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Rb-Sr dating and Sr isotope geochemistry by ID-TIMS

Our work is concentrated on material from the Alps, the Himalayan-Tibet orogenic belt and the wider Mediterranean area. The work encompasses:

  • Whole rock (wr) Rb-Sr ID-TIMS analysis of a variety of chemical composition, to elucidate protolith nature (initial Sr isotope composition), and age.
  • Mineral dating is mainly applied to micas (biotite, white mica) from igneous and metamorphic assemblages; however, other minerals (feldspars, apatite, amphibole, pyroxene) are also analyzed to control internal Sr isotope equilibration.
  • Precise Sr IC (isotope composition) measurements for marine carbonates, including microfossils (foraminifers).
  • Sr IC of igneous provinces (basalts, gabbros, granites s.l.), and from mantle rocks, to elucidate the geochemical nature of the protolith magma.
  • Sr IC of water samples.

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Sm-Nd mineral dating and Nd isotope geochemistry by ID-TIMS

The Sm-Nd method is a vital isotope analytical tool to approach both petrogenetic as well as geochronological questions. However, precise and accurate Sm-Nd isotope analysis for low-concentration material is only possible by ID-TIMS.

Fields of interest
Over the past 15 years, in our lab samples were analyzed mainly from the Alps and the Himalaya-Tibet orogenic belts, but also from other parts of the world (Mediterranean area, Africa, S-America).

Epsilon (e) Nd values and Sm-Nd model ages (DM, Depleted Mantle model ages) are used to discriminate whole-rock protolith materials which are derived from more juvenile or from long-term LILE depleted vs enriched sources. In addition, and mostly in combination with mineral dating, DM model ages from whole rocks are used to estimate a "mean crustal residence time" for rocks of the continental crust (clastic sediments or their metamorphic derivates: schists, gneisses).

Our main interest, however, is focused on mineral dating. The most important prerequisite for successful Sm-Nd mineral dating are ultrapure mineral separates. Depending on Nd concentration, sample sizes of some 10 - 100 mg are generally required (hence time-consuming hand-picking under the binocular microscope is inevitable!). To improve sample purity and to eliminate optically undetected micro-inclusions, acid leaching experiments (mainly for garnet) are also successfully performed.

Garnet has variable, but generally high Sm/Nd ratios if compared with other minerals (the highest Sm/Nd ratios measured so far are > 16), and therefore it yields important time information in metamorphic rocks, but also in (meta-)igneous rocks. Because of its prominent importance for P-T-d estimation, Sm-Nd data from this mineral have been frequently determined in order to link P-T-d and geochronological information directly (i.e., for one and the same material). In addition, garnet often shows conspicuous grain-internal trace element zonation resulting in strong variation of Sm/Nd, thus allowing "internal garnet isochrons" to be calculated. In our laboratory, Sm-Nd garnet analyses have been performed during the past fifteen years for a range of P-T conditions and on various lithologies (metabasites, metapelites, meta-pegmatites, dykes), with emphasis on the Eastern Alps.

Despite the well-known low-fractionation behavior of REE, the apparent "spread" in Sm/Nd of the different minerals pertaining to one and the same assemblage - most importantly major and trace mineral phases - is generally sufficient to allow internal time resolution to be made. The following materials have been analyzed:

  • Plagioclase-clinopyroxene (+ wr, ?orthopyroxene) pairs from gabbros (magmatic crystallsation age)
  • Garnet-omphacite (+ zoisite, + amphibole, + phengite) pairs from eclogite (high-P age)
  • Garnet-feldspar (+ wr, + mica, + staurolite) pairs from schists, paragneisses (metamorphic crystallization)
  • Garnet-feldspar (wr) pairs from (meta-)pegmatite / meta-granite, combined with precise ID-TIMS analysis of the co-existing high-REE accessory phases monazite, apatite, xenotime, titanite (and zircon) (emplacement age).

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U-Pb dating of accessory minerals by conventional ID-TIMS and in-situ U-Th-Pb dating by laser-ablation MC-ICP-MS

The following U-Pb dating and isotope geochemical techniques are used in our laboratory:

  • U-Pb dating of single zircons, monazite, xenotime, sphene, etc. using conventional digestion and ion-exchange procedures.
  • U-Pb dating of bisected single zircons using a step-wise vapour digestion technique (in combination with cathodoluminescence and back-scattered-electron imaging).
  • Whole rock and feldspar Pb isotope geochemistry using conventional digestion and ion-exchange procedures
  • In-situ U/Pb and Th/Pb dating by laser-ablation MC-ICP-MS

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Osmium isotope analysis

The Os isotope analysis was currently established at the Department of Lithospheric Research. The chemical and analytical procedure includes

  • high pressure asher digestion of the sample (with a mixed Re, Os, Ir, Pt spike),
  • solvent extraction using CCl4,
  • back extraction into HBr and
  • microdistillation.

