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Larnite (β-Ca2SiO4) has previously been reported as an inclusion in sub-lithospheric diamonds and is generally interpreted as a retrograde reaction product of calcium silicate perovskite. In this study, we review the controls on the stability of the Ca2SiO4 polymorphs and show that phosphorus is likely essential for the preservation of β-Ca2SiO4. We also report a detailed study of the solubility of water and its incorporation mechanisms in γ-Ca2SiO4 and phosphorus-doped β-Ca2SiO4 using FTIR spectroscopy on high-pressure experiments quenched from 4–9.5 GPa and 1000–1200 °C combined with ab initio calculations. The experimentally determined water solubilities are in the range of 107–178 ppm. Our FTIR spectra and ab initio calculations indicate that for phosphorus-free γ-Ca2SiO4 the incorporation mechanism involves protonated Si and Ca1 vacancies. For phosphorus-bearing β-Ca2SiO4, our preferred incorporation mechanism involves one Si4+ ion replaced by one P5+ ion with a single protonated Ca2 vacancy. The low water solubility observed here for larnite implies that if primary calcium silicate perovskite inclusions trap high water concentrations during diamond growth from a volatile-rich fluid, measurements of the concentration of water in larnite will not provide a useful record of the initial volatile concentration. Instead, water would be hosted in other retrograde reaction products, possibly including exsolved fluids. [ABSTRACT FROM AUTHOR]

Interactions between crustal and mantle reservoirs dominate the surface inventory of volatile elements over geological time, moderating atmospheric composition and maintaining a lifesupporting planet (1). While volcanoes expel volatile components into surface reservoirs, subduction of oceanic crust is responsible for replenishment of mantle reservoirs (2,3). Many natural, 'superdeep' diamonds originating in the deep upper mantle and transition zone host mineral inclusions, indicating an affinity to subducted oceanic crust (4-7). Here we show that the majority of slab geotherms will intersect a deep depression along the melting curve of carbonated oceanic crust at depths of approximately 300 to 700 kilometres, creating a barrier to direct carbonate recycling into the deep mantle. Low-degree partial melts are alkaline carbonatites that are highly reactive with reduced ambient mantle, producing diamond. Many inclusions in superdeep diamonds are best explained by carbonate melt-peridotite reaction. A deep carbon barrier may dominate the recycling of carbon in the mantle and contribute to chemical and isotopic heterogeneity of the mantle reservoir., Altered oceanic crust incorporates appreciable carbon, which is added by magmatic and hydrothermal processes (8), and by addition of C[O.sub.2] during interaction of basalt with sea water (9). Together, these [...]

The origin of sulfite ([S.sup.4+]) species in silicate glasses was evaluated using XANES at the S K-edge. Systematic investigations show that the presence of [S.sup.4+] species in silicate glasses is an analytical artifact related to changes in the sulfur species caused by irradiation with an electron beam during EMPA or by irradiation with an intense focused X-ray beam during synchrotron analysis. The data shown here indicate that [S.sup.2-] and [S.sup.6+] are the only significant sulfur species occurring in silicate glasses synthesized under geologically relevant conditions. Keywords: XAS (XAFS, XANES), sulfur K-edge, glass properties, sulfur oxidation state, beam damage, sulfur speciation

Two series of leucite group materials, with K, Rb, and Cs as extra-framework cations, have been synthesized by ion exchange from a natural well-ordered analcite and a natural disordered leucite. 29Si and 27Al MAS NMR data for the analcite-derived series provide complementary information on tetrahedral cation ordering. The ordering in terms of the number of Al next-nearest neighbors, Q4(nAl) (short-range order), does not change significantly during ion exchange, indicating that Al and Si remain essentially fixed in their original positions. In contrast, the ordering of Al over T1, T2, and T3 (long-range order) for the analcite-derived series changes dramatically with changing alkali cation; the Al occupancies for the three analcite-derived samples expressed as T1:T2:T3 are approximately 0.25:0.50:0.25 for KAl[Si.sub.2][O.sub.6], 0.40:0.20:0.40 for RbAl[Si.sub.2][O.sub.6], and 0.15:0.70:0.15 for CsAl[Si.sub.2][O.sub.6]. During the ion exchange, at temperatures above the cubic-tetragonal phase transitions, only one symmetrically distinct T site is present. It is proposed that on cooling through the cubic-tetragonal phase transition the structure collapses around the non-framework cations to give the lowest energy Si-Al distribution over the three T sites irrespective of the original T-site ordering in the starting material. Our data suggest that the identity of the cation in the W site affects the orientation of the framework distortions associated with the cubic-tetragonal phase transition and leads to the possibility that a particular tetrahedral cation site can take on the characteristics of a T1, T2, or T3 site. The data and their interpretation have important implications for the mechanism of this type of structural phase transition.

