high-Ca Pyroxene Research Articles

Case study of magmatic differentiation trends on the Moon based on lunar meteorite Northwest Africa 773 and comparison with Apollo 15 quartz monzodiorite

Pyroxene and feldspar compositions indicate that most clasts from the Northwest Africa 773 (NWA 773) lunar meteorite breccia crystallized from a common very low-Ti (VLT) mare basalt parental magma on the Moon. An olivine cumulate (OC), with low-Ca and high-Ca pyroxenes and plagioclase feldspar formed during early stages of crystallization, followed by pyroxene gabbro, which is characterized by zoned pyroxene (Fe#=molar Fe/(Fe+Mg)×100 from ∼35 to 90; Ti#=molar Ti/(Ti+Cr)×100 from ∼20 to 99) and feldspar (∼An90–95Ab05–10 to An80–85Ab10–16). Late stage lithologies include alkali-poor symplectite consisting of fayalite, hedenbergitic pyroxene and silica, and alkaline-phase-ferroan clasts characterized by K-rich glass and/or K,Ba-feldspar with fayalite and/or pyroxene. Igneous silica only occurs with the alkaline-phase-ferroan clasts. This sequence of clasts represents stages of magmatic evolution along a ferroan–titanian trend characterized by correlated Fe# and Ti# in pyroxene, and a wide range of increase in Fe# and Ti# prior to crystallization of igneous silica.Clasts of Apollo 15 quartz monzodiorite (QMD) also have pyroxene co-existing with silica, but the QMD pyroxene has more moderate Fe# (∼70). Thus, in AFM components (A=Na2O+K2O, M=MgO, F=FeO), the QMD clasts are similar to the terrestrial calc-alkaline trend (silica-enrichment at moderate Fe#), whereas the ferroan–titanian trend is similar to the terrestrial tholeiitic trend (silica-enrichment only after strong increase in Fe#). However, the variations in SiO2-contents of QMD clasts are due to variable mixing of SiO2-rich and FeO-rich immiscible liquids (i.e., not a progressive increase in SiO2). Immiscibility occurred after fractionation of a KREEP-rich parent liquid.A third trend is based on zoning relations within the NWA 773 OC, where pyroxene Ti# increases at constant Fe# with proximity to intercumulus, incompatible element-rich pockets rich in K,Ba-feldspar and Ca-phosphates. This type of fractionation (increasing refractory trace elements at constant Fe#) in a cumulate parent rock may have been important for generating lunar rocks that combine low Fe# with high incompatible trace element concentrations, such as KREEP basalts and the magnesian suite.MELTS (Ghiorso and Sack, 1995; Asimow and Ghiorso, 1998) models of one VLT, one low-Ti and two high-Ti mare basalts and one KREEP basalt all show evolution from low to high Fe# residual liquids during fractional crystallization; however strong enrichments in FeO-concentrations are limited to the VLT and low-Ti liquids. In the high-Ti liquids, crystallization of Fe–Ti-oxides prevents enrichment in FeO, and the increases in Fe# are due to depletion of MgO. Fe–Ti-oxide fractionation results in steady silica-enrichment in the high-Ti mare compositions. Intervals of FeO-enrichment on the VLT and low-Ti mare liquid lines of descent are linked to shifts from olivine to pyroxene crystallization. The onset of plagioclase feldspar crystallization limits the depletion of FeO during crystallization of one high-Ti mare basalt and of the KREEP basalt composition modeled.

An experimental study of trace element partitioning between augite and Fe-rich basalts

Aluminous titanium-rich hedenbergite and Fe-rich augite were experimentally produced at 1050–1220°C and 0.8–2.2GPa. Major element compositions are analogous to clinopyroxene from layered intrusions, angrites, nakhlites, and late-stage lunar magma ocean cumulates. Trace element concentrations in pyroxene and coexisting melt were measured by laser ablation inductively coupled plasma mass spectrometry for rare earth elements (REE), high field strength elements (HFSE), transition metals, and large-ion lithophile elements. REE, HFSE, and transition metal partition coefficients (Ds) are tightly correlated with the cation abundance of Fe on the pyroxene M1 site (XFeM1), and also correlated with Al on the tetrahedral site (XAlT). Parameterized lattice-strain models were developed to predict REE and HFSE partition coefficients as functions of temperature and pyroxene composition (XAlT or XFeM1). The parameterized models can be used to calculate REE and HFSE partition coefficients for Fe-rich high-Ca pyroxene. We calculated partition coefficients for clinopyroxene derived from modeled end-stage lunar magma ocean cumulate compositions and observe a factor of 3 variation in clinopyroxene-melt REE partition coefficients. Using representative nakhlite clinopyroxene core and rim compositions we calculated partition coefficients and observed a factor of 4 variability in clinopyroxene-melt REE partition coefficients. HFSE partition coefficients are even more sensitive to composition, showing variations of a factor of 4 for lunar late cumulates and an order of magnitude for nakhlites. The strong dependence of REE and HFSE partitioning on composition necessitates careful selection of appropriate partition coefficients for Fe-rich systems.

