Ca-rich Clinopyroxene Research Articles

Two discrete stages of fenitization in the Lizhuang REE deposit, SW China: Implications for REE mineralization

Fenitization is an alkali metasomatism that widely occurs in and around the alkaline-carbonatite complex, and is closely associated with rare earth elements (REE) mineralization. However, the origin and nature of the fenitizing fluids and its role in REE mineralization are poor understood. The Lizhuang REE deposit in the Panxi region, SW China, is hosted in a syenite-carbonatite complex with low- and high-grade fenitization. In this study, we conducted detailed petrographic, compositional and isotopic studies on the fresh/unaltered syenite and low- and high-grade fenites to reveal the origin and nature of the fenitizing fluids and their effects on REE mineralization. Our results show that the whole-rock Na/K ratios gradually increase from the fresh syenite, low-grade fenites to high-grade fenites, as well as mineral replacements, such as K-feldspar is replaced by albite and Ca-rich clinopyroxene is replaced by Na-rich clinopyroxene, indicating that sodic fenitization occurred in the Lizhuang syenite system. In situ U-Pb dating of the unaltered and altered apatite shows that the hydrothermal alteration processes are nearly simultaneous with the emplacement of syenite and carbonatite during 27.4–25.3 Ma, maybe lasting for ~2 Ma. In addition, the altered regions of feldspar, clinopyroxene and apatite have in situ 87Sr/86Sr ratios ranging from 0.70580 to 0.70674, which is consistent in error with those of the unaltered regions and host rock syenite-carbonatite complex (0.70566–0.70666). It is suggested that the fenitizing fluids were exsolved from the syenite-carbonatite complex in a relatively closed system. As the increase of alteration degree, the proportion of hydrothermal minerals (e.g., barite, calcite, fluorite and bastnäsite) in fenite increased gradually, indicating that the fenitizing fluids are enriched in SO42−, CO32− and F−. This is also supported by the elevated F and SO3 contents from the unaltered to altered apatite. The increasing REE concentrations, from the fresh syenite, low-grade fenite to high-grade fenite, indicate that REE minerals such as bastnäsite were precipitated from the fenitizing fluids during fenitization processes. In-situ analyses on the altered clinopyroxene and apatite show variable REE losses compared to the unaltered precursors, which indicates the successive remobilization and transferring of REE by fenitization fluids. Therefore, two discrete stages of fenitization have been identified in the Lizhuang REE deposit, resulted from fluids exsolved from the syenite and carbonatite, respectively. Meanwhile, such fenitization fluids are effective extracting agents of REE in post-magmatic stage, which is crucial to REE remobilization and precipitation to form economic deposit.

The interior and the fusion crust in Sariçiçek howardite: Study using X-ray diffraction, magnetization measurements and Mössbauer spectroscopy.

The meteorite Sariçiçek, a 2015 howardite fall in Turkey, was analyzed using various physical techniques. Both the interior and the fusion crust were studied by optical and scanning electron microscopy with energy dispersive spectroscopy, X-ray diffraction, magnetization measurements and Mössbauer spectroscopy. The main and minor iron-bearing phases such as orthopyroxene, Ca-poor and Ca-rich clinopyroxene, chromite with hercynite, Fe2+ and Fe3+ ilmenite, troilite, α-Fe(Ni, Co), α2-Fe(Ni, Co) and γ-Fe(Ni, Co) phases were identified. The ratios of Fe2+ occupancies in the M1 and M2 sites in the silicate phases as well as the equilibrium Fe2+ and Mg2+ cations distribution temperatures (Teq) for orthopyroxene were estimated using X-ray diffraction and Mössbauer spectroscopy, which appeared to be in a good agreement: for example, Teq were 886 and 878K, respectively. The glass-like fusion crust of Sariçiçek consists of orthopyroxene with ferrous and ferric compounds that are likely products of combustion and melting.

Near-infrared spectroscopy of Ca-rich clinopyroxenes revisited: A new interpretation of anomalous type-B spectra and implications for remote sensing of inner solar system bodies

Four Ca-rich clinopyroxene samples (3 terrestrial clinopyroxenes and a druse clinopyroxene from the D’Orbigny meteorite) have been studied by near-infrared (NIR) spectroscopy and X-ray structure refinement in order to address the issue of anomalous type-B NIR spectra seen in some terrestrial and extraterrestrial Ca-rich clinopyroxenes. The D’Orbigny druse clinopyroxene shows a typical type-B NIR spectrum, with two absorption bands located at ~ 1.1 and 2.3 μm. Based on previous Mössbauer and X-ray structure refinement results on synthetic clinopyroxenes, the 1.1 μm band is assigned to Fe2+ at the M1 site in diopside and high-pigeonite (i.e. pigeonite with C2/c symmetry), and the 2.3 μm band to Fe2+ at the M2 site in high-pigeonite. A comparison of NIR spectral type and X-ray structure refinement of the studied samples indicates that the average structure refinement of all type-B clinopyroxenes show a residual electron density peak in the difference-Fourier synthesis, corresponding to the split M2’ site which is mainly occupied by Fe2+. The results present a new interpretation for type-B NIR spectra of Ca-rich clinopyroxenes.

