Uwe Wiechert

Oxygen isotope evidence for rapid mixing of the HED meteorite parent body

Abstract The 16 O, 17 O and 18 O abundances of howardites, eucrites, and diogenites have been used to assign them to a single ‘HED’ parent body, thought to be asteroid 4 Vesta. We report the first evidence of oxygen isotopic heterogeneity among HED meteorites indicating incompletely mixed sources. New high-precision oxygen isotope measurements of 34 HED meteorites reveal that most have the same Δ 17 O′, consistent with a very rapid early history of large-scale mixing on Vesta. However, howardites are on average very slightly enriched in 16 O, whereas Ibitira, Caldera, Pasamonte, and ALHA78132 are 16 O-depleted compared to other investigated eucrites. The Δ 17 O′ of Ibitira is completely different from all other HEDs measured. Some of the results for eucrites and diogenites can be explained by partial melting and rapid mixing of the interior of Vesta. Others require a separate parent body or indicate that parts of the outer layer of Vesta retained some primary isotopic heterogeneity. The oxygen isotopic composition of howardites provides an upper limit for the amount of admixed carbonaceous chondritic material into the HED parent body regolith.

Hydrated sub-arc mantle: a source for the Kluchevskoy volcano, Kamchatka/Russia

Oxygen isotope ratios of olivine and clinopyroxene phenocrysts from the Kluchevskoy volcano in Kamchatka have been studied by CO2 and ArF laser techniques. Measured δ18O values of 5.8–7.1‰ for olivine and 6.2–7.5‰ for clinopyroxene are significantly heavier than typical mantle values and cannot be explained by crustal assimilation or a contribution of oceanic sediments. Positive correlations between δ18O and fluid-mobile elements (Cs, Li, Sr, Rb, Ba, Th, U, LREE, K) and a lack of correlation with fluid-immobile elements (HFSE, HREE) suggest that 18O was introduced into the mantle source by a fluid from subducted altered oceanic basalt. This conclusion is supported by radiogenic isotopes (Sr, Nd, Pb). Mass balance excludes simple fluid-induced mantle melting. Instead, our observations are consistent with melting a mantle wedge which has been hydrated by 18O-rich fluids percolating through the mantle wedge. 18O-enriched fluids are derived from the subducted oceanic crust and the Emperor seamount chain, which is responsible for a particularly high fluid flux. This hydrated mantle wedge was subsequently involved in arc magmatism beneath Kluchevskoy by active intra-arc rifting.

Content and isotopic composition of sulphur in ultramafic xenoliths from central Asia

Sulphur contents and isotopic compositions have been determined in 90 fresh mantle-derived garnet and spinel peridotite and pyroxenite xenoliths from six regions of Cenozoic alkali basaltic volcanism in southern Siberia and Mongolia. Sulphur contents in most of the peridotite nodules fall in the range 6–25 ppm, with largely positive δ34S values clustering between −1 and +7‰ (average +2‰). By contrast, the pyroxenites are richer in sulphur (25–140 ppm), and their δ34S values are close to 0‰ (−1.5 to +1.4‰). In the peridotite nodules, sulphur content correlates negatively with MgO, while their δ34S values correlate positively with MgO. Most of the fertile lherzolites (MgO= 37–39%, CaO= 3–4%) have δ34S values close to 0‰ (−1 to +2‰), similar to meteorite values, while in the harzburgites these values reach +3 to +5‰. These features apparently reflect sulphur depletion during partial melting of peridotite mantle accompanied by sulphur isotopic fractionation between residual peridotites and generated basaltic melts. Clinopyroxene-poor xenoliths from southern Mongolia yielded highest δ34S values of +5 to +7‰ accompanied by high Ba and K contents and high 87Sr/86Sr ratios; these may be a result of interaction with fluids derived from subducted oceanic crust which followed the partial melting and magmatic fractionation episodes. Low average sulphur concentrations ( < 50 ppm) and largely positive δ34S values may predominate in the continental lithospheric mantle worldwide. Sulphur isotope compositions typical of tholeiitic basalts and MORB (0 to +1‰ [1,2]) may be produced by melting of moderately depleted lherzolites. Primary melts with positive δ34S values may be generated from mantle peridotites with larger degrees of depletion and/or from rocks metasomatised by subduction-related fluids.

