Статьи, тезисы докладов
https://repository.geologyscience.ru/handle/123456789/1864
2024-03-29T10:13:24ZCoexistence of abyssal and ultra-depleted SSZ type mantle peridotites in a Neo-Tethyan Ophiolite in SW Turkey: Constraints from mineral composition, whole-rock geochemistry (major–trace–REE–PGE), and Re–Os isotope systematics
https://repository.geologyscience.ru/handle/123456789/42478
Coexistence of abyssal and ultra-depleted SSZ type mantle peridotites in a Neo-Tethyan Ophiolite in SW Turkey: Constraints from mineral composition, whole-rock geochemistry (major–trace–REE–PGE), and Re–Os isotope systematics
Uysal I.; Ersoy E.Y.; Karsli O.; Dilek Y.; Sadıklar M.B.; Ottley C.J.; Tiepolo M.; Meisel T.
We present new, whole-rock major and trace element chemistry, including rare earth elements (REE), platinum-group elements (PGE), and Re–Os isotope data from the upper mantle peridotites of a Cretaceous Neo-Tethyan ophiolite in the Muğla area in SW Turkey. We also report extensive mineral chemistry data for these peridotites in order to better constrain their petrogenesis and tectonic environment of formation. The Muğla peridotites consist mainly of cpx-harzburgite, depleted harzburgite, and dunite. Cpx-harzburgites are characterized by their higher average CaO (2.27 wt.%), Al2O3 (2.07 wt.%), REE (53 ppb), and 187Os/188Os(i) ratios varying between 0.12497 and 0.12858. They contain Al-rich pyroxene with lower Cr content of coexisting spinel (Cr#=13–22). In contrast, the depleted harzburgites and dunites are characterized by their lower average CaO (0.58 wt.%), Al2O3 (0.42 wt.%), and REE (1.24 ppb) values. Their clinopyroxenes are Al-poor and coexist with high-Cr spinel (Cr#=33–83). The 187Os/188Os(i) ratios are in the range of 0.12078–0.12588 and are more unradiogenic compared to those of the cpx-harzburgites. Mineral chemistry and whole rock trace and PGE data indicate that formation of the Muğla peridotites cannot be explained by a single stage melting event; at least two-stages of melting and refertilization processes are needed to explain their geochemical characteristics. Trace element compositions of the cpx-harzburgites can be modeled by up to ~10–16% closed-system dynamic melting of a primitive mantle source, whereas those of the depleted harzburgites and dunites can be reproduced by ~10–16% open-system melting of an already depleted (~16%) mantle. Thesemodels indicate that the cpx-harzburgites are the products of first-stage melting and low-degrees of melt–rock interaction that occurred in a mid-ocean ridge (MOR) environment. However, the depleted harzburgites and dunites are the product of second-stage melting and related refertilization which took place in a supra subduction zone (SSZ) environment. The Re–Os isotope systematics of the Muğla peridotites gives model age clusters of ~250 Ma, ~400 Ma and ~750 Ma that may record major tectonic events associated with the geodynamic evolution of the Neo-Tethyan, Rheic, and Proto-Tethyan oceans, respectively. Furthermore, >1000 Ma model ages can be interpreted as a result of an ancient melting event before the Proto-Tethys evolution.
2012-01-01T00:00:00ZProcesses controlling highly siderophile element fractionations in xenolithic peridotites and their influence on Os isotopes
https://repository.geologyscience.ru/handle/123456789/42456
Processes controlling highly siderophile element fractionations in xenolithic peridotites and their influence on Os isotopes
Liu J.; Rudnick R.L.; Walker R.J.; Gao S.; Wu F.; Piccoli P.M.