Mineral chemistry of MUSES‐C Regio inferred from analysis of dust particles collected from the first‐ and second‐touchdown sites on asteroid Itokawa

AbstractThe mineralogy and mineral chemistry of Itokawa dust particles captured during the first and second touchdowns on the MUSES‐C Regio were characterized by synchrotron‐radiation X‐ray diffraction and field‐emission electron microprobe analysis. Olivine and low‐ and high‐Ca pyroxene, plagioclase, and merrillite compositions of the first‐touchdown particles are similar to those of the second‐touchdown particles. The two touchdown sites are separated by approximately 100 meters and therefore the similarity suggests that MUSES‐C Regio is covered with dust particles of uniform mineral chemistry of LL chondrites. Quantitative compositional properties of 48 dust particles, including both first‐ and second‐touchdown samples, indicate that dust particles of MUSES‐C Regio have experienced prolonged thermal metamorphism, but they are not fully equilibrated in terms of chemical composition. This suggests that MUSES‐C particles were heated in a single asteroid at different temperatures. During slow cooling from a peak temperature of approximately 800 °C, chemical compositions of plagioclase and K‐feldspar seem to have been modified: Ab and Or contents changed during cooling, but An did not. This compositional modification is reproduced by a numerical simulation that modeled the cooling process of a 50 km sized Itokawa parent asteroid. After cooling, some particles have been heavily impacted and heated, which resulted in heterogeneous distributions of Na and K within plagioclase crystals. Impact‐induced chemical modification of plagioclase was verified by a comparison to a shock vein in the Kilabo LL6 ordinary chondrite where Na‐K distributions of plagioclase have been disturbed.

Oxygen three-isotope ratios of silicate particles returned from asteroid Itokawa by the Hayabusa spacecraft: A strong link with equilibrated LL chondrites

Oxygen three-isotope ratios of four mineral phases (olivine, low-Ca and high-Ca pyroxene, and plagioclase) in seven silicate particles from asteroid Itokawa were measured to investigate oxygen isotope systematics of asteroidal materials. In order to obtain highly precise and accurate oxygen isotope ratios (∼±0.3‰ in δ18O), we used a new indium mounting method that minimizes potential instrumental mass fractionation due to surface topography. The oxygen isotope data of the seven Itokawa particles are distributed above the terrestrial fractionation line with indistinguishable ΔO17(=δO17−0.52×δO18) values of +1.34±0.36‰ (2SD; n=22), which is similar to those of Itokawa particles analyzed by Yurimoto et al. (2011) and Nakamura et al. (2012). However, δ18O values of the seven Itokawa particles are tightly clustered (+3.9‰ to +5.9‰ VSMOW) compared to previous analyses of the Itokawa particles (+1.4‰ to +8.8‰).The average Δ17O values and 2SD of the two equilibrated ordinary chondrites (EOCs), which were analyzed for comparison, are +1.31±0.49‰ for St. Séverin (LL6) and +0.83±0.64‰ for Guareña (H6). The Δ17O values of the seven Itokawa particles are similar to those of St. Séverin. The average bulk δ18O value of the seven Itokawa particles is estimated as +4.8‰, which is within the δ18O range of bulk LL5 and LL6 chondrites. It is suggested that Itokawa particles are similar to those of equilibrated LL chondrites, which strengthens a link between asteroid Itokawa and equilibrated LL chondrites that fell to Earth.The seven Itokawa particles and two EOCs have δ18O values of high-Ca pyroxene that are lower than those of olivine, indicating that oxygen isotopes between the two mineral phases are not in equilibrium. Apparent oxygen isotope equilibrium temperatures estimated from a pair of low-Ca pyroxene and olivine are excessively higher than those from a pair of plagioclase and high-Ca pyroxene (∼800 °C) and peak metamorphic temperatures. Oxygen isotopes in olivine and low-Ca pyroxene do not represent equilibrium. On the other hand, oxygen isotope ratios in high-Ca pyroxene and plagioclase were mass-dependently fractionated accompanied by recrystallization (and possibly by diffusion) during thermal metamorphism. Values of δ18O in high-Ca pyroxene were lowered by exchange, which may have resulted in the reverse δ18O fractionation between olivine and high-Ca pyroxene.

