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1.
The main trends of water dissolution in Fe-bearing olivine have been investigated in the olivine–H2O–hydrocarbon fluid system in experiments at a pressure of 6.3 GPa, a temperature of 1200°C, and hydrogen fugacity ( fH2) buffered by the Mo–MoO2 equilibrium. The content and contribution of ОH defects of different types in Fe-bearing olivines depend on the composition of reduced fluids in the system. As the fraction of hydrocarbons in the fluid increases, the H2O content in olivine crystals decreases from 900 to 160–180 ppm, while the ОН absorption peaks become lower at high frequencies and occupy a larger part of the infrared spectrum in the low-frequency region. According to the experimental results, even the deepest seated mantle olivines with OH defects were not equilibrated with a fluid rich in light alkanes or oxygenated hydrocarbons.  相似文献   

2.
This study presents a new experimental approach for determining H2O solubility in basaltic melt at upper mantle conditions. Traditional solubility experiments are limited to pressures of ~600 MPa or less because it is difficult to reliably quench silicate melts containing greater than ~10 wt% dissolved H2O. To overcome this limitation, our approach relies on the use of secondary ion mass spectrometry to measure the concentration of H dissolved in olivine and on using the measured H in olivine as a proxy for the concentration of H2O in the co-existing basaltic melt. The solubility of H2O in the melt is determined by performing a series of experiments at a single pressure and temperature with increasing amounts of liquid H2O added to each charge. The point at which the concentration of H in the olivine first becomes independent of the amount of initial H2O content of the charge (added + adsorbed H2O) indicates its solubility in the melt. Experiments were conducted by packing basalt powder into a capsule fabricated from San Carlos olivine, which was then pressure-sealed inside a Ni outer capsule. Our experimental results indicate that at 1000 MPa and 1200 °C, the solubility of H2O in basaltic melt is 20.6 ± 0.9 wt% (2 × standard deviation). This concentration is considerably higher than predicted by most solubility models but defines a linear relationship between H2O fugacity and the square of molar H2O solubility when combined with solubility data from lower pressure experiments. Further, our solubility determination agrees with melting point depression determined experimentally by Grove et al. (2006) for the H2O-saturated peridotite solidus at 1000 MPa. Melting point depression calculations were used to estimate H2O solubility in basalt along the experimentally determined H2O-saturated peridotite solidus. The results suggest that a linear relationship between H2O fugacity and the square of molar solubility exists up to ~1300 MPa, where there is an inflection point and solubility begins to increase less strongly with increasing H2O fugacity.  相似文献   

3.
To investigate the solubility and the sites of incorporation of hydrogen in olivine as a function of point defect concentration, two-stage high-temperature annealing experiments have been carried out. The first annealing stage (the dry preannealing stage) was conducted at a total pressure of 0.1 MPa, a temperature of 1300° C and various oxygen fugacities in the range 10?11–10?4 MPa for times > 12 h. In these heat treatments, the samples were buffered against either orthopyroxene or magnesiowustite, or they remained unbuffered. The second annealing stage (the hydrothermal annealing stage) was performed at 300 MPa and 900–1050 ° C under a hydrogen fugacity of ～ 158 MPa for 1–5 h. Infrared spectra from the annealed samples revealed two distinct groups of bands. Group I bands occurred at wavenumbers in the range 3450–3650 cm?1, while Group II bands occurred in the range 3200–3450 cm?1. The hydrogen solubility associated with Group I bands is proportional to f O 2 to the 1/6 power for samples preannealed in contact with orthopyroxene, to the 1/3 power for samples preannealed in contact with magnesiowustite, and to the 1/13 power for samples preannealed in the absence of a solid-state buffer. The hydrogen concentration for Group II bands varies with f o 2 to the 1/3 power for opxbuffered samples, to the 1/2 power for mw-buffered samples, and to the 1/3 power for unbuffered samples. The dependence of hydrogen solubility on oxygen fugacity and orthopyroxene activity suggests that hydrogen is incorporated into the olivine structure via association with point defects. The presence of two distinct groups of absorption bands indicates that hydrogen is associated with two distinct lattice defects. The following point defect model for the mechanism of incorporation of hydrogen in olivine is consistent with these results: Hydrogen ions responsible for the Group I bands are associated with doubly charged oxygen interstitials, while hydrogen ions responsible for the Group II bands are associated with singly charged oxygen interstitials. Furthermore, the infrared bands observed in naturally derived olivines are present in spectra from our hydrothermally annealed crystals. Thus, the mechanisms of incorporation of hydrogen in olivine under geological conditions are the same as those operative under laboratory conditions. The maximum solubility reached in these experiments was ～ 360H/106Si, which corresponds to ～ 0.002 wt% of H2O. This value is a lower bound for the solubility of hydrogen in olivine under upper mantle conditions.  相似文献   

