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1.
Stability and phase relations of coexisting enstatite and H2 fluid were investigated in the pressure and temperature regions of 3.1–13.9 GPa and 1500–2000 K using laser-heated diamond-anvil cells. XRD measurements showed decomposition of enstatite upon heating to form forsterite, periclase, and coesite/stishovite. In the recovered samples, SiO2 grains were found at the margin of the heating hot spot, suggesting that the SiO2 component dissolved in the H2 fluid during heating, then precipitated when its solubility decreased with decreasing temperature. Raman and infrared spectra of the coexisting fluid phase revealed that SiH4 and H2O molecules formed through the reaction between dissolved SiO2 and H2. In contrast, forsterite and periclase crystals were found within the hot spot, which were assumed to have replaced the initial orthoenstatite crystals without dissolution. Preferential dissolution of SiO2 components of enstatite in H2 fluid, as well as that observed in the forsterite H2 system and the quartz H2 system, implies that H2-rich fluid enhances Mg/Si fractionation between the fluid and solid phases of mantle minerals.  相似文献   

2.
We experimentally investigated the dissolution of forsterite, enstatite and magnesite in graphite-saturated COH fluids, synthesized using a rocking piston cylinder apparatus at pressures from 1.0 to 2.1 GPa and temperatures from 700 to 1200 °C. Synthetic forsterite, enstatite, and nearly pure natural magnesite were used as starting materials. Redox conditions were buffered by Ni–NiO–H2O (ΔFMQ = ??0.21 to ??1.01), employing a double-capsule setting. Fluids, binary H2O–CO2 mixtures at the P, T, and fO2 conditions investigated, were generated from graphite, oxalic acid anhydrous (H2C2O4) and water. Their dissolved solute loads were analyzed through an improved version of the cryogenic technique, which takes into account the complexities associated with the presence of CO2-bearing fluids. The experimental data show that forsterite?+?enstatite solubility in H2O–CO2 fluids is higher compared to pure water, both in terms of dissolved silica (mSiO2?=?1.24 mol/kgH2O versus mSiO2?=?0.22 mol/kgH2O at P?=?1 GPa, T?=?800 °C) and magnesia (mMgO?=?1.08 mol/kgH2O versus mMgO?=?0.28 mol/kgH2O) probably due to the formation of organic C–Mg–Si complexes. Our experimental results show that at low temperature conditions, a graphite-saturated H2O–CO2 fluid interacting with a simplified model mantle composition, characterized by low MgO/SiO2 ratios, would lead to the formation of significant amounts of enstatite if solute concentrations are equal, while at higher temperatures these fluid, characterized by MgO/SiO2 ratios comparable with that of olivine, would be less effective in metasomatizing the surrounding rocks. However, the molality of COH fluids increases with pressure and temperature, and quintuplicates with respect to the carbon-free aqueous fluids. Therefore, the amount of fluid required to metasomatize the mantle decreases in the presence of carbon at high P–T conditions. COH fluids are thus effective carriers of C, Mg and Si in the mantle wedge up to the shallowest level of the upper mantle.  相似文献   

3.
In order to understand the role of aqueous fluid on the differentiation of the mantle, the compositions of aqueous fluids coexisting with mantle minerals were investigated in the system MgO-SiO2-H2O at pressures of 3 to 10 GPa and temperatures of 1000 to 1500°C with an MA8-type multianvil apparatus. Phase boundaries between the stability fields of forsterite + aqueous fluid, forsterite + enstatite + aqueous fluid, and enstatite + aqueous fluid were determined by varying the bulk composition at constant temperature and pressure. The composition of aqueous fluid coexisting with forsterite and enstatite can be defined by the intersection of these two phase boundaries. The solubility of silicate components in aqueous fluid coexisting with forsterite and enstatite increases with increasing pressure up to 8 GPa, from about 30 wt% at 3 GPa to about 70 wt% at 8 GPa. It becomes almost constant above 8 GPa. The Mg/Si weight ratio of these aqueous fluids is much higher than at low pressure (0.2 at 1.5 GPa) and almost constant (1.2) at pressures between 3 and 8 GPa. At 10 GPa, it becomes about 1.4. Aqueous fluid migrating upward through the mantle can therefore dissolve large amounts of silicates, leaving modified Mg/Si ratios of residual materials. It is suggested that the chemical stratification of Mg/Si in the Earth may have been formed as a result of aqueous fluid migration.  相似文献   

