Diopside Composition Research Articles

Activities of NiO, FeO, and O 2− in silicate melts

We have measured activity coefficients for NiO and FeO in a variety of silicate melts (SiO 2-CaO-MgO-Al 2O 3) using electrochemical methods similar to square wave voltametry. We report the activity of the oxide ion (a O 2−) in one composition. Based on these measurements, we have constructed a model that predicts the variations in activity we observe, and also variations in NiO activity reported in the literature. Activity of metal-oxide components such as NiO and FeO in silicate melts can be understood by considering contributions from both the activity of the oxide ion and the activity of the cation through expressions of the type: a NiO = ( a O 2- ) ( a Ni 2+ ) and a FeO = ( a O 2- ) ( a Fe 2+ ) Based on measurements of a O 2−, we find that a O 2− is effectively modeled by the expression a O 2− = 0.75 · NBO 2/BO (relative to a standard state of melt with the composition of diopside). Once the effects of variations in a O 2− on the metal-oxide activity coefficient are accounted for, cation activities (a Ni 2+, a Fe 2+) can be modeled by an ideal-solution-like model in which the activity of Ni 2+ or Fe 2+ is related to its concentration among a subset of other cations with which it mixes (primarily other divalent cations). This model is successful in predicting NiO activities over the range of compositions 0.3<X MgO + X CaO<0.55. Modeling contributions to the NiO activity from both a O 2− and a Ni 2+ accounts for virtually all of the variation in measured activity coefficients not arising from experimental uncertainty.

Petrography and oxygen isotopic compositions in refractory inclusions from CO chondrites

Fine-grained Ca-Al-rich inclusions (FGIs) in Yamato-81020 (CO3.0) and Kainsaz (CO3.1–CO3.2) chondrites have been studied by secondary ion mass spectrometry. The FGIs from Yamato-81020 consist of aggregates of hibonite, spinel, melilite, anorthite, diopside and olivine grains with no petrographic evidence of alteration. In contrast, the FGIs from Kainsaz commonly contain alteration products such as nepheline. From replacement textures and chemical compositions of altered and unaltered FGIs, we conclude that the alteration products formed by decomposition of melilite and anorthite. All phases in the Yamato-81020 FGIs are enriched in 16O, with δ 17, 18O = ∼−40‰ except for one FGI that experienced melting. Oxygen isotopic compositions of melilite, anorthite, some spinel and diopside in Kainsaz FGIs changed from δ 17, 18O = ∼−40‰ toward 0‰ by aqueous alteration. Alteration products in FGIs are depleted in 16O relative to primary phases, with δ 17, 18O = ∼0‰. These results show that FGIs in CO chondrites commonly had 16O-rich compositions in the solar nebula. The original 16O-rich FGIs were modified to 16O-poor compositions during aqueous alteration in the parent body.

Mineralogy of silicate inclusions of the Colomera IIE iron and crystallization of Cr-diopside and alkali feldspar from a partial melt