4.
We conducted a series of hydroxylation experiments using mm-sized cuboids cut from six different crystals of San Carlos olivine with a range of trace-element concentrations. The cuboids were pre-annealed and then hydroxylated under identical conditions, ensuring that variation in the amounts of H incorporated depended only on the compositional variables. The pre-anneal was at 1400 °C, atmospheric pressure and an oxygen fugacity equivalent to Δlog FMQ?+?1, with the subsequent hydroxylation at 800 °C and 1.5 GPa, for 3 days. Hydrogen was incorporated into all six crystals by the four main substitution mechanisms [Si], [Mg], [Ti] and [triv], with homogeneous H contents in the cores of the crystals, indicating H diffusion rates faster than 10??11 m2/s. Total H as H2O in the homogeneous cores calculated by summing all the infrared absorbance bands ranges from 13 to 27 wt. ppm. The total H2O in the six pre-annealed crystals is poorly correlated with any measured compositional variable. However, when the H2O associated with individual infrared bands is compared, clear trends emerge. The intensity of absorption bands at 3572 and 3525 cm??1 are strongly correlated with Ti concentrations, whose range in the six crystals exceeds an order of magnitude. Bands between 3400 and 3300 cm??1, correlate negatively with Na+, but are positively correlated with the difference between molar Cr3+ and Na+. This highlights a previously unrecognised role for Na in suppressing H incorporation in natural olivines. The results confirm the important role that the trace constituents of olivine play in H incorporation. Two of these trace elements, Na and Ti, tend to be similarly enriched or depleted by partial melting or metasomatism of the mantle, but have opposite effects on H incorporation, with Ti enhancing it but Na suppressing it. Models estimating the effect of H in olivine on mantle rheology must, therefore, consider carefully the availability of these trace elements.  相似文献   

5.
Using the secondary spinel standard, the authors have precisely measured the Fe3+/∑ Fe values of spinels in mantle xenoliths from Cenozoic basalts in eastern China, and estimated the oxygen fugacities recorded by 63 mantle xenoliths through olivine-orthopyroxene-spinel oxygen barometry. The results indicate that the oxygen fugacities of the lithospheric mantle in eastern China are higher in the south than in the north. Among them, the oxygen fugacity of the North China craton lithospheric mantle is the lowest, similar to that of the oceanic mantle, while that of Northeast and South China are the same as that of the global continental mantle. The variations of mantle redox state in eastern China are mainly controlled by the C-O-H fluids derived from the asthenospheric mantle. According to the mantle oxidation state, it can be concluded that the C-O-H fluids in the lithospheric mantle of eastern China consist mainly of CO2 and minor H2O, but CH4-rich fluids should come from the asthenosphere where the ox  相似文献   