Paleoproterozoic ($$\sim$$1.88–1.89 Ga) ultramafic–mafic sills, Cuddapah basin, India—revisited: Implications for interaction between mantle plume and metasomatized subcontinental lithospheric mantle

A number of mafic–ultramafic sills, which are supposed to be part of the widespread ~1.88–1.89 Ga Hampi–Bastanar Large Igneous Province of the Indian shield, are reported to intrude into sedimentary/meta-sedimentary rocks of the Proterozoic Cuddapah basin. Mineral chemistry of chrome-spinel, olivine and pyroxene from the cumulate ultramafic rocks of the Pulivendla sills, emplaced at the base of Tadpatri Formation of the Cuddpaha basin, is presented for better understanding on their nature and genesis. The intermediate Cr#, low Mg#, high Fe3+# and Ti (apfu) coupled with relatively low Al (apfu) content of studied spinel indicate their Alaskan-type nature. Moreover, the Ca-rich clinopyroxene together with classic variations of Al2O3, TiO2 and SiO2 follow the trend of arc type of intrusions. The calculated Al2O3 content of parental melt of the studied rocks also suggests arc-type characteristics. Whole-rock geochemistry show fractionated chondrite-normalized REE patterns, higher Ba/Nb and low Nb/La that suggest contribution from an enriched mantle source, whereas higher Th/Yb and negative Ta–Nb–Ti anomaly on the primitive mantle-normalized multi-element spidergrams emphasize the involvement of a subducted component in the lithospheric mantle source. Although, the mineral chemistry and geochemistry is akin to Alaskan-type intrusion, lack of concentric zoning of lithologies (olivine-rich ultramafic rocks in centre surrounded by mafic rocks), rarity of primary hornblende and abundance of orthopyroxene contradict it to be an Alaskan-type intrusion. It is suggested that the mantle source region of the Pulivendla ultramafic–mafic rocks was modified by a fluid-induced metasomatism during an ancient subduction event. Further, Al-in-olivine thermometry suggests crystallization temperature between 1410° and 1484°C, which is 260°C higher than the average temperature of MORB. Similarly, the estimated mantle potential temperature is also significantly higher (>1600°C) than the different secular cooling models of the earth at ~1.88–1.89 Ga and consistent with the thermal regime of a mantle plume. The existence of mantle plume during 1.88–1.90 Ga, which has played an important role in the genesis of mafic–ultramafic sills of the Cuddapah basin, is also well supported by a radiating mafic dyke swarm, domal uplift and magmatic underplating.

Comparison of the 57Fe hyperfine interactions in silicate phases in Sariçiçek howardite and some ordinary chondrites

Silicate crystals have different thermal history in non-differentiated and differentiated meteorites. This leads to some differences in the Fe2+ and Mg2+ distribution between the M1 and M2 sites in olivine, orthopyroxene and clinopyroxene crystals in stony meteorites resulting in small variations in the Fe local microenvironment. For this reason, a comparison of Mossbauer hyperfine parameters for the 57Fe in the M1 and M2 sites in orthopyroxene and Ca-rich clinopyroxene for non-differentiated NWA 6286 LL6, NWA 7857 LL6 and Tsarev L5 ordinary chondrites and differentiated Saricicek howardite was carried out. The results obtained demonstrated small variations in quadrupole splitting and isomer shift for the studied non-differentiated and differentiated stony meteorites.

Study of metallic Fe-Ni-Co alloy and stony part isolated from Seymchan meteorite using X-ray diffraction, magnetization measurement and Mössbauer spectroscopy

A detailed study of Fe-Ni-Co alloy and stony part extracted from Seymchan (main group pallasite) meteorite fragment using X-ray diffraction, magnetization measurements, and Mössbauer spectroscopy with a high velocity resolution was carried out. A complex phase composition in Fe-Ni-Co alloy was observed. The Mössbauer spectrum of this alloy was fitted using eight magnetic sextets which were related to α2-Fe(Ni, Co), α-Fe(Ni, Co) and γ-Fe(Ni, Co) phases. The stony part consists of the following iron-bearing phases: olivine, troilite, Ca-rich clinopyroxene, chromite and probably iron-magnesium chromite. Estimations of the Fe2+ partitioning between the M1 and M2 sites in olivine and Ca-rich clinopyroxene were based on X-ray diffraction and Mössbauer data. The distribution coefficients KD and the temperatures of Fe2+ and Mg2+ cations equilibrium distribution Teq in olivine were evaluated using X-ray diffraction data and Mössbauer parameters.