4.
《International Geology Review》2012,54(17):2184-2210
ABSTRACT

The Purang ultramafic massif, located in the Yarlung-Zangbo Suture Zone (YZSZ) of the Tibetan Plateau, consists mainly of harzburgites and minor lherzolites. The spinel-bearing lherzolites of the NW part of the massif display a granular texture, consisting of large olivine and pyroxene crystals with curvilinear grain boundaries. These lherzolites contain chromian spinel (Cr-spinel) of low Cr# [100 × Cr/(Cr +Al) = 24.7–30.2], enstatite with high Mg# [100 × Mg/(Mg + Fe2+) = 90.0–91.2] and relatively high Al2O3 content (3.3–4.1 wt%), and diopside with high Mg# (90.2–93.3) and Al2O3 content (4.6–5.0 wt%). These compositions are analogous to those of spinel and pyroxenes from residual peridotites. However, the Purang lherzolites show U-shaped primitive mantle (PM)-normalized rare earth element (REE)-profiles, which are not consistent with a potential origin as melting residues. The high LREE contents and positive Ti anomalies shown by the investigated lherzolites coupled with the low TiO2 content of their mineral constituents imply that these rocks possibly stored LREE- and Ti-bearing arc-related melts/fluids in their groundmass.

A mineral assemblage composed of diamond, super-reduced [(SuR) moissanite, native Cr] and crustal-derived minerals (zircon, corundum, rutile), has been separated from the Purang lherzolites. Uranium-Pb geochronological dating of zircons yielded an age range between 1718 and 465 Ma, indicating that they represent ancient crustal material delivered into the upper mantle via previous subduction events. Diamonds and old zircons (± crustal minerals) were carried to shallow mantle levels by asthenospheric magmas produced during a slab rollback-induced decompression melting process. The recovery of SuR minerals is consistent with fluid percolation and crystallization of alteration-related minerals in the lithospheric parts of a (hydrated) mantle wedge, resulting in the formation of highly reduced micro-environments.  相似文献   

5.
Halogens show a range from moderate (F) to highly (Cl, Br, I) volatile and incompatible behavior, which makes them excellent tracers for volatile transport processes in the Earth’s mantle. Experimentally determined fluorine and chlorine partitioning data between mantle minerals and silicate melt enable us to estimate Mid Ocean Ridge Basalt (MORB) and Ocean Island Basalt (OIB) source region concentrations for these elements. This study investigates the effect of varying small amounts of water on the fluorine and chlorine partitioning behavior at 1280?°C and 0.3 GPa between olivine and silicate melt in the Fe-free CMAS+F–Cl–Br–I–H2O model system. Results show that, within the uncertainty of the analyses, water has no effect on the chlorine partitioning behavior for bulk water contents ranging from 0.03 (2) wt% H2O (DCl ol/melt = 1.6?±?0.9 × 10?4) to 0.33 (6) wt% H2O (DCl ol/melt = 2.2?±?1.1 × 10?4). Consequently, with the effect of pressure being negligible in the uppermost mantle (Joachim et al. Chem Geol 416:65–78, 2015), temperature is the only parameter that needs to be considered for the determination of chlorine partition coefficients between olivine and melt at least in the simplified iron-free CMAS+F–Cl–Br–I–H2O system. In contrast, the fluorine partition coefficient increases linearly in this range and may be described at 1280?°C and 0.3 GPa with (R 2?=?0.99): \(D_{F}^{\text{ol/melt}}\ =\ 3.6\pm 0.4\ \times \ {{10}^{-3}}\ \times \ {{X}_{{{\text{H}}_{\text{2}}}\text{O}}}\left( \text{wt }\!\!\%\!\!\text{ } \right)\ +\ 6\ \pm \ 0.4\times \,{{10}^{-4}}\). The observed fluorine partitioning behavior supports the theory suggested by Crépisson et al. (Earth Planet Sci Lett 390:287–295, 2014) that fluorine and water are incorporated as clumped OH/F defects in the olivine structure. Results of this study further suggest that fluorine concentration estimates in OIB source regions are at least 10% lower than previously expected (Joachim et al. Chem Geol 416:65–78, 2015), implying that consideration of the effect of water on the fluorine partitioning behavior between Earth’s mantle minerals and silicate melt is vital for a correct estimation of fluorine abundances in OIB source regions. Estimates for MORB source fluorine concentrations as well as chlorine abundances in both mantle source regions are within uncertainty not affected by the presence of water.  相似文献   

