doi: 10.1111/j.1365-3121.2009.00912.x

Early Cretaceous migmatitic mafic granulites from the Sabzevar range (NE Iran): implications for the closure of the Mesozoic peri-Tethyan oceans in central Iran Federico Rossetti,1 Mohsen Nasrabady,2 Gianluca Vignaroli,1 Thomas Theye,3 Axel Gerdes,4 Mohammad Hossein Razavi2 and Hosein Moin Vaziri2 1

Dipartimento di Scienze Geologiche, Universita` Roma Tre, 00146 Roma, Italy; 2Department of Geology, Tarbiat Moalem University, Tehran, Iran; 3Institut fu¨r Mineralogie und Kristallchemie, Universita¨t Stuttgart, 70569 Stuttgart, Germany; 4Institut fu¨r Geowissenschaften, J. W. Goethe Universita¨t, D-60438 Frankfurt, Germany

ABSTRACT The ophiolitic me´lange of the Sabzevar Range (northern Iran) is a remnant of the Mesozoic oceanic basins on the northern margin of the Neotethys that were consumed during the Arabia–Eurasia convergence history. Occurrence of km-scale, dismembered mafic HP granulitic slices is reported in this study. Granulites record an episode of amphibole-dehydratation melting and felsic (tonalite ⁄ throndhjemite) melt segregation at c. 1.1 GPa and 800 °C. In situ U(-Th)–Pb geochronology of zircon and titanite grains hosted in melt segregations points to

Introduction The Iranian ophiolites are part of the orogenic sutures marking the diachronous closure of the Tethyan oceanic realms (Palaeotethys and Neotethys) along the Alpine–Himalayan convergent front running from the Mediterranean through East Europe, Middle East to Asia (Fig. 1a). In particular, various ophiolitic sutures surround the Central East Iranian Microcontinent (CEIM, Fig. 1b). These are remnants of the Mesozoic peri-Tethyan oceanic basins formed in the upperplate of the Neothethyan subduction and document a polyphase tectonic evolution during its Mesozoic–Cenozoic consumption along the Sanandaj–Sirjan Zone (Sto¨cklin, 1974; Sengo¨r et al., 1988; McCall, 1997; Stampfli and Borel, 2002; Bagheri and Stampfli, 2008). Although data from these ophiolites might provide key elements to better assess the palaeotectonic scenario along the Eurasia convergent margin, few modern petrological and geochronological studies exist. Correspondence: F. Rossetti, Dipartimento di Scienze Geologiche, Universita` Roma Tre, Largo S. L. Murialdo, 1, 00146 Rome, Italy. Tel.: +390657338043; fax: +390657338201; e-mail: rossetti@uniroma3. it 26

an Early Cretaceous (Albian) age for the metamorphic climax. Results of this study (i) impose reconsideration of the current palaeotectonic models of the Neothetyan convergent margin during the Early Cretaceous and (ii) argue that punctuated events of subduction of short-lived back-arc oceanic basins accompanied the long-lasting history of the Neotethyan subduction in the region.

Terra Nova, 22, 26–34, 2010

In this paper, we document the first report of migmatitic mafic granulites from the Palaeogene ophiolitic me´lange of the Sabzevar Range, located at the northern edge of the CEIM (Figs 1b–c). We asses their peak thermo-baric conditions and constrain timing of metamorphic climax by in situ laser ablation (LA)-ICPMS U–Pb dating of zircon and titanite occurring in felsic melt segregations. These data document an unknown episode of Early Cretaceous (c. 107 Ma) high-grade metamorphism linked to dehydratation melting of amphibole-bearing mafic protoliths. Results from this study impose reconsideration of the current geodynamic reconstructions of the Neotethyan palaeo-convergent margin in the region.