Trace element partitioning between mantle minerals and silico-carbonate melts at 6–12 GPa and applications to mantle metasomatism and kimberlite genesis

The partitioning of a number of trace elements (LILE, HFSE, REE, Cu, Pb, Co, Ni) between mantle minerals (olivine, pyroxenes and garnet) and silico-carbonate melts was experimentally studied at 6–12GPa and 1300–1700°C. The starting compositions were model kimberlitic with ~30wt.% SiO2, which differentiated to carbonatite-like melts with <10wt.% SiO2 depending on the degree of crystallization. The melts were rich in CO2 (up to ~30wt.%) and contained 0 to 30wt.% H2O. Trace elements were added to the starting mixtures to levels of ~100ppm. They were analyzed by LA ICP MS in the products of 18 experiments. The partition coefficients of Ba, La, Ta, and Nb are very low for all phases (<0.01). These elements are especially susceptible to contamination in the experimental products. We suspect that some relatively high values obtained for these elements in previous studies are overestimated. Generally, the partition coefficients (Ds/l) of the moderately incompatible and compatible elements increase in the sequence olivine–low-Ca pyroxene–high-Ca pyroxene. Garnet shows maximum fractionation of the trace elements, such that D values for the highly incompatible elements are lower than those for pyroxenes and highest for the HREE, Sc, and V. The partition coefficients of a number of incompatible elements are rather insensitive to pressure, temperature, and melt composition. There is no correlation between the partition coefficients and the content of H2O, CO2 and the overall content of trace elements. They are predominantly controlled by the composition of the crystalline phases. In particular, garnet–liquid partitioning is very sensitive to Ca in garnet, and pyroxene–liquid partitioning to Al in pyroxene.The comparison of the obtained partition coefficients with the compositions of likely kimberlite magmas shows that depleted and refertilized harzburgite from the continental lithospheric mantle is a viable candidate as the source material of kimberlites. The estimation of trace element characteristics of near-solidus mantle melts indicates that the most reliable indicator of carbonate melt metasomatism is depletion of Zr and Hf relative to Sm and Nd. Criteria based on the ratios of more incompatible elements (e.g. Nb/La) are very sensitive to the melting conditions and can be misleading.

Mineralogy of Mare Serenitatis on the near side of the Moon based on Chandrayaan-1 Moon Mineralogy Mapper (M3) observations

Spectral analysis of Mare Serenitatis has been carried out using Chandrayaan-1 Moon Mineralogy Mapper (M3) data in order to map the compositional diversity of the basaltic units that exist in the basin. Mare Serenitatis is characterized by multiple basaltic flows of different ages indicating a prolonged volcanism subsequent to the basin formation event. Reflectance spectra of fresh craters from the Mare Serenitatis have been analyzed to study the nature and location of the spectral absorption features around 1- and 2-μm respectively, arising due to the electronic charge transition of Fe2+ in the crystal lattice of pyroxenes and/or olivine. Chandrayaan-1 M3 data have been utilized to obtain an Integrated Band Depth (IBD) mosaic of the Serenitatis basin. Based on the spectral variations observed in the IBD mosaic, 13 spectral units have been mapped in the Mare Serenitatis. In the present study, we have also derived spectral band parameters, namely, band center, band strength, band area and band area ratio from the M3 data to study the mineralogical and compositional variations amongst the basaltic units of the studied basin. On the basis of spectral band parameter analysis, the pyroxene compositions of the basaltic units have been determined, which vary from low to intermediate end of the high-Ca pyroxene and probably represent a sub-calcic to calcic augite compositional range. Detailed spectral analyses reveal little variations in the mafic mineralogy of the mare basalts in terms of pyroxene chemistry. The uniformity in pyroxene composition across the basaltic units of Mare Serenitatis, therefore, suggest a probably stable basaltic source region, which might not have experienced large-scale fractionation during the prolonged volcanism that resulted in filling of the large Serenitatis basin.

Low-Mg rock debris in howardites: Evidence for KREEPy lithologies on Vesta?

The Northwest Africa (NWA) 1664 and 1769 howardites contain fragments of low-Mg lithologies. These clasts consist of plagioclase, Fe-rich pyroxenes (high-Ca pyroxene and an Fe-rich pigeonite (or pyroxferroite) partially to totally converted to a silica–fayalite–hedenbergite symplectite), K–Ba-feldspars (hyalophane), silica, troilite, phosphates (merrillite and apatite), ilmenite, baddeleyite and zircon. Textural considerations and phase compositions show that these clasts cannot be unrepresentative debris of mesostasis-rich areas from eucrites, but are instead remnants of evolved rocks. The occurrence of hyalophane and the modal abundances of phosphates in the largest of them suggest that their source rocks were much more K- and P-rich than regular HEDs. Moreover, the rare earth element abundances of the pyroxenes indicate that their parental melts were rich in incompatible trace elements (possibly at the level of 100×CI or much higher for heavy REEs), with variable negative Eu anomalies. Although, their petrogenesis is uncertain, they demonstrate that magmatic activity on Vesta has been able to generate a diversity of rocks greater than generally thought, and local lithologies with possible KREEP affinities.