An analysis of orthopyroxene from Tsarev L5 meteorite using X-ray diffraction, magnetization measurement and Mössbauer spectroscopy

The bulk Tsarev L5 ordinary chondrite powdered matter and chemically extracted orthopyroxene powder were studied using X-ray diffraction, magnetization measurement and Mössbauer spectroscopy. The unit cell parameters for orthopyroxene in the bulk material and in the extracted powder were the same. Magnetization data also showed similar behavior. However, X-ray diffraction and Mössbauer spectroscopy revealed residual iron-bearing phases in the extracted powder such as Ca-rich clinopyroxene, chromite and hercynite which were observed in the bulk matter. Additionally, the minor ferrous and ferric components were found in the chemically extracted orthopyroxene powder.

An Experimental Study of Chlorine Incorporation In Amphibole Synthesized Along the Pargasite–Ferro-Pargasite Join

Abstract Understanding how chlorine partitions between calcium amphiboles and an ambient fluid can be used to deduce the chlorine content of fluids involved in a wide range of geological processes, such as in high-grade metamorphism, deposition of economic minerals, chlorine metasomatism, and seawater-oceanic crust interactions. One of the most common correlations that is observed in calcium amphiboles is the increase in chlorine concentration with increasing Fe 2+ /(Fe 2+ + Mg) (= Fe#). To quantify this dependence, amphiboles with intended compositions along the pargasite–ferro-pargasite [NaCa 2 (Mg 4 Al)(Al 2 Si 6 )O 22 (Cl,OH) 2 –NaCa 2 (Fe 4 Al)(Al 2 Si 6 )O 22 (Cl,OH) 2 ] join were synthesized in the presence of NaCl brines ranging in concentration from 1 molal ( m ) NaCl to halite saturation. Syntheses were made over the range 700–950 °C and 0.2–0.5 GPa at oxygen fugacities ( f O 2 ) near the Co-CoO buffer and for durations of 46–576 h. The observed amphibole compositions varied between pargasite and hastingsite with increasing Fe# for an assumed constant Fe 3+ /(Fe 2+ + Fe 3+ ) ratio of 0.18. The most common additional phases were plagioclase and Ca-rich clinopyroxene with minor davyne in some syntheses. At any given bulk composition, amphiboles increase in Cl content with increasing brine concentrations, reaching a plateau in brines of about 0.1–0.2 mole fraction of NaCl ( X NaCl , or 5–15 m NaCl). Amphiboles synthesized in brines below halite saturation showed a fairly linear increase in Cl content from essentially 0.0 to 0.11 atoms per formula unit ( apfu ) with increasing Fe content. Syntheses in brines above halite saturation showed a distinct jump in Cl contents up to 0.14–0.30 apfu for the two most Fe-rich bulk compositions. Isopleths of constant NaCl concentration have been fitted to all of the data, and a thermochemical analysis of the dependence of the Cl/OH ratio of amphibole to the activity ratio of H 2 O/HCl in the brine has been offered for amphiboles synthesized in the 1 m NaCl brines. Comparison of the Cl contents of synthetic amphiboles made here with natural pargasitic amphiboles shows that even the highest Cl contents observed here are only about 20% of the maximum Cl contents found in amphiboles from some localities. The main implication of this study is that there is very limited Cl uptake in amphiboles made in this chemical system in the presence of aqueous brines of highly variable concentrations, even for the most Fe-rich amphiboles. The high Cl contents observed in natural assemblages are likely controlled by brine, or perhaps halide or silicate melt chemistry, or crystal-chemical factors ( e.g ., K content) that exert a stronger effect than just the Fe# of the amphibole.

An experimental study on the influence of fluorine and chlorine on phase relations in peralkaline phonolitic melts