Cosmogenic tungsten and the origin and earliest differentiation of the Moon

The decay of formerly live 182Hf with a half-life of 9 Myr results in variations in the abundance of 182W in early solar system objects. Here we demonstrate that major excesses in 182W in some lunar samples are the results of cosmogenic additions. Apollo 17 high-Ti mare basalts yield high 182W/184W of up to ϵw=+11±1. Even more extreme variations of up to ϵw=+22±1 are found for mineral separates, although these lavas were erupted more than 500 Myr after the start of the solar system. The measured 182W excess in the separated minerals is correlated with their Ta/W, confirming theoretical models that implicate the 181Ta(n,γ)182Ta(β−)182W reaction from cosmic irradiation as the most likely cause. In contrast, olivine–basalt 15555, which has a low cosmic ray exposure age, displays no internal 182W variations and defines an ϵw of +1.3±0.4. This is consistent with earlier conclusions that the Moon formed about 50 Myr after the start of the solar system. The high-Ti mare basalt source, with very high Hf/W, has a W isotopic composition that is not grossly different, from which a time limit of ∼70 Myr after the start of the solar system can be inferred for the formation of ilmenite-rich layers in the final stages of the lunar magma ocean.

Re-assessment of silicon isotope reference materials using high-resolution multi-collector ICP-MS

Silicon isotope ratios can now be measured to very high precision using high-resolution multi-collector ICP-MS. Based on this technique we report that the Si isotope composition of IRMM-018 is significantly lighter than the NBS28 standard, in direct contrast to previously published results. Our data are also inconsistent with recently published absolute Si isotope abundances for these standards by Valkiers et al. (2005) and Ding et al. (2005). Instead, our results are coherent with the certified values for NIST standard SRM990 that was used to determine the atomic weight of Si, with a 30Si/29Si ratio that is over 6 permil lower for the same atomic weight. In order to avoid problems with future assessments of stable Si isotope variations, the NBS28 silica sand standard (RM8546) should remain the zero point. Therefore, an inter-laboratory calibration of NBS28 and other references materials is recommended to solve the observed discrepancies and establish a reliable scale for reporting Si isotopes.

Excimer laser isotope-ratio-monitoring mass spectrometry for in situ oxygen isotope analysis

Abstract A technique using isotope-ratio-monitoring gas chromatography-mass spectrometry (irmGCMS) and excimer laser fluorination for in situ oxygen isotope analysis of silicates is described. The irmGCMS and oxygen extraction line is connected by a newly developed interface, reducing the time for a single analysis to less than 10 min. The precision obtained for δ18O is similar to what has been reported for excimer laser fluorination using dual inlet systems. δ18O values of two olivine standards had 1σ precision of ±0.14‰ (n=19 and n=10) and that of Dorentrup quartz had ±0.17‰. Eleven analyses of a large zircon crystal had a precision of ±0.12‰. However, between 300 and 600 nmol oxygen was liberated for a single analysis, equivalent to cylindrical laser holes 250 to 350 μm in diameter and depth. In the future it will be feasible to measure the isotope ratio of cylindrical volumes 150 μm in diameter simply by reducing the volume of the extraction line. While this is still significantly larger than what is possible with ion probes, the ratios obtained by excimer laser-irmGCMS are highly accurate and precise without correction. The value of this technique for in situ oxygen isotope measurements is demonstrated with two rock slabs from metamorphic rocks of the Dabie–Sulu ultra-high-pressure belt, China.

High-precision in situ oxygen isotope analysis of quartz using an ArF laser

Abstract High precision in situ oxygen isotope analyses of quartz using an ArF laser are described. The high precision is documented by comparing in situ ArF laser data with CO 2 laser and conventional analyses of silica and quartz. In contrast to the CO 2 and Nd:YAG laser, no oxygen isotope fractionation occurs during in situ ablation of quartz with high power ultra-violet ArF laser light pulses. Therefore, high oxygen yields are no prerequisite for accurate and precise δ 18 O measurements. No data correction are necessary when analyzing quartz. The high spatial resolution and in situ capabilities of the ArF laser technique are demonstrated by analyzing hydrothermally precipitated quartz, which shows a rhythmic growth history. A model is also presented that explains the behavior of quartz during ablation with ultra-violet laser.