6.
 The solubility of hydroxyl in the α, β and γ phases of (Mg,Fe)2SiO4 was investigated by hydrothermally annealing single crystals of San Carlos olivine. Experiments were performed at a temperature of 1000° or 1100 °C under a confining pressure of 2.5 to 19.5 GPa in a multianvil apparatus with the oxygen fugacity buffered by the Ni:NiO solid-state reaction. Hydroxyl solubilities were determined from infrared spectra obtained of polished thin sections in crack-free regions ≤100 μm in diameter. In the α-stability field, hydroxyl solubility increases systematically with increasing confining pressure, reaching a value of ∼20,000 H/106Si (1200 wt ppm H2O) at the α-β phase boundary near 13 GPa and 1100 °C. In the β field, the hydroxyl content is ∼400,000 H/106Si (24,000 wt ppm H2O) at 14–15 GPa and 1100 °C. In the γ field, the solubility is ∼450,000 H/106Si (27,000 wt ppm H2O) at 19.5 GPa and 1100 °C. The observed dependence of hydroxyl solubility with increasing confining pressure in the α phase reflects an increase in water fugacity with increasing pressure moderated by a molar volume term associated with the incorporation of hydroxyl ions into the olivine structure. Combined with published results on the dependence of hydroxyl solubility on water fugacity, the present results for the α phase can be summarized by the relation C OH = A(T)f nH2Oexp(−PΔV/RT), where A(T) = 1.1 H/106Si/MPa at 1100 °C, n = 1, and ΔV = 10.6×10–6 m3/mol. These data demonstrate that the entire present-day water content of the upper mantle could be incorporated in the mineral olivine alone; therefore, a free hydrous fluid phase cannot be stable in those regions of the upper mantle with a normal concentration of hydrogen. Free hydrous fluids are restricted to special tectonic environments, such as the mantle wedge above a subduction zone. Received: 10 February 1995 / Accepted: 23 October 1995  相似文献   

7.
The exsolutious of diopside and magnetite occur as intergrowth and orient within olivine from the mantle dunite, Luobusa ophiolite, Tibet. The dunite is very fresh with a mineral assemblage of olivine （〉95%） ＋ chromite （1%-4%） ＋ diopside （〈1%）. Two types of olivine are found in thin sections： one （Fo = 94） is coarse-grained, elongated with development of kink bands, wavy extinction and irregular margins; and the other （Fo = 96） is fine-grained and poly-angied. Some of the olivine grains contain minor Ca, Cr and Ni. Besides the exsolutions in olivine, three micron-size inclusions are also discovered. Analyzed through energy dispersive system （EDS） with unitary analytical method, the average compositions of the inclusions are： Na20, 3.12%-3.84%; MgO, 19.51%-23.79%; Al2O3, 9.33%-11.31%; SiO2, 44.89%-46.29%; CaO, 11.46%-12.90%; Cr2O3, 0.74%-2.29%; FeO, 4.26%- 5.27%, which is quite similar to those of amphibole. Diopside is anhedral f＇dling between olivines, or as micro-inclusions oriented in olivines. Chromite appears euhedral distributed between olivines, sometimes with apparent compositional zone. From core to rim of the chromite, Fe content increases and Cr decreases; and A! and Mg drop greatly on the rim. There is always incomplete magnetite zone around the chromite. Compared with the nodular chromite in the same section, the euhedral chromite has higher Fe3O4 and lower MgCr2O4 and MgAI2O4 end member contents, which means it formed under higher oxygen fugacity environment. With a geothermometer estimation, the equilibrium crystalline temperature is 820℃-960℃ for olivine and nodular chromite, 630℃-770℃ for olivine and euhedral chromite, and 350℃-550℃ for olivine and exsoluted magnetite, showing that the exsolutions occurred late at low temperature. Thus we propose that previously depleted mantle harzburgite reacted with the melt containing Na, Al and Ca, and produced an olivine solid solution added with Na^＋, Al^3＋, Ca^2＋, Fe^3＋, Cr^3＋. With temperature d  相似文献   

