High-temperature stability of San Carlos olivine

A suite of heat-treatment experiments have been performed to test the high-temperature stability of San Carlos olivine within the theoretical stability field at one atmosphere total pressure. Exsolution or contamination products did not form on the surfaces of samples which were surrounded by olivine, magnesia, or alumina. In contrast, silica-rich phases developed on the surfaces of samples which were in line-of-sight contact with silica or platinum. These silica-rich phases result from interaction of the olivine with silica or platinum in the furnace environment via vapor phase transport or surface diffusion, rather than from diffusion of excess silica from the bulk to the surface as suggested by Jaoul et al. (1984, 1985). This conclusion is consistent with the reported slow rate of diffusion of silicon in olivine and with the lack of internal precipitation of silica-rich phases. Consequently, it is concluded that San Carlos olivine does not contain silica in excess of the solubility limit under conditions which are within the -T stability field for this (Mg,Fe,Ni)-olivine.     

Surface destabilization and laboratory-induced non-stoichiometry in San Carlos olivine

Annealing experiments on natural olivine (Mg_1-x Fe _x )₂SiO₄ (with x≈0.11) crystals (San Carlos, Arizona, spinel-lherzolite context) have been performed between T=1,100° C and 1,500° C for oxygen partial pressures pO₂=10^?3 to 10^?13 bar and times of 1 to 140 h in CO/CO₂ or H₂/H₂O gas mixtures. Even specimens annealed within the T-pO₂ theoretical stability field (TSF) calculated for stoichiometric olivine (Nitsan 1974) show systematic alterations developed within the first few microns of the surface of the crystals. Pyroxene crystals or melt form on the original olivine surface even at T=1,100° C, with preference of pyroxene when T<1,350° C and SiO₂-rich glass if T>1,350° C. This glass (rhyolite-like) can concentrate calcium from the starting olivine, and aluminum when Cr-Al spinels are present as inclusions. These observations are in contradiction with the TSF. They are obviously due to the presence of platinum used as a container of our samples, even if the contact between olivine and platinum is very weak. Rapid surficial diffusion of iron toward platinum (or via the gas phase) induces a Fe-depleted surface. According to the TSF, this more forsteritic surface should have a wider pO₂ range of stability. This is not the case, just because this situation is largely out of equilibrium. This iron loss induces a departure from cationic stoichiometry: (Mg, Fe)_2(1?δ), SiO₄ with δ small and positive. We extended the model that Nakamura and Schmalzried (1983) (N.S.) developed for fayalite (x=1) to our natural olivine composition, under the assumption that the majority defects are magnesium vacancies, Fe³⁺ occupying octahedral and tetrahedral sites, and the more complex neutral defect corresponding to Coulombic attraction between neighboring Fe³⁺ ions. We have recalculated the olivine stability field in pO₂ vs. δ space at T=1,300° C using this model for x≈0.1 (its extreme limit of validity) and conclude that olivine is stable only in a very narrow range in pO₂ which depends on δ. The calculation shows also that when olivine has nearly cationic stoichiometry (δ=0) as we believe for our starting material, the pO₂ range of stability is narrower than indicated by the TSF. In particular, it explains why Fe precipitates from the olivine (δ=0) (in absence of any other precipitation of SiO₂-rich phases) between 10^?11 and 10^?13 bar at 1,300° C; this was not predicted by the TSF. Magnetite or iron precipitates also coexist with SiO₂-rich exsolutions or pyroxene when pO₂ is close to the upper or lower boundaries of the TSF, respectively. The N.S. model may have important implications for the interpretation of the existence of partial melting and/or the low-viscosity/low velocity zone in the upper mantle.     