The global distribution of calcium on the Moon: Implications for high-Ca pyroxene in the eastern mare region

The first unambiguous global distribution of Ca on the Moon has been revealed by the Kaguya Gamma-Ray Spectrometer. The observation of the 3737keV gamma rays from 40Ca with a high energy resolution Ge detector, corrected for fast neutrons, improved greatly our confidence in these remote sensing data. While the derived Ca abundance generally confirms the mare-highland dichotomy, we found that the eastern mare regions such as Crisium, Nectaris, and Fecunditatis exhibit unexpected high Ca abundances. This may be explained by regional concentrations of high-Ca clinopyroxene. Our Ca map also shows that the lowest CaO abundance found in the returned samples (∼8wt%) may actually be common in Oceanus Procellarum, suggesting the possible sampling bias of Apollo and Luna missions. These observational results on Ca serve as a complementary criterion for better characterization of mare regions on the Moon.

The Tafassasset primitive achondrite: Insights into initial stages of planetary differentiation

Tafassasset is an exceptional meteorite that has been linked to both the CR chondrites and the primitive achondrites. Because previous evidence suggests it might be a primitive achondrite from a known chondrite type, we have undertaken a study of the petrology, geochemistry, and formation history of the meteorite. Tafassasset is predominantly FeO-rich olivine (∼58%) yet contains abundant Fe,Ni-metal (∼10vol.%) and sulfide (∼3vol.%). Other phases include high- and low-Ca pyroxene, plagioclase, chromite, and phosphate. It has a recrystallized texture, containing equigranular grains that often meet at 120° triple junctions. There are no relict chondrules in the thin sections examined, although they have been reported previously. Electron microprobe analyses reveal homogeneous olivine (Fa28.6), both low-and high-Ca pyroxene (Fs23.6Wo3.7 and Fs12.2Wo39.3±1), a range of plagioclase composition (An23–47), Fe,Ni-metal (with 5.3–36.6wt.% Ni and 0.1–0.8wt.% Co), troilite, chromite, and Ca–phosphate. Bulk composition analyses reveal two chips depleted in refractory lithophile and some siderophile elements compared to CI chondrites. Exceptions are enrichments in Fe, Ni and Co. A third chip is essentially chondritic in bulk composition. Different stones of the meteorite have slightly different oxygen isotope composition, yet all lie in the CR chondrite trend with one in the acapulcoite–lodranite field. Thermodynamic calculations show that Tafassasset equilibrated at a temperature above the Fe,Ni–FeS eutectic and at an oxygen fugacity of ∼IW-1. The texture, heterogeneous distribution of mineral phases, plagioclase composition, two-mineral closure temperatures, and bulk composition all provide evidence that Tafassasset partially melted on its parent body. A comparison with the CR chondrites, the brachinites, and two anomalous achondrites indicates that Tafassasset is most similar to ungrouped primitive achondrites Lewis Cliff (LEW) 88763 and Divnoe, and to the brachinites in overall petrography, modal mineralogy, mineral compositions, oxidation state, and bulk composition. The comparison also excludes the possibility that Tafassasset formed by partial melting of a CR chondrite. Tafassasset is a primitive achondrite and likely evolved on a parent body that experienced incomplete melting, never reached isotopic homogeneity, and was from the same oxygen isotopic reservoir as the CR chondrite parent body.

Variability of diffusion of argon in albite, pyroxene, and olivine in shocked and unshocked samples

We analyzed the argon diffusion parameters of albite and pyroxene as a function of shock, as well as an olivine of unknown shock pressure. In albite, shock pressures of ∼30–60GPa all have an equal effect on the diffusion parameters, lowering the activation energy (E) from ∼45 to ∼25kcal/mol, and the logD0/a2 from ∼4 to ∼1s−1. In a mixture of low-Ca and high-Ca pyroxene, similar shock pressures also have an equal effect, lowering the E from ∼130 to ∼80kcal/mol, and the logD0/a2 from ∼14 to ∼7s−1. However, given that some shocked and unshocked pyroxene splits have similar release patterns, making a clear distinction between the two is difficult. Additionally, splits of pure unshocked high-Ca pyroxene have an E of ∼100kcal/mol and a logD0/a2 of ∼10s−1, intermediate to the values of mixed pyroxene when shocked and unshocked. The olivine (Fa18) has an E of ∼45kcal/mol and a logD0/a2 of ∼1s−1, somewhat similar to that of feldspar. These results will be useful in interpreting complex argon release patterns from shocked multi-mineral samples too fine-grained for easy mineral separation (such as chondrites).

