Stability of hydrous melt at the base of the Earth's upper mantle (original) (raw)
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Low-degree partial melting trends recorded in upper mantle minerals
Earth and Planetary Science Letters, 1998
The study of glass inclusions inside mantle minerals provides direct information about the chemistry of naturally occurring mantle-derived melts and the fine-scale complexity of the melting process responsible for their genesis. Minerals in a spinel lherzolite nodule from Grande Comore island contain glass inclusions which, after homogenization by heating, exhibit a continuous suite of chemical compositions clearly distinct from that of the host basanitic lava. The compositions range from silicic, with nepheline-olivine normative, 64 wt% SiO 2 and 11 wt% alkali oxides, to almost basaltic, with quartz normative, 50 wt% SiO 2 and 1-2 wt% alkali oxides. Within a single mineral phase, olivine, the inferred primary melt composition varies from 54 to 64 wt% SiO 2 for MgO content ranging from 8 to 0.8 wt%. An experimental study of the glass and fluid inclusions indicates that trapped melts represent liquids that are in equilibrium with their host phases at moderate temperature and pressure (T ³ 1230ºC and P ³ 1:0 Gpa for melts trapped in olivine). Quantitative modelling of the compositional trends defined in the suite shows that all of the glasses are part of a cogenetic set of melts formed by fractional melting of spinel lherzolite, with F varying between 0.2 and 5%. The initial highly silicic, alkali-rich melts preserved in Mg-rich olivine become richer in FeO, MgO, CaO and Cr 2 O 3 and poorer in SiO 2 , K 2 O, Na 2 O, Al 2 O 3 and Cl with increasing melt fractions, evolving toward the basaltic melts found in clinopyroxene. These results confirm the connection between glass inclusions inside mantle minerals and partial mantle melts, and indicate that primary melts with SiO 2 >60 wt%, alkali oxides >11%, FeO <1 wt% and MgO <1 wt% are generated during incipient melting of spinel peridotite. The composition of the primary melts is inferred to be dependent on pressure, and to reflect both the speciation of dissolved CO 2 and the effect of alkali oxides on the silica activity coefficient in the melt. At pressures around 1 GPa, low-degree melts are characterized by alkali and silica-rich compositions, with a limited effect of dissolved CO 2 and a decreased silica activity coefficient caused by the presence of alkali oxides, whereas at higher pressures alkali oxides form complexes with carbonates and, consequently, alkali-rich silica-poor melts will be generated.
Phase relations in hydrous MORB at 18–28GPa: implications for heterogeneity of the lower mantle
Physics of the Earth and Planetary Interiors, 2005
The phase relations in hydrous and anhydrous mid-ocean ridge basalt were determined at pressures of 18-28 GPa. Liquidus phase relations in hydrous and anhydrous MORB are different. Garnet is the liquidus phase at pressures below 21 GPa, Ca-Al (CAS) phase and stishovite are the liquidus phases at pressures of 22-27 GPa, and stishovite and Ca-perovskite are the liquidus phases above 27 GPa, whereas Ca-perovskite is a liquidus phase of anhydrous MORB at pressures above 23 GPa. Under subsolidus conditions, we have found that in the hydrous MORB system the stability fields of Al-bearing perovskite and Na-Al (NAL) phase might shift to lower pressure by about 1.5 GPa compared to the dry MORB system. This shift could be explained by oxidation of a garnet-bearing assemblage by hydrous fluid and formation of Fe 3+ -bearing aluminous perovskite at lower pressures relative to the anhydrous system and/or differences in water solubility of the phases existing in perovskite-bearing assemblages. Our data indicate that hydrous basaltic crust remains denser than peridotite along the geotherm of a subducting slab, i.e. there is no density crossover between peridotite and basalt. Therefore, in slabs going through the 660 km discontinuity, basalt would gravitationally sink into the lower mantle under relatively hydrous conditions. The delamination of former basaltic crust near the 660 km discontinuity might be possible under relatively dry conditions of subduction. There are no stable highly hydrous phases in MORB above 10 GPa even at lower temperatures corresponding to subducting slabs. Therefore, MORB cannot be an important carrier of water to the deep Earth interior. However, it can be constantly supplied by water-bearing fluid from the underlying peridotite part of the descending slab. Thus, it is plausible that water can control subduction of the oceanic crust into the lower mantle.