Fluorine and chlorine affect phase stabilities in magmatic rocks. We present phase equilibrium experiments investigating a peralkaline and iron-rich phonolitic composition with variable F and Cl contents. The starting composition represents a dyke rock, which is a possible parental melt to the peralkaline Ilimaussaq plutonic complex (South Greenland). Experiments were performed at 100 MPa, 1,000–650 °C and low oxygen fugacity adjusted with graphite-lined gold capsules in an internally heated argon pressure vessel and rapid quench cold seal pressure vessels. To cover this large T interval, we applied a two-step fractional crystallization strategy where glasses representing residual melt compositions at 800 °C were synthesized as starting material for consecutive experiments at lower T. In these experiments, oxidized starting glasses allocate oxygen by reduction of ferric iron and up to 1.2 wt% dissolved OH form through reaction with hydrogen provided by the pressure medium (H2O) in initially dry experiments. For OH determination, hydrated super-liquidus experiments in Au capsules were performed to calibrate the extinction coefficient for the fundamental OH stretching vibration using infrared spectroscopy (e 3,415 = 48 ± 3 L mol−1 cm−1). Observed mineral phases in our experiments are titanomagnetite, fayalitic olivine, clinopyroxene, aenigmatite (Na2Fe5TiSi6O20), alkali feldspar and nepheline (±native iron) coexisting with residual melt. Above 1.5 wt% Fmelt concentrations, fluorite (CaF2) and hiortdahlite (Ca6Zr2Si4O16F4) are stable in favor of Ca-rich clinopyroxene. Sodalite (Na8Al6Si6O24Cl2) and eudialyte (Na15Ca6Fe3Zr3Si26O73(OH)3Cl2) form at Clmelt concentrations of 0.2–0.5 wt% (depending on T) and ZrO2 melt concentrations >0.7 wt% are additionally needed to stabilize eudialyte and hiortdahlite. Therefore, F and Cl may become compatible in such systems and have the potential to influence F/Cl melt ratios in evolving magmas.

Local structure in C2/c clinopyroxenes on the hedenbergite (CaFeSi2O6)-ferrosilite (Fe2Si2O6) join: A new interpretation for the Mossbauer spectra of Ca-rich C2/c clinopyroxenes and implications for pyroxene exsolution

Three synthetic Ca-rich clinopyroxenes on the hedenbergite-ferrosilite join, with compositions (Hd 50 Fs 50 ), (Hd 60 Fs 40 ), and (Hd 70 Fs 30 ), were investigated by single-crystal X-ray diffraction and Mossbauer spectroscopy at room temperature. For all samples, there is no evidence for the presence of phases with symmetry other than C 2/ c . Structure refinement, using a split-atom model, shows the existence of two different local configurations in the M2 cavity, a hedenbergite-like arrangement and a C 2/ c clinoferrosilite-like arrangement that are centered by Ca and Fe 2+ , respectively. The Mossbauer spectra were analyzed by a quadrupole splitting distribution (QSD) method, and the hyperfine parameters of the QSD fit indicate the presence of Fe 2+ in three local environments that are similar to those of Fe 2+ at the M1 site in hedenbergite, and the M1 and M2 sites in C 2/ c clinoferrosilite, in agreement with the structure-refinement results. Our results are consistent with the calculated phase diagram for hedenbergite-ferrosilite, where a miscibility gap and a two-phase field of metastable hedenbergite and metastable pigeonite occur at low temperature. The coexistence of a hedenbergite-like configuration around Ca and a C 2/ c clinoferrosilite-like configuration around Fe can be considered as a precursor for pyroxene exsolution at low temperature. Our results support previous structure-refinement work on Ca-rich clinopyroxenes along the diopside-enstatite join, and provide a new insight in the interpretation of Mossbauer spectra of Ca-rich C 2/ c clinopyroxenes.

Columbia Hills — An exhumed layered igneous intrusion on Mars?

The rover Spirit has now performed a series of whole-rock chemical analyses on rock outcrops and boulders along its traverse across the Columbia Hills of Gusev crater. The wide-spread consensus that the Columbia Hills represent a mafic volcanic pyroclastic sequence has problems, however, explaining the compositional variation observed within the Columbia Hills analyses. The positive trend in Fe–Mg space exhibited by the rocks of the Columbia Hills contrasts markedly with volcanic trends controlled by crystal fractionation and no plausible crystal fractionation scheme can explain the co-variation of Fe and Mg, if the analyzed rocks are assumed to represent volcanic compositions defining a liquid line of descent. Positive arrays in Fe–Mg space are, however, common features in whole-rock data collected across cyclically-layered igneous intrusions. The repeated alteration of feldspar-rich versus mafic-rich cumulates in such intrusions results in whole-rock compositions that scatter along mixing lines between feldspar, near the origin, and the mafic cumulate phases, reflecting the magmatic sorting of cumulus crystals as opposed to crystal–liquid equilibria. There is a striking correspondence between the Fe–Mg trends observed in the Columbia Hills data and the cumulate stratigraphy of a number of terrestrial layered intrusions. This raises the possibility that stratification seen at many localities along Spirit's traverse may reflect magmatic sedimentation in a large layered igneous intrusion. According to this interpretation, the most magnesium rocks are harzburgite to olivine norite cumulates without cumulus magnetite. The majority of the rocks of the Columbia Hills, however, appear to be olivine norites to gabbronorites with cumulus magnetite, which are characterized by variable proportions of feldspar versus mafic silicates plus magnetite. Furthermore, phosphorus-rich rocks, which have previously been interpreted to be alkaline basalts, are better explained as leucocratic norites with cumulus apatite. The sequence of crystallization of the magma responsible for the Columbia Hills is therefore interpreted to be: olivine, followed by orthopyroxene and plagioclase, then magnetite, and finally apatite. Ca-rich clinopyroxene appears to have been a late post-cumulus phase. If this interpretation is correct, then the Columbia Hills likely represent a Noachian-age layered intrusion that was exhumed by rebound following the meteorite impact that formed Gusev crater in the Hesperian.

