epoxy) or as rectangular thin

sections with a thickness of ca 50 lm. This slightly higher-

than-normal thickness still allows detailed textural analysis

with a polarizing microscope while making it possible to

ablate for sufficient time before ablating through the

crystal. A polarizing microscope and BSE imaging were

used to find crack- and inclusion-free rutiles. Samples and

standards were inserted in an 8 cm round sample cell with

a small (0.5 mm) gas insert nozzle as described in Jackson

et al. (2004). Directly before U/Pb dating, samples were

cleaned by polishing the surface with c-alumina powder to

remove carbon coating, then put in an ultrasonic bath for

5 min with milli-Q water and finally dried with ethanol-

soaked kimwipe. Further cleaning (like cleansing with

nitric acid) was found to be ineffective since Pb-containing

surface contamination builds up during an analytical ses-

sion, probably caused by unavoidable accidental ablation

of epoxy and glass when drilling through crystals. Instead,

the most effective way to minimize contamination was

preablating the surface with 5 laser pulses 30 s before

starting each analysis with a beam diameter slightly larger

than during analysis (see Fig. 1). Each analysis measures

the background for 30 s before switching on the laser for

30–50 s. A fully automated analytical session consists of

acquiring data under identical measurement conditions

(e.g., spot sizes) of several blocks of 10 analyses of

unknowns each bracketed by 2–3 analyses of the rutile

standard R10. This setup allows acquisition of data for

more than 60 unknowns during an 8-h analytical session,

including sample changes, finding appropriate crystals,

programming the analytical protocol, and a washout time

of 120 s between each analysis. In Mainz, subsequent to

this study, we increased this output to[120 analysis in 8 h

by the installation of a large format cell (LFC; New Wave)

that allows fast washout (\20 s) and does not require

sample change.

U/Pb age data were collected by ablating rutiles with

laser beam diameters between 50 and 90 lm, a beam

energy density of ca. 3.5 J/cm2 and a repetition rate of

10 Hz. With an ablation rate of ca 0.1 lm/pulse, even 50 s

of ablation at the smallest beam diameter will produce an

ablation crater with an aspect ratio of\1 (diameter/depth),

therefore avoiding extreme element fractionation effects in

narrow craters (e.g., Eggins et al. 1998). The sample cell is

flushed by helium that carries the aerosol produced during

ablation to the ICP-MS. Before reaching the torch, the

carrier gas is mixed with argon, so that carrier gas flow

rates are ca. 0.75 l/min helium and ca 0.65 l/min argon.

Isotopes were measured in time-resolved mode. For

U/Pb dating, dwell times for each isotope for each mass

scan are 10 ms for (90Zr),232Th and 238U, 30 ms for 206Pb

and 50 ms for 207Pb and 208Pb. Adding the settling time (ca

2 ms, depending on size of mass jump) between each

isotope gives a total of ca 5 mass sweeps per second. Zr

was analyzed in selected sessions, and since dwell times

are only 10 ms, times per mass sweep were not signifi-

cantly compromised. Such rapid scanning allows a detailed

isotopic depth profile to be obtained for each spot analysis,

through which heterogeneous material (e.g., submicro-

scopic inclusions, surface contamination) can be detected

and excluded during data processing. Of special impor-

tance is the exclusion of 206Pb, 207Pb, and 238U spikes

during ablation that are found frequently (B10 spikes per

analysis) during rutile analysis. Spikes can be defined as

abnormal high count rates in a single time slice visible in

log-normalized plots (ca 3 times above the relative stan-

dard deviation of peak count rates; see Dunkl et al. 2008

for similar data rejection). These spikes are probably

caused by large particles produced during rutile ablation

(but e.g., not as frequent during zircon ablation) and seem

1000000a 238U

10000

100000 206Pb

207Pb

100

1000

coun

ts p

er s

econ

dco

unts

per

sec

ond

208Pb

1020 30 40 50 60 70

time in seconds

100000

1000000

b 238U

206Pb

1000

10000

54±6 cps 208Pb 86±22 cps 208Pb

207Pb

100

54±6 cps Pb

1020 30 40 50 60 70

time in seconds

Fig. 1 Time-resolved raw signals of typical rutile ablation signals

(a) without and (b) with preablation. [PH5]

Contrib Mineral Petrol

123

to be unavoidable at a laser wavelength of 213 nm due to

the good cleavage of rutile along {110} (Deer et al. 1992).

Zr, Th, and U concentrations, 206Pb/208Pb ratios, as well

as 207Pb/235U and 206Pb/238U ages are calculated offline

from time-resolved raw counts provided by the ICP-MS

with an in-house MS Excel spreadsheet. The problem of a

systematic change in element ratios due to fractionation

processes with the deepening of a laser crater (e.g., Eggins

et al. 1998) in U/Pb dating has been addressed previously

by (1) projecting the U/Pb ratios back to the beginning of

ablation by linear regression (Sylvester and Ghaderi 1997;

Horn et al. 2000) or (2) integrating exactly the same time

slices for standards and samples (Jackson et al. 2004). In

this study, we followed the latter strategy as fractionation is

not severe at the rather large spot sizes of 50–90 lm.

However, in contrast to Jackson et al., errors are not cal-

culated from counting statistics, but rather from standard

errors for selected isotopes of all selected time slices for

peak and background. Count rates measured on the rutile

standard R10 during a run under the exact same conditions

are used to calculate Zr, common Pb, Th, and U concen-

trations of the samples, taking the simplified assumption of

identical matrix effects between standard and unknown

rutiles. Under this assumption, it is not necessary to use an

internal standard element with known concentration to

correct for ablation yields. With sensitivities of ca

1,000 cps/ppm 90Zr and ca 2,000 cps/ppm 238U (calculated

from count rates on R10 with 790 ppm Zr and 50 ppm U;

Luvizotto et al. 2009) at laser spot sizes of 50 lm and

backgrounds of\40 cps for 90Zr,\20 cps for 206Pb, 207Pb,232Th, and 238U, and \30 cps for 208Pb, typical detection

limits (10 times the standard error on the background) are

\0.1 ppm for Zr,\0.02 ppm for Pb and\0.01 ppm for Th

and U. As Th in the standard rutile is extremely low,

sensitivities for 232Th are treated to be identical to 238U

sensitivities (valid within a 20% uncertainty; based on

sensitivities for NIST NBS 610 glass determined at the

beginning of each session).