Geochemistry, Geophysics, Geosystems, 2002
1] We describe a newly calibrated model for the thermodynamic properties of magmatic silicate liquid. The new model, pMELTS, is based on MELTS [Ghiorso and Sack, 1995] but has a number of improvements aimed at increasing the accuracy of calculations of partial melting of spinel peridotite. The pMELTS algorithm uses models of the thermodynamic properties of minerals and the phase equilibrium algorithms of MELTS, but the model for silicate liquid differs from MELTS in the following ways: (1) The new algorithm is calibrated from an expanded set of mineral-liquid equilibrium constraints from 2439 experiments, 54% more than MELTS. (2) The new calibration includes mineral components not considered during calibration of MELTS and results in 11,394 individual mineral-liquid calibration constraints (110% more than MELTS). Of these, 4924 statements of equilibrium are from experiments conducted at elevated pressure (200% more than MELTS). (3) The pMELTS model employs an improved liquid equation of state based on a third-order Birch-Murnaghan equation, calibrated from high-pressure sink-float and shockwave experiments to 10 GPa. (4) The new model employs a revised set of end-member liquid components. The revised components were chosen to better span liquid composition-space. Thermodynamic properties of these components are optimized as part of the mineral-liquid calibration. Comparison of pMELTS to partial melting relations of spinel peridotite from experiments near 1 GPa indicates significant improvements relative to MELTS, but important outstanding problems remain. The pMELTS model accurately predicts oxide concentrations, including SiO 2 , for liquids from partial melting of MM3 peridotite at 1 GPa from near the solidus up to 2525% melting. Compared to experiments, the greatest discrepancy is for MgO, for which the calculations are between 1 and 4% high. Temperatures required to achieve a given melt fraction match those of the experiments near the solidus but are 2560°C high over much of the spinel lherzolite melting interval at this pressure. Much of this discrepancy can probably be attributed to overstabilization of clinopyroxene in pMELTS under these conditions. Comparison of pMELTS calculations to the crystallization and partial melting experiments of Falloon et al. [1999] shows excellent agreement but also suffers from exaggerated calculated stability of clinopyroxene. Finally, comparison of pMELTS
Journal of Petrology, 2007
We have determined the near-solidus melt compositions for peridotite MM-3, a suitable composition for the production of mid-ocean ridge basalt (MORB) by decompression partial melting, at 1 and 1Á5 GPa. At 1 GPa the MM-3 composition has a subsolidus plagioclase-bearing spinel lherzolite assemblage, and a solidus at $ 12708C. At only $ 58C above the solidus, 4% melt is present as a result of almost complete melting of plagioclase. This melting behaviour in plagioclase lherzolite is predicted from simple systems and previous experimental work. The persistence of plagioclase to 40Á8 GPa is strongly dependent on bulk-rock CaO/Na 2 O and normative plagioclase content in the peridotite. At 1Á5 GPa the MM-3 composition has a subsolidus spinel lherzolite assemblage, and a solidus at $ 13508C. We have determined a near-solidus melt composition at $ 2% melting within 108C of the solidus. Near-solidus melts at both 1 and 1Á5 GPa are nepheline normative, and have low normative diopside contents; also they have the highestTiO 2 , Al 2 O 3 and Na 2 O, and the lowest FeO and Cr 2 O 3 contents compared with higher degree partial melts. Comparison of these near-solidus melts with primitive MORB glasses, which lie in the olivine-only field of crystallization at low pressure, indicate that petrogenetic models involving aggregation of near-fractional melts formed during melting at pressures of 1Á5 GPa or less are unlikely to be correct. In this study we use an experimental approach that utilizes sintered oxide mix starting materials and peridotite reaction experiments. We also examine some recent studies using an alternative approach of melt migration into, and entrapment within 'melt traps' (olivine, diamond, vitreous carbon) and discuss optimal procedures for this method.
Earth and Planetary Science Letters
We present results of an experimental study to determine the nature of minimum to near-minimum melt compositions in equilibrium with upper mantle peridotite mineralogy at 1 GPa. We confirm earlier conclusions that anhydrous melts of lherzolite at 1 GPa are basaltic with ; 15-20% normative diopside, ) 10% normative olivine and at low degrees of melting are Na O and K O-rich and nepheline-normative in 'fertile' mantle. The most extreme Na O-rich minimum melt 2 2 2 composition is in equilibrium with an albite-bearing harzburgite residue at 12208C. This melt composition is nephelinew x normative with ; 64% SiO and about ; 12% Na O. Our results disagree with recent reports 1,2 that peridotitic 2 2 minimum melt compositions have an 'andesitic' character at 1 GPa. We present reversal experiments showing that these latter melts are not in equilibrium with a spinel or plagioclase lherzolite upper mantle assemblage. We use our new data and Ž data from the literature to define minimum melts i.e. melts in equilibrium with olivineq orthopyroxeneq clinopyroxeneq . plagioclase" spinel for fertile or enriched to refractory lherzolite at 1 GPa. The minimum melt compositions are nephelineq olivine-normative for sodium-rich sources and hyperstheneq olivine-normative for refractory or depleted compositions with very calcic plagioclase or high CarCaq Na ratios in spinel lherzolite. It is not possible to derive quartz-normative basaltic or 'andesitic' melt compositions by partial melting of anhydrous lherzolite at 1 GPa. q 1997 Elsevier Science B.V. during mantle upwelling. However, it is very difficult to determine the chemical compositions of very small melt fractions by direct experiment in largely crystalline silicate systems. The proven approach in simple silicate systems is to use multiple bulk compositions to map out a eutectic or minimum melt w x composition 3,4 , but to do this in complex, natural systems requires a lot of work. Recently, the dia-0012-821Xr97r$17.00 q 1997 Elsevier Science B.V. All rights reserved.