Mechanical re-equilibration of fluid inclusions in San Carlos olivine by power-law creep

Fluid inclusions in San Carlos olivine stretch via plastic mechanisms when heat-treated at 1400° C and 0.1 MPa in controlled atmospheres for several days. Measurable changes in both inclusion dimensions and fluid densities occur; densities decrease from 1.0 to 0.7 g/cc. Stretching is fastest along 100, and slower along 001 and 010. The dislocation microstructure around the inclusions suggests that creep mechanisms operate. Uncertainties in the experimental determinations of stretching rates result from optical resolution limits, errors inherent in measuring homogenization temperatures, uncertainties in the fluid equation of state, and changes in fluid chemistry during the heat-treatment. Inclusion stretching by dislocation creep can be treated using a model developed for hot isostatic pressing. In this model, we assume spherical symmetry of plastic flow, that the material yields by steady-state power-law creep, and that the parameters for the constitutive law and fluid equation of state are known. Stretching rates are predicted to depend on the difference between the fluid pressure and the external pressure, the temperature, the constitutive law governing power-law creep, and geometry. Predicted stretching rates show fair, but not exact, agreement with experimentally measured rates. The amount of stretching predicted by the model is in rough agreement with estimates based on dislocation microstructures around natural inclusions, if xenolith ascent rates are of the order of 1 cm/s or faster.Now at: L-201, Lawrence Livermore Laboratory, P.O. Box 808 Livermore, CA 94550, USA     

Thermodynamic properties of San Carlos olivine at high temperature and high pressure

In this study, the thermal expansion and heat capacity of San Carlos olivine under high temperature and high pressure are reported. Combining accurate sound velocity data under different P–T conditions with density and heat capacity data at ambient pressure, the density, adiabatic bulk modulus, shear modulus, and most importantly, thermal expansion and heat capacity, of San Carlos are extracted to 14 GPa by a numerical procedure using classic thermodynamic relationships. These data are in agreement with published findings. To estimate the temperature gradient in the upper mantle, we also report the fitting equations of thermal expansion and heat capacity of San Carlos olivine as a function of both temperature and pressure to the P–T condition of the 410 km discontinuity, which provide the thermodynamic properties with increasing depth in the Earth’s interior.     

Experimental study of the incorporation of Li, Sc, Al and other trace elements into olivine

We performed a series of synthesis experiments at 1 atm pressure to investigate the substitution mechanisms of 1+ and 3+ ions into olivine. Forsterite crystals were grown from bulk compositions that contained the element of interest (e.g. Li) and different amounts of additional single trace elements. By working at constant (major element) liquid composition and temperature we eliminated all compositional effects other than those due to the trace elements. Mineral-melt pairs were then analysed to determine the compositional-dependence of the partition coefficient (D), which corresponds to , and where [element] refers to weight concentration of the element in the respective phase.We find that Li forms a stable coupled substitution with Sc and, at above ∼500 ppm Sc in the crystal, Li⁺ and Sc³⁺ ions form an ordered neutral complex ([LiSc]). This complex dissociates at lower trace element concentrations and a second, concentration-independent, mechanism begins to dominate. This second solution mechanism is most likely 2Li⁺ ⇔ Mg²⁺ where one of the Li atoms is in an interstitial position in the crystal lattice. Natural olivines show Li contents slightly greater than Sc (on an atomic basis), indicating that both substitution mechanisms are significant. Unlike Sc, Al does not appear to form a stable complex with Li in the olivine structure.Sodium is highly incompatible in olivine with of ∼0.00015-0.03. Olivine-liquid partitioning of Na⁺ is independent of Sc³⁺ or Al³⁺ concentration. This indicates that the coupled substitution of Na⁺ with any 3+ ions is unlikely. Instead, the relevant substitution mechanism appears to be 2Na⁺ ⇔ Mg²⁺. Although independent of 3+ ion concentration, is inversely correlated with the Li concentration of both melts and crystals, implying that Na competes (unsuccessfully) with Li to replace Mg in the olivine structure.Aluminium is highly incompatible in forsterite . Values of are similar for all phase pairs synthesised from starting materials containing between 10 and 100,000 ppm Al. This suggests that Al is principally incorporated in forsterite by replacing one Mg and one Si atom , where the Al atoms on octahedral (Mg) and tetrahedral (Si) sites are dissociated from one another.The incorporation of gallium into forsterite is influenced by the presence of Li. Where Li concentration in the crystal is much greater than that of Ga (on an atomic basis) we find an excellent correlation between and melt Li content. This relationship indicates that Ga³⁺ and Li⁺ replace 2Mg²⁺ on octahedral sites and that the Ga and Li atoms are, like Sc and Li, strongly associated in the crystal structure.The mechanism by which scandium is incorporated into forsterite is strongly governed by the presence Li. As discussed above, ordered complexes form readily in forsterite in Li-rich experiments. Under Li-absent but Sc-rich conditions (Sc in the crystal >∼500 ppm), is proportional to the concentration of Sc in the melt. This indicates that Sc incorporation is charge-balanced by the formation of magnesium vacancies , and that both species are associated . At lower Sc concentrations (<500 ppm in the crystal), the concentration-dependence of partitioning indicates that the complexes dissociate.Our results demonstrate that partitioning of 1+ and 3+ ions into olivine is complex and involves a range of point defects which yield strongly composition-dependent crystal-melt partition coefficients. Since physical and chemical properties of natural olivine, such as diffusion of ⁶Li and ⁷Li and H₂O solubility, depend on the concentrations of the defects identified in this study, our results provide an important insight into how determining substitution mechanisms can improve our understanding of large-scale mantle processes and properties.     