For low Th phases like rutile, common Pb can be

accurately calculated by measuring 208Pb (Pbmeas), know-

ing that most 208Pb is not derived from radioactive decay of232Th within the phase. In an identical way as common Pb

(Pbcom) is calculated by measuring 204Pb, the radiogenic Pb

component of 206Pb and 207Pb (Pbcorr) from an analysis is

calculated from a given 206Pb/208Pb and 207Pb/208Pb ratio,

respectively and the 208Pb count rate:

206Pbcorr ¼206 Pbmeas �208 Pbmeas � ð206Pb=208

PbÞcomÞ ð1Þ207Pbcorr ¼207 Pbmeas �208 Pbmeas � ð207

Pb=208PbÞcomÞ ð2Þ

For young samples, we use (206Pb/208Pb)com and

(207Pb/208Pb)com ratios of 0.474 and 0.413, respectively.

Allowing an uncertainty of 2% (from 0.465 to 0.483 and

from 0.405 to 0.421, respectively), this spans Pb ratios

from crustal ages between 1,000 Ma and today using the

Stacey and Kramers (1975) model.

The corrected 206Pb and 207Pb count rates are then used

to calculate 207Pb/206Pb, 206Pb/238U and 207Pb/235U ratios

(235U = 238U/137.88). For standardizing U/Pb rutile anal-

yses, a correction factor for these ratios is calculated from

an average of all R10 analyses at a given spot size of one

run, using average TIMS ratios for R10 corresponding to a

concordia age of 1,090 ± 5 Ma; Luvizotto et al. (2009).

We note that drift within a short run (less than 2 h) is

always less than the scatter between consecutive analysis,

so that average ratios of the standard R10 were used to

calculate fractionation errors. Significant drift is only

observed at the beginning of an analytical session. We

therefore did not start analysing unknown samples for the

first 2 h after starting the ICP-MS. With spot sizes of

50 lm, standard deviations (2 s) on 207Pb/206Pb,206Pb/238U and 207Pb/235U ratios for R10 are typically less

than 3.5% within one run. Reported errors on ratios and

ages for a single analysis are calculated by propagating

errors on the peak and background of standards and sam-

ples, a 0.5% TIMS error on the R10 standard as well as a

2% error on the 206Pb/208Pb and 207Pb/208Pb ratios for

common Pb correction (see above). Decay constants used

for calculating 206Pb/238U and 207Pb/235U ages are238U = 0.155125 9 10-9 y-1 and for 235U = 0.98485 9

10-9 y-1, respectively (Steiger and Jager 1977). The same

decay constants were also used for calculating TIMS and

ion microprobe ages (see below). Concordia and weighted206Pb/238U ages for a given population of rutiles were

calculated and graphically presented using Isoplot/Ex 3

(Ludwig 2003).

For concentration measurements, the isotopes 28Si, 49Ti,51V, 53Cr, 90Zr, 93Nb, 120Sn, 181Ta, 184W, 208Pb, 232Th, and238U are analyzed (dwell times 10 ms each). The glass

NIST SRM 610 (concentrations taken from Jochum and

Stoll 2008) is used as a standard which is analyzed twice

with a laser beam diameter of 50 lm for every 20 unknown

analyses. It has been found that even with 15 s of ablation

at a laser beam size of 12 lm, all determined trace ele-

ments are above the detection limit in rutile (except Si, Pb,

and Th, which are only monitored to detect and exclude

contamination caused by cracks or inclusions and to notice

if unusually Th-rich rutiles are encountered). Therefore,

analytical time for concentration measurements for rutiles

can be minimized, so that at 30 s for one analysis (15 s

collection of background and 15 s collection of sample)

and 30 s wash out between analysis, ca 50 unknown

samples and 8 standards can be analysed in an hour. Ti is

used as the internal standard element, with TiO2 concen-

trations in rutile set as 100 wt%. The commercially avail-

able software Glitter is used for quantification. The validity

Contrib Mineral Petrol

123

of non-matrix matching for concentration measurements

employing different spot sizes for standard and samples has

been evaluated by comparing results of the rutile standard

R10 with published values (Luvizotto et al. 2009). We

would like to add that non-matrix matching is valid as long

as accuracy of within 20–30% is sufficient. However,

accuracy better than 10% requires calibration with help of

rutile standards (Zack, unpublished data).

TIMS U–Pb analyses were carried out at the University

of Kansas (Department of Geology). Single fragments

from the R19 crystal were dissolved employing the pres-

sure-vessel HF–HNO3 dissolution methods of Krogh

(1973) and Parrish (1987). Elemental separation was per-

formed with HCl anion column chemistry to eliminate Ti

and separate both Pb and U. Isotopic analyses were

determined on a VG Sector multicollector thermal ioni-

zation mass spectrometer on separate Pb and U fractions.

A mass fractionation correction of 0.16 ± 0.05%/amu, as

determined by standard runs on NBS 981 (common Pb)

and NBS 982 (equal atom Pb), was applied to the Pb data.

A mass fractionation of 0.10 ± 0.05%/amu, as determined

by standard runs on U500, was applied to all U data.

Samples were spiked with a mixed 205Pb/235U spike.

Errors on 206Pb/204Pb were minimized by the use of a

Daly multiplier and are typically on the order of 1% or

less. Common lead corrections were made using values

determined from Stacey and Kramers (1975) for the

interpreted crystallization age.