Formation of primary kimberlite melts – Constraints from experiments at 6–12 GPa and variable CO2/H2O

Liquidus phase relations were experimentally studied in model kimberlite melts at 6–12GPa. Four starting materials were used with different CO2 and H2O contents but almost identical proportions of other components (~35wt.% SiO2, 3wt.% Al2O3, 10wt.% FeO, 33wt.% MgO, 15wt.% CaO, 0.3wt.% Na2O, and 1wt.% K2O on a 100% volatile-free basis). The liquidus phase of the CO2 richest mixture (33wt.% CO2 and no H2O) is coesite at P>6–8GPa and low-Ca pyroxene at lower pressures. In an H2O-free mixture with lower CO2 content (18wt.%) low-Ca pyroxene is the main liquidus phase and coesite was never observed in the experimental products. The addition of water (mixtures with 10wt.% H2O+9wt.% CO2 and 12wt.% H2O+5wt.% CO2) depresses the liquidus temperature and expands the crystallization field of olivine. At high pressures (>8GPa), garnet crystallization dominates near-liquidus phase relations. Based on the experimental data, possible conditions of the simultaneous saturation of kimberlite melt with olivine+low-Ca pyroxene±garnet were estimated. High-Ca pyroxene was never found in the near-liquidus assemblages despite the high-CaO compositions of the starting mixtures. It was shown that the supposed primary kimberlite melt can be in equilibrium with the garnet harzburgite assemblage at 6–8GPa and variable volatile contents, from ~15wt.% CO2 under anhydrous conditions to 20wt.% H2O and low CO2. These conditions correspond to possible parameters of kimberlite magma generation by partial melting of carbonated garnet harzburgite in the lithospheric mantle. The primary CO2 content could be higher if the kimberlite magma was poorer in SiO2 (more ‘carbonatitic’). The maximum CO2 content is limited by the appearance of magnesite on the liquidus at 20–22wt.%, which is much lower than CO2 solubility in kimberlite melt at 7–8GPa (>30wt.%). The content of H2O in primary kimberlite magmas depends on the bulk H2O content in the source, because no hydrous phases are stable near the kimberlite liquidus.

The origin of Vesta’s crust: Insights from spectroscopy of the Vestoids

High quality VNIR spectra of 15 Vestoids, small asteroids that are believed to originate from Vesta, were collected and compared to laboratory spectra and compositional data for selected HED meteorites. A combination of spectral parameters such as band centers, and factors derived from Modified Gaussian Model fits (band centers, band strengths, calculation of the low to high-Ca pyroxene ratio) were used to establish if each Vestoid appeared most like eucrite or diogenite material, or a mixture of the two (howardite). This resulted in the identification of the first asteroid with a ferroan diogenite composition, 2511 Patterson. This asteroid can be used to constrain the size of diogenite magma chambers within the crust of Vesta. The Vestoids indicate that both large-scale homogeneous units (>5 km) and smaller-scale heterogeneity (<1 km) exist on the surface of Vesta, as both monomineralogic (eucrite or diogenite material alone) and mixed (both eucrite and diogenite) spectra are observed. The small-scale of the variation observed within the Vestoid population is predicted by the partial melting model, which has multiple intrusions penetrating into the crust of Vesta. It is much more difficult to reconcile the observations here with the magma ocean model, which would predict much more homogeneous layers on a large-scale both at the surface and with depth.

Decomposition of mineral absorption bands using nonlinear least squares curve fitting: Application to Martian meteorites and CRISM data

This study advances curve-fitting modeling of absorption bands of reflectance spectra and applies this new model to spectra of Martian meteorites ALH 84001 and EETA 79001 and data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). This study also details a recently introduced automated parameter initialization technique. We assess the performance of this automated procedure by comparing it to the currently available initialization method and perform a sensitivity analysis of the fit results to variation in initial guesses. We explore the issues related to the removal of the continuum, offer guidelines for continuum removal when modeling the absorptions and explore different continuum-removal techniques. We further evaluate the suitability of curve fitting techniques using Gaussians/Modified Gaussians to decompose spectra into individual end-member bands. We show that nonlinear least squares techniques such as the Levenberg–Marquardt algorithm achieve comparable results to the MGM model ( Sunshine and Pieters, 1993; Sunshine et al., 1990) for meteorite spectra. Finally we use Gaussian modeling to fit CRISM spectra of pyroxene and olivine-rich terrains on Mars. Analysis of CRISM spectra of two regions show that the pyroxene-dominated rock spectra measured at Juventae Chasma were modeled well with low Ca pyroxene, while the pyroxene-rich spectra acquired at Libya Montes required both low-Ca and high-Ca pyroxene for a good fit.

Mineralogy and thermal properties of V-type Asteroid 956 Elisa: Evidence for diogenitic material from the Spitzer IRS (5–35 μm) spectrum