Regional Geology The NW–SE trending ophiolitic belt of the Sabzevar Range formed at the expenses of the Late Cretaceous Sabzevar ocean, a part of the marginal basins that originally segmented the CEIM northward of the active margin of Neotethys (McCall, 1997) (Figs 1b–c). The structural architecture of the Sabzevar Range consists of a ductile-to-brittle, S ⁄ SW-verging accretionary complex, made of a dismembered ophiolitic suite with a tectonised and partially serpentinised

mantle section and a volcano-sedimentary sequence, upper Late Cretaceous (Campanian; c. 84 Ma Baroz et al., 1984) to Palaeocene in age (Shojaat et al., 2003). These rock types occur dispersed as centimetreto kilometre-size blocks into a highly sheared serpentinite matrix to form a major ophiolitic tectonic me´lange. Variably-sized, foliated metabasic rocks (blueschists, greenschists and amphibolites) are also involved in the tectonic me´lange (Lench et al., 1977; Macaudier, 1983; Baroz et al., 1984). A further me´lange unit underlies the serpentinite me´lange and consists of SW-verging embricated thrust slices of red limestones, cherts, and volcanicvolcaniclastic rocks forming the frontal part of the range. Late tectonic, sheeted granites intrude the ophiolitic me´lange in the inner sector of the chain. Available radiometric data, derived from K–Ar (muscovite) and Rb–Sr (whole rock and muscovite) methods, constrain the tectono-metamorphic structure of the Sabzevar Range to the Early Eocene (at about 50–55 Ma; Baroz et al., 1984).

The Sabzevar granulites Two exposures of km-scale (c. 10 km long and 1 km wide), variably retrogressed (amphibolitised) mafic granulitic bodies were recognized in  2009 Blackwell Publishing Ltd

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F. Rossetti et al. • Closure of the mesozoic tethyan oceans, Iran

............................................................................................................................................................. (b)

(a)

(c)

Fig. 1 (a) Distribution of the remnants of the Tethyan oceanic realm along the Alpine–Himalayan convergence zone. (b) Simplified geological map showing the main tectonic domains in Iran, with the main ophiolitic belts (in white) indicated (modified after Shojaat et al., 2003; Bagheri and Stampfli, 2008). CEIM: Central East Iranian Microcontinent. (c) Geological map of the Sabzevar Range (modified and readapted after Lench et al., 1977), with location of the granulite-facies rocks. The location of the sample NG353 studied for U–Pb geochronology together with its geographical coordinates are also indicated.

the frontal part of the range, to the northwest of the Sabzevar city (Fig. 1c). They occur as dismembered, NW ⁄ SE-striking tectonic slivers embedded within the ophiolitic me´lange. Contacts with the surrounding rocks are obscured by intense brittle deformation because of the late Neogene to Quaternary faulting.

Texture, petrography and mineral compositions The granulite bodies are dark, medium to fine-grained rocks showing granoblastic groundmass or weak foliation. Texture is characterized by occurrence of submillimetric to milli 2009 Blackwell Publishing Ltd

metric leucocratic patches interlayered within the granoblastic mineral matrix made of Am + Grt + Cpx + Pl ± Qtz, with Ilm, Rt, Ap, Zr (abbreviation after Bucher and Frey, 2002) as main accessory phases (Fig. 2a). Both garnet and clinopyroxene form porphyroblasts, which typically occur within the leucocratic domains; garnets are poikiloblastic, hosting multiphase and single inclusion assemblages made of Am, Pl, Qtz, Rt, Ilm, Ttn (Figs 2a–c). The leucocratic patches invariably consist of Qtz + Pl-rich segregations of broadly tonalitic ⁄ trondhjemitic composition (Qtz ⁄ Pl modal proportions 50 ⁄ 35– 50 ⁄ 65). They show a systematic intra-

granular connectivity, with Pl and Qtz showing a coarse granoblastic and, usually, strain-free texture (Fig. 2d). The Pl + Qtz associations also form film-like intergrowths surrounding matrix amphibole, with quartz usually showing xenomorphic habit (Fig. 2e). Titanite and zircon are the main accessory phases in the leucocratic segregations (Fig. 2f). Representative mineral compositions are shown in Table 1. Garnet is essentially almandine-grossularpyrope and spessartine poor (Alm53–48 Prp21–12Sps6–3Grs21–31), commonly characterized by flat chemical profiles. Zoning is only seldom evident at the garnet rim, with a general increase in 27

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.............................................................................................................................................................