Ion microprobe U/Pb analyses were carried out at the

University of Edinburgh (School of Geosciences) on the

Cameca ims-1270. Analytical conditions are similar to

those used for zircon (Kelly et al. 2008) except that no O2

flooding was employed. Analyses were made using a pri-

mary beam of 11nA 16O2- with net impact energy of

22 keV, Kohler illumination and approximately 25–30 lm

spot size. As usual a field aperture was inserted to reduce

the number of secondary ions coming from the outer part of

the sputtered area. It was found that for rutile the O2

flooding strongly enhanced the ionisation of the surface

contamination at the periphery of the sputtered area,

especially that of common Pb. Since the enhancement of

the Pb signal with O2 flooding was only 25%, yet the

potential contamination from common Pb could be 1–2

orders of magnitude higher with it present, no O2 flooding

was employed. Under the analytical conditions used, 204Pb

was typically less than 0.05 ppb whereas with flooding

could be up to 0.5–1 ppb. U/Pb ratios were determined

using R10 rutile as standard. We used a correction proce-

dure based on ln Pb/U vs ln UO2/UO, resulting in a much

smaller slope than is observed for zircon (0.18 compared to

2.6 used for zircon, Kelly et al. 2008). Reproducibility of

the standard was about 1% point to point. Similar count

rates were observed at masses 196 and 212 (HfO and Zr2O2

species) for both standards and unknowns, the former peak

being used to check both mass calibration and correcting

for changes to the beam position from point to point.

Absolute concentrations are approximate and assume R10

rutile contains 50 ppm U. The detector background during

these analyses (measured at mass 204.5) was 0.008 cps,

equivalent to about 0.05 ppb Th. Because common Pb was

dominated in the ion microprobe analyses by surface

contamination, rather than that from the mineral, correc-

tions were made for present-day lead.

Results

For this study, 141 single spot U/Pb analysis of 8 samples

(1) were measured by LA-ICP-MS during the course of

18 months. From this data set, we excluded 9 analyses due

to irregular ablation signals. The remaining rutile analyses

share some remarkable properties. As can be observed

from Supplementary Material 1, more than 80% of all

analyses have Th concentrations below detection limit (at

least\0.05 ppm). Even for analyses with laser spot sizes of

90 lm, Th concentrations are often below the detection

limit of ca 0.003 ppm. With U concentrations in the ana-

lyzed rutiles between 6 and 274 ppm, Th/U ratios are

between less than 0.0001 and 0.05 (90% of the analyses

give Th/U ratios of less than 0.003). To our knowledge,

these are the lowest Th/U ratios of any datable U-bearing

mineral. Furthermore, more than 90% of all rutile analyses

show common Pb concentrations, calculated from count

rates of 208Pb, below 0.2 ppm (total range from less than

0.01–1.2 ppm). Depending on age and U concentration in a

given grain, this often translates to favorable 206Pb/208Pb

ratios, with 90% of the data showing ratios over 10 (total

range from 1 to more than 2,000). Please note that in

crustal lead, 208Pb is ca. 40 times more abundant than204Pb, so that e.g., a 206Pb/208Pb ratio of 10 translates into a206Pb/204Pb ratio of ca. 400. As explained below, all U/Pb

ages are reported with an ‘‘expanded error’’ of 2% (2 s).

Rutile mineral standards

R10

Traverses through the ca 1 cm large rutile standard R10

(from Gjerstad, Norway; see detailed description in

Luvizotto et al. 2009) performed by HR-SIMS in Edin-

burgh and LA-ICP-MS in Mainz reveal no detectable age

zoning, at least not at distances C200 lm from the rim of

the crystal (Table 1; Fig. 2). This allows us to claim that

Contrib Mineral Petrol

123

R10 is not only a favorable secondary mineral standard for

several important trace elements but is also suitable as a

standard for U/Pb rutile dating (Luvizotto et al. 2009).

Uranium concentrations are sufficiently high (ca 50 ppm;

Luvizotto et al. 2009) and ages sufficiently old (1,090 Ma)

to give favourably high count rates on 207Pb even for

50 lm spots by LA-quadrupole-ICP-MS. Additionally,206Pb/208Pb ratios typically of [400 make common Pb

corrections for this sample negligible.

R19

The sample R19 is a ca 500 mm3 sized single crystal from

Blumberg, Australia and has been studied in detail for a

range of trace elements and Hf isotopes (Luvizotto et al.

2009). For this study, we obtained TIMS ages on four

crystal fragments (Table 2; Fig. 3). A weighted mean206Pb/238U age of 489.5 ± 0.9 Ma was obtained. This age

is consistent with the exhumation history of the area

around Blumberg. Here, the Delamerian orogeny was the

last major tectonic episode where exposed rocks cooled

below 400�C between 485 and 490 Ma as recorded by Ar–

Ar ages of biotite and hornblende (Turner et al. 1996;

Foden et al. 2006). U/Pb ages for rutile determined by LA-

ICP-MS during the course of this study give an average

concordia as well as a weighted mean 206Pb/238U age of

493 ± 5 Ma (MSWD = 0.0088; Fig. 4). This is within

error of the TIMS data. As discussed below, multi-day

reproducibility reveals larger uncertainties.