橄榄石微量元素原位分析的现状及其应用

随着高精度EMPA和LA-ICP-MS分析技术的发展和矿物微量元素测试精度的提高,利用橄榄石中的微量元素示踪地幔部分熔融、地幔交代作用、岩浆早期结晶过程等地质问题成为近年来一个新兴的研究方向。一系列开拓性的研究发现也被陆续的发表,主要涉及橄榄石中Ni、Co、Al、Cr、Zn、Ti、Li、V、Sc、Mn、Ca和P等元素的示踪使用。一些卓有成效的示踪方法为:Ca、Al、Ti、Ni及Mn能够很好的用于区分橄榄石捕掳晶和斑晶;橄榄石-尖晶石地幔演化趋势线(OSMA:olivine-spinel mantle array)图解可以用于表征岩浆源区的亏损程度;玄武岩中橄榄石斑晶的Li同位素及Li含量可以很有效地指示岩浆源区是否存在地壳物质再循环及地幔交代作用;橄榄石斑晶中Ni、Ca、Mn、Cr和Al协变关系图解可以识别岩浆的辉石岩源区;利用橄榄石捕掳晶中Zr和Sc的含量差异特征可将橄榄岩中三种最主要的类型(尖晶石橄榄岩、石榴石橄榄岩以及尖晶石-石榴石橄榄岩)区分开来;一些元素的比值或组合(例如Ni/Co、Fe/Mn、V/Sc、Zr和Sc、Ca和Ti)可以指示源区交代作用、岩浆作用过程及氧化状态;基于橄榄石中Al、Cr及Ca的地质温度计可以为推算地幔热状态提供新方法;基于橄榄石分离结晶Fo-NiO演化线的原始岩浆计算模型可以较好的推算原始岩浆成分;利用橄榄石的环带及微量元素的扩散机制可以判别更多岩石成因信息,如识别交代介质、熔体类型以及地质构造背景等。基于上述最新研究的相关资料和已有成果,本文对橄榄石微量元素的地球化学示踪方法做系统性的归纳整理,并对橄榄石微量元素赋存状况、橄榄石微量元素测试方法、橄榄石微量元素的使用条件及需注意的问题等进行讨论,为读者在做相关研究时提供参考。     

Mg tracer diffusion in synthetic forsterite and San Carlos olivine as a function of P,T and fO2