8.
The ophiolitic peridotites in the Wadi Arais area, south Eastern Desert of Egypt, represent a part of Neoproterozoic ophiolites of the Arabian-Nubian Shield (ANS). We found relics of fresh dunites enveloped by serpentinites that show abundances of bastite after orthopyroxene, reflecting harzburgite protoliths. The bulk-rock chemistry confirmed the harzburgites as the main protoliths. The primary mantle minerals such as orthopyroxene, olivine and chromian spinel in Arais serpentinites are still preserved. The orthopyroxene has high Mg# [=Mg/(Mg + Fe2+)], ~0.923 on average. It shows intra-grain chemical homogeneity and contains, on average, 2.28 wt.% A12O3, 0.88 wt.% Cr2O3 and 0.53 wt.% CaO, similar to primary orthopyroxenes in modern forearc peridotites. The olivine in harzburgites has lower Fo (93?94.5) than that in dunites (Fo94.3?Fo95.9). The Arais olivine is similar in NiO (0.47 wt.% on average) and MnO (0.08 wt.% on average) contents to the mantle olivine in primary peridotites. This olivine is high in Fo content, similar to Mg-rich olivines in ANS ophiolitic harzburgites, because of its residual origin. The chromian spinel, found in harzburgites, shows wide ranges of Cr#s [=Cr/(Cr + Al)], 0.46?0.81 and Mg#s, 0.34?0.67. The chromian spinel in dunites shows an intra-grain chemical homogeneity with high Cr#s (0.82?0.86). The chromian spinels in Arais peridotites are low in TiO2, 0.05 wt.% and YFe [= Fe3+/(Cr + Al + Fe3+)], ~0.06 on average. They are similar in chemistry to spinels in forearc peridotites. Their compositions associated with olivine’s Fo suggest that the harzburgites are refractory residues after high-degree partial melting (mainly ~25?30 % partial melting) and dunites are more depleted, similar to highly refractory peridotites recovered from forearcs. This is in accordance with the partial melting (>20 % melt) obtained by the whole-rock Al2O3 composition. The Arais peridotites have been possibly formed in a sub-arc setting (mantle wedge), where high degrees of partial melting were available during subduction and closing of the Mozambique Ocean, and emplaced in a forearc basin. Their equilibrium temperature based on olivine?spinel thermometry ranges from 650 to 780 °C, and their oxygen fugacity is high (Δlog ?O2?=?2.3 to 2.8), which is characteristic of mantle-wedge peridotites. The Arais peridotites are affected by secondary processes forming microinclusions inside the dunitic olivine, abundances of carbonates and talc flakes in serpentinites. These microinclusions have been formed by reaction between trapped fluids and host olivine in a closed system. Lizardite and chrysotile, based on Raman analyses, are the main serpentine minerals with lesser antigorite, indicating that serpentines were possibly formed under retrograde metamorphism during exhumation and near the surface at low T (<400 °C).  相似文献   

9.
The infrared (IR) spectra of gem-quality olivine crystals from Pakistan, formed in serpentinised dunitic rocks, are characterised by strongly pleochroic absorption bands at 3,613, 3,597, 3,580 and 3,566 cm?1. These bands are assigned to O-H stretching vibrations of OH point defects corresponding to H2O concentrations of about 35 wt ppm. Unlike other olivine spectra, the dominating bands are strongly polarised parallel to the b-axis. The unusual spectra type, excludes the presence of planar defects. This finding is supported by transmission electron microscopy. The 3,613 cm?1 band is related to vacant Si sites, the slightly lower energetic bands preferentially to vacant M2 sites. The exclusive presence of these bands is not only a characteristic feature of olivines treated under high P,T conditions equivalent to mantle environment, the presence of these bands in untreated natural olivine also indicates formation conditions equivalent to crustal rocks.  相似文献   

10.
Based on the experiments on dehydration-melting of solid samples of Al-rich gneiss (H029) and biotite granulitite (H013), the fugacities of O2, H2O and H2 have been calculated. It is recognized that the fugacities of O2, H2O and H2 vary regularly, but the fugacity of H2O shows a tendency of abrupt increasing at about 700°C and 800°C. According to the above fact, the melting mechanism of biotite can be well documented. Under relatively low temperatures (< 750°C), part of the water can be liberated and induce plagioclase to melt, which may mark the beginning of migmatization. At high temperatures (> 800°C), biotite can be dissociated and a larger amount of water can be released, which would result in a bigger degree of melting, hence leading to the formation of granitic magma.  相似文献   