WH-1

A third rutile sample for which TIMS data are available

(2,625 Ma determined by L.M. Heaman; cited in Clark

et al. 2000) was extracted from the Windmill Hill quartz-

ites, Jimperding metamorphic belt, Western Australia. The

pooled U/Pb concordia age of 2,640 ± 13 Ma

(MSWD = 1.07) determined by LA-ICP-MS (Fig. 5) has a

2 s error of only 0.4% due to unusually high UTa

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24

2.7

0.0

00

10

.00

00

03

86

,47

31

.91

00

.03

30

.18

20

0.0

02

30

.73

0.0

76

12

0.0

00

32

1,0

85

11

1,0

78

13

1,0

98

8

34

0.3

\0

.00

01

\0

.00

00

02

34

,84

51

.88

20

.03

40

.18

21

0.0

02

50

.76

0.0

74

96

0.0

00

30

1,0

75

12

1,0

78

14

1,0

67

8

43

3.3

0.0

02

60

.00

00

82

3,0

39

1.9

03

0.0

33

0.1

83

50

.00

23

0.7

00

.07

52

00

.00

04

31

,08

21

21

,08

61

21

,07

31

2

54

2.1

\0

.00

01

\0

.00

00

02

31

,65

81

.87

70

.03

30

.18

22

0.0

02

40

.73

0.0

74

73

0.0

00

36

1,0

73

12

1,0

79

13

1,0

61

9

64

1.5

\0

.00

01

\0

.00

00

02

39

,48

71

.94

30

.03

80

.18

62

0.0

02

80

.76

0.0

75

70

0.0

00

46

1,0

96

13

1,1

01

15

1,0

87

12

aR

awd

ata

bC

orr

ecte

dfo

rco

mm

on

Pb

(fro

m206P

b/2

08P

bra

tio

s)an

dfo

rm

ach

ine-

ind

uce

dfr

acti

on

atio

n

1120

1160

1200

R10, SIMS, LA-ICP-MS

1000

1040

1080

0 1 2 3 4 5 6 7 8

age

in M

a

diameter in mm

Fig. 2 206Pb/238U age profile for the entire rutile crystal R10. Crystal

rims at 0 and 8 mm. Please notice that standarization for LA-ICP-MS

(red) and SIMS (black) is achieved via normalization using TIMS age

from the same grain. No age variation is visible within the grain

outside the analytical uncertainty

Contrib Mineral Petrol

123

concentrations in this sample (ca. 200 ppm; see Supple-

mentary Material 1). The deviation from the TIMS age is

about 0.6%, surprisingly good especially if an error for the

TIMS age of at least 0.2% is taken into account.

Error propagation and need for an expanded error

In this study, we chose to rigorously propagate errors from

standard errors for each isotope (here 206Pb, 207Pb, 208Pb

and 238U) of all selected time slices (in cps) of peaks and

backgrounds. However, a very low MSWD for Z05-724-3

and PD-1 (see below) points to an overestimation of errors

for single spots. Possible reasons are a change of signal

intensity during an analysis and fractionation of 206Pb/238U

ratios during ablation. Small spot sizes favour such effects

and may be the reason for small MSWD for sample Z05-

724-3 (all laser spots are 50 lm in size). Still, especially

for large spots sizes (70–90 lm) such changes during 30 s

of ablation are minor. Furthermore, the chosen approachTa

ble

2T

IMS

dat

afo

rru

tile

stan

dar

dR

19

Wei

gh

t

(mg

)

U (pp

m)

Pb

*/

Pb

c

Pb

com

(pg

)

Rat

io±

2s

(%)

Rat

io206P

b/2

38U

±2

s(%

)R

atio

207P

b/2

06P

b±

2s

(%)

Err

corr

(rh

o)

Ag

e(i

nM

a)207P

b/2

35U

Ag

e(i

nM

a)206P

b/2

38U

Ag

e(i

nM

a)207P

b/2

06P

b206P

b/2

04P

b208P

b/2

04P

b207P

b/2

35U

10

.12

11

7.0

6.4

81

8.6

74

74

37

.80

.61

83

0.8

10

.07

87

90

.39

0.0

56

91

0.6

50

.61

48

9.6

48

8.9

49

2

20

.10

81

0.3

5.1

91

4.2

23

83

38

.00

.62

19

0.8

50

.07

92

00

.43

0.0

56

95

0.7

00

.58

49

2.1

49

1.4

49

5

30

.19

61

5.1

36

.65

.87

25

81

39

.90

.61

51

0.4

10

.07

85

50

.32

0.0

56

79

0.2

40

.81

48

6.9

48

7.4

48

4

40

.14

91

6.6

14

.97

.95

10

67

36

.60

.61

90

0.7

30

.07

89

50

.34

0.0

56

87

0.5

50

.71

48

9.6

48

9.8

48

8

Av

g0

.61

86

0.0

78

87

0.0

56

88

48

9.6

48

9.4

48

9.8

2s

Std

0.0

05

60

.00

05

40

.00

01

44

.23

.39

.3

4920.0792

0.0796

/238U

R19, TIMS

488

0.0784

0.0788

206 P

b/

Concordia Age = 488.9 ±0.9 Ma

MSWD (of concordance) = 1.8,

Probability (of concordance) = 0.18

0.07800.608 0.612 0.616 0.620 0.624 0.628 0.632

207Pb/235U

Fig. 3 Concordia plot for TIMS data of rutile standard R19

0.11

R19, LA-ICP-MS650

600

0.08

0.09

0.10

206 P

b/23

8 U550

Concordia Age = 493 ± 5 MaMSWD (of conc.) = 0.0088,Probability (of conc.) = 0.93

400

0.05

0.06

0.07

0 0 0 2 0 4 0 6 0 8 1 0 1 2 1 4

450

350

. . . . . . . .207Pb/ 235U

Fig. 4 Concordia plot for LA-ICP-MS data of rutile standard R19

Contrib Mineral Petrol

123

guarantees that any instability of the ablation process and

any sample heterogeneity during a single analysis is

accounted for and also allows for a robust common Pb

correction (see Eqs. 1 and 2).

Large numbers of laser ablation ages are often pooled

(e.g., with help of Isoplot; Ludwig 2003), resulting in

apparent statistically meaningful errors of significantly less

than 1%. If applied to all results for rutile standard R19, we

obtain an error for concordia ages of ca 1% (493 ± 5 Ma);

this seems to indicate that accuracy of the laser-based U/Pb

dating method in this study would be in the order of 1%.

