E. Jagoutz

Sm–Nd and Rb–Sr isotopic chronology and cooling history of ultrahigh pressure metamorphic rocks and their country rocks at Shuanghe in the Dabie Mountains, Central China

Abstract Ultrahigh pressure metamorphic (UHPM) rocks at Shuanghe area in the Dabie Mountains occur as a UHPM block within the regional granitic gneiss. A Sm–Nd isochron defined by garnet + omphacite + rutile from coesite-bearing eclogite yield an age of 226.3 ± 3.2 Ma. Another Sm–Nd isochron defined by garnet + two phengites form UHP gneiss yield a similar age of 226.5 ± 2.3 Ma. The two consistent Sm–Nd ages defined by three UHPM minerals suggest that Nd isotopic equilibrium between UHPM minerals in these rocks during UHP metamorphism has been achieved. This may correspond to the age of peak metamorphism with an average metamorphic temperature of 800°C. It is demonstrated that the retrograde metamorphism of UHPM rocks occurred in an open chemical system, whereas the Nd and Sr isotopic systematics of UHPM minerals may have remained closed. Nd and Sr isotopic disequilibrium between UHPM minerals and retrograde metamorphic minerals has been observed; therefore, the tie line of garnet or phengite and whole rock containing retrograde metamorphic minerals gives an old Sm–Nd age and a young Rb–Sr age, respectively, with no geologic significance. However, a Rb–Sr ages of 219.0 ± 6.6 Ma defined by phengite + garnet from a UHPM gneiss indicates that the UHPM rocks at Shuanghe cooled down to 500°C at that time. It suggests that the UHPM rocks at Shuanghe experienced the first rapid cooling during 226–219 Ma. On the other hand, the Rb–Sr ages of 174 ± 7.8 to 169.2 ± 3.3 Ma defined by retrograde minerals (amphibole or biotite) with closure temperature ranging from 450° to 300°C and intensely retrograded metamorphic rocks reflect a second rapid cooling during this time interval. This is consistent with the “rapid cooling” time of 190–170 Ma of the orthogneiss in the Dabie–Su–Lu UHPM belt obtained by Ar–Ar dating method. In contrast to UHPM rocks, the Nd isotopic composition of the garnet in granitic gneiss has been reset during retrograde metamorphism and is in equilibrium with those of retrograde epidote and biotite, which yields a Sm–Nd isochron age of 213 ± 5 Ma indicating the retrometamorphic time corresponding to amphibolite facies. In addition, the biotites from the granitic gneiss yield Rb–Sr ages of 171–173 Ma similar to those of the UHPM rocks. These data suggest that the country rocks (granitic gneiss) may have a similar cooling history to the UHPM rocks at Shuanghe. Two stages of rapid cooling of UHPM rocks at Shuanghe may correspond two stages of fast uplift: the initial rapid uplifting and cooling of UHPM rocks during 226–219 Ma may be caused by compression tectonics during subducting time of the continental crust; whereas the later rapid cooling may reflect the exhumation of the entire subducted continental crust by extension during the early–middle Jurassic.

Some new aspects of dating eclogites in orogenic belts: Sm-Nd, Rb-Sr, and Pb-Pb isotopic results from the Austroalpine Saualpe and Koralpe type-locality (Carinthia/Styria, southeastern Austria)