We present the thermal infrared (5–35 μm) spectrum of 956 Elisa as measured by the Spitzer Infrared Spectrograph (“IRS”; Houck, J.R. et al. [2004]. Astrophys. J. Suppl. 154, 18–24) together with new groundbased lightcurve data and near-IR spectra. From the visible lightcurve photometry, we determine a rotation period of 16.494 ± 0.001 h, identify the rotational phase of the Spitzer observations, and estimate the visible absolute magnitude ( H V ) at that rotational phase to be 12.58 ± 0.04. From radiometric analysis of the thermal flux spectrum, we find that at the time of observation 956 Elisa had a projected radius of 5.3 ± 0.4 km with a visible albedo p V = 0.142 ± 0.022, significantly lower than that of the prototype V-type asteroid, 4 Vesta. (This corresponds to a radius of 5.2 ± 0.4 km at lightcurve mean.) Analysis with the standard thermal model (STM) results in a sub-solar temperature of 292.3 ± 2.8 K and beaming parameter η = 1.16 ± 0.05. Thermophysical modeling places a lower limit of 20 J m - 2 K - 1 s - 1 2 on the thermal inertia of the asteroid’s surface layer (if the surface is very smooth) but more likely values fall between 30 and 150 J m - 2 K - 1 s - 1 2 depending on the sense of rotation. The emissivity spectrum, calculated by dividing the measured thermal flux spectrum by the modeled thermal continuum, exhibits mineralogically interpretable spectral features within the 9–12 μm reststrahlen band, the 15–16.5 μm Si–O–Si stretching region, and the 16–25 μm reststrahlen region that are consistent with pyroxene of diogenitic composition: extant diogenitic pyroxenes fall within the narrow compositional range Wo 2±1En 74±2Fs 24±1. Spectral deconvolution of the 9–12 μm reststrahlen features indicates that up to ≈20% olivine may also be present, suggesting an olivine-diogenite-like mineralogy. The mid-IR spectrum is inconsistent with non-cumulate eucrite as the major component on the surface of 956 Elisa, although cumulate eucrite material may be present at abundances lower than that of the diogenite component. Analysis of new near-IR spectra of 956 Elisa with the Modified Gaussian Model (MGM; Sunshine, J.M., Pieters, C.M., Pratt, S.F. [1990]. J. Geophys. Res. 95 (May), 6955–6966) results in two pyroxene compositions: 75% magnesian low-Ca pyroxene and 25% high-Ca pyroxene. High-Ca pyroxene is not evident in the mid-IR data, but may belong to a component that is underrepresented in the mid-IR spectrum either because of its spatial distribution on the asteroid or because of its particle size. High-Ca pyroxenes that occur as exsolution lamellae may also be more evident spectrally in the NIR than in the mid-IR. In any case, we find that the mid-IR spectrum of 956 Elisa is dominated by emission from material of diogenite-like composition, which has very rarely been observed among asteroids.

Spectral evidence of volcanic cryptodomes on the northern plains of Mars

The composition and detailed morphology of dome-shaped features located in western Arcadia Planitia and just west of Utopia Planitia were examined in this study utilizing data from Mars Reconnaissance Orbiter and Mars Odyssey sensors. The domes have diameters averaging 1.5 km and heights averaging 160 m, and are generally dark-toned, although some are lighter toned or have split dark and light-toned surfaces. The domes are surrounded by annular deposits comprising, with increasing distance from the domes, dark-toned aprons, light-toned aureoles, and dark-toned aureoles. Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) data over several areas in the western Arcadia region show that spectra from the flanks of several domes have 1 and 2 μm absorption features consistent with the presence of olivine and a high-Ca pyroxene, nominally augite. Modified Gaussian Model (MGM) analysis of these spectra indicates Fe-rich olivine compositions. The tops of domes and the aprons surrounding many domes have negative sloping flat spectra in the near infrared, which is consistent with tachylite-rich, glassy compositions. High Resolution Imaging Science Experiment (HiRISE) images over several domes indicate that relatively high thermal inertia values associated with the tops of domes can be attributed to boulder strewn surfaces. HiRISE images also reveal that light-toned aureoles around domes consist of crenulated ground resembling “brain terrain” textures previously described for ice-rich concentric crater fill elsewhere on the northern plains. The plains surrounding the domes also display lineations that are interpreted to be lava channels or tubes. The combination of volcanic and ice-related features are consistent with the domes having formed as cryptodomes in the near sub-surface. We suggest that the domes could be basaltic in composition if the magmas were degassed and/or highly crystallized, and thus more viscous than typical basaltic magmas. The intrusion of these magmas into an ice-rich horizon would have produced a pervasively jointed chilled margin on the domes, which, once the domes were exposed, would have mechanically weathered to form the dark aprons. The domes could have served as local centers for ice accumulation during periods of high orbital obliquity, which ultimately would have led to the formation of the “brain terrain” surrounding the features. The domes represent late stage volcanic products on the northern plains of Mars and associated features provide more evidence for the role that ice accumulation and modification has played in recent martian history.