6.
Lherzolite xenoliths containing fluid inclusions from the Ichinomegata volcano, located on the rear-arc side of the Northeast Japan arc, may be considered as samples of the uppermost mantle above the melting region in the mantle wedge. Thus, these fluid inclusions provide valuable information on the nature of fluids present in the sub-arc mantle. The inclusions in the Ichinomegata amphibole-bearing spinel–plagioclase lherzolite xenoliths were found to be composed mainly of CO2–H2O–Cl–S fluids. At equilibrium temperature of 920 °C, the fluid inclusions preserve pressures of 0.66–0.78 GPa, which correspond to depths of 23–28 km. The molar fraction of H2O and the salinity of fluid inclusions are 0.18–0.35 and 3.71 ± 0.78 wt% NaCl equivalent, respectively. These fluid inclusions are not believed to be fluids derived directly from the subducting slab, but rather fluids exsolved from sub-arc basaltic magmas that are formed through partial melting of mantle wedge triggered by slab-derived fluids.  相似文献   

7.
We present data on volatile (S, F and Cl) and major element contents in olivine-hosted melt inclusions (MIs) from alkaline basaltic tephras along the Quaternary Payenia backarc volcanic province (~34°S–38°S) of the Andean Southern Volcanic Zone (SVZ). The composition of Cr-spinel inclusions and host olivines in Payenia are also included to constrain any variations in oxygen fugacity. The variation of potassium, fluorine and chlorine in MIs in Payenia can be modelled by partial melting (1–10%) of a variously metasomatised mantle. The high chlorine contents in MIs (up to 3200 ppm) from Northern Payenia require addition of subduction-related fluids to a mantle wedge, whereas volatile signatures in the southern Payenia are consistent with derivation from an enriched OIB source. Cl and Cl/K ratios define positive correlations with host olivine fosterite content (Fo80-90) that cannot be explained by olivine fractionation, degassing and/or degree of mantle melting. Neither can the correlation between SiO2 and TiO2 in the MIs and host olivine Fo-content be explained by magmatic differentiation processes. Instead these correlations essentially require a south to north mantle source transition from a low Mg# pyroxenite (from recycled eclogite) to a high Mg# fluid metasomatised peridotite. The Cl/K and S/K ratios in Payenia MIs extend from enriched OIB-like signatures (south) to Andean SVZ arc like signatures (north). We show that the northward increase in S, Cl and S/K is coupled to a northward increase in melt oxidation states and thus in Fe3+/Fetot ratios in the magmas. The increase in oxidation state also correlates with an increase of Mn/Fe (olivine) ratios. We calculate that 25% of the apparent north–south pyroxenite–peridotite source variation in Payenia (based on olivine Mn/Fe ratios) can be explained by the south to north variation in melt oxidation states.  相似文献   