(a) (b)

(c)

(e)

(d)

(f)

Fig. 2 Textures and mineral assemblages from the Sabzevar mafic granulites (sample NG353) (a) Rock slab showing the overall texture of the rock. Coarse-grained, dark green amphibole forms the main matrix assemblage with porphyroblastic garnet and clinopyroxene. Leucocratic Qtz–Pl segregations enclose garnet porphyroblasts. The dashed white circle indicates the rock slab used for the in situ U–Pb dating. (b) Thin section showing microstructures. Garnet is typically euhedral, but also xenoblastic grains are observed. Straight boundaries occur with the matrix amphibole and the leucocratic segregations (plane polarized light). (c) Poikiloblastic garnet hosting Ilm–Am–Qtz–Pl composite inclusions and rutile needles (plane polarized light). (d) Leucocratic Qtz– Pl segregation showing well preserved igneous texture (crossed polars). (e) Interstitial Qtz–Pl segregations and xenomorphic quartz surrounding matrix amphibole (crossed polars). (f) Back scattered electron (BSE) image showing coexisting zircon and titanite in the Qtz–Pl segregations.

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............................................................................................................................................................. Ca and Fe ⁄ (Fe+Mg). Clinopyroxene is diopside-rich with minor hedenbergite, orthopyroxene and Ca-Tschermaks components (Di49– 60Hd10–30Opx7–15Ca-Ts2–18). Core-torim decrease of the Ca-ts component is systematically observed. Plagioclase is andesine (An43–51Ab50–56Or0–1), either occurring in Pl–Qtz segregations or as inclusion in garnet. Amphibole (either inclusion in garnet or in the matrix) shows Mg ⁄ (Mg + Fe2+)

values ranging between 0.6 and 0.8, with Si4+between 6.0 and 6.7 a.p.f.u. It can be classified as tschermakite transitional to Mg-hornblende (Leake et al., 2004).

Peak P–T estimates The peak mineral assemblage (Grt + Cpx + Pl ± Am ± Qtz) is indicative of the Opx-free, high-pressure (HP) granulite facies (Pattison, 2003). Textures

Table 1 Representative microprobe analyses and structural formulae of equilibrium mineral phases at the metamorphic peak in the Sabzevar granulites*. Mineral Grt Analysis #36-c SiO2 TiO2 Al2O3 Cr2O3 FeOTot MnO MgO CaO Na2O K2O Total

37.55 0.12 21.99 0.01 25.82 2.12 4.74 8.56 0.04 0.00 100.95 12 O

Cations Si Ti Al Cr Fe3+ Fe2+ Mn Mg Ca Na K Sum X_Grs X_Prp X_Alm X_Sps X_Di X_Hd X_Ts X_Opx X_An X_Ab X_Or

2.91 0.01 2.01 0.00 0.17 1.51 0.14 0.55 0.71 0.01 0.00 8.00 0.24 0.19 0.52 0.05

Grt #42-r

Cpx Cpx Amp #107-i #28-m #44-i

37.47 51.89 0.20 0.23 21.51 2.93 0.01 0.01 25.12 10.38 1.98 0.31 3.71 14.00 10.38 20.32 0.02 0.58 0.00 0.02 100.40 100.67 12 O 2.93 0.01 1.98 0.00 0.14 1.50 0.13 0.43 0.87 0.00 0.00 8.00 0.30 0.15 0.51 0.04

6O

49.61 0.40 4.11 0.00 12.31 0.40 10.29 21.25 0.84 0.01 99.51 6O

1.92 0.01 0.13 0.00 0.07 0.25 0.01 0.77 0.80 0.04 0.00 4.00

1.89 0.01 0.18 0.00 0.08 0.31 0.01 0.58 0.87 0.06 0.00 4.00

0.54 0.18 0.07 0.15

0.49 0.26 0.10 0.07

41.88 1.38 11.62 0.02 18.09 0.14 8.89 10.96 1.90 0.70 95.58 23 O 6.43 0.16 2.10 0.00 0.26 2.07 0.02 2.04 1.80 0.57 0.14 15.67

Amp #24-m 42.61 1.53 12.25 0.03 16.53 0.21 10.42 11.58 1.70 0.73 97.59 23 O 6.35 0.17 2.15 0.00 0.34 1.72 0.03 2.31 1.85 0.49 0.14 15.67