We studied the mineralogy, mineral chemistry, and compositions of 48 interior silicate inclusions and a large K-rich surface inclusion from the Colomera IIE iron meteorite. Common minerals in the interior silicate inclusions are Cr diopside and Na plagioclase (albite). They are often enclosed by or coexist with albitic glasses with excess silica and minor Fe-Mg components. This mineral assemblage is similar to the “andesitic” material found in the Caddo County IAB iron meteorite for which a partial melt origin has been proposed. The fairly uniform compositions of Cr diopside (Ca 44Mg 46Fe 10) and Na plagioclase (Or 2.5Ab 90.0An 7.5 to Or 3.5Ab 96.1An 0.4) in Colomera interior inclusions and the angular boundaries between minerals and metal suggest that diopside and plagioclase partially crystallized under near-equilibrium conditions from a common melt before emplacement into molten metal. The melt–crystal assemblage has been called “crystal mush.” The bulk compositions of the individual composite inclusions form an array between the most diopside-rich inclusion and plagioclase. This is consistent only with a simple mechanical mixing relationship, not a magmatic evolution series. We propose a model in which partly molten metal and crystal mush were mixed together by impact on the IIE parent body. Other models involving impact melting of the chondritic source material followed by growth of diopside and plagioclase do not easily explain near equilibrium growth of diopside and Na plagioclase, followed by rapid cooling. In the K-rich surface inclusion, K feldspar, orthopyroxene, and olivine were found together with diopside for the first time. K feldspar (sanidine, Or 92.7Ab 7.2An 0.1 to Or 87.3Ab 11.0An 1.7) occurs in an irregular veinlike region in contact with large orthopyroxene crystals of nearly uniform composition (Ca 1.3Mg 80.5Fe 17.8 to Ca 3.1Mg 78.1Fe 18.9) and intruding into a relict olivine with deformed-oval shape. Silica and subrounded Cr diopside are present within such K-feldspar regions. Some enrichments of the albite component have been detected at the end of curved elongated nodules of K feldspar intruded into the mafic silicates. The textural relationships suggest that a K-rich melt was present. A K-rich melt is neither the first melt of a chondritic system nor a differentiation product of a Na-rich partial melt of chondritic material. The K-rich material may have originated as a fluid phase that leached K from surrounding materials and segregated by a mechanism similar to that proposed for the Na-rich inclusions.

Magmatic crystallisation of Cr-Al diopside and Al-Fe3+ diopside from the ancient alkaline basalts (Mt. Etna, Sicily)

The petrography of basalts from Ancient Alkaline Centres of Mt. Etna (Sicily) shows several disequilibrium textures, particularly in pyroxene and plagioclase. As regards the former, this disequilibrium is shown by complex zoning: Cr-Al diopside (Di) composition for the core and Al-Fe 3+ Di for the rim, with different thicknesses. More rarely, homogeneous Al-Fe 3+ Di phenocrysts are found. MELT calculations indicate that the only pyroxene to be in equilibrium with the bulk rock composition is Al-Fe 3+ Di, in agreement with petrographic observations. A crystal-chemical study performed on zoned and unzoned phenocrystic pyroxene suggests similar conditions of crystallisation pressure, and a magmatic origin for both compositions. The clinopyroxene geobarometer of Nimis (1999) was applied and maximum values around 0.5 (± 0.2) GPa were obtained. A similar crystallisation pressure is suggested both by link-band deformation of olivine, and olivine-hosted CO 2 inclusions in one of the studied basalts. Excluding variations of crystallisation pressure as responsible for the disequilibrium between the two pyroxenes, this may be ascribed to time-related changes in the geochemical character of the melt. Clinopyroxene crystal chemistry, host-rock petrology, and fluid-inclusion data are also consistent with seismic data suggesting a complex reservoir system feeding Etnean activity at depths around 15–20 km.

Spectra of extremely reduced assemblages: Implications for Mercury

Abstract— We investigate the possibility that Mercury's crust is very reduced with FeO concentrations of less than ˜0.1 wt%. We believe that such a surface could have a composition of enstatite, plagioclase, diopside, and sulfide, similar to the mineral assemblages found in aubritic meteorites. To test this hypothesis, we investigated the spectra of aubrites and their constituent minerals as analogs for the surface of Mercury. We found that some sulfides have distinctive absorption features in their spectra shortwards of ˜0.6 μm that may be apparent in the spectrum of such an object. Determination of the surface composition of Mercury using orbital x‐ray spectroscopy should easily distinguish between a lunar highlands and enstatite basalt composition since these materials have significant differences in concentrations of Al, Mg, S, and Fe. The strongest argument against Mercury having an enstatite basalt composition is its extreme spectral redness. Significant reddening of the surface of an object (such as Mercury) is believed to require reduction of FeO to nanophase iron, thus requiring a few percent FeO in the material prior to alteration.