New Sm-Nd, Rb-Sr, and Pb-Pb isotope data on eclogites and metagabbros from the Austroalpine Koralpe and Saualpe basement nappes of the eastern Alps are presented. These rocks are encased in polymetamorphic gneisses and micaschists that yield tNdCHUR ages of between 1.04 and 1.81 Ga. ϵ0Nd values from seven eclogite whole rocks range between +7.0 and +10.8; 147Sm144Nd is close to modern DM In a 208pb206Pb diagram all samples plot very close to the MORB field. Most analyses of the major mineral components, garnet, clinopyroxene, zoisite/epidote, phengite, amphibole and rutile, show disequilibrium in all three isotopic systems. Internal Sm-Nd and Rb-Sr mineral isochron ages range between 53 and 151 Ma. A minimum age of around 100 Ma is estimated for the crystallization of the high-P paragenesis garnet + omphacite + zoisite + kyanite + amphibole + quartz + rutile ± phengite + accessories, on the basis of these results. Later thermal overprint, fluid activity, and retrogression during exhumation of the eclogites involved (re-)crystallization of amphibole and garnet, thus leading in part to geochronologically poorly interpretable isochrons, without strict time significance. The last (eo-Alpine) thermal climax, involving static (re-)crystallization of garnet, staurolite and kyanite within the eclogite host rocks, is defined by concordant Sm-Nd and Rb-Sr isochrons on garnet, white mica, and staurolite at around 90 ± 3 Ma. Biotite Rb-Sr ages from these rocks range between 57 and 92 Ma. Plagioclase, pyroxene, and whole rock, analyzed from a relic gabbro core that shows continuous transition into eclogite from the southern Koralpe, yielded a Sm-Nd isochron of 275 ± 18 Ma, and an initial 143Nd144Nd ratio of 0.51271 ± 2 (ϵtNd = + 8.4 ± 0.5). This age is interpreted to date primary magmatic crystallization, thus setting also an uppermost time limit for eclogite metamorphism in the study area. The same outcrop yields a Sm-Nd isochron age for garnet and whole rock from the eclogitized gabbro of 93 ± 15 Ma; clinopyroxene from the same assemblage, however, lies clearly off this isochron. Whereas the 93 Ma figure may be regarded as a lower age limit for the eclogite event, an upper age limit of ca. 150 Ma may also be inferred for this metamorphism on the basis of these results. Taking the isotopic data from both Saualpe and Koralpe together, two basically different processes may be responsible for the data scatter and the partly unrealistically young isochrons in the Saualpe eclogites: 1. 1) Incomplete isotopic resetting, even on the grain scale, during the eclogitization of the igneous rocks, leaving cpx partly as a closed system. 2. 2) Ongoing crystallization (primarily of amphibole and/or garnet) after the peak of high-P metamorphism, probably combined with the introduction of a fluid phase with a strongly different isotopic signature from the eclogite host rocks. The results show that the Alpine evolution within the Austroalpine domain began very early (already in Permian times), with continental fragmentation, crustal thinning, and oceanic magmatic activity, close to the northern border of Gondwana. Ongoing extensional processes led to extensive production of basaltic melts close to or within a province of disrupted and strongly reduced continental basement. With the onset of collision at the western end of the Tethys ocean in Upper Mesozoic times, basement and young ocean floor were involved in Alpine subduction and, finally, in nappe tectonics, in forming the “root zone” of the present Austroalpine basement nappes.

Diamondiferous eclogites from Siberia: Remnants of Archean oceanic crust

We have investigated eight diamond-bearing bimineralic eclogite xenoliths from the Udachnaya Mine, Yakutia, Siberia, in terms of major elements, 87Sr86Sr−, 143Nd144Nd and oxygen isotopic ratios. The β18O-values, measured with the new laser-fluorination technique, are different from mantle values and range between 5.19 and 7.38%. with an average error of 0.08%.. Strontium and neodymium initial isotopic ratios for cpx are between 0.70226 and 0.70699 and 0.51170 and 0.51257, respectively. Chemically and petrographically, the Siberian eclogites are very similar to the South African eclogite suite from Roberts Victor or Bellsbank, the most important similarities being the late Archean age (2.76 Ga) and the δ18O values that deviate from mantle values. However, differences exist in detail, as no samples with δ18O values lower than mantle values have yet been reported from Siberia and the cesium concentrations of the Siberian eclogites are generally lower than those of the Roberts Victor eclogite suite. The data obtained from the studied sample suite are best explained by a model proposing an origin from Archean oceanic crust that was intensely altered prior to subduction to mantle depths. Using oxygen isotopic values, the effects of seawater alteration can be shown and the composition of the unaltered protolith qualitatively estimated. We propose that mantle eclogites from kimberlites were generated by a globally operating subduction process during the late Archean and that differences between samples from different cratons are small compared to their similarities.