We present new experimental data on Mg tracer diffusion in oriented single crystals of forsterite (Fo₁₀₀) and San Carlos olivine (Fo₉₂) between 1000–1300° C. The activation energies of diffusion are found to be 400 (±60) kJ/mol (96 kcal/mol) and 275 (±25) kJ/mol (65 kcal/ mol) in forsterite and San Carlos olivine, respectively, along [001] at a fO₂ of 10^–12 bars. There is no change in activation energy of Mg tracer diffusion within this temperature range. Mg tracer diffusion in a nominally pure forsterite is found to be anisotropic (D_c > D_a > D _b) and a function of fO₂. This fO₂ dependence is different from that in olivine containing Fe as a major element, which suggests that the diffusion mechanism of Mg in forsterite is different from that in Fe-bearing olivine at least over some range of fO₂. The diffusion mechanism in nominally pure forsterites may involve impurities present below the limits of detection or alternately, Si or Fe³⁺ interstitial defects, Fe being present as impurity (ppm level) in forsterite. Pressure dependence of Mg tracer diffusivity in forsterite measured to 10 GPa in a multianvil apparatus yields an activation volume of approximately 1–3.5 cm³/ mol. It is found that presence of small amounts of hydrogen bearing species in the atmosphere during diffusion anneal (f_H2 0.2 bars, f_H20 0.24 bars) do not affect Mg tracer diffusion in forsterite within the resolution of our measurement at a total pressure of 1 bar. The observed diffusion process is shown to be extrinsic; hence extrapolation of the diffusion data to lower temperatures should not be plagued by uncertainties related to change of diffusion mechanism from intrinsic to extrinsic.     

Insights into the petrogenesis of the West Kimberley lamproites from trace elements in olivine

The Miocene lamproites of the West Kimberley region, Western Australia include olivine-leucite lamproites (≤10 wt% MgO) containing olivine and leucite microphenocrysts, and diamondiferous olivine lamproites (20–30 wt% MgO) containing olivine phenocrysts and larger (1–10 mm) olivine as mantle xenocrysts and dunite micro-xenoliths. Olivine phenocrysts and thin (<100 μm) magmatic rims define trends of decreasing Cr and Ni, and increasing Ca and Mn, with decreasing olivine Mg#, consistent with fractional crystallisation of olivine (and minor chromite). Many phenocrysts are zoned, and those with cores of similar Mg# and trace element abundances to the mantle xenocrysts may be xenocrysts overgrown by later olivine crystallised from the lamproite magma. Magmatic olivines Mg#_91–92 are estimated to have been in equilibrium with olivine lamproite magma(s) containing ~22–24 wt% MgO. The xenocrystic mantle olivines Mg_90–92.5 in the olivine lamproites are inferred from trace element abundances to be mostly derived from garnet peridotite with equilibration temperatures estimated from the Al-in-olivine thermometer (Bussweiler et al. 2017) to be ~1000–1270 °C at depths of 115–190 km. Olivines from the deeper lithosphere are less depleted (lower Mg#, higher Na, Al, P, Ti, Zr etc) than those at shallower depths, a feature suggested to reflect the combined effects of metasomatic re-enrichment of the craton roots (Ti, Fe, Zr etc) and increasing temperature with depth of origin (Na, Al, Ca). The West Kimberley lamproite olivines are not enriched in Li, as might be expected if their source regions contained continental sedimentary material as has been previously inferred from lamproite large-ion-lithophile trace elements, and Sr and Pb isotopes.

     

Thin amorphous films (1–2 nm) at olivine grain boundaries in mantle xenoliths from San Carlos,Arizona

 Olivine grain boundaries and phase boundaries in xenoliths from San Carlos have been investigated by high-resolution transmission electron microscopy (HREM) and analytical electron microscopy (AEM). Thin amorphous intergranular layers with variable width (1–2 nm) were detected along olivine grain boundaries. The Al₂O₃, TiO₂ and CaO concentrations of the amorphous layers increase with increasing width of the layer. The composition of the amorphous intergranular layers depends on the interface type – grain or phase boundary. Morphology, amorphous state and chemical composition of the intergranular layer suggest the presence of a melt film at olivine grain boundaries. Since the composition of the amorphous phase strongly depends on the type of interface, the melt must have been generated at the grain boundary. Also, the melt chemistry is different from the composition of partial melts produced from possible hydrous phases, such as phlogopite or amphibole, and from the host basanite. The mobility of very thin melt films is assumed to be very limited due to the strong interface forces between the melt and the grain boundary. It is concluded that grain boundary melting occurred at the interfaces due to decompression during uplift. The melt wetted olivine grain boundaries as well as olivine-opx phase boundaries. The thin amorphous layers formed melt microsystems. Mixing of melts from different microsystems is suggested to occur in wider melt films, melt veins or melt pockets thus creating a magmatic melt that could be extracted from its source. Received: 6 November 1995 / Accepted: 24 January 1996     