Pl #46-i

Pl Ttn #48-m #72-i

56.51 56.25 0.01 0.00 27.63 28.77 0.00 0.00 0.00 0.00 0.02 0.02 0.02 0.01 8.89 9.36 6.43 6.48 0.22 0.16 99.73 101.05 8O

Ttn #33-m

29.70 38.93 1.12 0.01 0.90 0.09 0.04 28.07 0.00 0.00 98.81

29.81 38.00 1.04 0.02 0.98 0.02 0.01 28.46 0.09 0.04 98.47

1.00 0.99 0.04 0.00 0.02

1.00 0.96 0.04 0.00 0.02

0.00 0.00 1.01 0.00

0.00 0.00 1.02 0.01

3.06

3.05

8O

2.54 0.00 1.46 0.00 0.02 0.00 0.00 0.00 0.43 0.56 0.01 5.01

2.50 0.00 1.51 0.00 0.01 0.00 0.00 0.00 0.45 0.56 0.01 5.03

0.43 0.56 0.01

0.44 0.55 0.01

*Mineral compositions were measured with a CAMECA SX100 electron probe at the Institut fu¨r Mineralogie, Universitat Stuttgart (15 kV, 15 nA beam conditions; WDS mode). Structural formula normalization (to number of oxygen (O) or to Si4+= 1 for Ttn) and estimate of the Fe3+content are derived from the AX2000 software included in the THERMOCALC package. Am-i, amphibole inclusion in garnet; Am-m, amphibole in matrix; Cpx-i, clinopyroxene inclusion in garnet; Cpx-m, clinopyroxene in matrix; Grt-c, garnet core; Grt-r, garnet rim; Pl-i, plagioclase inclusion in garnet; Pl-m, plagioclase in matrix; Ttn-i, titanite inclusion in amphibole; Ttn-m, titanite in matrix.

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such as those described above show strong similarities with those reported in Hartel and Pattison (1996), who interpreted the Qtz + Pl segregations as remnants of melt. In particular, the skeletal nature of the quartz and the Qtz + Pl films around amphibole argue for an in situ origin of such melts (e.g. Brown, 2002), and hence product of migmatisation of a basic protolith. A likely scenario is amphibole dehydratation melting during prograde granulite facies metamorphism according the following generalized reaction (Hartel and Pattison, 1996): Am þ Pl ¼ Grt þ Cpx þ Ttn þ meltðtrondhjemiteÞ

ð1Þ

The flat chemical profiles in porphyroblastic garnets are here interpreted as the effect of the hightemperature chemical homogenization attained at the metamorphic climax (e.g. Spear, 1993; Ganguly, 2002). The rimward zoning can be interpreted as due garnet growth in presence of a Ca-rich melt phase, coupled with partial post-peak P–T re-equilibration (e.g. Spear and Kohn, 1996; Kohn and Spear, 2000). Mineral core composition of large crystals showing textural equilibria are then combined with those of mineral inclusions (amphibole and plagioclase) hosted in garnet and used for estimating peak conditions. The P–T estimates and phase reaction calculations were obtained using the THERMOCALC3.26 software (Powell and Holland, 2008). Considering the coexisting phases Grt + Cpx + Pl + Am + Qtz, results running THERMOCALC in the average P–T mode are 811 ± 81 C and 1.09 ± 0.13 GPa (Table 2). Phase reaction calculations considering the Grt-Cpx Fe–Mg exchange thermometry and the equilibria 2Grs + Prp +3Qtz = 3An + 3 Di (GADS) and 2Grs + Alm +3Qtz = 3An + Hd (GAHS) for barometry provided an intersection at 800 C and 1.1 GPa. These data were complemented with the zirconium-in rutile thermometry, which yields consistent results ranging between 721 to 810 C (Table 2). Calculated peak P–T conditions thus provide further evidence for partial melting of amphibolite as they locate above the H2O-saturated basaltic solidus (Fig. 3). 29

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............................................................................................................................................................. Table 2 Mineral assemblages, activity models and average P–T results as obtained from the THERMOCALC calculations (average P–T mode), together with results from Zr-in rutile thermometry. THERMOCALCv3.26

(+ Ttn, Rt, Ilm)

Mineral

Grt

Cpx

Pl

Am

Mineral activities

aGrs 0.033 aPrp 0.089 aAlm 0.110 aSps 0.001

aDi 0.490 aHd 0.310

aAn 0.570 aAb 0.560

aTs 0.001 afact 0.001 aTr 0.036 aparg 0.015

P (GPa ± 1r)

T (C ± 1r)

corr.

1.09 ± 0.13

811 ± 81

0.869

Zr-in rutile thermometry Calibration n = 28

Zr(n) (ppm) 777–1606

W06 727–803 C

F&W07 721–802 C

T07 716-810 C*

n, number of analysed rutile grains; Zr(n): range of the Zr-in rutile content for the n analysed grains; W06, Watson et al. (2006); F&W07, Ferry and Watson (2007); T07, Tomkins et al. (2007); *At 1.1 GPa.