Xenolithic peridotites having a similar range of major element compositions from two nearby localities in the Trans-North China Orogen, North China Craton, provide a rare opportunity to explore effects resulting from both primary partial melting and secondary processes on Os isotopes and highly siderophile element (HSE) abundances. HSE patterns of peridotites from Hannuoba are similar to those of orogenic peridotite massifs worldwide, but are rare for xenolithic peridotites. These patterns can be explained by relatively low degrees of melt depletion, coupled with long-term preservation of sulfides. By contrast, peridotites from Yangyuan have major element compositions similar to or slightly more depleted than Hannuoba xenoliths, but are characterized by distinct, highly fractionated HSE patterns with lower total HSE abundances and Os, Pd and Re depletions relative to Ir. Some of the latter HSE characteristics must reflect secondary processes. The low S and Se contents of Yangyuan peridotites, coupled with scarcity of observable sulfides, suggest that they experienced sulfide breakdown, possibly as a result of interaction with a S-undersaturated melt/fluid. This may have occurred under oxidizing conditions, as suggested by the somewhat higher ƒO2 recorded in the Yangyuan peridotites compared to the Hannuoba peridotites, as well as the metal-deficient composition of rare, mono-sulfide-solid solution (mss) sulfides within the Yangyuan peridotites. We speculate that under such conditions, Os, Pd, and possibly Re, more strongly partition into a sulfide liquid, or the oxidizing medium (melt or fluid), than Ir and Pt and, thus, become depleted. These effects would have been imposed on original patterns that were similar to those in the Hannuoba suite. The good correlation between 187Os/188Os and major element indices of melt depletion in the Yangyuan rocks, coupled with the poor correlation between 187Os/188Os and 187Re/188Os, suggests that the S, Os, Pd and Re removal was recent. Hence, the long-term Re–Os isotopic systematics of these rocks would not have been affected, and Re depletion model ages, based on Os isotopes, remain viable to constrain the timing of melt deletion in these peridotites. The similarity of model age distributions between Yangyuan and Hannuoba peridotites (TRD = 0 to 1.7 and 0 to 1.5 Ga, respectively) is consistent with this, and further indicates that these peridotites formed in the Paleoproterozoic.
2010-01-01T00:00:00ZGeochemistry, Re–Os isotopes and highly siderophile element abundances in the Eastern Pontide peridotites (NE Turkey): Multiple episodes of melt extraction–depletion, melt–rock interaction and fertilization of the Rheic Ocean mantle
https://repository.geologyscience.ru/handle/123456789/42455
Geochemistry, Re–Os isotopes and highly siderophile element abundances in the Eastern Pontide peridotites (NE Turkey): Multiple episodes of melt extraction–depletion, melt–rock interaction and fertilization of the Rheic Ocean mantle
Dokuz A.; Uysal I.; Dilek Y.; Karsli O.; Meisel T.; Kandemir R.
We report on the structure, geochemistry, Re–Os isotopes and relative abundances of highly siderophile elements (HSEs) of the Paleozoic peridotite–basalt occurrences in the Eastern Pontide belt of northeastern Turkey. These peridotites and the associated basaltic rocks are the remnants of the Rheic oceanic lithosphere, incorporated into the Eurasian continental margin during the Variscan (Hercynian) orogeny. The peridotites display a complex record of multiple magmatic, metasomatic and metamorphic events in different tectonic settings during the evolution of the Rheic upper mantle. The Beyçam harzburgite (BH) contains low Al2O3 (0.51–1.88 anhydrous wt.%) and high MgO (41.35–42.34 wt.%) contents, and its bulk-rock trace element compositions are less than 10% of the primitive upper mantle (PUM) values. The platinum, Pd and Re contents of the Beyçam harzburgite are highly depleted, whereas its Os, Ir, and Ru contents are slightly enriched relative to the PUM values. Its Pd and Re contents that are higher than those of the fertile Pulur lherzolite (PL) to the south and the absence of an isochronous relationship between its 187Os/188Os and 187Re/188Os show that the trace element distribution and the isotope ratios of the Beyçam harzburgite were significantly modified after the first melt-extraction episode. The first melt extraction occurred beneath the Rheic mid-ocean ridge spreading center, whereas the second melt extraction occurred in a mantle wedge above a Rheic subduction zone. The primary magmatic phases of the Pulur lherzolite show the geochemical characteristics typical for fertile lherzolite, formed in the early stages of oceanic lithosphere generation subsequent to the continental break up. The Pulur lherzolite also contains a secondary magmatic phase in the form of networks of clinopyroxene veins and channels, which are interpreted as an evidence for solid state melt–rock reactions between the lherzolite and a percolating basaltic melt above a subduction zone. This clinopyroxene addition resulted in the formation of variable concentrations of Al2O3 (2.47–4.33 wt.