Vapor-condensed phase processes in the early solar system

Abstract– Equilibrium thermodynamic calculations of the sequence of condensation of phases from a cooling gas of solar composition at total pressures thought to have prevailed in the inner part of the solar nebula successfully predict the primary mineral assemblages of refractory inclusions in CM2 and CV3 chondrites. Many refractory inclusions in CM2 chondrites contain a relatively SiO2-poor assemblage (spinel, hibonite, grossite, perovskite, corundum) that represents a high-temperature stage of condensation, and some may be pristine condensates that escaped later melting. Compact Type A and Type B refractory inclusions, consisting of spinel, melilite, perovskite, Ca-rich clinopyroxene ± anorthite, in CV3 chondrites are more SiO2-rich and equilibrated with the solar nebular gas at a slightly lower temperature. Textures of many of these objects indicate that they underwent melting after condensation, crystallizing into the same phase assemblage as their precursors. The Ti3+/Ti4+ ratio of their pyroxene indicates that this process occurred in a gas whose oxygen fugacity () was approximately 8.5 log units below that of the iron-wüstite buffer, making them the only objects in chondrites known to have formed in a system whose composition was close to that of the sun. Relative to CI chondrites, these inclusions are uniformly enriched in a group of elements (e.g., Ca, REE, Zr, Ta, Ir) that are chemically diverse except for their high condensation temperatures in a system of solar composition. The enrichment factor, 17.5, can be interpreted to mean that these objects represent either the first 5.7 wt% of the condensable matter to condense during nebular cooling or the residue after vaporization of 94.3% of a CI chondrite precursor. The Mg and Si isotopic compositions of Types A and B inclusions are mass-fractionated by up to 10 and 4 ‰/amu, respectively. When interpreted in terms of Rayleigh fractionation during evaporation of Mg and Si from the inclusions while they were molten, the isotopic compositions imply that up to 60% of the Mg and up to 25% of the Si were evaporated, and that approximately 80% of the enrichment in refractory (CaO+Al2O3) relative to more volatile (MgO+SiO2) in the average inclusion is due to initial condensation and approximately 20% due to subsequent evaporation. The mineralogical composition, including the Ti3+/Ti4+ ratio of the pyroxene, in Inti, a particle sampled from Comet Wild 2 by the Stardust spacecraft, is nearly identical to that of a Type B inclusion, indicating that comets contain not only the lowest-temperature condensates in the form of ices but the highest-temperature condensates as well. The FeO/(FeO+MgO) ratios of olivine and pyroxene in the matrix and chondrules of carbonaceous and ordinary chondrites are too high to be made in a system of solar composition, requiring s only 1 or 2 log units below iron-wüstite, more than 105 times higher than that of a solar gas. Various ways have been devised to generate cosmic gases sufficiently oxidizing to stabilize significant FeO in olivine at temperatures above those where Fe-Mg interdiffusion in olivine ceases. One is by vertical settling of dust toward the nebular midplane, enriching a region in dust relative to gas. Because dust is enriched in oxygen compared to carbon and hydrogen relative to solar composition, a higher results from total vaporization of the region, but the factor by which theoretical models have so far enriched the dust is 10 times too low. Another is by transporting icy bodies from the outer part of the nebula into the hot, inner part where vaporization of water ice occurs. Not only does this method fail to make the needed by a factor of 30–1000 but it also ignores simultaneous evaporation of carbon-bearing ices that would make the even lower.

Resolved spectroscopy of Mercury in the near-IR with SpeX/IRTF

We present resolved near-infrared spectra of Mercury scanning 70% of the surface in latitude and longitude from three separate observations, allowing us to perform a compositional investigation of its surface. By scanning the surface we find that all spectra in our sample are remarkably similar suggesting overall compositional homogeneity. We do, however, observe a slope difference between the spectra. These slope changes are most likely due to differences in the emission angle over different parts of the surface. We confirm the presence of a 1.1 μm feature that had been previously detected (Warell, J. et al. [2006]. Icarus 180, 281–291) and attributed to Ca-rich clinopyroxene. Finally, we investigated Mercury’s surface composition by comparing its spectrum with ground-based lunar spectra, lunar soil spectra collected in the laboratory, and analysis with a simple linear mixing model using various minerals as end-members. The result of this compositional investigation reveals that Mercury’s surface composition is likely to be quite different from the Moon’s. While low-Ca iron-rich pyroxenes are main surface components on the Moon (abundance varying from ∼5% to ∼35%), their abundance on Mercury may not exceed 5%. We also find that a Ca-rich clinopyroxene (in the hedenbergite–diopside series) is likely to be a main component of Mercury’s surface whereas this mineral is almost absent on the Moon. Our analysis also suggests the possible presence of olivine. We find that Mercury’s slope is less red than that of the Moon, in agreement with results from MESSENGER (McClintock, W.E., and 12 colleagues [2008]. Science 321, 62–65), and composition rather than variation of space weathering is likely the cause of this difference.