Mantle Chemistry and Accretion History of the Earth

The chemical composition of the Earth’s primitive mantle (present mantle + crust) yields important information about the accretion history of the Earth. For the upper mantle reliable data on its composition have been obtained from the study of primitive and unaltered ultramafic xenoliths (Jagoutz et al. 1979). Normalized to C 1 and Si the Earth’s mantle is slightly enriched in refractory oxyphile elements and in magnesium. It might be that this enrichment is fictitious and only due to the normalization to Si and that the Earth’s mantle is depleted in Si, which partly entered the Earth’s core in metallic form. Alternatively, the depletion of Si may only be valid for the upper mantle and is compensated by a Si enrichment of the lower mantle.

Sm/Nd, Rb/Sr, and 40Ar/39Ar Isotopic Systematics of the Ultrahigh-Pressure Metamorphic Rocks in the Dabie-Sulu Belt, Central China: A Retrospective View

Because of a complicated metamorphic history, the isotopic systematics of the ultrahigh-pressure (UHP) metamorphic rocks in the Dabie-Sulu belt, east China, appear to be rather different from what were expected. Depending on the degree of retrograde metamorphism and on the retentivity of isotopes, the radiogenic isotopic systematics in the UHP metamorphic rocks yielded a wide range of radiometric ages. Some of these ages are geologically meaningful, but others may not be. In some fine-grained UHP metamorphic rocks, Sm/Nd isotopic systematics appear to be in equilibrium among the UHP phases, showing the best estimate for the age of peak metamorphism at 226 ± 3 Ma. On the other hand, retrograde overprinting often makes the interpretation of isotopic data more difficult. It is common to find that the Sm/Nd and Rb/Sr isotopic systematics among the UHP phases and retrograde phases are not in equilibrium. Regression of isotopic data involving both UHP and retrograde minerals in isotopic correlation diagrams oft...

Nd and Sr systematics in an eclogite xenolith from Tanzania: Evidence for frozen mineral equilibria in the continental lithosphere

Combined isotopic (Nd, Sr) and petrographic analyses are reported for an eclogitic xenolith from the Vissury Mine, Tanzania. An exceedingly large difference in measured ϵNd values in garnet and clinopyroxene (>200 ϵ units) suggest a major mineral equilibration at 1.75 ± 0.014 Ga. Nd isotopic heterogeneities are demonstrated in different generations of garnet and zonation in Sr and Nd isotope compositions is present within the clinopyroxene. The variation in measured Nd isotope compositions in different generations of garnet corresponds to 16 ϵ units. Exsolution of garnet from clinopyroxene occurred in an essentially closed system with respect to Sr and Nd. Petrographically different generations of garnet are present. One garnet with rutile inclusions as well as the rims of clinopyroxene are clearly off the isochron. Features like this can originate either if a system is not closed (e.g. affected by a migrating fluid) or if a remanent isotopic heterogeneity survived the homogenisation at 1.75 Ga. The subsolidus partitioning of Sm and Nd between garnet and clinopyroxene results in higher fractionation of the SmNd ratio in garnet than in the liquid-crystal partitioning. Since the closure temperature for the Sm-Nd exchange between garnet and clinopyroxene may be around 850°C in a dry system, many of the lithospheric ultramafic rocks may exhibit frozen mineral equilibria. The isotopic data suggest that this eclogite was emplaced at 1.75 Ga into the lithosphere mantle, at temperatures of about >1040°C, and then cooled as a closed system at a rate of 6–50°C/my down to 840°C, where all the exchange reactions were efficiently frozen in at 1.75 Ga ago. The coincidence of the emplacement of granites (1.7–1.8 Ga) into the central part of the Tanzanian craton with the age of the eclogite suggests that lower crustal melting was triggered by a thermal event in the upper lithosphere.