APPLICATION OF DYNAMIC NEUTRON IMAGING IN THE EARTH SCIENCES TO DETERMINE VISCOSITIES AND DENSITIES OF SILICATE MELTS

The use of dynamic neutron imaging in combination with the falling sphere technique to measure viscosities and densities of silicate melts at high temperatures and moderate pressures is explored. The experimental set up of this technique at the neutron radiography facility of the Orphe reactor in Saclay (France) is described. First falling sphere experiments with a melt of diopside (CaMgSi206) composition show that it is possible to determine viscosities and densities with this technique. Absorption measurements of a bulk rock show that the contrast between different mineral species is sufficient for their detection, This allows the observation of processes like partial melting, melt segregation and mixing in inhomogeneous melts, which can be monitored in real time through a video system.

Equilibrium density and expansivity of silicate melts in the glass transition range

Equilibrium volumes and expansivities of three liquids in the system anorthite (CaAl2Si2O8)–diopside (CaMgSi2O6) have been derived from dilatometric measurements of the equilibrium length of samples in the glass transition range. The typical temperature range of 40 K for the measurements is limited at low temperature by the very long times necessary to reach structural equilibrium and at high temperature by the penetration of the rod used to measure sample dilatation. Despite such narrow intervals, the expansivities are determined to better than 3% thanks to the high precision with which length changes are measured. The coefficient of volume thermal expansion (1/V dV/dT) of the fully relaxed liquid just above the glass transition is found to decrease linearly from diopside composition (139 ± 4 × 10−6 K−1) to anorthite composition (59 ± 2 × 10−6 K−1). These values are greater than those determined for the same liquids at superliquidus temperatures, demonstrating that expansivities of silicate melts may decrease markedly with increasing temperature. A predictive model based upon partial molar volumes which vary as a linear function of the logarithm of temperature is proposed.

Evaluation of diamond potential from the composition of peridotitic chromian diopside

Evaluation of diamond potential from the composition of peridotitic chromian diopside

Thermal and shock metamorphism of the Tenham chondrite: A TEM examination

During the early episode Of the solar system, the L6 chondrite Tenham has been affected by intense thermal metamorphism. Microanalytical data reveal homogeneous compositions Of Olivine (F O 75 Fa 25), enstatite (En 79Fs 19W O 2 ), and diopside (En 47Fs 8W O 45 ). Using these data, empirical pyroxene thermometers yield temperature estimates for this thermal metamorphism, ranging from 810 to 870°C. Due to the presence of thin shock veins, which contain the high-pressure phases majorite and ringwoodite, the L6 chondrite Tenham is an instructive example for strong shock metamorphism. In contrast to previous transmission electron microscopy (TEM) studies, which concentrated on these shock veins, we also systematically characterized the shock signature of the silicates occurring in the bulk Of Tenham. Plagioclase is either pervaded by thin (200 nm), amorphous lamellae, so-called planar deformation features (“PDFs”), or it is transformed to maskelynite, a diaplectic glass Of feldspar composition. In olivine, shock deformation has caused the formation of irregular and planar fractures and the activation of numerous (2 × 10 14 m −2) c dislocations in the glide planes (100) and {110}; energetically favorable but less mobile a dislocations are totally absent. Fracturing in olivine is interpreted as the cause of dislocation formation. A low dislocation density (<10 12 m −2) and clinoenstatite lamellae have been detected in orthoenstatite. Although other formation mechanisms are known for the ortho-/clino-enstatite inversion, a shock origin is most reasonable in this case because of the presence of strong shock damage in the other silicates. Diopside displays the greatest diversity of shock defects: mechanical twins parallel to (100) and (001), numerous dislocations, and PDFs. The predominant glide system of dislocations is (100)[001], but the {110}[001] glide system is also present to a lesser extent. To our knowledge, we report here on the first evidence of thin (≤50 nm), amorphous lamellae in naturally shocked diopside. These PDFs are oriented parallel to {221} and {221}. Fine-grained (<2–3 μm), polycrystalline aggregates of the high-pressure spinels, majorite and ringwoodite, were observed in a thin shock vein. Majorite is defect-free, whereas ringwoodite contains several stacking faults parallel to {110} planes. Microanalyses show that both phases are less homogeneous than olivines and pyroxenes. This and the small grain sizes suggest a rapid crystallization of majorite and ringwoodite from a high-pressure melt. The stacking faults in ringwoodite are, hence, interpreted as growth defects. The results of this study substantiate that the shock pressure in Tenham is heterogeneously distributed, ranging from approximately 25 to 45 GPa. The formation Of shock veins is not Only ascribed to pressure excursions, but mainly to the high shear stresses resulting from the different shock impedances of Tenham's mineral constituents.