%) and MgO (29.76–40.10 wt.%) in the lherzolite. The rhenium, Pd and Pt concentrations of the Pulur lherzolite are depleted relative to the PUM values, whereas the Os, Ir and Ru concentrations are in the range of the PUM values as commonly observed in peridotites with a melt depletion history. The high suprachondritic 187Os/188Os is, however, inconsistent with a simple melt depletion history, and can be explained by the addition of radiogenic Os-bearing sulfide phases into the lherzolite as a result of melt–rock reactions. Basaltic rocks with an island arc tholeiitic composition from the Beyçam area represent the partial melting product of the moderately depleted Beyçam harzburgite and the basaltic parental melt from which the clinopyroxene precipitated. The covariation between the 187Re/188Os and 187Os/188Os of these basaltic rocks defines an isochron age of 377 ± 8 Ma (late Devonian). The combined structural, geochemical and isotope data indicate a prolonged history of multiple episodes of melt extraction–depletion, and melt–rock interaction and fertilization of the mantle lithosphere of the Rheic Ocean.
2014-01-01T00:00:00ZTrace-element composition of Fe-rich residual liquids formed by fractional crystallization: Implications for the Hadean magma ocean
https://repository.geologyscience.ru/handle/123456789/42454
Trace-element composition of Fe-rich residual liquids formed by fractional crystallization: Implications for the Hadean magma ocean
Lee C.A.; Yin Q.; Lenardic A.; Agranier A.; O'Neill C.J.; Thiagarajan N.
New isotopic studies of 142Nd, the daughter product of the short-lived and now extinct isotope 146Sm, have revealed that the accessible part of the silicate Earth (e.g., upper mantle and crust) is more radiogenic in 142Nd/144Nd than that of chondritic meteorites. The positive 142Nd anomaly of the Earth’s mantle implies that the Sm/Nd ratio of the mantle was fractionated early in Earth’s history and that the complementary low 142Nd reservoir has remained isolated from the mantle since its formation. This has led to the suggestion that an early enriched reservoir, formed within Earth’s first hundred million years (the Hadean), resides permanently in the deep interior of the Earth. One hypothesis for a permanently isolated reservoir is that there may be an Fe-rich, and hence intrinsically dense, chemical boundary layer at the core-mantle boundary. The protoliths of this chemical boundary layer could have originated at upper mantle pressures during extreme fractional crystallization of a global magma ocean during the Hadean but testing this hypothesis is difficult because samples of this early enriched reservoir do not exist. This hypothesis, however, is potentially refutable. Here, we investigate a post-Archean magnetite-sulfide magma formed by extreme magmatic differentiation to test whether residual Fe-rich liquids of any kind have the necessary trace-element signatures to satisfy certain global geochemical imbalances. The magnetite-sulfide magma is found to have high Pb contents (and low U/Pb ratios), high Re/Os ratios, and anti-correlated Sm/Nd and Lu/Hf fractionations. Permanent segregation of such a magma would (1) provide a means of early Pb sequestration, resulting in the high U/Pb ratio of the bulk silicate Earth, (2) be a source of radiogenic 187Os in the source regions of plumes, and (3) provide an explanation for decoupled Hf and Nd isotopic evolution in the early Archean, which is not easily produced by silicate fractionation. However, the magnetite-sulfide magma is not highly enriched in K, and thus, at face value, this magma analog would not serve as a repository for all of the heat producing elements. Nevertheless, other Fe–O–S liquids reported elsewhere are enriched in apatite, which carries high concentrations of K, U and Th. Given some promising geochemical fractionations of the Fe-rich liquids investigated here, the notion of a Hadean Fe-rich residual liquid deserves continued consideration from additional experimental or analog studies.
2007-01-01T00:00:00Z