Laser-ablation microprobe (LAM)-ICPMS unravels the highly siderophile element geochemistry of the oceanic mantle
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Abstract Experiments on partial melting of mantle lherzolite have been realized at 0.6 and 1.0 GPa and the chemical compositional variations of melts during different melting stages have been first discussed. The results show that the trends of variations in SiO 2 , CaO, Al 2 O 3 , Na 2 O and TiO 2 are different at different melting stages. The melts produced at lower pressure are richer in SiO 2 than those at higher pressure. The mantle‐derived silica‐rich fluids (silicate melts) are polygenetic, but the basic and intermediate‐acid silicate melts in mantle peridotite xenoliths from the same host rocks, which have equivalent contents of volatile and alkali components and different contents of other components, should result from in‐situ (low‐degree) partial melting of mantle peridotite under different conditions (e.g. at different depths, with introduction of C‐O‐H fluids or in the presence of metasomatic minerals). The intermediate‐acid melts may be the result of partial melting (at lower pressure) Opx + Sp + K‐Na‐rich fluid ± (Amphi) ± (Phlog) = Ol + melt. But the intermediate‐acid magmas cannot be produced from the partial melting of normal mantle peridotite unless the crustal materials are introduced to some extent.
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Near-solidus Melting of the Shallow Upper Mantle: Partial Melting Experiments on Depleted Peridotite
We present the results of melting experiments on a moderately depleted peridotite composition (DMM1) at 10 kbar and 1250–1390°C. Specially designed experiments demonstrate that liquids extracted into aggregates of vitreous carbon spheres maintained chemical contact with the bulk charge down to melt fractions of ∼0·02–0·04 and approached equilibrium closely. With increasing melt fraction, SiO2, FeO*, and MgO contents of the partial melts increase, Al2O3 and Na2O contents decrease, and CaO contents first increase up to clinopyroxene-out at a melt fraction of 0·09–0·10, then decrease with further melting. A linear fit to melt fraction vs temperature data for lherzolite-bearing experiments yields a solidus of 1272 ± 11°C. The melting reaction is 0·56 orthopyroxene + 0·72 clinopyroxene + 0·04 spinel = 0·34 olivine + 1 liquid. Above clinopyroxene-out, the reaction is 1·24 orthopyroxene = 0·24 olivine + 1 liquid. Near the solidus, DMM1 glass compositions have lower SiO2, TiO2, Na2O, and K2O contents, higher FeO*, MgO, and CaO contents, and higher CaO/Al2O3 ratios compared with glasses from low-degree melting of fertile peridotite compositions. Recent computational models predict partial melting trends generally parallel to our experimental results. We present a parameterization of 10 kbar peridotite solidus temperatures suggesting that K2O and P2O5 have greater effects on solidus depression than Na2O, consistent with theoretical expectations. Our parameterization also suggests that abyssal peridotites have 10 kbar solidi of ∼1278–1295°C.
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This article reviews studies on fundamental question, whether oceanic peridotite can be the source of oceanic crust or the residue left after formation of oceanic crust, using a radiogenic isotopic composition from the 1990s. The Sr-Nd isotopic compositions of the oceanic peridotite are highly heterogeneous compared to those of the oceanic crust. The wide isotopic variation cannot be explained by simple partial melting or interaction between melt and peridotite. It requires several stages of ancient partial melting and interaction with melt. It has been suggested that the Sr-Nd isotopic compositions of the oceanic crust are homogenized by partial melting of the heterogeneous mantle peridotite. Accumulation of Sr-Nd isotopic data of the oceanic peridotites will be useful to further decipher the processes of partial melting and/or melt-peridotite interaction which occurred through the Earth's history.