In situ U–Pb geochronology Sample NG353 (see Fig. 1c for sample location) was chosen for U–Pb geo-

chronology as it provides the best preserved example of the peak granulite metamorphism. The same rock section used for the petrographical

study (Fig. 2a) was used for in situ dating of zircon and titanite occurring in melt segregations (Fig. 4a). After careful petrographical investigation, a circular (1 inch in diameter), 100 lm thick rock slab was cored from the section and prepared for both backscattered electron (BSE) and cathodoluminescence (CL) imaging. Zircons are typically rounded in shape and fine-grained (20 lm as average). BSE and cathodoluminescence images reveal a homogeneous zircon population characterized by oscillatory- to sector zoning (Fig. 4b), a texture that is typical of magmatic crystallization (e.g. Harley et al., 2007). Titanite is instead relatively coarse grained (commonly 60–100 lm) with no compositional zoning (Fig. 4c). Selected spots of 16–40 lm in diameter were then analysed for U–Th–Pb isotopic compositions using a laser ablation ICPMS system. Plots and age calculations were made using the ISOPLOT software (Ludwig, 2003). Results are shown Fig. 4d and listed in Table 3, which also provides details on the analytical methods. Six spots on five zircon grains yielded a monomodal age distribution with a concordia age of 107.4 ± 2.4 Ma. Ten spots on six titanite grains provided a nearly identical well defined concordant cluster at 105.9 ± 2.3 Ma. These results point to an Early Cretaceous (Albian) age for felsic melt segregation and peak metamorphism in the Sabzevar granulites.

A working hypothesis for the closure of the Mesozoic peri-Tethyan oceans in central Iran

Fig. 3 Peak P–T estimates for the Sabzevar granulites as obtained from the THERMOCALC software. The ellipse quotes the errors at 1r level (average P–T mode calculations). Metamorphic facies boundaries are after Bucher and Frey (2002). The grid showing regimes of melting for basaltic system is after Vielzeuf and Schmidt (2001). Key to symbols: A, amphibolite facies; EA, epidote amphibolite facies; G, granulite facies; Ecl, eclogite facies. 30

The HP granulite facies metamorphic conditions such those documented in this study are diagnostic of crustal thickening in collisional belts (OÕBrien and Ro¨tzler, 2003). Peculiarity here is the fact that granulite metamorphism is reported from a basic protolith and both the mineral assemblages and the high-grade conditions are in principle compatible with those reported from subophiolitic dynamothermal soles (e.g. Williams and Smyth, 1973; Jamieson, 1986). Textures suggest deep melting of mafic rocks to form felsic melt (tonalite ⁄ trondhjemite) and garnetclinopyroxene residues (HP granulite). Formation environments of mafic garnet granulite residues in orogenic  2009 Blackwell Publishing Ltd

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............................................................................................................................................................. (a)

(b)

(c) (d)

Fig. 4 (a) Rock slab used for the U–Pb geochronology and localization of the dated zircon and titanite grains. (b) Representative back-scattered electron (BSE) (top) and cathodoluminescence (bottom) images of zircons. The grains show homogeneous growth with oscillatory zoning. Locations of the LA-ICPMS spots (white circles) are also indicated. (c) Representative BSE images of some of the dated titanite grains, with laser spots (white circles) and U–Pb isotope data indicated. (d) Conventional concordia diagrams showing all data. All ages are concordia ages, with errors quoted at 2r level.

(a)

(b)

(c)

Fig. 5 Tentative palaeotectonic reconstruction of the sequence of events linked to the Neotethyan closure along the Eurasian margin of Iran (source of data and readapted after Ghasemi and Talbot, 2006; Agard et al., 2007; Moghadam et al., 2009). Relative motion of the various crustal blocks making up the upper-plate of the Neothetyan subduction should be eventually contemplated. Not to scale; location of structures is only indicative.  2009 Blackwell Publishing Ltd