11.
A series of liquidus determinations is reported for a primitive arc basalt (15.4 wt % MgO, 45.5 wt % SiO2) from Grenada, Lesser Antilles, at anhydrous, H2O-undersaturated and H2O-saturated conditions in the pressure range 1 atm to 1.7 GPa. \(\hbox{Fe}^{3+}/\Upsigma\hbox{Fe}\) of high-pressure experimental glasses as measured by μXANES ranges from 0.44 to 0.86, corresponding to oxygen fugacities (fO2) between 3.2 and 7.8 log units above the nickel–nickel oxide redox buffer (NNO). 1-atm experiments conducted from NNO ? 2.5 to + 3.8 show that increasing fO2 mainly increases the forsterite content (Fo) of olivine and has little effect on phase relations. The crystallisation sequence at lower crustal pressures for all water contents is forsteritic olivine + Cr-rich spinel followed by clinopyroxene. The anhydrous liquidus is depressed by 100 and 120 °C in the presence of 2.9 and 3.8 wt % H2O, respectively. H2O-undersaturated experiments at NNO + 3.2 to + 4.5 produce olivine of equivalent composition to the most primitive olivine phenocrysts in Grenadan picrites (Fo91.4). We conclude that direct mantle melts originating beneath Grenada could be as oxidised as ~NNO + 3, consistent with the uppermost estimates from olivine–spinel oxybarometry of high Mg basalts. μXANES analyses of olivine-bearing experimental glasses are used to develop a semi-empirical oxybarometer based on the value of \({{K}_{D}}_{\rm ol-melt}^{\rm Fe-Mg}\) when all Fe is assumed to be in the Fe2+ state (\({K}_{D}^{{\rm Fe}_T}\)). The oxybarometer is tested on an independent data set and is able to reproduce experimental fO2 to ≤1.2 log units. Experiments also show that the geochemically and petrographically distinct M- and C-series lavas on the island can be produced from hydrous melting of a common picritic source. Low pressures expand the olivine stability field at the expense of clinopyroxene, enriching an evolving melt in CaO and forcing differentiation to take place along a C-series liquid line of descent. Higher pressure conditions allow early and abundant clinopyroxene crystallisation, rapidly depleting the melt in both CaO and MgO, and thus creating the M-series.  相似文献   

12.
The natural occurrence of hydroxide in olivine   总被引：1，自引：0，他引：1  
Polarized infrared (IR) spectra of olivine single crystals from 17 different localities show a tremendous variability in both mode and abundance of hydroxide (OH) incorporation. Kimberlitic olivines contain the most total OH at an estimated concentration level of 976 H/106Si, whereas olivines from basalts contain the least at 3 H/106Si. Olivines of metamorphic and hydrothermal origin have widely varying concentration levels intermediate between those of basalts and kimberlites. Over 30 distinct OH absorption bands have been identified. Most of these bands are not unique to individual localities but may be found in samples from several different localities. Pleochroism is consistent among localities, but relative band intensities vary. No evidence is found for molecular H2 in olivine. Hydrous minerals have been identified in olivine by their characteristic OH absorption bands. Serpentine is commonly found and is clearly distinguishable from intrinsic OH. Talc is present in one sample. Prominent OH bands at 3572 and 3525 cm?1 are attributed to humite group minerals. San Carlos, Arizona, olivines annealed in the presence of H2O develop absorption bands which are found in natural samples, however the OH absorption spectra of these annealed olivines are not identical to those of any single natural crystal. Sharp-band OH abundances in annealed samples are an order of magnitude lower than the maximum measured in natural specimens. The mechanical properties determined from these annealed olivines may not be directly applicable to mantle olivine because both the OH sites and concentrations are different.  相似文献   