Chemical systematics of the shergotty meteorite and the composition of its parent body (Mars)

Abstract We report chemical data for 60 elements by INAA and RNAA in two bulk samples, for 30 elements in various mineral separates of Shergotty, and results of leaching experiments with 1M HCl on powdered aliquots of Shergotty and BETA 79001, lithologies A and B. Shergotty is homogeneous in major element composition but heterogeneous with respect to LIL trace elements (~20%). The heterogeneity is even greater for volatile and siderophile trace elements. The mineral data, including three clinopyroxene fractions with variable FeO contents, maskelynite and minor phases (Ti-magnetite, ilmenite, quartz, K-rich phase), show that major minerals do not account for the rare earth elements (REE) in the bulk meteorite. Instead, the REE are to a large extent concentrated in accessory whitlockite and apatite (shown by leaching with 1M HCl): together with the majority of REE (La, 96%, Yb 70%), Cl and Br are quantitatively dissolved by leaching. The REE patterns of the leachate of Shergotty and EETA 79001 are different. The Shergotty leachate may consist of two components. Component l is similar to that of EETA 79001 leachate (whitlockite), component 2 is enriched in light REE and may be responsible for the higher LREE contents of Shergotty in comparison to the other shergottites. There is some evidence that Shergotty was an open system and component 2 was introduced after crystallization. The REE patterns of the residues of Shergotty and EETA 79001 are identical indicating that the parent magmas of both meteorites are compositionally similar. Based on cpx separates with the lowest REE content, the REE pattern in the Shergotty parent magma was calculated. It is enriched in LREE and has a subchondritic Nd Sm ratio. The negative Eu anomaly in the phosphates indicates that at least some plagioclase crystallized before phosphate. Based on several element correlations in SNC meteorites, it was suggested (Dreibus and Wanke, 1984) that both the Shergotty parent body (SPB, very probably Mars), and the Earth accreted from the same two chemically different components: component A, highly reduced and devoid of volatile elements and an oxidized component B containing also volatile elements. The SPB (Mars) mantle is 2–4 times richer in volatile and moderately siderophile elements than the Earth, indicating a higher portion of component B in the SPB. The concentrations of chalcophile elements in the SPB mantle are low, reflecting equilibration with a sulfide phase and subsequent segregation of sulfide into the core. Unlike the Earth (Wanke, 1981), the SPB (Mars) may therefore have accreted almost homogeneously.

Further Sr and Nd isotopic results from peridotites of the Ronda Ultramafic Complex

Abstract Clinopyroxenes derived from peridotites of the spinel and garnet facies of the Ronda Ultramafic Complex yield Sr and Nd isotopic ratios which extend the range of compositions found in the massif to values as depleted as 0.70205 for Sr and 0.51363 for Nd. Large-amplitude, short-wavelength isotopic variations are found to be ubiquitous throughout the massif. In the garnet facies, some of these variations are shown to be produced by the tectonic disaggregation of mafic layers in an isotopically depleted peridotite matrix. Ages obtained from garnet-clinopyroxene Sm Nd isochrons (about 22 m.y.) agree with previous determinations of the time of crustal emplacement. In the plagioclase facies, where the Sr and Nd isotopic compositions have been very strongly affected by recent cryptic metasomatism, detailed study of one sample reveals that intermineral Nd isotopic equilibrium exists between clinopyroxene, orthopyroxene, and plagioclase. This indicates that the metasomatism occurred at high temperatures, and thus probably within the mantle. A rough correlation between 143 Nd/ 144 Nd and 147 Sm/ 144 N , with an apparent “age” of 1.3 b.y. and an initial e Nd (0) value of +6.0, is observed among clinopyroxenes derived from river sediments from throughout the massif. This age is interpreted as the time that the massif left the convecting mantle and became incorporated into the sub-continental lithosphere.