settings have been generally ascribed to two-end member processes: arc maturation, i.e. formation in consequence of magmatic loading at the mature arc stage (e.g. Garrido et al., 2006; Berger et al., 2008), or slab melting in high heat-flow subduction settings, and hence remnants of a former oceanic crust (e.g. Garcı´ a-Casco et al., 2008). Most of reconstructions on the closure of the Neotethys propose that formation of the active margin along the Eurasian margin started since the Late Triassic–Early Jurassic (e.g. Berberian and King, 1981; Besse et al., 1998; Stampfli and Borel, 2002; Arvin et al., 2007). This oceanic subduction was accompanied by formation of a cordilleran-type margin along the Sanandaj–Sirjan Zone during the Jurassic–Cretaceous (e.g. Berberian and Berberian, 1981; Ghasemi and Talbot, 2006) and by formation of various marginal oceans in the backarc domain (Inner Mesozoic Oceans of McCall, 1997). In particular, recent geochronological studies from the CEIM ophiolites have documented that such oceanic basins formed in 31

32

393 423 330 388 398 324 915 724 529 725 1062 1069 924 591 457 593

Zr Zr Zr Zr Zr Zr Ttn Ttn Ttn Ttn Ttn Ttn Ttn Ttn Ttn Ttn

105 106 156 181 86 237 60 72 86 107 108 108 130 135 107 133

‡

1.7 1.7 2.5 2.9 1.4 3.9 1.8 1.4 1.6 1.8 3.3 2.7 2.9 2.4 2.0 2.6

Pb (ppm)

‡

0.01 0.01 0.02 0.01 0.01 0.02 0.25 0.30 0.04 0.05 0.14 0.22 0.66 0.14 0.17 0.20

Th/U

‡

3364 1365 2188 2509 2780 1297 752 1414 929 2118 625 613 1914 3690 1068 1328

Pb/

206

Pb

204

U

238 §

0.01715 0.01634 0.01690 0.01668 0.01724 0.01641 0.01558 0.01639 0.01750 0.01685 0.01596 0.01700 0.01661 0.01728 0.01675 0.01694

Pb/

206

4.3 4.7 4.5 4.6 4.0 4.7 3.9 3.6 5.0 3.4 3.1 3.2 3.2 4.7 3.6 4.4

±2r (%)

U

235 §

0.1061 0.1022 0.1087 0.1164 0.1182 0.1114 0.1069 0.1094 0.1086 0.1137 0.1072 0.1098 0.1096 0.1200 0.1106 0.1122

Pb/

207

9.6 9.8 9.8 9.3 10 9.8 8.8 8.7 9.7 9.0 5.1 5.1 5.0 9.7 5.2 8.1

±2r (%)

Pb

206

0.0449 0.0453 0.0467 0.0506 0.0497 0.0492 0.0498 0.0484 0.0450 0.0490 0.0487 0.0468 0.0479 0.0504 0.0479 0.0480

Pb/

207

§

8.6 8.6 8.6 8.0 9.2 8.6 7.9 7.9 8.3 8.4 4.0 3.9 3.8 8.4 3.8 6.9

0.45 0.48 0.47 0.49 0.40 0.48 0.45 0.42 0.52 0.38 0.61 0.64 0.64 0.49 0.69 0.54

±2r (%) Rho

**

110 105 108 107 110 105 100 105 112 108 102 109 106 110 107 108

5 5 5 5 4 5 4 4 6 4 3 3 3 5 4 5

Pb/238U ±2r (Ma)

Age (Ma) 206

102 99 105 112 113 107 103 105 105 109 103 106 106 115 106 108

9 9 10 10 11 10 9 9 10 9 5 5 5 11 5 8

Pb/235U ±2r (Ma)

207

Closure of the mesozoic tethyan oceans, Iran • F. Rossetti et al.

Diameter of laser spot was 16, 20 or 30 lM for zircon (Zr) and 30 or 40 lm for titanite (Ttn); depth of crater 10–15 lM. *Uranium, thorium and lead isotopes were analysed using a Thermo-Scientific Element 2 sector field ICP-MS coupled to a New Wave Research UP-213 ultraviolet laser system at Goethe University Frankfurt (Gerdes and Zeh, 2006, 2009). Data were acquired in time resolved – peak jumping – pulse counting mode over 810 mass scans, with a 16 s background measurement followed by 28 s sample ablation. Laser spot-sizes varied from 16 to 30 lM with a typical penetration depth of 15–20 lM. Signal was tuned for maximum sensitivity for Pb and U while keeping oxide production, monitored as 254UO ⁄ 238U, well below 1%. A teardrop-shaped, low volume (