Structural environment of nickel in silicate glass/melt systems: Part 1. Spectroscopic determination of coordination states

Nickel environments in silicate and aluminosilicate glasses have been investigated using X-ray Absorption Near Edge Structure (XANES) spectroscopy, Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy, and optical absorption spectroscopy. Glass compositions included nickel end-members such as Ni-diopside (CaNiSi 2O 6) and Ni-alkali silicates (Na 2NiSi 3O 8 and K 2NiSi 3O 8) as well as glasses with compositions of diopside, anorthite, potassium trisilicate, and sodium trisilicate and disilicate containing 1–3 wt% NiO. Optical spectroscopy, XANES, and EXAFS show that nickel occurs as [4]Ni and [51]Ni in all the glasses investigated, [4]Ni being predominant in potassic glasses as [51]Ni prevails in glasses with higher field strength cations (Na, Ca, Mg). [4]Ni occurs in a distorted tetrahedral site and [5]Ni in a slightly distorted trigonal bipyramid. The geometry of [4]Ni and [5]Ni sites does not vary much among the glasses investigated, as shown by the constant position of [4]Ni and [5]Ni optical absorption bands. XANES parameters are intermediate between those expected for [4]Ni and [5]Ni. EXAFS-derived mean Ni-O distances and number of oxygen neighbors are 1.96–1.99 Å and 3.8–4.7 (±0.5), respectively, depending on glass composition. They also suggest a mixing of [4]Ni and [5]Ni with [4]Ni-O = 1.95 A ̊ and [5]Ni-O = 2.00 A ̊ , respectively. Including the data of the literature, spectroscopic data on Ni are independent on the total Ni content of the glass investigated (0.1–27 wt% NiO). However, the relative proportion of the two Ni coordination states depends upon glass composition. Silicon second neighbors were determined by EXAFS in all glasses at Ni-Si distances of 3.2–3.3 Å, depending on glass composition, but the number of Si neighbors is higher in the potassic glasses. Finally, there is no indication of significant amounts of [6]Ni in silicate glasses, which is consistent with the structural data and models on the sites occupied in silicate glasses by other small divalent cations, such as Mg and divalent transition metal cations.

Crystallization characteristics of iron-containing spodumene-diopside glasses

Abstract The crystallization characteristics of glasses based on a stoichiometric spodumene (LiAlSi2O6)-diopside (CaMgSi2O6) composition with addition of Fe2O3 replacing Al2O3 are described. The effects of compositional variation due to the Al2O3/Fe2O3 replacement and thermal treatment on the nature, type and stability fields of crystallizing phases as well as the resulting microstructure are traced by DTA, X-ray diffraction (XRD) and scanning electron microscopy (SEM). In most cases, intense uniform bulk crystallization with fine-grained microstructure were achieved on increase of iron oxide at the expense of alumina. The crystallizing phases, β-eucryptite solid solution (ss), β-spodumene ss, varieties of pyroxene solid solution of diopside, hedenbergite and augite nature are mostly formed together, in some cases, with α-quartz, lithium silicates, lithium ferrate (LiFeO2) and iron oxide. The type, stability and compatibility of the crystallized phases are discussed in relation to the compositional variation of the glass and thermal treatment.

