Triassic high-Mg andesitic magmatism induced by sediment melt-peridotite interactions in the central Tibetan Plateau
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Peridotite
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Abstract Melt‐peridotite interaction has the ability to modify the δ 56 Fe of peridotite. However, iron isotopic fractionation during melt‐peridotite interaction is not well understood at present. In this study, we present high‐precision iron isotopic data for serpentinized and carbonated peridotite from the Yushigou supra‐subduction zone ophiolite in the North Qilian orogen of northern Tibet, to provide insights into iron isotope behavior during melt‐peridotite interaction in the mantle wedge. High‐Cr and high‐Al dunites coexist with harzburgite in the Yushigou ophiolite. The high‐Cr and high‐Al dunites were produced by interaction of peridotite with boninitic melt and tholeiitic melt, respectively. Serpentinization and carbonation have negligible influence on δ 56 Fe of the peridotite. The harzburgite mostly has δ 56 Fe ranging from −0.055 ± 0.029‰ to 0.056 ± 0.030‰, which overlaps with δ 56 Fe of abyssal peridotite. The high‐Al dunite is enriched in FeO, but it has δ 56 Fe (0.004 ± 0.034‰–0.083 ± 0.064‰) similar to the harzburgite. In contrast, the high‐Cr dunite displays a large range in δ 56 Fe from −0.173 ± 0.017‰ to 0.173 ± 0.057‰. Moreover, δ 56 Fe of the high‐Cr dunite is negatively correlated with its FeO content, which is ascribed to kinetic iron isotopic fractionation during melt‐peridotite interaction. Surprisingly, distinct iron isotope behavior during melt‐peridotite interaction recorded by the Yushigou peridotite can well explain the large variation of FeO and δ 56 Fe in abyssal peridotite and mantle wedge peridotite on a global scale.
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Dynamic metasomatism experiments were performed by reacting a lamproite melt with garnet peridotite by drawing melt through the peridotite into a vitreous carbon melt trap, ensuring the flow of melt through the peridotite and facilitating analysis of the melt. Pressure (2–3 GPa) and temperature (1050–1125 °C) conditions were chosen where the lamproite was molten but the peridotite was not. Phlogopite was formed and garnet and orthopyroxene reacted out, resulting in phlogopite wehrlite (2 GPa) and phlogopite harzburgite (3 GPa). Phlogopites in the peridotite have higher Mg/(Mg + Fe) and Cr2O3 and lower TiO2 than in the lamproite due to buffering by peridotite minerals, with Cr2O3 from the elimination of garnet. Compositional trends in phlogopites in the peridotite are similar to those in natural garnet peridotite xenoliths in kimberlites. Changes in melt composition resulting from the reaction show decreased TiO2 and increased Cr2O3 and Mg/(Mg + Fe). The loss of phlogopite components during migration through the peridotite results in low K2O/Na2O and K/Al in melts, indicating that chemical characteristics of lamproites are lost through reaction with peridotite so that emerging melts would be less extreme in composition. This indicates that lamproites are unlikely to be derived from a source rich in peridotite, and more likely originate in a source dominated by phlogopite-rich hydrous pyroxenites. Phlogopites from an experiment in which lamproite and peridotite were intimately mixed before the experiment did not produce the same phlogopite compositions, showing that care must be taken in the design of reaction experiments.
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Thesis. 1977. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences.
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The Earth is unique in our solar system in having a buoyant, highland-forming continental crust with a differentiated, andesitic composition; thus, it can be referred to as an "andesite planet." Andesitic magmatism is associated with convergent plate margins such as subduction zones, leading to a broad consensus that this setting has been the major site of continental crust formation. However, while andesites are dominant in mature continental arcs, they are subordinate in juvenile oceanic arcs, resulting in a great conflict regarding the creation of the continental crust. We focused on the Izu-Bonin-Mariana arc to assess this problem, as it is a juvenile intra-oceanic arc with a mid-crustal layer that has a seismic velocity identical to that of the bulk continental crust. Petrological modeling of the production of andesitic melts by the mixing of mantle-derived basalt with crust-derived, rhyolite magmas successfully reproduced the crust/mantle structure observed in seismic profiles of the Izu-Bonin-Mariana arc. As a result, we presented a challenging hypothesis: the continent was created in the ocean. One key mechanism that differentiates initial basaltic arc crust to evolved, andesitic continental crust may be the delamination of SiO2-depleted residues of crustal melting, termed "anti-continent," from the arc crust.
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We have performed reaction experiments between 1, 4, and 5 wt % CO2-bearing MORB-eclogite (recycled oceanic crust)-derived low-degree andesitic partial melt and fertile peridotite at 1375°C, 3 GPa for infiltrating melt fractions of 25% and 33% by weight. We observe that the reacted melts are alkalic with degree of alkalinity or Si undersaturation increasing with increasing CO2 content in reacting melt. Consequently, an andesite evolves through basanite to nephelinite owing to greater drawdown of SiO2 from melt and enhanced precipitation of orthopyroxene in residue. We have developed an empirical model to predict reacted melt composition as a function of reacting andesite fraction and source CO2 concentration. Using our model, we have quantified the mutual proportions of equilibrated melt from andesite-peridotite (+ CO2) hybridization and subsequent peridotite (± CO2)-derived melt required to produce the major element composition of various ocean island basalts. Our model can thus be applied to characterize the source of ocean islands from primary alkalic lava composition. Accordingly, we determined that average HIMU source requires 24 wt % of MORB-eclogite-derived melt relative to peridotite containing 2 wt % CO2 and subsequent contribution of 45% of volatile-free peridotite partial melt. We demonstrate that mantle hybridization by eclogite melt-peridotite (± CO2) reaction in the system can produce high MgO (>15 wt %) basaltic melts at mantle potential temperature (TP) of 1350°C. Therefore, currently used thermometers to estimate TP using MgO content of primary alkalic melts need to be revised, with corrections for melt-rock reaction in a heterogeneous mantle as well as presence of CO2.
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