Limestone assimilation by basaltic magmas: an experimental re-assessment and application to Italian volcanoes

The results of an experimental study of limestone assimilation by hydrated basaltic magmas in the range 1,050–1,150°C, 0.1–500 MPa are reported. Alkali basalts doped with up to 19 wt% of Ca, Mg-carbonates were equilibrated in internally heated pressure vessels and the resulting phase relationships are described. The major effects of carbonate incorporation are: (1) generation of CO2-rich fluid phases; (2) change in liquidus phase equilibria; the crystallization of Ca-rich clinopyroxene is favored and the other phases (e.g. olivine, plagioclase), present in the absence of carbonate assimilation, are consumed. As a consequence of the massive clinopyroxene crystallization, the residual melt is strongly silica-depleted and becomes nepheline-normative. Compositional and mineralogical evolutions observed in Mt. Vesuvius eruptive products match those documented in our experiments with added carbonates, suggesting the possibility that carbonate assimilation increased during the last 25 ka of activity. In Central-Southern Italy, carbonate assimilation at shallow levels probably superimposes on deeper source heterogeneities.

Host rock solid-state transformation in a shock-induced melt vein of Tenham L6 chondrite

The host-rock fragments entrained in a 580-μm-wide melt vein of the Tenham L6 chondrite were investigated using field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) to better understand the solid-state transformation mechanisms and the shock conditions. The melt vein consists of a matrix of silicate plus metal-sulfide that crystallized from immiscible melts, and sub-rounded host-rock fragments that have been entrained in the melt and transformed to polycrystalline high-pressure silicates by solid-state transformation mechanisms. These high-pressure phases include ringwoodite, low-Ca majorite, clinoenstatite, hollandite-structured plagioclase and Ca-rich majorite. The Ca-rich majorite occurs as a symplectitic intergrowth with a Ca-poor amorphous silicate phase in a 200 μm-diameter chondrule in the vein. This intergrowth seems to be the result of a disproportionate breakdown of a Ca-rich clinopyroxene precursor into Ca-rich majorite and (FeMg)SiO 3 perovskite, which subsequently vitrified upon pressure release. The TEM observations suggest that most solid-state transformations in the Tenham are reconstructive. The transformation of olivine to polycrystalline ringwoodite appears to involve incoherent intracrystalline nucleation and interface-controlled growth. Lamellae in partially transformed olivine are not continuous coherent lamellae, but rather lamellae of polycrystalline ringwoodite, which is inconsistent with a coherent lamellar transformation mechanism. Growth rate calculations based on published kinetic data suggest that the time required to grow 1 μm ringwoodite crystal is ∼ 100 ms at 1600 K, suggesting that the minimum shock pulse of approximately 100 ms.

Geochemical evidence in clinopyroxenes from gabbroic sequence for two distinct magmatisms in the Oman ophiolite

In the Oman ophiolite, one of the best preserved and most studied ophiolites in the world, two distinct petrogenetic suites of gabbroic rocks from the layered gabbro sequence of the Wadi Haymiliyah section is established using trace element chemistry of Ca-rich clinopyroxenes. The earlier GB1 suite is characterized by plagioclase with lower An (Ca/(Ca + Na)) content and clinopyroxene with low large-ion lithophile elements (LILE) concentrations. The later DWGB2 suite contains plagioclase with rather high An content and clinopyroxene with high LILE. This difference in clinopyroxene chemistry can be extended to the extrusive rocks in this section: lower, (earlier) HV1 suite with low LILE clinopyroxene and upper (later) HV2 suite with high LILE clinopyroxene. Difference in LILE concentration of clinopyroxenes is essentially due to geochemical difference in parental magmas. The GB1/HV1 suites formed at fast-spreading MOR setting and DWGB2/HV2 suites at SSZ setting, supporting a model of transition from mid-oceanic ridge to supra-subduction zone settings of the Oman ophiolite. Our results indicate that geochemical signature of clinopyroxene is a very strong tool for identification of tectonic setting of ophiolites.