Dielectric constants of crystalline and amorphous spodumene, anorthite and diopside and the oxide additivity rule

The dielectric constants and dissipation factors of LiAlSi_2O_6, CaAl_2Si2O_8 and CaMgSi_2O_6 in both the crystalline (α-spodumene, anorthite, and diopside) and amorphous forms were determined at 1 MHz using a two-terminal method and empirically determined edge corrections. The results are: spodumene κ'_(11)=7.30 tan δ= 0.0007 κ′_(22)=8.463 tan δ= 0.0002 κ′_(33) =11.12 tan δ= 0.0007 anorthite κ′_a^*=5.47 tan δ= 0.0009 κ′_b^*=8.76 tan δ= 0.0010 κ′_c^*=7.19 tan δ= 0.0013 diopside κ′_(11)=9.69 tan δ= 0.0016 κ′_(22) = 7.31 tan δ= 0.0007 κ′_(33)=7.29 tan δ= 0.00019 LiAlSi_2O_6 κ′=8.07 tan δ= 0.047 amorphous CaAl_2Si_2O_8 κ′=7.50 tan δ= 0.0024 amorphous CaMgSi_2O_6 κ′=8.89 tan δ= 0.0021 amorphous The dielectric properties of a spodumene glass, progressively crystallized at different conditions, were also determined. As the crystallization temperature was increased from 720 to 920° C, κ′ increased from 6.22 to 6.44. The dissipation factor, tan δ, remained constant at 0.020. Similarly, as the crystallization time at 750° C increased from 0.5 hr to 6.0 hr, κ′ increased from 6.28 to 6.35. The deviations of the measured dielectric polarizabilities as determined from the Clausius-Mosotti equation from those calculated from the sum of oxide polarizabilities according to α_D(mineral, glass) = σ α D(oxides) are +7.4% for α-spodumene, +1.2% for diopside, and +28.0, +19.6 and +15.9% for amorphous spodumene, anorthitie and diopside compositions, respectively. Positive deviations in α-spodumene and anorthite are consistent with lower than normal apparent cation bond valence sums and are believed to be evidence for loosely bonded “rattling” Li and Ca ions. Diopside, with Ca and Mg ions having normal bond valence sums, exhibits no abnormal deviation from additivity. Larger positive deviations in amorphous SiO_2, LiAlSi_2O_6, CaAl_2Si_2O_8 and CaMgSi_2O_6 are postulated to arise from a combination of loosely bonded cations and disordered O= ions where the oxygen dielectric polarizability increased from its normal value of 2.0 A^3 in well-behaved oxides to 2.2–3.0 A_3 in the amorphous phases.

Open system melting and temporal and spatial variation of peridotite and basalt at the Atlantis II Fracture Zone

In situ ion microprobe analysis of trace and rare earth elements in discrete diopsides in abyssal peridotites from nine transform dredge hauls from the Atlantis II Fracture Zone (All FZ) shows that these samples have a wide range of trace element contents close to the total range found for the entire Southwest Indian Ridge. Though the spread in analyses is large, the average composition of the peridotites is close to that reported for the All FZ by Johnson et al. (1990) and lies at the relatively undepleted end of the spectrum for SW Indian Ridge residual mantle peridotites. A sharp break in peridotite diopside composition and modal mineralogy occurs across the transform, suggesting that it acts as a boundary for different melting regimes and initial mantle compositions. The difference in peridotite compositions is mirrored in spatially associated basalts, which lie on separate parallel liquidus trends in the normative ternary pyroxene‐olivine‐plagioclase. Basalts from the east side of the transform have higher normative plagioclase contents, indicating that they may be products of lower degrees of mantle melting than basalts from the western side, consistent with greater depletion of peridotites from the western wall or a more depleted initial composition. Basalts from the eastern wall also have consistently lower Fe8.0 and higher Na8.0 than basalts from the western wall and lie parallel to the global along‐ridge Fe8.0 − Na8.0 trend (Klein and Langmuir, 1987) and orthogonal to the local melting paths of Klein and Langmuir (1989). Our data provide strong evidence for segmentation of the melting regime, with major mantle discontinuities occurring at transform offsets at slow spreading ridges. Peridotites analyzed along the eastern wall of the fracture zone also show a systematic change in composition with latitude and, with the older peridotites from the median tectonic ridge, define a systematic change in the degree of melting of the mantle occurring beneath the paleoridge axis over the last 11 m.y. Emplacement of mantle showing the lowest degree of melting, or the least depleted parental mantle composition, corresponds roughly to the time of crystallization of Ocean Drilling Program site 735B gabbros. Melting is modeled as a non‐steady state, discontinuous process with 0.1–0.5 vol % aggregated melt retained in the porous residue (open system melting). The range in degree of open system melting for the combined suite of All FZ peridotites is 8–20%. Such a large systematic variation would appear to require a dynamically significant change with time, either in the initial temperature and/or a large compositional difference of the mantle beneath the paleoridge axis. This in turn suggests that in the relative reference frame of the ridge axis, mantle flow was non‐steady state. This could reflect episodic mantle diapirism beneath the ridge axis or, alternatively, that the ridge axis has moved over a zone of enhanced upflow in the underlying mantle that was fixed in the absolute hotspot mantle reference frame.