13.
The thermodynamic model of Doukhan and Trepied (1985) for water solubility in quartz, based on the (4H)Si substitution, has been developed to take into account the variation in oxygen fugacity under different buffering conditions, as well as the solubility of the quartz in the water. An evaluation of the thermodynamic parameters has then been made using analogous data on grossular and hydrogrossular. This evaluation, although approximate, leads to predicted solubilities that are lower than some published experimental values but are consistent with other observations indicating that the solubility is relatively low, less than 100H/106 Si at most, even at the highest temperatures and pressures. The predictions give a monotonically increasing solubility with increasing temperature at given pressure, and an increase in solubility with increasing pressure at given temperature up to a maximum, beyond which the solubility decreases with further increase in pressure. The variation of solubility with oxygen fugacity at fixed pressure and temperature broadly mirrors the observations of Ord and Hobbs (1986) although not predicting the finer details. An interstitial H2O model is also considered and shown to be probably less important as a mechanism for water solubility than the (4H)Si mechanism. Hydrogen/alkali exchange depends on hydrogen fugacity rather than water fugacity and is predicted to be more important at lower oxygen fugacities.  相似文献   

14.
The influence of oxygen fugacity and water on phase equilibria and the link between redox conditions and water activity were investigated experimentally using a primitive tholeiitic basalt composition relevant to the ocean crust. The crystallization experiments were performed in internally heated pressure vessels at 200 MPa in the temperature range 940–1,220°C. The oxygen fugacity was measured using the H2-membrane technique. To study the effect of oxygen fugacity, three sets of experiments with different hydrogen fugacities were performed, showing systematic effects on the phase relations and compositions. In each experimental series, the water content of the system was varied from nominally dry to water-saturated conditions, causing a range of oxygen fugacities varying by ~3 log units per series. The range in oxygen fugacity investigated spans ~7 log units. Systematic effects of oxygen fugacity on the stability and composition of the mafic silicate phases, Cr–spinel and Fe–Ti oxides, under varying water contents were recorded. The Mg# of the melt, and therefore also the Mg# of olivine and clinopyroxene, changed systematically as a function of oxygen fugacity. An example of the link between oxygen fugacity and water activity under hydrogen-buffered conditions is the change in the crystallization sequence (olivine and Cr–spinel) due to a change in the oxygen fugacity caused by an increase in the water activity. The stability of magnetite is restricted to highly oxidizing conditions. The absence of magnetite in most of the experiments allows the determination of differentiation trends as a function of oxygen fugacity and water content, demonstrating that in an oxide-free crystallization sequence, water systematically affects the differentiation trend, while oxygen fugacity seems to have a negligible effect.  相似文献   

15.
地幔的力学性质主要受橄榄石流变性的控制,含水对橄榄石流变性质的影响很大,而橄榄石的水溶性受到温度和铁含量的影响,因此,本文进行了不同铁含量橄榄石在不同温度下的水溶性实验研究。实验使用的样品为天然橄榄石单晶Fa_(17)和Fa_(24.7)(Fe_(No.)=100×molar Fe/(Mg+Fe))以及人工合成的橄榄石单晶Fa_(22);橄榄石单晶的水溶性实验在300MPa围压和1273~1473K的温度条件下进行,每隔50K进行一组实验,氧逸度被控制在Ni NiO水平上。实验结束后,对橄榄石单晶沿b面进行双面研磨抛光,用电子探针分析确定橄榄石单晶成分,采用EBSD精确测量橄榄石的单晶方向,使用红外光谱仪(FTIR)的非偏振光路测试橄榄石单晶在b轴上的吸收光谱。对FTIR吸收光谱进行积分得到富铁橄榄石的水溶性实验结果:当温度由1273K升至1473K时,橄榄石单晶Fa_(17)的水溶性变化为600~1200H/10^(6) Si,橄榄石单晶Fa_(24.7)的水溶性变化为1000~1300H/10^(6) Si,人工合成的橄榄石单晶Fa_(22)的水溶性变化为500~900 H/10^(6) Si。因此,相同铁含量橄榄石单晶的水溶性随温度的增加而增加,相同温度条件下,天然形成的橄榄石的水溶性随着铁含量的增加而增加,百分之一的铁含量的增加,可以导致约百分之十的水溶性的增加。本文所研究的不同铁含量的橄榄石可以为更好地估算上地幔水溶性提供依据。  相似文献   