8.
We present new partition coefficients for various trace elements including Cl between olivine, pyroxenes, amphibole and coexisting chlorine-bearing aqueous fluid in a series of high-pressure experiments at 2 GPa between 900 and 1,300 °C in natural and synthetic systems. Diamond aggregates were added to the experimental capsule set-up in order to separate the fluid from the solid residue and enable in situ analysis of the quenched solute by LA–ICP–MS. The chlorine and fluorine contents in mantle minerals were measured by electron microprobe, and the nature of OH defects was investigated by infrared spectroscopy. Furthermore, a fluorine-rich olivine from one selected sample was investigated by TEM. Results reveal average Cl concentrations in olivine and pyroxenes around 20 ppm and up to 900 ppm F in olivine, making olivine an important repository of halogens in the mantle. Chlorine is always incompatible with Cl partition coefficients D Cl olivine/fluid varying between 10?5 and 10?3, whereas D Cl orthopyroxene/fluid and D Cl clinopyroxene/fluid are ~10?4 and D Cl amphibole/fluid is ~5 × 10?3. Furthermore, partitioning results for incompatible trace element show that compatibilities of trace elements are generally ordered as D amph/fluid ≈ D cpx/fluid > D opx/fluid > D ol/fluid but that D mineral/fluid for Li and P is very similar for all observed silicate phases. Infrared spectra of olivine synthesized in a F-free Ti-bearing system show absorption bands at 3,525 and ~3,570 cm?1. In F ± TiO2-bearing systems, additional absorption bands appear at ~3,535, ~3,595, 3,640 and 3,670 cm?1. Absorption bands at ~3,530 and ~3,570 cm?1, previously assigned to humite-like point defects, profit from low synthesis temperatures and the presence of F. The presence of planar defects could not be proved by TEM investigations, but dislocations in the olivine lattice were observed and are suggested to be an important site for halogen incorporation in olivine.  相似文献   

9.
Clinochlore, which is, within the limits of error, the thermally most stable member of the Mg-chlorites, breaks down at = P tot to the assemblage enstatite+forsterite+spinel+H2O along a univariant curve located at 11 kb, 838 ° C; 15kb, 862 ° C; and 18 kb, 880 ° C (±1 kb ±10 ° C). At water pressures above that of an invariant point at 20.3 kb and 894 ° C involving the phases clinochlore, enstatite, forsterite, spinel, pyrope, and hydrous vapor, clinochlore disintegrates to pyrope+forsterite+spinel+H2O. The resulting univariant curve has a steep, negative dP/dT slope of –930 bar/ °C at least up to 35 kb.Thus, given the proper chemical environment, Mg-chlorites have the potential of appearing as stable phases within the earth's upper mantle to maximum depths between about 60 and 100 km depending on the prevailing undisturbed geotherm, and to still greater depths in subduction zones. However, unequivocal criteria for mantle derived Mg-chlorites are difficult to find in ultrabasic rocks.  相似文献   

10.
Samples of enstatite and forsterite were crystallized in the presence of a hydrous fluid at 15 kbar and 1100 °C. Water contents in quenched samples were measured by 1H MAS NMR and by FTIR. If the samples were prepared in the same way, similar water concentrations were obtained by both methods. There is no evidence that one or the other method would severely over or underestimate water contents in nominally anhydrous minerals. However, measured water contents vary by orders of magnitude depending on sample preparation. The lowest water contents are measured by polarized FTIR spectroscopy on clear, inclusion-free single crystals. These water contents probably reflect the real point defect solubility in the crystals. Polycrystalline material shows much higher total water concentrations, presumably due to hydrous species on grain boundaries, growth defects, and in submicroscopic fluid inclusions. Grinding the sample in air further increases water concentration. This effect is even more pronounced if the sample is ground in water and subsequently dried at 150 °C. Polarized FTIR measurements on clear single crystals of enstatite saturated at 15 kbar and 1100 °C give 199 ± 25 ppm by weight of water. The spectra show sharp and strongly polarized bands. These bands are also present in spectra measured through turbid, polycrystalline aggregates of enstatite. However, in these spectra, they are superimposed on much broader, nearly isotropic bands resulting from hydrous species in grain boundaries, growth defects, and submicroscopic fluid or melt inclusions. Total water contents for these polycrystalline aggregates are between 2000 and 4000 ppm. Water contents measured by FTIR on enstatite powders are 5300 ppm after grinding in air and 12 600 ppm after grinding under water und subsequent drying at 150 °C. Received: 25 June 1999 / Revised, accepted: 4 October 1999  相似文献   

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