1,721,093 research outputs found
A record of Antarctic surface temperature between 25 and 50 million years ago
No full textWe present the fi rst Antarctic terrestrial record of climate variations through the Cenozoic,
based on the hydrogen isotope composition of hydrothermally altered minerals of intrusive
rocks. This new record provides an independent geochemical proxy for continental climatic
conditions; whereas, most land surface temperature proxies are biological. The temperature
record is consistent with the range predicted by global climate models and proxy records for
glacial and pre-glacial conditions in the Ross Sea region of Antarctica. The combined stable
isotope (O and H) and age (40Ar-39Ar) determinations of hydrous mineral from Cenozoic igneous
plutons and dikes show that the protracted time scale of magmatic activity and extensive
hydrothermal exchange with local meteoric waters has preserved a semiquantitative climate
signal of intervals in which atmospheric temperatures signifi cantly fl uctuated. These data also
reveal that glacial episodes comparable with current polar conditions occurred repeatedly
prior to geographical and thermal isolation of the Antarctic continent
Petrogenesis of post-Variscan olivine-bearing cumulates and associated basalt-dykes from Bocca di Tenda (northern Corsica): implications for mantle sources.
No full textThe Oxygen Isotopic Composition of Xenoliths from Tallante (Southern Spain): Evidence for Crust Recycling into the Mantle
No full textMantle xenoliths from Tallante (Betic Cordillera, Spain) include samples recording a peculiar distinct style of metasomatism that induced orthopyroxene, plagioclase, phlogopite and amphibole crystallization and generated “hydrous” opx-rich mantle domains. The latter are locally crosscut by felsic veinlets containing plagioclase and orthopyroxene ± quartz ± phlogopite ± amphibole. The observed parageneses and available Sr-Nd-Hf isotopic data suggest that metasomatic agents were related to recycling of crust components within the mantle, plausibly in connection with subduction processes occurred during the Cenozoic Betic orogenic cycle.
In this study we investigated representative samples of composite xenoliths consisting of peridotite crosscut by felsic veins (varying in size from centimetric to millimetric) and unveined peridotites, measuring the 18O/16O ratios of the constituent minerals by laser fluorination. Results show that the narrow O-isotope compositional “typical” of mantle rocks, and the limited oxygen isotope fractionation at mantle temperatures, make oxygen isotopes a powerful tool for identifying recycled crustal material in the mantle. Orthopyroxene and plagioclase of the centimetric vein show delta18O values of +9.8 and +10.6‰, respectively, whereas clinopyroxene of the surrounding peridotite country rock has delta18O= +6.2‰. Plagioclase of two distinct millimetric felsic veins show delta18O of 7.6 and 7.3‰. The delta18O values significantly higher than typical mantle ones provide insights to the genesis of the Cenozoic subduction-related magmas of the Betic region that include silica-oversaturated calcalkaline (s.l.) and lamproite products, possibly resolving source vs. shallow level crustal contamination of the magmatic rocks. Moreover, the different O-isotope composition recorded in veinlets characterized by different thickness could provide insights into diffusion-assisted O-isotope requilibration of mantle rocks, thus constraining the time for “crust digestion” into the mantle
Fluid-inclusion and stable-isotope evidence for fluid infiltration and veining during metamorphism in marbles and metapelites
No full textSyn-metamorphic quartz + carbonate veins within lower-amphibolite-facies marbles and greenschist-facies metapelites of the O'Kane Canyon (northern Victoria Land, Antarctica) are characterised by fluids of different composition: H2O-CO2-CH4 mixtures (34-44 H2O, 50-61 CO2, 3-6 CH4, in mole %) (Type A), or H2O-CO2-N-2 fluids (35-43 H2O 48-54 CO2, 9-11 N-2, in mole %) (Type B). In metapelites, quartz + carbonate veins contain H2O-CO2-CH4 (Type C) fluids, which have a composition 30-68 H2O, 29-64 CO2, 2-6 CH4, in mole %) similar to Type A fluids in marbles. Isochores for both Type A and Type B fluids intersect the estimated peak conditions of 500 degrees C and 3-3.5 kbar, indicating that fluids with different compositions were trapped in the marbles during metamorphic veining. In greenschist-facies metapelites, isochores for Type C fluids cross the metamorphic P-T conditions at 2 kbar and 400 degrees C. Overall isochore distribution indicates early cooling (500-400 degrees C) without significant uplift. For the N-2-bearing Type B fluid inclusions, the delta(13)C value of the CO2 is 2.5 parts per thousand PDB and the delta D of H2O is 0 parts per thousand. The delta(18)O of host vein-quartz is 20.3 parts per thousand, and delta(13)C Of calcite within the host vein and in marble are both -1.5 parts per thousand. Tn contrast, the delta(13)C values of carbonate veins containing Type A H2O-CO2-CH4 fluids are lighter (-5.3 parts per thousand) than in the host marble (-2.9 to -1.5 parts per thousand). Type A fluid inclusions have light delta(13)C CO2 values (0.1 parts per thousand) and delta D H2O values (-84 parts per thousand). Type C inclusions also have light delta(13)C CO2 values (-3.1 parts per thousand), and variable delta D H2O (-30 and 98 parts per thousand). Fluid-inclusion and stable-isotope studies indicate that the analysed quartz-calcite veins formed by local processes in a similar structural context. The veins preserved distinct fluid-inclusion and isotopic compositions, indicating diverse sources of fluids: internally derived metamorphic H2O-CO2-N-2 mixtures (Type B), and externally derived H2O-CO2 fluids (Types A and C), interpreted in terms of mixing with a magmatic component, probably derived from local granitic rocks
Geochemical and O-isotope constraints on the evolution of lithospheric mantle in the Ross Sea rift area (Antarctica)
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Mantle sources and crustal input as recorded in High-Mg Deccan Trap basalts of Gujarat (India)
No full textNear-primitive picritic basalts in the northwestern Deccan Traps have MgO > 10 wt.% and consist of two groups (low-Ti and high-Ti) with markedly different incompatible element and Nd-Sr-Pb isotope characteristics. Many elemental characteristics of the low-Ti picritic basalts are similar to those of transitional or normal ocean ridge basalts. However, values of ratios like Ba/Nb (13-30) and Ce/Pb (4-11), and isotopic ratios e.g., epsilon(Nd)(t) +0.3 to -6.3 (Pb-207/Pb-204), 15.63-15.75 at (Pb-206/Pb-204 18.19-18.84, delta O-18(olivine) as high as +6.2 parts per thousand) are far-removed from ocean-ridge-type values, indicating a significant contribution from continental crust. The crustal signature could represent crustal contamination of ascending magmas; alternatively, it could represent a minor component within the Indian lithospheric mantle of anciently subducted sedimentary material or fluids derived from subducted material. In contrast, the high-Ti picritic basalts are chemically and isotopically rather similar to recent shield lavas of the Reunion hotspot (e.g., epsilon(Nd)(t) +2 to +4) and to volcanic rocks along the postulated pre-Deccan track of this hotspot in Pakistan. Neither type of picritic basalt is parental to the voluminous flows comprising the bulk of the Deccan Traps. However, many of the Deccan primary magmas could have been derived from mixtures of a high-Ti-type, Reunion-like source component and a component more similar to, or even more incompatible-element-depleted than, average ocean-ridge mantle
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