High precision SIMS oxygen three isotope study of chondrules in LL3 chondrites: Role of ambient gas during chondrule formation

We report high precision SIMS oxygen three isotope analyses of 36 chondrules from some of the least equilibrated LL3 chondrites, and find systematic variations in oxygen isotope ratios with chondrule types. FeO-poor (type I) chondrules generally plot along a mass dependent fractionation line (Δ 17O ∼ 0.7‰), with δ 18O values lower in olivine-rich (IA) than pyroxene-rich (IB) chondrules. Data from FeO-rich (type II) chondrules show a limited range of δ 18O and δ 17O values at δ 18O = 4.5‰, δ 17O = 2.9‰, and Δ 17O = 0.5‰, which is slightly 16O-enriched relative to bulk LL chondrites (Δ 17O ∼ 1.3‰). Data from four chondrules show 16O-rich oxygen isotope ratios that plot near the CCAM (Carbonaceous Chondrite Anhydrous Mineral) line. Glass analyses in selected chondrules are systematically higher than co-existing minerals in both δ 18O and Δ 17O values, whereas high-Ca pyroxene data in the same chondrule are similar to those in olivine and pyroxene phenocrysts. Our results suggest that the LL chondrite chondrule-forming region contained two kinds of solid precursors, (1) 16O-poor precursors with Δ 17O > 1.6‰ and (2) 16O-rich solid precursors derived from the same oxygen isotope reservoir as carbonaceous chondrites. Oxygen isotopes exhibited open system behavior during chondrule formation, and the interaction between the solid and ambient gas might occur as described in the following model. Significant evaporation and recondensation of solid precursors caused a large mass-dependent fractionation due to either kinetic or equilibrium isotope exchange between gas and solid to form type IA chondrules with higher bulk Mg/Si ratios. Type II chondrules formed under elevated dust/gas ratios and with water ice in the precursors, in which the ambient H 2O gas homogenized chondrule melts by isotope exchange. Low temperature oxygen isotope exchange may have occurred between chondrule glasses and aqueous fluids with high Δ 17O (∼5‰) in LL the parent body. According to our model, oxygen isotope ratios of chondrules were strongly influenced by the local solid precursors in the proto-planetary disk and the ambient gas during chondrule melting events.

Analysis of ordinary chondrites using powder X-ray diffraction: 1. Modal mineral abundances

Abstract– Powder X-ray diffraction (XRD) is used to quantify the modal abundances (in wt%) of 18 H, 17 L, and 13 LL unbrecciated ordinary chondrite falls, which represents the complete petrologic range of equilibrated ordinary chondrites (types 4–6). The XRD technique presents an effective alternative to traditional methods for determining modal abundances, such as optical point counting and electron microprobe phase (EMP) mapping. The majority of chondrite powders in this study were previously prepared for chemical characterization from 8 to 20 g of material, which is consistent with the suggested mass (10 g) necessary to provide representative sampling of ordinary chondrites. Olivine and low-Ca pyroxene are the most abundant phases present, comprising one-half to three-fourths of total abundances, while plagioclase, high-Ca pyroxene, troilite, and metal comprise the remaining XRD-measured mineralogy. Pigeonite may also be present in some samples, but it is fitted using a high-Ca pyroxene standard, so exact abundances cannot be measured directly using XRD. Comparison of XRD-measured abundances with calculated Cross, Iddings, Pirsson, Washington (CIPW) normative abundances indicates that systematic discrepancies exist between these two data sets, particularly in olivine and high-Ca pyroxene. This discrepancy is attributed to the absence of pigeonite as a possible phase in the CIPW normative mineralogy. Oxides associated with pigeonite are improperly allocated, resulting in overestimated normative olivine abundances and underestimated normative high-Ca pyroxene abundances. This suggests that the CIPW norm is poorly suited for determining mineralogical modal abundances of ordinary chondrites.

Hf–W thermochronometry: II. Accretion and thermal history of the acapulcoite–lodranite parent body

Acapulcoites and lodranites are highly metamorphosed to partially molten meteorites with mineral and bulk compositions similar to those of ordinary chondrites. These properties place the acapulcoites and lodranites between the unmelted chondrites and the differentiated meteorites and as such acapulcoites–lodranites are of special interest for understanding the initial stages of asteroid differentiation as well as the role of 26Al heating in the thermal history of asteroids. To constrain the accretion timescale and thermal history of the acapulcoite–lodranite parent body, and to compare these results to the thermal histories of other meteorite parent bodies, the Hf–W system was applied to several acapulcoites and lodranites. Acapulcoites Dhofar 125 and NWA 2775 and lodranite NWA 2627 have indistinguishable Hf–W ages of Δ t CAI = 5.2 ± 0.9 Ma and Δ t CAI = 5.7 ± 1.0 Ma, corresponding to absolute ages of 4563.1 ± 0.8 Ma and 4562.6 ± 0.9 Ma. Closure temperatures for the Hf–W system for acapulcoites and lodranites, estimated from numerical simulations of W diffusion in high-Ca pyroxene, are 975 ± 50 °C and 1025 ± 50 °C, respectively. Owing to these high closure temperatures, the Hf–W ages provide information on the earliest high-temperature evolution, and combined with thermal modeling indicate that the acapulcoite–lodranite parent body accreted ~ 1.5–2 Ma after CAI formation, was internally heated by 26Al decay, and reached its thermal peak ~ 3 Ma after CAI formation. Cooling rates for acapulcoites decreased from ~ 120 °C/Ma just below the thermal peak to ~ 50 °C/Ma at ~ 600 °C. Over the same temperature interval the cooling rate for lodranites decreased from ~ 100 °C/Ma to ~ 40 °C/Ma. These thermal histories may reflect cooling in the uppermost ~ 10 km of a parent body with a radius of ~ 35–100 km. Acapulcoites and lodranites evolved with a 180Hf/ 184W ratio of ~ 0.64, which is indistinguishable from that of H chondrites but significantly lower than 180Hf/ 184W~ 1.23 for carbonaceous chondrites. The low 180Hf/ 184W ratios of acapulcoites–lodranites were established before ~ 2 Ma and, hence, prior to partial melting in the parent body at ~ 3 Ma. Thus, they must reflect Hf–W fractionation of the precursor material by processes in the solar nebula. Combined with Hf–W ages of Δ t CAI < 1 Ma for differentiation of the parent bodies of magmatic iron meteorites and an Hf–W age of Δ t CAI ~ 2.5 Ma for the accretion of the H chondrite parent body, the Hf–W results for acapulcoites and lodranites reveal an inverse correlation between accretion age of asteroids and peak temperature in their interiors. The different thermal histories of most meteorite parent bodies, therefore, primarily reflect variations in their initial 26Al abundance, which is determined by their accretion time.