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Thermodynamic calculation of partial melting of peridotite using the MELTS algorithm has the potential to aid understanding of a wide range of problems related to mantle melting. We review the methodology of MELTS calculations with special emphasis on the features that are relevant for evaluating the suitability of this thermodynamic model for simulations of mantle melting. Comparison of MELTS calculations with well–characterized peridotite partial melting experiments allows detailed evaluation of the strengths and weaknesses of the algorithm for application to peridotite melting problems. Calculated liquid compositions for partial melting of fertile and depleted peridotite show good agreement with experimental trends for all oxides; for some oxides the agreement between the calculated and experimental concentrations is almost perfect, whereas for others, the trends with melt fraction are comparable, but there is a systematic offset in absolute concentration. Of particular interest is the prediction by MELTS that at 1 GPa, near–solidus partial melts of fertile peridotite have markedly higher SiO2 than higher melt fraction liquids, but that at similar melt fractions, calculated partial melts of depleted peridotites are not SiO2 enriched. Similarly, MELTS calculations suggest that near–solidus partial melts of fertile peridotite, but not those of depleted peridotite, have less TiO2 than would be anticipated from higher temperature experiments. Because both experiments and calculations suggest that these unusual near–solidus melt compositions occur for fertile peridotite but not for depleted peridotite, it is highly unlikely that these effects are the consequence of experimental or model artifacts. Despite these successes of the results of calculations of peridotite melting using MELTS, there are a number of shortcomings to application of this thermodynamic model to calculations of mantle melting. In particular, calculated compositions of liquids produced by partial melting of peridotite have more MgO and less SiO2 than equivalent experimentally derived liquids. This mismatch, which is caused by overprediction of the stability of orthopyroxene relative to olivine, causes a number of other problems, including calculated temperatures of melting that are too high. Secondarily, the calculated distribution of Na between pyroxenes and liquid does not match experimentally observed values, which leads to exaggerated calculated Na concentrations for near–solidus partial melts of peridotite. Calculations of small increments of batch melting followed by melt removal predict that fractional melting is less productive than batch melting near the solidus, where the composition of the liquid is changing rapidly, but that once the composition of the liquid ceases to change rapidly, fractional and batch melting produce liquid at similar rates per increment of temperature increase until the exhaustion of clinopyroxene. This predicted effect is corroborated by sequential incremental batch melting experiments (Hirose & Kawamura, 1994, Geophysical Research Letters, 21, 2139–2142). For melting of peridotite in response to fluxing with water, the calculated effect is that melt fraction increases linearly with the amount of water added until exhaustion of clinopyroxene (cpx), at which point the proportion of melt created per increment of water added decreases. Between the solidus and exhaustion of cpx, the amount of melt generated per increment of water added increases with temperature. These trends are similar to those documented experimentally by Hirose & Kawamoto (1995, Earth and Planetary Science Letters, 133, 463–473).
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Thermodynamic calculation of partial melting of peridotite using the MELTS algorithm has the potential to aid understanding of a wide range of problems related to mantle melting. We review the methodology of MELTS calculations with special emphasis on the features that are relevant for evaluating the suitability of this thermodynamic model for simulations of mantle melting. Comparison of MELTS calculations with well–characterized peridotite partial melting experiments allows detailed evaluation of the strengths and weaknesses of the algorithm for application to peridotite melting problems. Calculated liquid compositions for partial melting of fertile and depleted peridotite show good agreement with experimental trends for all oxides; for some oxides the agreement between the calculated and experimental concentrations is almost perfect, whereas for others, the trends with melt fraction are comparable, but there is a systematic offset in absolute concentration. Of particular interest is the prediction by MELTS that at 1 GPa, near–solidus partial melts of fertile peridotite have markedly higher SiO2 than higher melt fraction liquids, but that at similar melt fractions, calculated partial melts of depleted peridotites are not SiO2 enriched. Similarly, MELTS calculations suggest that near–solidus partial melts of fertile peridotite, but not those of depleted peridotite, have less TiO2 than would be anticipated from higher temperature experiments. Because both experiments and calculations suggest that these unusual near–solidus melt compositions occur for fertile peridotite but not for depleted peridotite, it is highly unlikely that these effects are the consequence of experimental or model artifacts. Despite these successes of the results of calculations of peridotite melting using MELTS, there are a number of shortcomings to application of this thermodynamic model to calculations of mantle melting. In particular, calculated compositions of liquids produced by partial melting of peridotite have more MgO and less SiO2 than equivalent experimentally derived liquids. This mismatch, which is caused by overprediction of the stability of orthopyroxene relative to olivine, causes a number of other problems, including calculated temperatures of melting that are too high. Secondarily, the calculated distribution of Na between pyroxenes and liquid does not match experimentally observed values, which leads to exaggerated calculated Na concentrations for near–solidus partial melts of peridotite. Calculations of small increments of batch melting followed by melt removal predict that fractional melting is less productive than batch melting near the solidus, where the composition of the liquid is changing rapidly, but that once the composition of the liquid ceases to change rapidly, fractional and batch melting produce liquid at similar rates per increment of temperature increase until the exhaustion of clinopyroxene. This predicted effect is corroborated by sequential incremental batch melting experiments (Hirose & Kawamura, 1994, Geophysical Research Letters, 21, 2139–2142). For melting of peridotite in response to fluxing with water, the calculated effect is that melt fraction increases linearly with the amount of water added until exhaustion of clinopyroxene (cpx), at which point the proportion of melt created per increment of water added decreases. Between the solidus and exhaustion of cpx, the amount of melt generated per increment of water added increases with temperature. These trends are similar to those documented experimentally by Hirose & Kawamoto (1995, Earth and Planetary Science Letters, 133, 463–473).
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