橄榄石中H的结合机制及扩散行为

名义上无水矿物(nominally anhydrous minerals,NAMs)中以点缺陷形式存在的结构水,因其对于矿物物理化学性质的显著影响而受到越来越广泛的关注。橄榄石是上地幔中含量最丰富的矿物,水在橄榄石中的存在形式、扩散机制和速率对上地幔的流变学和电导性质有着重要的影响。因此,对于橄榄石中H的结合机制及其扩散机制和速率的了解有助于理解地球深部的水循环,也有助于构建上地幔流变结构以及解释电导测量结果。本文总结了近几十年来学界对于H在橄榄石中存在的缺陷类型与OH红外光谱吸收峰之间的对应关系,以及H在矿物晶格中的扩散机制、扩散速率等重要问题的研究成果,并探讨了现有研究中依然存在的问题。     

Distribution and nature of incorporation of trace elements in modern aragonitic corals*

The objective of this study is to locate as closely as possible the sites of strontium, magnesium, sodium, and potassium in modern aragonitic corals, specifically whether these cations are adsorbed, or are substituted in the carbonate lattice or are incorporated in organic components. In addition to locating the sites of each of these four elements we wanted to find out quantitatively how much of each element occurs at each site. The experiments in this study are based on the dissolution rate of aragonite in distilled water and on the substitution of strontium and magnesium by calcium and sodium. Special attention has been given to the occurrence of strontium, magnesium, sodium, and potassium in the organic components of the corals. The main site for strontium in the corals is in the aragonite lattice. Twenty-five per cent of the total magnesium occurs in adsorbed sites and in organic compounds. The rest of the magnesium may be located in the aragonite lattice, but it is easily removed by repeated leaching or by replacement with calcium ions. Another possibility is that magnesium may occur in a dispersed mineral phase more soluble than aragonite because magnesium was released at a higher Mg to Ca ratio than is found in the solid coral; also because no local concentration of magnesium could be detected with an electron microprobe. About 12% of the total sodium is in adsorbed sites and is included in the organic compounds. The rest of the sodium might be in the lattice replacing calcium, but the low total exchange capacity is not enough to provide the needed charge balance. Another possibility is that sodium is located in a proposed mineral phase. Potassium is in adsorbed sites and incorporated in the organic compounds to an extent greater than all the other elements studied (30% of the total potassium), but again the evidence suggests that the remaining potassium is in a proposed mineral phase. Calcite is detected on the surfaces of aragonite corals after 5 months in the substitution experiment. The change of argonite to calcite took place after the inhibitor magnesium was exchanged from the surface sites and replaced by calcium. The organic compounds in corals contain small amounts of strontium, magnesium, sodium and potassium. Strontium is preferentially enriched in the organic compounds over magnesium.     