Osmium isotopes are measured with the TRITON thermal ionization mass spectrometer in negative mode, whereas Re, Pt and Ir concentrations are measured using MC-ICPMS. Ongoing research is dominantly dedicated (but not restricted) to imapct science:

The Re-Os isotope system is based on the beta decay of 187Re to 187Os (half-live 42.3 Byr). During partial melting of mantle rocks, Os remains in the residue, but Re is enriched in the melt. Thus, crustal rocks have high Re and low Os concentrations and the 187Os/188Os ratios of crustal rocks increase rapidly with time. The present day 187Os/188Os ratio of mantle rocks is about 0.13. Meteorites also exhibit low 187Os/188Os ratios of about 0.11-0.18. Osmium is much more abundant in meteorites than Re, leading to only small changes in the meteoritic 187Os/188Os ratio with time, whereas old continental crust has 187Os/188Os ratios of about 0.67 to 1.61, distinctly different from the meteoritic values.

This allows the use of Re-Os isotope systematics for the study of impact craters and ejecta. Impact melts, breccias, and different materials in ejecta consist of terrestrial target rocks, in some cases mixed with a very small (<1%) admixture of recondensed projectile material, the so called meteoritic component. Because of the high Os abundances in meteorites, the admixture of only a very small meteoritic component to crustal target material will drastically change the Os isotope characteristics of the resulting breccias or impact melt rocks.

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Hf-W dating and isotope geochemistry

The Hf-W chronometer is based on the decay of 182Hf into 182W. The corresponding half-live of 8.9 Myr is sufficiently short to resolve the timescales of asteroidal differentiation processes within the first ~50 Myr of solar system history. Hafnium and W can be assumed to occur in chondritic proportions in undifferentiated solar system matter because both elements are refractory (both elements condensed at nearly the same temperatures during cooling of the protosolar cloud). During metal-silicate separation the lithophile, silicate loving Hf preferentially partitions into the silicates, whereas the siderophile, metal-loving W is enriched in the metal phase (Fig. 1).

Fig. 1: Partitioning of Hf and W under oxidizing conditions during asteroidal differentiation. Tungsten is moderately siderophile and therefore partitions strongly into the core, whereas lithophile Hf partitions into silicates. The W remainder in the mantle can be enriched in the crust during partial mantle melting as a result of the incompatible behaviour of W. Enrichments in W in crustal rocks so obtained can even exceed the W concentration of the bulk asteroid.

The extinct 182Hf/182W isotope system therefore provides the unique opportunity to make constraints on the relative timing of metal-segregation (e.g. core segregation). Early segregated metals in asteroids will have a relatively unradiogenic tungsten composition, depending on the exact timing of the metal-silicate differentiation (Fig. 2).

Fig. 2: Radiogenic ingrowth of 182W during asteroidal differentiation. Metals that segregate from a chondritic precursor during the lifetime of 182Hf (approximately the first 50 Myr of solar system history) have a constant 182W deficit (compared to the present day chondritic value), whose exact magnitude depend on the timing of the metal segregation (e.g. core formation). In contrast, silicates evolve to highly radiogenic W isotope compositions depending on the time of the metal-silicate separation event. The Epsilon notation is defined as the deviation of the 182W/184W ratio in a specific reservoir relative to a terrestrial standard in parts per 10.000.

Tungsten is only moderately siderophile and thus partly retained in the mantle. Silicate melts produced by subsequent internal differentiation of the silicate mantle are enriched in W relative to the residue, because W is more incompatible than Hf during silicate melting as long as the conditions remain oxidizing. If the segregation of Hf from W occurs during the parent nuclides life-time, the differentiated materials will have a time dependent excess or deficit in their 182W abundances.
The 182Hf-182W chronometer can thus be applied to both, metals (via measurements of their W isotope compositions) and to silicates (via the construction of internal isochrones; see Fig. 3).

Fig. 3: 182Hf-182W isochron diagram, showing two reservoirs with different Hf/W ratios and a single initial tungsten isotope composition. During the parent nuclides lifetime the two reservoirs evolve to different radiogenic tungsten compositions. After the cessation of 182Hf decay, these reservoirs define a final stage isochron (t=1) with the slope m, yielding the parent nuclides initial concentration (which can be used to obtain a relative time information, or an absolute by anchoring onto an absolute time scale).
Department of Lithospheric Research
University of Vienna
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1090 Vienna

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