However, close inspection of the data reveals that average

ages for each analytical session (Fig. 6) are different from

day-to-day (a student t-test for two populations with

485 ± 7, n = 5 and 499 ± 9, n = 5 gives a significantly

positive result). Furthermore, one analytical session is just

within the error of the expected TIMS age of ca 490 Ma

(499 ± 9 Ma from 12/05/2007). Although subtle, results

for the rutile standard R19 from day-to-day (‘‘long-term

reproducibility’’) are not perfectly consistent with expected

TIMS ages. We therefore conclude at this stage that a

conservative estimate of the accuracy of this method is

±2% (based on the difference between 490 and 499 Ma);

that is, an uncertainty of this magnitude should be added

after pooling. Run-to-run variations within this limit may

arise due to hitherto uncorrected fluctuations, matrix effects

and/or undetected inhomogeneities in the mineral standards

(R10, R19). This means that no age can be determined with

an accuracy of better than 2%, even if counting statistics

and stability of the signal results in precisions of sometimes

\1% (e.g., data from Ivrea, see below). We will therefore

‘‘expand’’ errors calculated from pooled ages, so that

reported errors are always at least 2%. For rutile standard

R19, we therefore report an LA-ICP-MS age of

493 ± 10 Ma and for WH-1 an age of 2,640 ± 50 Ma.

Further studies are needed to better understand and mini-

mize systematic variations within the 2% (for laser-induced

fractionation fluctuations see Jackson 2008).

Rutile in metamorphic rocks

All samples from metamorphic rocks analysed in this

study have more than 10 large rutiles ([100 lm) within

one rectangular thin section suitable for dating. Results

from concentration measurements gave no U concentra-

tions of \5 ppm, so that no bias is introduced in age

determination from neglecting rutiles with certain com-

positions. In the following, results from this study are

reported as pooled concordia ages together with an

expanded error of 2%, so that errors are larger than stated

in the figures.

IVZ-R25b

This sample is from a grt-sil-ksp ± bt-gneiss from the

granulite-facies area of the Ivrea-Verbano Zone, Southern

Alps, Italy. Locations, sample description and data on rutile

chemistry are described in Luvizotto and Zack (2009). For

this study, we have only included data from the largest

analyzed rutiles ([200 lm). An age dependence with

0 55 2700

206 P

b /

238U

2800

2700

2600

0.49

0.51

0.53

.

6P

b/23

8 U a

ge

in M

a

2620

2660

WH-1

207Pb / 235U

Mean Age = 2645±9 Ma [0.4%] MSWD = 0.47, probability = 0.91

2500

0.45

0.47

10 11 12 13 14 15

20

2540

2580Concordia Age = 2640±10 MaMSWD (of conc.) = 1.07,

Probability (of conc.) = 0.30

Fig. 5 Concordia plot and206Pb/238U ages from LA-ICP-

MS data for rutile standard

WH1

530

550

90 µm499 ±±10 Ma 499 ± 9 Ma 485± 7 Ma

70 µm70 µm

510

530

8 U

490206 P

b/23

450

470

12/06/07 03/18/08 11/13/08

Fig. 6 206Pb/238U ages for rutile standard R19 over time. Boldhorizontal line at 489 Ma represents the TIMS age (2 s error within

thickness of line)

Contrib Mineral Petrol

123

relation to grain size (see e.g., Schoene and Bowring 2007;

Kooijman et al. 2010) will be discussed elsewhere (Zack

et al. in preparation).

A pooled U/Pb rutile age of 181 ± 4 Ma (MSWD =

0.66; Fig. 7a) is significantly younger than the U/Pb ages of

ca. 300 Ma for zircons and monazites interpreted to have

grown during peak metamorphic conditions (Vavra and

Schaltegger 1999; Schaltegger et al. 1996, 1999), but

comparable to Rb/Sr biotite ages of 184 ± 15 Ma from

granulites of the same area (Graeser and Hunziker 1968).

These ages are bracketed by Ar–Ar hornblende ages of

202–211 Ma and K–Ar biotite ages of 156–159 Ma from

samples of the upper Val Strona Valley (Siegesmund et al.

2008).

0.05 200a Ivrea IVZ-R25b

300

250

150

0.02

0.03

0.04

6 Pb

/238 U

ag

e in

Ma

170

180

190

Conc. Age = 181.2±1.5 Ma

206 P

b/2

38U

200

0.010.05 0.10 0.15 0.20 0.25 0.30 0.35

206

160

400

Mean = 181.3±1.5 Ma [0.8%] MSWD = 1.01, probability = 0.44

MSWD (of conc.) = 0.66probability (of conc.) = 0.42

207Pb/235U

100

206 P

b/23

8 U

/238U

ag

e in

Ma

320

360450

250

0.05

0.07

0.09

b Saidenbach ERG-R2b

400

207Pb/235U

2

206 P

b/

240

280Mean Age = 339±3 Ma [0.9%] MSWD = 1.3, probability = 0.15

Conc. Age = 339±4 Ma MSWD (of conc.) = 0.20

probability (of conc.) = 0.66

250

150

0.01

0.03

0.1 0.2 0.3 0.4 0.5 0.6 0.7

200

Pb/ U440

t ft Z05 724 3

6 Pb

/238U

ag

e in

Ma

360

380

400

420550

450

350

0.07

0.05

0.09

206 P

b/2

38U

C A 386±5 M

c Fjørtoft Z05-724-3

206

320

340 Mean Age = 386±5 Ma [1.3%]MSWD = 0.66, probability = 0.79

250

0.030.2 0.4 0.6 0.8

207Pb/235U

Conc. Age = 386±5 Ma MSWD (of conc.) = 0.057

probability (of conc.) = 0.81

650

750

0.10

0.12

0.14

b/23

8 U

640

680

ag

e in

Mad Andrelandia PD-1

450

550

0.06

0.08

0.4 0.6 0.8 1.0 1.2

206 P

520

560

206 P

b/23

8 U

Mean Age = 606±5 Ma [0.9%]MSWD = 1.6, probability = 0.054

Conc. Age = 606±5 Ma MSWD (of conc.) = 0.042

probability (of conc.) = 0.84

207Pb/235U207Pb/235U

600

Fig. 7 Concordia plot and206Pb/238U ages for LA-ICP-MS

data of rutiles from (a) Ivrea

(sample IVZ-R25b),

(b) Erzgebirge (sample ERG-

R2b), (c) Western Gneiss

Region (sample Z05-724-3) and

(d) Andrelandia (sample PD-1).