Pressure dependence on partition coefficients for trace elements between olivine and the coexisting melts

Partition coefficients between olivine and melt at upper mantle conditions, 3 to 14 GPa, have been determined for 27 trace elements (Li, Be, B, Na, Mg, Al, Si, P, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Rb, Sr, Y, Zr, Cs, Ba, La and Ce) using secondary-ion mass-spectrometry (SIMS) and electron-probe microanalysis (EPMA). The general pattern of olivine/melt partitioning on Onuma diagrams resembles those reported previously for natural systems. This agreement strongly supports the argument that partitioning is under structural control of olivine even at high pressure. The partition coefficients for mono- and tri-valent cations show significant pressure dependence, both becoming larger with pressure, and are strongly correlated with coupled substitution into cation sites in the olivine structure. The dominant type of trace element substitution for mono- and tri-valent cations into olivine changes gradually from (Si, Mg)↔(Al, Cr) at low pressure to (Si, Mg)↔(Al, Al) and (Mg, Mg)↔(Na, Al) at high pressure. The change in substitution type results in an increase in partition coefficients of Al and Na with pressure. An inverse correlation between the partition coefficients for divalent cations and pressure has been observed, especially for Ni, Co and Fe. The order of decreasing rate of partition coefficient with pressure correlates to strength of crystal field effect of the cation. The pressure dependence of olivine/melt partitioning can be attributed to the compression of cation polyhedra induced by pressure and the compensation of electrostatic valence by cation substitution. Received: March 6, 1997 / Revised, accepted: March 12, 1998     

Mechanisms of hydrogen incorporation and diffusion in iron-bearing olivine

The incorporation and diffusion of hydrogen in San Carlos olivine (Fo₉₀) single crystals were studied by performing experiments under hydrothermal conditions. The experiments were carried out either at 1.5 GPa, 1,000°C for 1.5 h in a piston cylinder apparatus or at 0.2 GPa, 900°C for 1 or 20 h in a cold-seal vessel. The oxygen fugacity was buffered using Ni–NiO, and the silica activity was buffered by adding San Carlos orthopyroxene powders. Polarized Fourier transform infrared (FTIR) spectroscopy was utilized to quantify the hydroxyl distributions in the samples after the experiments. The resulting infrared spectra reproduce the features of FTIR spectra that are observed in olivine from common mantle peridotite xenoliths. The hydrogen concentration at the edges of the hydrogenated olivine crystals corresponds to concentration levels calculated from published water solubility laws. Hydrogen diffusivities were determined for the three crystallographic axes from profiles of water content as a function of position. The chemical diffusion coefficients are comparable to those previously reported for natural iron-bearing olivine. At high temperature, hydrogenation is dominated by coupled diffusion of protons and octahedrally coordinated metal vacancies where the vacancy diffusion rate limits the process. From the experimental data, we determined the following diffusion laws (diffusivity in m² s⁻¹, activation energies in kJ mol⁻¹): for diffusion along [100] and [010]; for diffusion along [001]. These diffusion rates are fast enough to modify significantly water contents within olivine grains in xenoliths ascending from the mantle.     

Noble gases in ultramafic xenoliths from San Carlos,Arizona

This work reports the results of noble gas (Ne, Ar, Kr, Xe) analyses of accidental mantle xenoliths from San Carlos, Arizona. Except for the addition of radiogenic ⁴⁰Ar and mass fractionation effects, the isotopic structures of these gases are indistinguishable from atmospheric composition. The absence of ¹²⁹Xe excesses in these rocks may reflect indirect mixing of atmospheric gases with the source region of the xenoliths. The dominant influence on the noble gas abundances in the San Carlos xenoliths appears to have been diffusive gas loss, which may have occurred in a mantle metamorphic event or during contact with the host basanite magma. Evidence is presented for the partitioning of significant amounts of the heavy noble gases into fluid inclusions in the xenolith minerals; the proportion of each gas in the inclusions increases with increasing atomic weight of the gas, possibly reflecting solubility effects. The noble gases are present in greater concentration in pyroxenes than in olivine, similar to the behavior of other incompatible elements.     