Quantification of transient signals in multiple collector inductively coupled plasma mass spectrometry: accurate lead isotope ratio determination by laser ablation of individual fluid inclusions

This work establishes the analytical protocol for accurate Pb isotopic analysis of fast transient signals by multiple-collector ICP-MS instruments. Individual synthetic fluid inclusions of known Pb and Tl isotopic compositions (dissolved SRM 981 with or without SRM 997 from NIST, enclosed in quartz by a hydrothermal crack annealing technique) were liberated by 193 nm UV laser ablation (LA). Data were recorded on Faraday detectors, for which correction schemes for bias in amplifier response (“tau correction”) are presented and evaluated. tau-Corrected Pb isotope data reveal LA-induced isotope fractionation amounting to ∼0.5% amu−1 for Pb isotopes over the course of an entire fluid inclusion ablation. Instrumental mass bias correction was effected within-run using Tl provided by the fluid inclusion itself or admixed to the ablation aerosol via desolvated nebulization. Isotope ratios derived from the transient signals were either based on individual readings or on bulk signal integration, of which the latter produces significantly more accurate data. The external precision achieved by ablating SRM 610 glass with a 60 µm beam is ±0.011% (2 SD, relative) for 208Pb/206Pb and 207Pb/206Pb ratios and ±0.032% for Pb isotope ratios normalized to mass 204 (n = 18). Inclusion-to-inclusion reproducibilities (n = 11; ∼0.1 ng Pb per inclusion) are ±0.05% (2 SD; 208Pb/206Pb and 207Pb/206Pb) and ±0.13% (20xPb/204Pb), respectively; inclusions containing as little as 0.005 ng Pb returned ±0.1% and ±0.8%. These results are accurate as demonstrated by analysis of synthetic fluid inclusions containing SRM 981 Pb. The analytical protocol presented here for measuring isotope ratios on minute analyte quantities by multiple-collector ICP-MS in fast transient signal mode has great potential for applications to geochemical, archaeological, environmental and possibly biochemical problems.

The rates of accretion, core formation and volatile loss in the early Solar System

Nuclides with half–lives of 105–108 yr permit the elucidation of nebula time–scales and the rates of accretion of planetesimals. However, the 182Hf–182W system with a half–life of 9_2 Myr also provides new and very useful constraints on the formation of the terrestrial planets. This technique allows one to address the timing of metal–silicate equilibration in objects as different as chondrites and the Earth. With improvements in sensitivity and precision, very small time differences in metal segregation in asteroids should be resolvable from measuring iron meteorites. It is already clear that the formation and differentiation of some asteroidal–sized objects was completed in less than 10 Myr. Accretion and core formation were protracted in the case of the Earth (greater than 50 Myr) relative to Mars (probably less than 20 Myr). Indeed, the Martian mantle appears to retain both chemical and isotopic heterogeneities that are residual from the process of core formation. Such early features appear to have been eliminated from the Earths mantle presumably because of 4.5 Gyr of relatively efficient convective mixing. Tungsten isotope data provide compelling support for the ‘giant impact’ theory of lunar origin. The Moon is a high Hf/W object that contains a major component of chondritic W. This is consistent with a time of formation of greater than 50 Myr after the start of the Solar System. New highly precise oxygen isotope data are unable to resolve any difference between the source of components in the Earth and Moon. Therefore, the giant impact itself may have produced some of the differences in moderately volatile element budgets between these objects. This finds support in precise Sr isotopic data for early lunar samples. The data are consistent with the proto–Earth and Theia (the impactor) having Rb/Sr ratios that were not very different from that of present day Mars. Therefore, the extended history of accretion, rather than nebular phenomena, may be responsible for some of the major differences between the terrestrial planets.

Interlaboratory comparison of magnesium isotopic compositions of 12 felsic to ultramafic igneous rock standards analyzed by MC-ICPMS

To evaluate the interlaboratory mass bias for high-precision stable Mg isotopic analysis of natural materials, a suite of silicate standards ranging in composition from felsic to ultramafic were analyzed in five laboratories by using three types of multicollector inductively coupled plasma mass spectrometer (MC-ICPMS). Magnesium isotopic compositions from all labs are in agreement for most rocks within quoted uncertainties but are significantly (up to 0.3‰ in 26Mg/24Mg, >4 times of uncertainties) different for some mafic samples. The interlaboratory mass bias does not correlate with matrix element/Mg ratios, and the mechanism for producing it is uncertain but very likely arises from column chemistry. Our results suggest that standards with different matrices are needed to calibrate the efficiency of column chemistry and caution should be taken when dealing with samples with complicated matrices. Well-calibrated standards with matrix elements matching samples should be used to reduce the interlaboratory mass bias.