Non-linear temperature dependence of liquid volumes in the system albite-anorthite-diopside

The temperature-dependent thermal expansivities of glasses and liquids in the ternary albite-anorthite-diopside have been determined using a combination of calorimetry, dilatometry and Pt and Ir double bob Archimedean densitometry. Supercooled liquid volumes and molar thermal expansivities were determined across the glass transition using a combination of scanning calorimetry and dilatometry, based upon the equivalence of relaxation of volume and enthalpy in the vicinity of the glass transition. Superliquidus volumes were determined using double Pt bob Archimedean densitometry at temperatures up to 1,650°C and double Ir bob densitometry at 1,800°C. Experimental access to liquid volumes near the glass transition temperatures (680–920°C) and at superliquidus temperatures (1,400–1,800°C) for these compositions results in the observation of a nonlinear temperature dependence of molar volume, i.e., temperature-dependent thermal expansivities. The diopside composition wxhibits the largest temperature dependence of thermal expansivity, decreasing by ∼50% between 800 and 1,500°C. Linear extrapolation of the high-temperature volume data of diopside to 810°C would result in a 3% overestimation of the molar voltime. The temperature dependence of the molar volume of anorthite is approximately linear. The thermal expansivities of the liquids in the albite-anorthite-diopside system appear to converge at high temperature. This study uses a combination of methods that allows interpolation rather than extrapolation of the extant melt-volume data into the petrologically meaningful (subliquidus) temperature range.

Temperature-dependent thermal expansivities of silicate melts: The system anorthite-diopside

The temperature-dependent thermal expansivities of melts along the join anorthite-diopside have been determined on glassy and liquid samples using a combination of calorimetry, dilatometry, and Pt double bob Archimedean densitometry. Supercooled liquid volumes and molar thermal expansivities were determined using scanning calorimetric and dilatometric measurements of properties in the glass region and their behavior at the glass transition. The extraction of low-temperature liquid molar expansivities from dilatometry /calorimetry is based on an assumed equivalence of the relaxation of volume and enthalpy at the glass transition using a method developed and tested by Webb et al. (1992). This method corrects for transient effects at the glass transition which can lead to serious overestimates of liquid thermal expansivity from “peak” values. Superliquidus volumes were determined using double Pt bob Archimedean densitometry at temperatures up to 1650°C. The resulting data for liquid volumes near glass transition temperatures (810–920°C) and at superliquidus temperatures (1400–1650°C) are combined to yield thermal expansivities over the entire supercooled and stable liquid range. The molar expansivities are, in general, temperature dependent. The temperature-dependence of thermal expansivity increases from anorthite to diopside composition. The thermal expansivity of anorthite is essentially temperature independent, whereas that of diopside decreases by ≅ 50% between 800 and 1500°C, with the consequence that the thermal expansivities of the liquids in the anorthite-diopside system converge at high temperature.