16.
Titanium- and water-rich metamorphic olivine (Fo 86–88) is reported from partially dehydrated serpentinites from the Voltri complex, Ligurian Alps. The rocks are composed of mostly antigorite and olivine in addition to magnetite, chlorite, clinopyroxene and Ti-clinohumite. In situ secondary ion mass spectrometry (SIMS) data show that metamorphic olivine has very high and strongly correlated H2O (up to 0.7 wt%) and TiO2 contents (up to 0.85 wt%). Ti-rich olivine shows colourless to yellow pleochroism. Olivine associated with Ti-clinohumite contains low Ti, suggesting that Ti-rich olivine is not the breakdown product of Ti-clinohumite. Fourier transform infrared spectroscopy (FTIR) absorption spectra show peaks of serpentine, Ti-clinohumite and OH-related Si vacancies. Combining FTIR and SIMS data, we suggest the presence of clustered planar defects or nanoscale exsolutions of Ti-clinohumite in olivine. These defects or exsolutions contain more H2O (x ~ 0.1 in the formula 4Mg2SiO4·(1?x)Mg(OH,F)2·xTiO2) than Ti-clinohumite in the sample matrix (x = 0.34–0.46). In addition to TiO2 and H2O, secondary olivine contains significant Li (2–60 ppm), B (10–20 ppm), F (10–130 ppm) and Zr (0.9–2.1 ppm). It is enriched in 11B (δ11B = +17 to +23 ‰). Our data indicate that secondary olivine may play a significant role in transporting water, high-field strength and fluid-mobile elements into the deeper mantle as well as introduce significant B isotope anomalies. Release of hydrogen from H2O-rich olivine subducted into the deep mantle may result in strongly reduced mantle domains.  相似文献   

17.
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   

18.
Major and trace elements and water contents were analyzed in 16 peridotite xenoliths embedded by the Cenozoic basalts in Pingnan (southeastern Guangxi Province), to constrain the chemical composition and evolution of the lithospheric mantle located in the central part of the South China Block (SCB). The peridotites are mainly moderately refractory harzburgites and lherzolites (Mg#-Ol?=?90.3–91.7) and minor fertile lherzolites (Mg#-Ol?=?88.9–89.9). Clinopyroxenes in the peridotites show LREE-depleted pattern, and commonly exhibit negative anomalies in Nb and Ti, suggesting the peridotites probably represent residues after 1–10% of partial melting without significant mantle metasomatism. Water contents range from 146 to 237 ppm wt. H2O in clinopyroxene, and from 65 to 112 ppm wt. H2O, in orthopyroxene but are below detection limit (2 ppm wt. H2O) in olivine. Calculated bulk water contents, based on the mineral modes and partition coefficient, range from 14 to 83 ppm wt. H2O (average 59 ppm wt. H2O). There is a correlation between melting indices (such as Mg#-Ol, Ybn in clinopyroxene) and water contents in clinopyroxene and orthopyroxene, but no correlation is observed between the whole-rock water contents and the redox state (Fe3+/∑Fe ratios in spinel), suggesting that water contents in the peridotites are mainly controlled by the degree of partial melting rather than by oxygen fugacity. The lithospheric mantle beneath the interior of the SCB may not be compositionally stratified; fertile and moderately refractory mantle coexist at the similar depths. Geochemical data and water contents of the studied peridotites are similar to the proposed MORB source and indicate that the ancient refractory lithospheric mantle was irregularly eroded or reacted by the upwelling asthenosphere, and eventually replaced by juvenile fertile accreted mantle through the cooling of the asthenosphere.  相似文献   