Leachable trace elements in San Francisco bay sediments: Indicators of sources and estuarine processes

Geographic variations of leachable Fe, Mn, Cu, Zn, Co, and Ni in San Francisco Bay sediments indicate that Fe, Mn, Co, and Ni are all predominantly supplied to bay sediments from the San Joaquin-Sacramento River system, with little evidence for direct contributions from municipal and industrial sources. In contrast, both Cu and Zn have significant sources within the Bay system, probably municipal and industrial discharges. Precipitation and coagulation of Fe oxides in the low-salinity region of the estuary results in significantly greater concentrations of that element in the most landward portion of the estuary. Co and Ni appear to be actively coprecipitating with Fe but their distributions are also influenced by other factors. Mn is not a major geochemical agent in this system. Its pattern is different from the other elements and it does not account for any of the other elemental variance. The behavior of Cu and Zn is affected not only by the presence of nonriverine sources but also through surface-active processes and organic complexing, coupled with the transportation of fine-grained sediment.     

Geochemical factors controlling accumulation of major and trace elements in Greenland coastal and fjord sediments

 The major (Al, Ti, Ca, Mg, Fe, Mn, Si) and trace element (Cd, Cr, Cu, Hg, Li, Ni, Pb, V, Zn) concentrations in surficial (<20 cm) sediments from fjords and open coastal waters around Greenland have been determined. Regionally, high concentrations of Fe, Ti, Mg, Cr, Cu, Ni, and V occur in some west and east coast sediments, but they appear to be natural in origin, as there is no indication of anthropogenic influence. Chemical partition indicates that most of the heavy metals are structurally bound in various silicate, oxide, and sulfide minerals. These host minerals occur more or less equally in the coarse and fine sediment fractions (material >63 μm and <63 μm) and have accumulated at the same rate as other detrital clastic material. Provenance and glaciomarine deposition are the main factors controlling the abundance and distribution of the major and trace elements. The chemical composition reflects the mineralogical differences in the provenance of glacial marine material deposited by water and ice adjacent to Greenland. The main source of the sediments enriched in Ti, Fe, Mg, Cr, Cu, Ni, and V appears to be material derived from the volcanic rocks of the Mesozoic-Tertiary Provinces of Greenland by glacial erosion. Received: 26 June 1995 · Accepted: 11 August 1995     

The distribution of trace elements in carbonaceous chondrites

12 carbonaceous chondrites, amongst them representatives of nearly all known petrologic types were analyzed for twenty trace elements by spark source mass spectrography combined with the isotope dilution method. Data on different element groups (refractory, moderately volatile and volatile) show that the distribution of the trace elements in the carbonaceous chondrites, with the exception of Renazzo, can be well explained by Anders' two-component model. This is also valid for the highly metamorphosed CV5 chondrite Karoonda.Furthermore, it is observed that the $\text{Zr}\text{Hf}$-ratios in the carbonaceous chondrites increase with increasing petrologic type which is interpreted as the result of mixing two components with different $\text{Zr}\text{Hf}$-ratios     

The role of CO2 in the genesis of olivine melilitite

The nature of the near-liquidus phases for a mantle-derived olivine melilitite composition have been determined at high pressure under dry conditions and with various water contents. Olivine and clinopyroxene occur on or near the liquidus and there are no conditions where orthopyroxene crystallizes in equilibrium with the olivine melilitite. We have determined the effect on the liquidus temperature and liquidus phases of substituting CO₂ for H₂O on a mole for mole basis at 30 kb, using olivine melilitite + 20 wt% H₂O at = 0 and = (CO₂)/(H₂+CO₂) (mole fraction) = 0.25, 0.5, 0.75 and 1.0 (i.e. olivine melilitite + 38 wt% CO₂). Experiments were buffered by the MH or NNO buffers. At 30 kb, CO₂ is only slightly less soluble than water for <0.5 as judged by the slight increase in liquidus temperature on mole-for-mole substitution of CO₂ for H₂O and at 30 kb, 1200° C, = = 0.5 the olivine melilitite contains 8.8 wt% H₂O and 21 wt% CO₂ in solution. For 1 the CO₂ saturated liquidus is depressed 70 ° C below the anhydrous liquidus and the magma dissolves approx. 17% CO₂ at 30kb, 1400 ° C, 1, 0. Infrared spectra of quenched glasses have absorption bands characteristic of CO ₃ ⁼ and OH- molecules and no evidence for HCO ₃ ^- . The effect of CO ₃ ⁼ molecules dissolved in the olivine melilitite at high pressure is to suppress the near-liquidus crystallization of olivine and clinopyroxene and bring orthopyroxene and garnet on to the liquidus. We infer that olivine melilitite magmas may be derived by equilibrium partial melting (<5%) of pyrolite at 30 kb, 1150–1200 ° C, provided that both H₂O and CO₂ are present in the source region in minor amounts. Preferred conditions are 0< <0.5, 0.5< <1, and at low oxygen fugacities (相似文献   