High-temperature annealing of amoeboid olivine aggregates: Heating experiments on olivine–anorthite mixtures

Amoeboid olivine aggregates (AOAs) are composed of forsteritic olivine, Fe,Ni-metal, and Ca,Al-rich nodules consisting mainly of Al-diopside, spinel, and anorthite. Although the textures, shapes, and mineralogy of AOAs are consistent with their being aggregates of nebular condensates, some textures are in conflict with equilibrium condensation calculations, indicating that AOAs were not produced by a simple one-stage condensation. To examine the origin of the constituent minerals within AOAs and their textural relationships, we performed heating experiments using mineral mixtures analogous to those in AOAs. Isothermal and cooling experiments on forsterite + anorthite mixtures reveal that a high-Ca pyroxene phase forms via the incipient melting of the two minerals. Comparative studies of heating experiments performed using the mineralogy of AOAs suggest that Al-diopside in AOAs can be produced from a small degree of melting of forsterite and anorthite. The formation of Al-diopside in this way is consistent with the annealing textures observed in AOAs, and it may account for the discrepancy between the observed mineralogy of AOAs and the results of equilibrium condensation calculations, the occurrence of two types of diopside (Al,Ti-rich diopside and Al-diopside), and the variable Al 2O 3 content of Al-diopside.

Can pyroxenes be liquidus minerals in the kimberlite magma?

Clinopyroxene and orthopyroxene are generally rare in kimberlites, and believed to originate from disintegrated mantle and crustal xenoliths. We report occurrence of inclusions of low-Ca and high-Ca pyroxenes in the olivine phenocrysts in the Udachnaya–East hypabyssal kimberlite (Yakutia, Russia), and make inferences on their relationships to the kimberlite magma. Pyroxenes are only found as either resorbed macrocrysts or small inclusions in olivine; both types are being very rare and volumetrically insignificant. All clinopyroxene and majority of orthopyroxene inclusions are hosted in the olivine cores (Fo 86–92) that are compositionally similar to olivine macrocrysts. Major and trace element compositions of clinopyroxene inclusions in olivine phenocrysts and macrocrysts are similar, and resemble compositions of clinopyroxene from lherzolite xenoliths hosted in the Udachnaya–East kimberlite. Their high Na 2O and Cr 2O 3 abundances (0.65–2.55 and 0.6–2.8 wt.%, respectively) suggest deep mantle origin (> 4.5 GPa). The majority of clinopyroxene inclusions have convex-upward trace element patterns that imply kimberlite-like compositions for the hypothetical equilibrium melts. Re-equilibration of clinopyroxene inclusions with the host kimberlite liquid through micro-cracks in olivine is our preferred explanation in this case, however, we cannot exclude their high-pressure crystallisation from the protokimberlite melt. Orthopyroxene inclusions show significant compositional overlap with the low-Al orthopyroxene from the Udachnaya peridotite nodules. Where such inclusions occur in olivine fragments and contact with the kimberlite groundmass, they are strongly resorbed and partially replaced by monticellite. The mantle origin of orthopyroxene and its host olivine and disequilibrium relationships with the kimberlite melt is most likely. Both clinopyroxene and orthopyroxene are completely absent in the kimberlite groundmass. We conclude that the parental melt of the Udachnaya–East kimberlite was not saturated in either clinopyroxene or orthopyroxene at low pressure. This argues against a perceived mafic-ultramafic lineage of the kimberlite primary melt, and provides further support for its essentially carbonate-chloride composition.