Please note that errors, MSWD,

and degree of concordance are

directly taken from the Isoplot

results. Hence, errors are not

expanded

Contrib Mineral Petrol

123

ERG-R2b

This sample from the Saidenbach Reservoir (Erzgebirge,

Germany) is the only eclogite in this study that bears rutiles

with sufficient U content (12–20 ppm; see Supplementary

Table 1). Rutiles from eclogites from the few other local-

ities (identical with list of investigated eclogites in Zack

and Luvizotto 2006) investigated so far have U concen-

trations of \2 ppm (Zack et al. 2002, unpublished data).

The Saidenbach eclogites are unusual as whole rocks are

highly enriched in incompatible trace element contents,

e.g., (La/Sm)N = 1.2–3.0 and (Nb)N = 20–170 (Massonne

and Czambor 2007), most likely representing within-plate

basalt protoliths. The highly trace element enriched nature

of these eclogites is also apparent from unusually high

Cr/Nb ratios of greater than one in rutiles from this locality,

otherwise atypical for rutiles from mafic sources (Triebold

et al. 2007).

Pooled U/Pb ages of rutile give 339 ± 6 Ma

(MSWD = 0.20; Fig. 7b) that is identical within error to

U/Pb ages of metamorphic zircons from nearby diamond-

bearing gneisses (TIMS ages of 340.5 ± 1.1 Ma from

Kroner and Willner 1998 and SHRIMP ages of diamond-

bearing zircon zones of 336.8 ± 2.8 Ma from Massonne

et al. 2007). Rapid cooling in this area is indicated by

Ar/Ar cooling ages of white micas of ca 336 Ma (Werner

and Lippolt 2000), although these authors also report

resetting of white mica Ar/Ar ages as young as ca 330 Ma

(Werner and Lippolt 2000), coincident with overgrowth

zones in Saidenbach zircons (330.2 ± 5.8 Ma; Massonne

et al. 2007). Considering all the evidence, our new U/Pb

rutile ages fit into a model for the cooling history of the

area around Saidenbach (the Gneiss Eclogite Unit) where

rapid exhumation from more than 90 km to mid crustal

levels occurred in less than 4 Ma (340–336 Ma), with a

rapid heat pulse ca 6 Ma afterward.

Z05-724-3

This sample is from the only known locality (Fjørtoft) in

the Western Gneiss Region (WGR, Norway) where dia-

mond has been reported in metapelites (Dobrzhinetskaya

et al. 1995). The mineral assemblage is only slightly ret-

rograded (biotite, phengite and plagioclase formation) and

consists mostly of the primary phases garnet–kyanite–

quartz–rutile. Rutile is always very fresh and occurs as

inclusions in garnet and as a matrix phase.

Rutile U/Pb ages give a pooled age of 386 ± 6 Ma

(MSWD = 0.057; Fig. 7c). This result can be compared

with other geochronological data, since this unique locality

has spurred extensive investigations. U/Pb ages determined

by SHRIMP are 398 ± 6 Ma for matrix monazites (Terry

et al. 2000), while the youngest U/Pb ages for zircon

determined by TIMS are 398 ± 2 Ma (Krogh et al. 2003)

and Ar/Ar ages on biotite are 380 ± 4 Ma (Root et al. 2005).

We can therefore conclude that U/Pb rutile ages from this

study (ca. 386 Ma) are bracketed by published U/Pb mon-

azite/zircon ages on the one side (ca. 398 Ma) and Ar/Ar

biotite ages on the other side (ca. 380 Ma). This is consistent

with previous studies which found the same age relation-

ships when U/Pb rutile ages have been determined by TIMS

and strongly indicate typical (depending on cooling rate and

grain size) closure temperatures of around 400�C (e.g.,

Mezger et al. 1989; Moller et al. 2000). Recent U/Pb rutile

ages dated by TIMS from other localities within the WGR

range from 385 to 392 Ma (Kylander-Clark et al. 2008) and

are in excellent agreement with results from this study.

PD-1

This sample is a kyanite-garnet-ternary feldspar-bearing

felsic granulite from the Andrelandia Complex, Brazil

(Moraes et al. 2003). Descriptions of this sample can be

found in Zack et al. (2004a, b). Zircon U/Pb ages show

two peaks at 650 ± 4 Ma and 605 ± 6 Ma, the latter one

is interpreted to correspond with emplacement ages of

the Andrelandia Complex nappe stack (Reno et al. 2006).

The U/Pb rutile age of 606 ± 12 Ma (MSWD = 0.042;

Fig. 7d) from this study corresponds to this younger peak

in zircon ages, consistent with cooling contemporaneous

with the nappe emplacement.

Rutile in provenance studies

EGC-4

This sample is collected from a modern stream sediment

from the Gawler Craton (South Australia), already used in

a reconnaissance study on trace elements, U/Pb and Hf

isotopes of detrital zircons (Belousova et al. 2006, 2009).

Analyzed detrital rutiles are from the same mineral fraction

(non-magnetic, heavy mineral fraction of grain sizes from

150 to 250 lm) from which zircons have been separated.

This study allows a comparison of U/Pb ages derived from

detrital zircons and rutiles within a given sediment (see

also Okay et al. in press). Seventy-two rutile grains have

been analyzed for trace elements with a 10-lm laser spot

(Supplementary Material 2), from which 28 grains (ca

40%) have been chosen for further U/Pb analysis with a

50-lm spot based on their high U concentrations (here

C20 ppm). All 28 analyzed rutiles plot on the concordia,

spanning an age range between 1,660 and 2,100 Ma, with

one single grain plotting on the concordia at ca 550 Ma

(Fig. 8). Belousova et al. (2006, 2009) argue that all zir-

cons with ages\1,000 Ma have to have been derived from

Contrib Mineral Petrol

123

outside the Gawler Craton perhaps introduced by aeolian

transport, as no tectonic activity is reported for the Gawler

Craton younger than 1,000 Ma, with ages of between 500

and 600 Ma most likely stemming from the Adelaide Fold

Belt. We concur with this conclusion and argue that the

single 550 Ma rutile grain also derives from the Adelaide

Fold Belt. More than 40% of all dated grains fall within a

narrow age range of 1,675 ± 40 Ma.