19.
The onset of hydrous partial melting in the mantle above the transition zone is dictated by the H2O storage capacity of peridotite, which is defined as the maximum concentration that the solid assemblage can store at P and T without stabilizing a hydrous fluid or melt. H2O storage capacities of minerals in simple systems do not adequately constrain the peridotite water storage capacity because simpler systems do not account for enhanced hydrous melt stability and reduced H2O activity facilitated by the additional components of multiply saturated peridotite. In this study, we determine peridotite-saturated olivine and pyroxene water storage capacities at 10–13 GPa and 1,350–1,450°C by employing layered experiments, in which the bottom ~2/3 of the capsule consists of hydrated KLB-1 oxide analog peridotite and the top ~1/3 of the capsule is a nearly monomineralic layer of hydrated Mg# 89.6 olivine. This method facilitates the growth of ~200-μm olivine crystals, as well as accessory low-Ca pyroxenes up to ~50 μm in diameter. The presence of small amounts of hydrous melt ensures that crystalline phases have maximal H2O contents possible, while in equilibrium with the full peridotite assemblage (melt + ol + pyx + gt). At 12 GPa, olivine and pyroxene water storage capacities decrease from ~1,000 to 650 ppm, and ~1,400 to 1,100 ppm, respectively, as temperature increases from 1,350 to 1,450°C. Combining our results with those from a companion study at 5–8 GPa (Ardia et al., in prep.) at 1,450°C, the olivine water storage capacity increases linearly with increasing pressure and is defined by the relation C\textH2 \textO\textolivine ( \textppm ) = 57.6( ±16 ) ×P( \textGPa ) - 169( ±18 ). C_{{{\text{H}}_{2} {\text{O}}}}^{\text{olivine}} \left( {\text{ppm}} \right) = 57.6\left( { \pm 16} \right) \times P\left( {\text{GPa}} \right) - 169\left( { \pm 18} \right). Adjustment of this trend for small increases in temperature along the mantle geotherm, combined with experimental determinations of D\textH2 \textO\textpyx/olivine D_{{{\text{H}}_{2} {\text{O}}}}^{\text{pyx/olivine}} from this study and estimates of D\textH2 \textO\textgt/\textolivine D_{{{\text{H}}_{2} {\text{O}}}}^{{{\text{gt}}/{\text{olivine}}}} , allows for estimation of peridotite H2O storage capacity, which is 440 ± 200 ppm at 400 km. This suggests that MORB source upper mantle, which contains 50–200 ppm bulk H2O, is not wet enough to incite a global melt layer above the 410-km discontinuity. However, OIB source mantle and residues of subducted slabs, which contain 300–1,000 ppm bulk H2O, can exceed the peridotite H2O storage capacity and incite localized hydrous partial melting in the deep upper mantle. Experimentally determined values of D\textH2 \textO\textpyx/\textolivine D_{{{\text{H}}_{2} {\text{O}}}}^{{{\text{pyx}}/{\text{olivine}}}} at 10–13 GPa have a narrow range of 1.35 ± 0.13, meaning that olivine is probably the most important host of H2O in the deep upper mantle. The increase in hydration of olivine with depth in the upper mantle may have significant influence on viscosity and other transport properties.  相似文献   

20.
A suite of eclogites from the Roberts Victor kimberlite has been extensively characterized in terms of petrology and geochemical compositions (Gréau et al. in Geochim Cosmochim Acta 75(22):6927–6954, 2011; Huang et al. in Lithos 142–143:161–181, 2012a). In the present study, the water contents of eclogitic garnet and omphacite were analyzed by Fourier transform infrared spectrometry. Garnet does not contain measureable OH in any sample. The water content of omphacite in the studied eclogites ranges from 211 to 1,496 ppm. Mantle metasomatism has modified the water content of some of the eclogites, while others retain water contents characteristic of their original environment. The OH contents of the metasomatized eclogites may be mainly controlled by the H2O fugacity and mineral compositions. The OH contents of the non-metasomatized samples are interpreted to be more sensitive to their mantle equilibration temperature, pressure, and the local fugacities of H2O and O2. The calculated water content of the metasomatic medium is similar to that of carbonatitic–kimberlitic melts/fluids. Eclogites contain more water than peridotites recorded in the literature (341 ± 161 vs 122 ± 54 ppm) and represent an important water reservoir in the lithospheric mantle wherever they occur. This is an important parameter to be considered in the interpretation of mantle processes and geophysical data such as seismic wave speeds and electrical conductivity, and in geodynamic modeling.  相似文献   

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