2008
We have determined the near-solidus melt compositions for peridotite MM-3, a suitable composition for the production of mid-ocean ridge basalt (MORB) by decompression partial melting, at 1 and 1Á5 GPa. At 1 GPa the MM-3 composition has a subsolidus plagioclase-bearing spinel lherzolite assemblage, and a solidus at $ 12708C. At only $ 58C above the solidus, 4% melt is present as a result of almost complete melting of plagioclase. This melting behaviour in plagioclase lherzolite is predicted from simple systems and previous experimental work. The persistence of plagioclase to 40Á8 GPa is strongly dependent on bulk-rock CaO/Na 2 O and normative plagioclase content in the peridotite. At 1Á5 GPa the MM-3 composition has a subsolidus spinel lherzolite assemblage, and a solidus at $ 13508C. We have determined a near-solidus melt composition at $ 2% melting within 108C of the solidus. Near-solidus melts at both 1 and 1Á5 GPa are nepheline normative, and have low normative diopside contents; also they have the highestTiO 2 , Al 2 O 3 and Na 2 O, and the lowest FeO and Cr 2 O 3 contents compared with higher degree partial melts. Comparison of these near-solidus melts with primitive MORB glasses, which lie in the olivine-only field of crystallization at low pressure, indicate that petrogenetic models involving aggregation of near-fractional melts formed during melting at pressures of 1Á5 GPa or less are unlikely to be correct. In this study we use an experimental approach that utilizes sintered oxide mix starting materials and peridotite reaction experiments. We also examine some recent studies using an alternative approach of melt migration into, and entrapment within 'melt traps' (olivine, diamond, vitreous carbon) and discuss optimal procedures for this method.
We performed laser-heated diamond anvil cell experiments on bulk compositions in the systems MgO–SiO 2 –H 2 O (MSH) and MgO–Al 2 O 3 –SiO 2 –H 2 O (MASH) that constrain the stability of hydrous phases in Earth's lower mantle. Phase identification by synchrotron powder diffraction reveals a consistent set of stability relations for the high-pressure, dense hydrous silicate phases D and H. In the MSH system phase D is stable to ~50 GPa, independent of temperature from ~1300 to 1700 K. Phase H becomes stable between 35 and 40 GPa, and the phase H out reaction occurs at ~55 GPa at 1600 K with a negative dT/dP slope of ~−75 K/GPa. Between ~30 and 50 GPa dehydration melting occurs at ~1800 K with a flat dT/dP slope. A cusp along the solidus at ~50 GPa corresponds with the intersection of the subsolidus phase H out reaction, and the dT/dP melting slope steepens to ~15 K/GPa up to ~85 GPa. In the MASH system phase H is stable in experiments between ~45 and 115 GPa in all bulk compositions studied, and we expect aluminous phase H to be stable throughout the lower mantle depth range beneath ~1200 km in both peridotitic and basaltic lithologies. In the subsolidus, aluminous phase D is stable to ~55 GPa, whereas at higher pressures aluminous phase H is the stable hydrous phase. The presence of hydrogen may sharpen the bridgmanite to post-perovskite transition. The ambient unit cell volume of bridgmanite increases systematically with pressure above ~55 GPa, possibly representing an increase in alumina content, and potentially hydrogen content, with depth. Bridgmanite in equilibrium with phases D and H has a relatively low alumina content, and alumina partitions preferentially into the hydrous phases. The melting curves of MASH compositions are shallower than in the MSH system, with dT/dP of ~6 K/GPa. Phase D and H solid solutions are stable in cold, hy-drated subducting slabs and can deliver water to the deepest lower mantle. However, hydrated lithologies in the lower mantle are likely to be partially molten at all depths along an ambient mantle geotherm.