We note that 28 dated rutiles are sufficient to conclude that

the U/Pb age spectrum for rutiles from sample EGC-4 forms

only one pronounced peak (at ca 1,675 Ma). According to

Vermeesch (2004), there is a 95% confidence that no fraction

of more than 15% abundance is missed. We cannot exclude

that rutiles of\20 ppm U will have a different age spectrum.

Still, U contents of [20 ppm are not confined to exotic

sources, but rather do occur in a range of typical rock types

like metapelites and hydrothermal environments.

An important result of this study is that U/Pb ages of

rutile will provide new insights for provenance studies

when combined with U/Pb ages of zircon. This becomes

clear when the differences are compared for rutile and

zircon in the age distribution (Fig. 9). While rutile ages

show only one marked peak at 1,675 Ma, zircon ages of

[1,000 Ma show three equally marked peaks at 1,150,

1,733, and 2,243 Ma (Fig. 9 and Belousova et al. 2006).

We argue that the majority of zircon ages reflect magmatic

episodes (related to Grenvillian, Kimban, and Sleafordian

orogenic activities, respectively), while rutile ages reflect

cooling of this part of the craton below ca 400�C. Sample

EGC-4 is from the middle of the Christie Domain. Here,

Tomkins et al. (2004) dated the last ductile deformation at

around 1,670 Ma, consistent with rutile U/Pb ages. It is

remarkable that none of the analyzed rutiles is reset during

the last magmatic episode at ca 1,150 Ma, implying that

intrusions/extrusions in this part of the Christie Domain

were localized and did not lead to large-scale thermal

perturbations. Alternatively, it cannot be completely ruled

out that the 1,150 Ma age zircon population represents

material transported from the nearby Musgrave Block

(Belousova et al. 2009). This is however unlikely as the

Musgrave Block should also have provided detrital rutile.

The majority of all analyzed rutiles have Zr concentra-

tions consistent with peak temperatures at granulite-facies

conditions ([700�C; see Fig. 10). Such high temperatures

are found both for the 1,675 Ma age group and for the older

rutiles. We argue that temperature and U/Pb age informa-

tion of single rutiles are decoupled if derived from areas

with slow cooling histories as diffusion rates of Zr and Pb

are substantially different (closure temperatures for

500-lm-sized rutiles are ca 700�C for Zr and 400�C for Pb;

Cherniak et al. 2007; Mezger et al. 1989; Luvizotto and

Zack 2009; but see Cherniak 2000 for a different per-

spective). What can be concluded from the Zr data of

detrital rutiles from EGC-4, however, is that most rutiles

likely originated at granulite-facies conditions, with no

direct age information on their origin. Even more specific,

0.4

0.5

Gawler Craton EGHC-42200

1800

10000.2

0.3

206 P

b/23

8 U

1400

0.0

0.1

86420207Pb/ 235U

600

Fig. 8 Concordia plot for detrital rutiles from the northern Gawler

Craton (sample EGHC-4)

0

2

4

6

8

10

12

14

16

0 500 1000 1500 2000 2500 3000 3500207Pb/235U age

rela

tive

prob

abili

ty

0

2

4

6

8

10

12

14

0 500 1000 1500 2000 2500 3000 3500

rela

tive

prob

abili

ty

rutile

207Pb/235U age

zircon

500

550

1150

1733

2443

1675

Fig. 9 Comparison of age histograms for detrital rutiles and zircons

from the northern Gawler Craton (sample EGHC-4). Gray areaindicates age range interpreted to be produced only by detrital grains

transported from outside the Gawler Craton

Contrib Mineral Petrol

123

maximum peak temperatures of almost 1,100�C may be

deduced (see Fig. 10). This is not unlikely for rocks from

the Christie Domain, as granulite-facies conditions of ca

800�C have been recorded here by Tomkins et al. (2004).

Sapphirine–quartz assemblages indicative of minimum

temperatures of [950�C have indeed been found in the

neighboring Nawa Domain (see Tomkins et al. 2004).

Uranium and Th incorporation in rutile

One of the most striking features regarding the geochem-

ical properties of rutile is its extremely low Th content (see

Mezger et al. 1989). As noted,[80% of all analyzed rutiles

have Th concentrations of \0.05 ppm and Th/U ratios of

\0.003. Several explanations have been given for varying

Th/U ratios in accessory minerals: (1) Availability of Th

and U during crystal growth may be an important factor for

low Th/U ratios, especially if other coexisting Th-rich

accessory phases like monazite are present in the reaction

domain (Harley et al. 2007); (2) Th/U ratios can also be

strongly influenced by the precursor phase (Romer 2001)

and hence inherit this ratio from its reaction site rather than

being in equilibrium with apparently coexisting nearby

phases; (3) The commonly (but see exceptions in Moller

et al. 2002) observed change from high Th/U ratios of

magmatic zircons to low Th/U ratios in metamorphically

recrystallized zircons has been explained in the framework

of the lattice strain model, with high temperatures favoring

incorporation of larger elements and low temperatures

accentuating differences in cation sizes (Th4? is about 4%

larger than U4?; Hoskin and Black 2000).

Results from this study indicate that explanations (1)

and (2) cannot be the dominant process of producing the

extremely low Th/U ratios in rutile. As demonstrated,

all investigated rutiles show low Th/U ratios. However,

they come from very different lithologies. For example,

ERG-R2b is an eclogite where no monazite is found, while

IVZ-R6, Z05-724-3 and PD-1 are from metapelites with

abundant monazite coexisting with rutile. Furthermore

rutiles in IVZ-R6 formed from biotite breakdown

(Luvizotto and Zack 2009), while the hydrothermal rutiles

R10 and R19 most likely precipitated directly out of a fluid.