Experimental study of a low-alkali tholeiite at 1–5 kbar: optimal condition for the crystallization of high-An plagioclase in hydrous arc tholeiite

We conducted melting experiments on a low-alkali tholeiite (SiO2 ~52 wt%, MgO ~6.5 wt%, CaO/Na2O~4.4, Al2O3/SiO2 ~0.33) under both H2O-undersaturated and H2O-saturated conditions to investigate the effect of H2O on the Ca–Na partitioning between plagioclase and melt. Experiments were performed in the temperature and pressure ranges of 1,000–1,300°C and 1–5 kbar, respectively, with varying H2O contents of 0–12wt%. Redox condition was 0–2 log unit above NNO (nickel–nickel oxide) buffer. Temperature-bulk H2O diagrams for the low-alkali tholeiite are constructed at 1, 2, and 5 kbar, and compositions of near-liquidus plagioclase and coexisting melt are determined. To exclude the effect of melt composition (CaO/Na2O and Al2O3/SiO2 ratios) on plagioclase composition and to reveal the effect of H2O on An (=100×Ca/(Ca+Na)) content and $$K_{D}^{{\rm Ca-Na}}$$ (=(Ca/Na)pl/(Ca/Na)melt), we focused on the composition of near-liquidus plagioclases which crystallized from melts with nearly constant CaO/Na2O and Al2O3/SiO2 ratios. Our experimental results show that, at each experimental pressure, An content of the near-liquidus plagioclase and the K Ca-Na almost linearly increases as H2O content in melt increases. Each of the An content and the $$K_{D}^{{\rm Ca-Na}}$$ variations in a low-alkali tholeiitic system (CaO/Na2O~4.0–4.5, Al2O3/SiO2 ~0.27–0.33) can be described by one equation using temperature, pressure, and melt H2O content as parameters. An content and $$K_{D}^{{\rm Ca-Na}}$$ of liquidus plagioclase increases with increasing melt H2O and with decreasing pressure, elucidating that nearly H2O-saturated conditions of 2–3 kbar is optimal for the crystallization of the most An-rich plagioclase (>An88). We suggest this pressure condition of 2–3 kbar, corresponding to depth of 7–11 km, plays an important role for the origin of An-rich plagioclase in H2O-rich low-alkali tholeiite. At pressures more than ca. 4 kbar, crystallization of liquidus Ca-rich clinopyroxene decreases the CaO/Na2O ratio of liquid, thus prohibiting the crystallization of high-An plagioclase from hydrous tholeiite.

Magma plumbing system of the 2000 eruption of Miyakejima Volcano, Japan

During the 2000 eruption at Miyakejima Volcano, two magmas with different compositions erupted successively from different craters. Magma erupted as spatter from the submarine craters on 27 June is aphyric basaltic andesite (<5 vol% phenocrysts, 51.4–52.2 wt% SiO2), whereas magma issued as volcanic bombs from the summit caldera on 18 August is plagioclase-phyric basalt (20 vol% phenocrysts, 50.8–51.3 wt% SiO2). The submarine spatter contains two types of crystal-clots, A-type and A′-type (andesitic type). The phenocryst assemblages (plagioclase, pyroxenes and magnetite) and compositions of clinopyroxene in these clots are nearly the same, but only A′-type clots contain Ca-poor plagioclase (An < 70). We consider that the A′-type clots could have crystallized from a more differentiated andesitic magma than the A-type clots, because FeO*/MgO is not strongly influenced during shallow andesitic differentiation. The summit bombs contain only B-type (basaltic type) crystal-clots of Ca-rich plagioclase, olivine and clinopyroxene. The A-type and B-type clots have often coexisted in Miyakejima lavas of the period 1469–1983, suggesting that the magma storage system consists of independent batches of andesitic and basaltic magmas. According to the temporal variations of mineral compositions in crystal-clots, the andesitic magma became less evolved, and the basaltic magma more evolved, over the past 500 years. We conclude that gradually differentiating basaltic magma has been repeatedly injected into the shallower andesitic magma over this period, causing the andesitic magma to become less evolved with time. The mineral chemistries in crystal-clots of the submarine spatter and 18 August summit bombs of the 2000 eruption fall on the evolution trends of the A-type and B-type clots respectively, suggesting that the shallow andesitic and deeper basaltic magmas existing since 1469 had successively erupted from different craters. The 2000 summit collapse occurred due to drainage of the andesitic magma from the shallower chamber; as the collapse occurred, it may have caused disruption of crustal cumulates which then contaminated the ascending, deeper basalt. Thus, porphyritic basaltic magma could erupt alone without mixing with the andesitic magma from the summit caldera. The historical magma plumbing system of Miyakejima was probably destroyed during the 2000 eruption, and a new one may now form.