Structural environment around Th 4+ in silicate glasses: Implications for the geochemistry of incompatible Me 4+ elements

The structural environment around Th 4+ in several silicate glasses containing 1–3 wt% Th 4+ was investigated as a function of melt composition and polymerization using Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. One set of glasses had the bulk chemical composition of monticellite (MO), diopside (DI), anorthite (AN), albite (AL), olivine basalt (O-BAS), tholeiitic basalt (T-BAS), calc-alkaline dacite (DAC), and calc-alkaline rhyolite (RHY). A second set of Th-bearing glasses had the bulk AL composition but also contained either 5.5 wt% F or 2000 ppm Cl. In glasses containing 3 wt% Th, Th-coordination varies from 8 ( d[ viiiTh-O]≈ 2.41 ± 0.02 Å in MO and DI) to a mixture of 8 and 6 coordinations ( d[ vi+viiiTh-O]≈ 2.35−2.36 ± 0.02 Å in all other compositions). The presence of viiiTh is related to the thorianite (ThO 2)-saturation in the glass. In AL glasses containing 1 wt% Th, 6-fold coordinated Th dominates ( d[ viTh-O]≈ 2.32 ± 0.02 Å). No clear evidence for F or Cl complexes around Th is observed in any of the halogen-containing glasses. Weak but significant interactions of Th with tetrahedral {Si, Al} network and/or alkali elements are observed, especially in highly polymerized compositions (RHY), in which Th-{Si, AI, Na, K} contributions are detected between 3.25 and 3.50 ± 0.05 A ̊ . These results suggest that, in this set of glasses, Th does not form “complexes” but rather acts as a weak network modifier. The presence of viTh in silicate glasses, an unusual low coordination for Th, suggests that the ionic radius of Th in magmatic systems is lower than previously reported ( r Th = 0.94 A ̊ ). The resulting high Th-O bond strength (≈0.7 vu), unknown in minerals, may explain the observed low crystal-melt partition coefficients of Th 4+, i.e., its incompatible character during magmatic differentiation. Increasing polymerization is predicted to favor higher coordination around Th (VIII, IX) as a result of a shortage of NBO to which Th preferentially bonds. Hence, dramatic changes in the crystal chemistry of Th are predicted, i.e., the incorporation of Th in minerals. Comparison with U 4+ suggests that viTh is less abundant in similar glass/melt systems, compared with viU. This may explain the U/Th fractionation often observed in zircon, as well as the preferential incorporation of Th in accessory minerals containing large sites in which Th 4+ can be substituted (e.g., the Ca 2+ site in titanite, apatite, or zirkelite).

Phase relations in the transition zone

Phase relations in the systems MgO‐Al2O3‐SiO2 and CaO‐MgO‐SiO2 have been investigated experimentally at 166–226 kbar pressures and 1350°–2500°C temperatures with a split sphere anvil apparatus (USSA 2000). MgSiO3 ilmenite is stable to lower pressures (175 kbar) than found previously. CaSiO3 perovskite forms at P(bar)=15T(°C)+151,000. Three new high‐pressure phases are reported: a phase with the diopsidic composition (the CM phase), a high‐pressure polymorph of Al2O3, and superphase B with the tentative composition Mg10Si3O14 (OH, F)4. The invariant point for coexisting majorite (Mg4Si4O12), perovskite (MgSiO3), beta phase (Mg2SiO4), and liquid has been located at 224 kbar, 2430°C, with the eutectic melt composition on the enstatite‐forsterite join at 40–44 wt% of forsterite. The melting temperature of MgSiO3 perovskite at 225 kbar is predicted to be 2600°C. The new data made possible the calculation of a temperature‐pressure phase diagram for the system CaO‐MgO‐Al2O3‐SiO2 (CMAS) in the pressure range 0–280 kbar. The mineralogy of the Earth's mantle and the possible mechanisms for producing large seismic velocity gradients in the transition zone are discussed on the basis of the derived phase relations.