Instead, we favor explanation (3) proposed by Hoskin and

Black (2000) as the lattice strain model should affect Th

and U incorporation in rutile to an even greater degree

when compared to zircon. While Th4? is only 30% larger

than Zr4?, it is more than 50% larger than Ti4?. Therefore,

the misfit will strongly favor U4? over Th4? in rutile in all

lithologies and at all temperature conditions (with lower

temperatures accentuating this effect). This conclusion is

supported by experiments by Klemme et al. (2005) where

Drutile/melt partition coefficients for U are about 3 orders of

magnitude higher compared to Th, even at temperatures of

1,200�C. Nevertheless, we notice that other mechanisms

may be important in certain environments, as Dunkl and

von Eynatten (2009) observe rutiles with unusually high

Th/U ratios (4 ppm U and 35 ppm Th) in diagenetically

overprinted sandstones, emphasizing the need for a careful

characterization of Th in all rutile dating efforts.

Common Pb and 208Pb correction

Due to the extremely low Th concentrations in all rutiles

analyzed in this study, all 208Pb measured was treated as

initial common Pb. Using the measured 208Pb as well as the206Pb/208Pb and 207Pb/208Pb ratios of crustal Pb from Sta-

cey and Kramers (1975), the amount of common Pb can be

calculated, as described in Analytical Methods. As noted,

more than 90% of all rutiles analyzed show initial Pb

concentrations of less than 0.2 ppm. Despite the frequent

description of rutile as being a common Pb-rich phase, the

low common Pb concentrations measured in this study are

consistent with low concentrations from other accessory

minerals that lack Ca2? as a major constituent (e.g., zircon,

monazite) compared to high initial common Pb contents in

Ca-rich phases (e.g., apatite, titanite, allanite). This seems

to indicate that most Pb incorporated in typical accessory

minerals during growth is divalent.

Especially for young and/or low-U rutiles, it is essential

to apply the 208Pb correction, even at the 100 s of ppb

1000

1100

C

1675 Ma

700

800

900

T in

°C

granulite-facies

hib lit f i

600

700

1500 1700 1900 2100207Pb/ 235U ages

amp o e-facies

Fig. 10 207Pb/235U ages vs calculated apparent temperatures (calcu-

lated from Zr-in-rutile with the calibration of Tomkins et al. (2007)

assuming 0.5 Gpa) [SJ33] for detrital rutiles from the northern

Gawler Craton (sample EGHC-4). Gray areas indicate that both

rutiles around 1,675 ages and rutiles significantly older have a

granulite-facies ([750�C) component

Contrib Mineral Petrol

123

level. Sample IVZ-R6b (from the Ivrea Verbano Zone) has

the youngest rutiles in this study and is therefore particu-

larly well suited to illustrate the common Pb correction

based on 208Pb. Rutiles from this sample display a wide

array of 206Pb/208Pb ratios (from 2 to 35). To illustrate the

power of 208Pb correction, uncorrected data are shown in a

Tera-Wasserburg plot in Fig. 11. All data points are above

the reverse concordia, forming a mixing line between a

concordia age of 183 ± 4 Ma and a 207Pb/206Pb ratio of

0.83 ± 0.15 for zero U concentration. The 207Pb/206Pb

ratio is within error of today’s crustal Pb (207Pb/206Pb ratio

of 0.836 from Stacey and Kramers 1975). It can also be

observed that the degree of discordance is directly corre-

lated to the 206Pb/208Pb ratio (see Fig. 11 for details). Using

these 206Pb/208Pb ratios for the common Pb correction, all

analyzed rutiles plot on the concordia (Fig. 7a). This

clearly shows that the 206Pb/208Pb ratio gives robust

information about the presence of common Pb component

in rutiles.

Concluding remarks

We have to stress that the 208Pb correction allows evaluation

of concordance of single spots. This is in striking contrast to

the mixing line method (as in Fig. 11) where two assump-

tions have to be made: that all analyzed crystals (1) belong to

one population and (2) are concordant (an assumption that

has to be made for titanites; see e.g., Storey et al. 2006).

Instead, the 208Pb correction method will be invaluable in

future studies in which each spot analysis can be evaluated

independently. Examples for such applications are found in

provenance studies, with each individual detrital rutile

having its own protolith. Also, spot dating will make it

possible to investigate age zoning within single mineral

grains. Rutile may prove to be the best candidate for studying

age zoning within single crystals (see Kooijman et al. 2010),

as it is (1) the only nearly spherical mineral with (2) rela-

tively fast diffusion (monazite and zircon have closure

temperatures higher than formation temperatures) and (3)

low initial common Pb (single spot common Pb correction is

difficult for titanite and apatite). Finally, texturally con-

trolled age dating of rutile will become a fruitful field of

research (for zircon see Moller et al. 2003), potentially

allowing us to date growth of minerals by measuring rutile

inclusions (e.g., garnet) and to date breakdown of minerals

that produce rutile (e.g., biotite). However, so far no fully

shielded rutile has been found. Rutiles included in garnet and

from the matrix always yielded identical ages for Z05-724-3

and PD-1. Finally, we note that in provenance studies, rutile

dating will complement information gathered by other

methods from the geochemist’s toolbox, e.g., zircon dating,

adding another new layer of information, in this case

knowledge about the timing of the last metamorphic episode.

Acknowledgments We wish to thank Dirk Frei, Paul Hoskin and

Simon Jackson for detailed and constructive reviews. We would also

like to thank Dorrit Jacob, Klemens Link, Jasper Berndt-Gerdes, Ellen

Kooijman, and Klaus Mezger for discussions. Andreas Moller is

thanked for providing grains of WH-1. This project was supported by

grants from the DFG (Za285/7-1) and by NERC funding (for the

Edinburgh Ion Microprobe Facility). This is contribution 704 from the

ARC National Key Centre for the Geochemical Evolution and Met-

allogeny of Continents (http://www.es.mq.edu.au/GEMOC/).

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