The structural phase transitions of monoclinic tridymite that occur at elevated temperatures, as well as the structure of the high-temperature modifications, have been studied by 29Si MAS NMR. The sample, originating from the surface of a refractory silica brick, displays sharp discontinuities in the first moment ([M.sub.1]), second moment ([M.sub.2]), or both, of the 29Si MAS NMR spectrum at the following phase transitions: 108 [+ or -] 1 [degrees] C for the monoclinic (MC) to orthorhombic (OP) transition, 165 [+ or -] 5 [degrees] C for the transition from OP to a second orthorhombic phase (OS), and 206 [+ or -] 4 [degrees] C for the transition from OS to a third orthorhombic phase (OC). [M.sub.1] for the OC phase becomes more negative with increasing temperature up to [approximately]323 [degrees] C, at which there is also a small discontinuity in [M.sub.2], indicating a transition from OC to a hexagonal phase (LHP). The spectrum of the MC phase consists of 12 narrow lines of equal area, consistent with the known crystal structure. The spectrum of the OP phase is broad and asymmetric, indicating a distribution of local environments at the Si sites. It can be simulated assuming an incommensurate structural modulation with a maximum amplitude of [approximately]5 x [10.sup.-2] [Angstrom], but it is also consistent with a recent domain-formation model, provided that most disorder occurs at the Si6 site. The spectrum of the OS phase is asymmetric, indicating a nonlinear incommensurate modulation with a maximum amplitude of [approximately]2 x [10.sup.-2] [Angstrom]. The spectra of higher temperature phases are consistent with the single Si site of the known crystal structures, but [M.sub.1] values imply significantly smaller mean Si-O-Si angles e.g., = 152.6 [degrees] at 220 [degrees] C from NMR, compared with 168.0 [degrees] from X-ray diffraction data. This difference is consistent with disorder of O atoms around Si-Si axes.

A synthetic analog of leucite was analyzed by magic angle spinning spinning nuclear magnetic resonance (MAS NMR) for silicon (Si) and aluminum (Al) ordering and was compared with MAS NMR observations of natural leucite. The synthetic sample was made from ion exchange treatment of analcite. The synthetic sample had 45-50% of the total Al on T2 crystal sites and the rest divided equally on the T1 and T3 sites. Models based on this finding were then applied to the natural sample. Al in the natural sample is probably ordered on the T2 crystal site.

Eight glasses with molar Mg/2Al [approximately equals] 1 in the system MgO-[Al.sub.2][O.sub.3]-Si[O.sub.2] have been studied by magic angle spinning (MAS) NMR spectroscopy. Using triple quantum (3Q) NMR techniques we find evidence for significant concentrations of Al coordinated to five O atoms in all glasses, the proportion increasing with decreasing Mg/Al and decreasing silica content. In glasses with Mg/2Al = 1, up to 6% of the Al is estimated to be coordinated to five rather than four O atoms. Calculations of the polymerization state of these liquids made assuming that all aluminum is in tetrahedral coordination charge balanced by magnesium are thus seriously in error. Such errors may be of even greater importance at the high temperatures and pressures relevant to the Earth and materials sciences.

We present new experimental results on the fluorine storage capacity of olivine and orthopyroxene in the Earth’s mantle. Experiments were performed in the system MgO-SiO 2 + MgF 2 at temperatures between 1350 °C and 1700 °C and pressures up to 17 GPa. Electron microprobe measurements show that fluorine concentrations in olivine reach up to 5100 µg/g. The storage capacity of fluorine in olivine shows only a small pressure dependence but a strong temperature dependence with a positive correlation between increasing temperature and fluorine storage capacity. Fluorine concentrations found in enstatite are one order of magnitude smaller and reach up to 670 µg/g. Our data show that concentrations of fluorine in fluorine-saturated olivine are in the same range as water concentrations in olivine. Nevertheless, fluorine and water solubility in olivine show opposing behavior with increasing pressure and temperature. The fluorine solubility in olivine increases with increasing temperature but is not much affected by pressure. In contrast, water solubility in olivine has previously been shown to decrease with increasing temperature and increase with increasing pressure. Our experiments show that nominally fluorine-free minerals like forsterite and enstatite are capable of storing the entire fluorine budget of the upper mantle, without the need to invoke accessory phases such as apatite or amphibole. [ABSTRACT FROM AUTHOR]