Determination of trace element mineral/liquid partition coefficients in melilite and diopside by ion and electron microprobe techniques

The use of the ion microprobe for quantitative analysis of Sr, Y, Zr, La, Sm, and Yb in melilite and pyroxene is evaluated. Three trace element-doped synthetic glasses of composition Ak 40, Ak 80, and Di 2AbAn were analyzed by ion microprobe (IMP) using ion yields determined from Corning glass standards. IMP-determined oxide concentrations in the Di 2AbAn glass agree well with electron microprobe (EMP) analyses (to within 6%), but IMP analyses of the melilite glasses deviate from EMP averages by up to 19%. The deviations are due to erroneous SiO 2 estimates caused by suppression of Si ion intensities by the enhanced concentrations of Ca and Al in the melilite glasses compared to the standards. Thus, in order to determine compositions of melilite, diopside, and glass from subliquidus experiments on each of the three starting compositions, we adopted a new set of ion yields such that IMP analyses of the three starting glasses reproduce the EMP average compositions. Further IMP and EMP comparisons of the subliquidus assemblages show that quantitative analyses of melilite, diopside, and glass can be obtained by IMP that are within 10% of the concentrations obtained by EMP, when ion yields determined from glass starting compositions are used. EMP-IMP comparison of crystal and glass analyses also suggests that a structural matrix effect may result in overestimation of SrO (10–12%) in melilite by IMP. Comparison of our data for Ak 12 and Ak 90 melilite compositions with literature results shows that melilite/liquid D- values for REE 3+ determined by IMP decrease with increasing X AK ( Ak 90: D la = 0.038, D Sm = 0.032, D Yb = 0.0086 ; Ak 12: 0.67, 0.75, 0.25 , respectively) while that for Sr (=Eu 2+) changes only slightly (0.99 to 0.78, respectively). Since X Ak increases with decreasing temperature for all melilite with X Ak < 0.7 , a progressively larger positive Eu anomaly is predicted for melilite as it crystallizes with falling temperature. Our diopside/liquid data are characterized by a large degree of scatter on most interelement correlation plots of apparent partition coefficients. The data cannot be understood in terms of simple models of boundary layer formation but require a complex surface partitioning explanation. Nevertheless, estimates of diopside/liquid D- values are in excellent agreement with literature data.

Sample preparation and aging effects on the dissolution rate and surface composition of diopside

This study shows that the initial dissolution rate of ground diopside powders depends on sample preparation history, in general agreement with the findings of previous studies. However, our attempts to reproduce our own dissolution rate experiments were unsuccessful. Instead, we have found that the initial dissolution rate of all diopside powders used in this study decrease with the time elapsed since the sample was ground. This aging effect, measured from samples stored in air and at room temperature, was followed over a period of several months. These results suggest that diopside surfaces created by grinding gradually relax toward a less-reactive state under these conditions. In addition, aging also affects the surface composition (measured using X-ray photoelectron spectroscopy) of the pretreated ground diopside powders. Surface reaction controlled dissolution of silicate minerals is a complicated function of variables both internal and external to the dissolving crystal. The internal variables ( e.g. dislocations, structural distortion, microcracks, surface composition, etc.) are affected by mechanical or chemical pretreatments. Various mechanisms by which pretreatments and aging may affect these variables and thereby affect initial dissolution rates and surface composition are discussed.

Surface tension of melts in the system CaMgSi2O6-CaAl2Si2O8 and its structural significance

The surface tension between silicate melts and air has been measured for melt compositions lying on the diopside-anorthite (Di-An) join from 1300° C to 1580° C. It ranges from 300 dyne/cm to 400 dyne/cm, and decreases with increasing temperature, except for a pure diopside composition. At relatively high temperatures, the surface tension decreases as the anorthite content increases, whereas at lower temperature it is almost constant. These results suggest that diopside melt has a more discrete structure at higher temperatures, whereas, anorthite-bearing melts do not dissociate in the temperature range studied. They also suggest that the structure of both the surface and interior parts of the melt are almost identical at lower temperatures, but at higher temperatures, the surface part has a more polymerized structure with Al2O3 enrichment. The surface energy, obtained from the relationship between surface tension and temperature, increases from 294 erg/cm2 (Di composition) to 1013 erg/cm2 (Di40An60) with increasing anorthite content.