碳酸盐矿物的阴极发光性与微量元素的关系

碳酸盐矿物的阴极发光特征与其成分有关。笔者用阴极发光与电子探针微区分析法对砂岩中碳酸盐矿物进行测试分析,其结果表明碳酸盐矿物的阴极发光与微量元素含量有如下规律:①碳酸盐矿物在铁含量高于猝灭下限或锰含量低于激活下限时,不具有阴极发光性;②铁的猝灭下限约为0.04 mol,锰的激活下限为小于7×10^-5mol;③铁/锰比值越高,越不利于碳酸盐矿物阴极发光;但是铁/锰比值小于1的碳酸盐一定具有阴极发光;④铁或锰二者之一含量很少时不利于碳酸盐矿物的阴极发光。     

活体珊瑚微量元素对硼同位素组成影响及硼掺入珊瑚的形式

为确定元素 B 掺入进珊瑚的形式、B(OH)与 B(OH)3间的分馏系数及珊瑚中微量元素对硼同位素?4组成的影响,对北海涠洲岛、海南三亚和雷州半岛灯楼角三地活体珊瑚的 Mg、Sr、Na、Ca、B 浓度及硼同位素组成(δ11Bcoral)进行了测定.结果表明,涠洲岛、三亚和灯楼角三地珊瑚的 Mg、Sr、Na 和 Ca 浓度平均值分别为40.1 mmol/L、86.1 mmol/L、449 mmol/L 和12.1 mol/L.珊瑚中 B 浓度的变化范围为4.4~8.4 mmol/L,平均值为5.9 mmol/L.Ca、Sr 在珊瑚中明显富集,而 Mg 在珊瑚中贫化.珊瑚 B 浓度的变化主要反映了珊瑚生长时海水 pH 值的变化.δ11Bcoral 的变化范围为22.8‰~27.9‰,平均为25.2‰.除与 B 浓度呈弱正相关关系外,δ11Bcoral 与其他四种元素不相关.应用珊瑚硼同位素组成恢复古海水 pH 值时选择同属种珊瑚和判别是否有 Mg(OH)2的存在是必要的.涠洲岛、灯楼角和三亚三地珊瑚与海水间的硼同位素分馏系数αcoral-sw 分别为0.9839、0.9847和0.9850.计算得到的珊瑚与海水 B(OH)3间的分馏系数αcoral-3的变化范围为0.9772~0.9800,平均值为0.9788.该新的α4-3值是准确的,可以用来反演古海水 pH 值.αcoral-sw 和αcoral-3随pH 值的升高分别呈现出增大和减小的变化趋势.珊瑚的平均δ11Bcoral 位于理论计算的δ11B4和δ11B3曲线之间,而且都低于原始合成海水的δ11B.这些都表明 B 以 B(OH)3和B(OH)两种形式以变化的比例同时掺入进?4珊瑚,并以B(OH)优先掺入为主.计算得到有0.1%~5.5%(平均值为2.2%)的 B(OH)3掺入进珊瑚中.由于?4 B(OH)3同时掺入进生物碳酸盐,δ11Bcarb=δ11B4的假设不能成立,由所测生物碳酸盐δ11Bcarb 值计算的海水 pH值将产生误差,使δ11B-pH 技术变得更为复杂.通过无机碳酸盐沉积或有孔虫或珊瑚的养殖实验(或者野外观测实验)建立用于重建古海水 pH 值的δ11Bcarb-pH 经验方程是今后的一项重要任务.