Experimental and theoretical evidence for the coupled substitution of B and H in synthetic forsterite and a natural olivine is presented. The intensities of OH bands at 3704 cm-1 (//z), 3598 cm-1 (//x,y), and 3525 cm-1 (//x) in a heterogeneous B-doped synthetic forsterite crystal matches the zoning of B concentration measured by ion probe. The two anti-symmetric stretching vibrations of BO3 groups occur at 1301 cm-1 (//x) and 1207 cm-1 (//z) for the 10B and at 1256 and 1168 cm-1 for the 11B isotope. A microscopic model of the mixed (B,H) defect that accounts for experimental observations is obtained from quantum mechanical calculations. The BO3 group lies on the (O3-O1-O3) face of the vacant Si site and the H atom is bonded to the O2 atom at the remaining apex. The occurrence of the same OH bands associated with v3 BO3 vibrations in a natural olivine sample from Pakistan confirms the occurrence of this defect in nature. The three diagnostic OH bands can be used as a signature of H associated with boron substitution in olivine and forsterite, leading to a quantitative analysis of their contribution to H-defects. [ABSTRACT FROM AUTHOR]

Pressure-temperature melting curves in two carbonate-bearing systems of relevance to Earth's mantle have been determined using the laser-heated diamond-anvil cell (LH-DAC). The solidus along the MgSiO3-MgCO3 join in the MgO-SiO2-CO2. system (MS-CO2) was defined from 15 to 80 GPa using in situ melting criteria, reaching a maximum temperature of ~2340 K at 80 GPa. The occurrence of melting has been confirmed with ex-situ textural and chemical analysis of recovered samples. The melting curve has a negative dT/dP slope at pressures between ~15 and 23 GPa resulting from the subsolidus phase transition of ilmenite- to perovskite-structured MgSiO3. The shallow slope of the melting curve at pressures higher than this transition indicate that for plausible mantle geotherms carbonate-bearing silicate lithologies will melt throughout the lower mantle. The solidus of a mixture along the MgCO3-CaCO3 join was determined as a proxy for alkali-free carbonate lithologies. Melting temperatures increase from 1860 K at 16 GPa to ~2100 K above 35 GPa, where the melting curve flattens. The melting reaction magnesite + post-aragonite (high-pressure CaCO3) = melt was confirmed using an in situ experiment. We conclude that crystalline Mg and Ca carbonate mixtures are unstable with respect to molten carbonate at conditions of the convective lower mantle. The flat melting curves at high pressures in both systems suggests that subducted carbonates will undergo melting at lower mantle conditions, a process that may be important for superdeep diamond formation and carbon storage in the deep mantle. [ABSTRACT FROM AUTHOR]

The partitioning of phosphorus between olivine and melt was measured by conducting partitioning experiments within the system Mg2SiO4-Ca2Al2Si2O9-NaAlSi3O8-Mg3(PO4)2, using olivines crystallized with cooling rates in the range 1 to 10 °C/h. Partition coefficients, DPol/melt, vary from 0.05 to 1.41 and are strongly influenced by melt composition, with high partition coefficients observed for more polymerized melts. Correlations between elemental abundances in the olivines demonstrate that phosphorus substitutes on a near one for one basis for Si, charge-balanced by Mg vacancies; no correlation between P and Al concentration in the olivine was observed. A complete understanding of the controls on DPol/melt is required to model quantitatively the zoning of P in olivine that has recently been shown to be a common feature of igneous olivine crystals. The strong melt compositional dependence observed in this study implies that both changing concentrations of P in the melt during fractionation, and changing values of DPol/melt can contribute to core-to-rim variations in P when the surface composition of a growing olivine crystal is in equilibrium with the melt. The common observation of zones in natural olivines where high P is correlated with high Al and Cr must be explained by processes other than equilibrium growth and, as suggested previously, such zones are probably related to solute trapping during episodes of rapid growth. [ABSTRACT FROM AUTHOR]

The partitioning of phosphorus between olivine and melt was measured by conducting partitioning experiments within the system Mg2SiO4-Ca2Al2Si2O9-NaAlSi3O8-Mg3(PO4)2, using olivines crystallized with cooling rates in the range 1 to 10 °C/h. Partition coefficients, DPol/melt, vary from 0.05 to 1.41 and are strongly influenced by melt composition, with high partition coefficients observed for more polymerized melts. Correlations between elemental abundances in the olivines demonstrate that phosphorus substitutes on a near one for one basis for Si, charge-balanced by Mg vacancies; no correlation between P and Al concentration in the olivine was observed. A complete understanding of the controls on DPol/melt is required to model quantitatively the zoning of P in olivine that has recently been shown to be a common feature of igneous olivine crystals. The strong melt compositional dependence observed in this study implies that both changing concentrations of P in the melt during fractionation, and changing values of DPol/melt can contribute to core-to-rim variations in P when the surface composition of a growing olivine crystal is in equilibrium with the melt. The common observation of zones in natural olivines where high P is correlated with high Al and Cr must be explained by processes other than equilibrium growth and, as suggested previously, such zones are probably related to solute trapping during episodes of rapid growth. [ABSTRACT FROM AUTHOR]