Near-infrared reflectance spectroscopy of Ca-rich clinopyroxenes and prospects for remote spectral characterization of planetary surfaces

We present results of laboratory near-infrared reflectance studies of a set of calcic pyroxenes with comparable calcium contents (Wo 45–50) but variable iron content and oxidation states. This new dataset complements earlier published data (Cloutis and Gaffey, 1991, J. Geophys. Res. 96, 22809–28826, and references therein). In particular, our new spectra extend the scarce available spectral data on chemically analyzed Fe-rich high-Ca clinopyroxenes. We attempted to interpret the spectral behavior of our samples in terms of chemistry and coordination site occupancies. Tentatively, we conclude that Fe-rich calcic pyroxenes have very low contents of Fe 2+ in the M2 sites and belong to the spectral type A lacking the 2-μm band. This may be due to high Ca and Mn contents in these pyroxenes. Fe-poor high-Ca pyroxenes are more spectrally variable. In general, they tend to belong to the spectral type B with two major bands near 1 and 2 μm, unless the samples have high Fe 3+/Fe 2+ ratios or are rich in Mn and Ca. Some of them (including unusual meteorite Angra dos Reis) are of type B despite very high Ca contents. We applied the Modified Gaussian Model (MGM) to characterize three major Fe 2+ absorption bands in the 1-μm region of the spectra of Ca-rich pyroxenes. Only the band due to Fe 2+ in the M1 coordination site near 1.15 μm may be potentially useful to estimate the Fe content in calcic pyroxenes on remotely-sensed surfaces of Solar System bodies. The spectral variability of basaltic meteorites (angrites) that are rich in calcic pyroxenes is also discussed. The presence of spectral type A calcic pyroxenes in these meteorites complicates unambiguous identification of olivine in asteroid spectra.

Experimental investigation of mixing properties of (Ca,Fe,Mg)&lt;sub&gt;2&lt;/sub&gt;SiO&lt;sub&gt;4&lt;/sub&gt; olivine: Fe-Mg exchange with Ca-rich clinopyroxene and phase relations in olivine quadrilateral

To constrain the mixing properties of (Ca,Fe,Mg)2SiO4 olivine, Fe-Mg exchange experiments have been carried out on the assemblages of low-Ca olivine+clinopyroxene, low-Ca olivine+high-Ca olivine+clinopyroxene and high-Ca olivine+clinopyroxene with bulk Fe/(Fe+Mg) in the range of 0.4 to 0.6, mainly about 0.5, at pressures from 1 atm to 2.0 GPa and temperatures from 1100°C to 1300°C, mainly at 1 atm and 1150°C. The phase relations of (Ca,Fe,Mg)2SiO4 olivine have been also determined at 1 atm and 1150°C. Experiments under 1 atm were performed in an electric furnace equipped with CO2/H2 control device. The oxygen fugacity around the sample space in the furnace was kept to that realized with the iron-wustite buffer. High-pressure experiments were carried out using a piston-cylinder apparatus. The Fe-Mg distribution coefficient between olivine and clinopyroxene, KD [=(XMgCpx XFeOl)/(XFeCpx XMgOl)], increases with bulk Ca/(Ca+Fe+Mg) content, but insensitive to pressure. The solvus between low-Ca and high-Ca olivines becomes narrow with pressure and Fe/(Fe+Mg) of the system, but the pressure effect on the solvus is very small. Thermodynamic analyses were made on the present and previous data to refine the compositional dependence on KD of olivine-clinopyroxene pair and the solvus of quadrilateral (Ca,Fe,Mg)2SiO4 olivine using the Fe-Mg mixing data, and the free energy changes of the Fe-Mg exchange reactions between these phases and in olivine. The symmetric and asymmetric two-site regular solution models were applied to (Ca,Fe,Mg)2SiO4 olivines in terms of the Fe-Mg intracrystalline exchange between M1 and M2 sites and the excess energies of Ca-Fe-Mg mixing. In the present model, it is assumed that substitution of Ca is limited to M2 sites alone whereas Fe and Mg substitute on both M1 and M2 sites. The mixing parameters of the asymmetric Ca-Mg and symmetric Fe-Mg and Ca-Fe models are evaluated as WCaMg=36.31±0.19, WMgCa=32.73±0.18, WCaFe=21.34±0.06, WCaFeMg=6.12±0.98, δ1=2.24±1.30 and δ2=−8.45±0.94 (unit in kJ), whereas the symmetric Ca-Fe-Mg mixing model (WCaFeMg=0) resulted as WCaMg=WMgCa=34.20±0.18, WCaFe=23.75±0.58, δ1=−1.50±2.35 and δ2=−10.20±1.37. The present results are consistent with the previous analyses of miscibility gaps in Ca-Fe-Mg olivines. The Ca-Mg mixing is slightly less non-ideal than the previous estimates on the forsterite-monticellite solvus. Considerations about the Fe-Mg intracrystalline exchange show a slight preference of Fe in M1 site in Ca-free olivines, and indicate preferential order of Fe to M2 site with increase of Ca contents of olivine.