Chronology of SNC meteorites

In this paper a model is presented for the geochemical evolution of Mars which is constrained by the isotope systematics of Pb, Nd, and Sr determined for SNC meteorites (SNCs). The young magmatic crystallization ages (internal or mineral ages) of SNCs may indicate that these meteorites indeed stem from Mars. Internal ages and U-Pb and Pb-Pb systematics strongly suggest that they are the result of two magmatic processes. In addition, shock metamorphism is implied from observed petrographic shock features. For ALHA 77009 a shock-age < 15 Ma is obtained which is within uncertainty identical to the independently determined cosmic ray exposure age. It is therefore plausible that shock and exposure ages are identical for all SNCs. The Rb/Sr data of all common (non-SNC) meteorites form a 4.55 Ga isochron as do the Pb-Pb data (geochron). The SNC data fall close to these two isochrons. The Sr and Pb isotopic compositions in SNCs suggest that they formed in a recent (1.3-0.15 Ga) melting event from reservoirs which had been magmatically differentiated 4.3 ± 0.2 Ga ago. In a concordia diagram (U-Pb evolution plot) the SNC data reflect recent increase of the U/Pb ratio and the same two stage magmatic history as suggested by the other isotopic systems. The oxygen isotopic composition as well as the Nd isotopic systematics strongly suggest that the SNCs stem from one common reservoir which chemically differentiated 4.3 ± 0.2 Ga ago and then formed sub-reservoirs. In contrast to common meteorites, SNCs experienced an early magmatic differentiation where the Sm/Nd, U/Pb and Rb/Sr ratios have been strongly fractionated. In the recent magmatic process (1.3-0.15 Ga ago), in which the SNCs were formed as rocks, Sm/Nd and U/Pb were fractionated, while Rb/Sr remained similar to that of the source from which the magmas originated. During these melting events, mixing of components from different sub-reservoirs might have had occurred. At least three subreservoirs are necessary to explain the isotopic variations observed in SNCs. In contrast to the isotopic evolution of the Earth, Mars conserved remnants of the primary differentiation, a fact, which places important constraints on the tectonic evolution of Mars.

Sr and Nd isotopic systematics of Shergotty meteorite

Abstract Strontium and neodymium isotopic measurements are reported on mineral separates, whole rock samples, and their leachates and residues of Shergotty meteorite. A very large variation of the Sm Nd ratio in the phosphates of Shergotty is observed. Two sources of Nd are observed which are not mixed completely within Shergotty, resulting in two “ages”: (1) an apparent age of 147 ± 20 m.y. from the whole rock leachateresidue Nd trend; and (2) 360 ± 16 m.y. from a well-defined Nd isochron of minerals and their leachates and residues. About 60% of the Sr in the rock resides in the plagioclase and mesostasis, and this was reset at 167 m.y. For the pyroxenes this resetting event had a more complex but also a more localized effect. From the general pattern of this resetting a shock event at this time seems possible. The 360 m.y. whole rock leachate-residue age in the Rb-Sr isotopic system coincides with the Sm-Nd isochron age. We think that the 360 m.y. age represents the crystallization of Shergotty. The apparently younger Nd age of the phosphates which are indicated by the leaching experiments might be caused by late or post-magmatic contamination of the whitlockites with “radiogenic” Nd. This same contamination process must also have affected the Sr isotopic system. The 360 m.y. crystallization age implies that the plagioclase and phosphates behaved as an open system to the contaminant after the crystallization of the pyroxenes. The Sr of the contaminant affected both the plagioclase and whitlockite, but Nd influenced only the whitlockites, reflecting the partitioning of Sr and Nd between these phases. The general chemical pattern of this contamination process is different from contamination patterns observed in terrestrial rocks. This pattern may represent the finger-print of localized fluid-rock interaction on Mars. Since a similar trend is observed in other SNC meteorites, it could indicate that a potassium phase could be a mantle mineral on Mars. This would have important effects on the chemical evolution of a planet especially in respect to isotopic tracers like Ar, Pb, Sr.