Ca Pyroxene Research Articles

Evaluation of two (opx-cpx) pyroxene Geothermometer

In the last few decades, for estimation of the original equilibrium conditions of the mantle and deep crustal rocks, several empirical as well as synthetic thermometers have been proposed. Various types of rocks from earth, moon or meteorites contain two coexisting pyroxenes. Petrologists have long recognised the potential of coexisting high Ca and low Ca pyroxenes to yield thermometric calculations. Several models have been proposed for two pyroxene thermometers in the last few decades. The authors have compared ten models of two pyroxene thermometers proposed since 1973. Sixty one (61) sample data of granulites from the global literature were collected and processed through the “Opx-Cpx.EXE” software. We conclude that three models are the most valid and reliable of this kinds of thermometers: Kretz (1982), Bertrend and Mercier (1985) and Nickel and Green (1985).

A new compositional, mineralogical and chronological study of the Leibnitz crater within the SPA basin

Leibnitz is an impact crater 237 ​km in diameter located on the lunar farside within the South Pole–Aitken (SPA) basin. It shares much of its southern rim with the Von Kármán crater (186 ​km in diameter), which is currently the focus of in situ exploration by the Yutu-2 rover, part of the Chang'e−4 mission. This complementary study analyzes Leibnitz's surface materials mineralogy and age using remote sensing data. We focused on the spectral information from 95 fresh craters within Leibnitz to minimize the effects of weathering on the exposed materials. Subsequently, we employed the Modified Gaussian Model to derive a distribution map for the high to low Ca pyroxene ratio values. Finally, we derived absolute model age estimations of the mare basalts using the lunar crater chronology method. A new geologic map of the Leibnitz crater summarizes the results of this study. The results show that the interiors of Von Kármán and Leibnitz share a degree of similarity in mineral composition, with strong absorption features around the 1 ​μm and 2 ​μm bandcenters, due to the spectral dominance of mafic minerals. The ejecta from the adjacent Finsen crater have higher Ca/Fe and HCP/LCP ratios than the exposed materials of the Leibnitz crater. This study also finds evidence to support an older age of formation of the crater than age estimates to date from previous studies of the Leibnitz crater.

Mineralogy, petrology, and oxygen isotopic compositions of aluminum-rich chondrules from unequilibrated ordinary and the Dar al Gani 083 (CO3.1) chondrite

Understanding the genetic relationship between different chondritic components will help to decipher their origin and dynamical evolution within the protoplanetary disk. Here, we obtain insight into these processes by acquiring O-isotope data from 17 Al-rich chondrules from unequilibrated ordinary chondrites (OCs, petrologic type ≤ 3.2) and four Al-rich chondrules from the CO3.1 carbonaceous chondrite Dar al Gani (DaG) 083. These particular kinds of chondrules are of special interest, as it is suggested that their precursors may have contained refractory material related to Ca,Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs).The four investigated Al-rich chondrules from the CO3.1 chondrite Dar al Gani 083 consist of olivine, low-Ca pyroxene, Ca pyroxene, and spinel phenocrysts embedded in mostly Na-rich glassy mesostasis. Two chondrules have a homogeneous O-isotopic composition and two are heterogeneous in their O-isotopic composition. One chondrule contains relict spinel grains with a Δ17O value of −24.3 ± 1.3‰, indicative of 16O-rich precursor refractory material, similar to constituents of CAIs and AOAs. The presence of CAI-like precursors for the Al-rich chondrules from CO chondrites is consistent with their previously reported presence of 50Ti excesses (Ebert et al., 2018).The Al-rich chondrules in the ordinary chondrites studied consist of olivine, low-Ca pyroxene, Ca pyroxene, and, occasionally, spinel phenocrysts embedded in mostly Na-rich glassy mesostasis. Hibonite is present in one Al-rich chondrule. The vast majority of these chondrules have heterogeneous O-isotopic compositions: Chondrule glasses are 16O-depleted compared to chondrule phenocrysts; the Δ17O values of the former approach those of aqueously formed fayalite and magnetite grains in type 3 OCs, ∼ +5‰. We infer that the chondrule glasses experienced O-isotope exchange with an aqueous fluid on the OC parent asteroids.Chondrule phenocrysts, like spinel, olivine, low-Ca pyroxene, and Ca pyroxene, were not affected by this isotope exchange and preserved their initial O-isotope compositions. The phenocrysts within individual chondrules have similar Δ17O, whereas the inter-chondrule Δ17O values range from −4.5 to +1.4‰, i.e., they are in general 16O enriched relative to the majority of ferromagnesian type I and type II porphyritic chondrules in OCs having Δ17O of ∼ +1‰. Because no relict grains were identified in the Al-rich chondrules from ordinary chondrites, the original O-isotopic composition of the refractory precursor material remains unknown.Additional detailed Na measurements within olivine grains show no major changes in the Na content of the chondrule melt during their crystallization. This implies either that the Na was part of the precursor material or that the Na was enriched in the chondrule melt/glass after crystallization of the olivines.

Dating Clinopyroxene Phenocrysts in Submarine Basalts Using 40Ar/39Ar Geochronology

AbstractDating submarine basalts using 40Ar/39Ar geochronology is often hindered by a lack of potassium‐bearing phenocrystic phases and severe alteration in the groundmass. Clinopyroxene is a common phenocrystic phase in seafloor basalts and is highly resistive to low‐temperature alteration. Here we show that clinopyroxene phenocrysts separated from marine basalts are a viable phase for 40Ar/39Ar incremental heating age determinations. We provide results from a pilot study comprising 16 age experiments from nine clinopyroxene separates, five of which from samples with dated coeval phases. The clinopyroxene ages range from 11.5 to 112 Ma with relatively high uncertainties (ranging from 0.8% to 7.1%; median of 1.9%) compared to more traditional phases. The clinopyroxene age plateaus form at low to moderate temperature steps and are characterized by relatively elevated K/Ca of 0.002–0.4, suggesting that other K‐bearing phases hosted within the clinopyroxene are likely degassing to yield the 40Ar/39Ar age information. There are three possible origins for the K and corresponding 40Ar* including films of trapped melt/nanomineral inclusions along grain defects, secondary melt inclusion bands, or variations in degassing behaviors between lower and higher crystalline Ca pyroxene phases. Regardless of the source of the K, the age determinations are successful with 75% of the experiments producing long plateaus (>60% 39Ar released) with mean square of the weighted deviations ranging from 0.6 to 1.5 and probability of fit values >0.05. We conclude that clinopyroxene dating by the 40Ar/39Ar method has the potential to provide a wealth of information for previously undated, altered seafloor lithologies and continental equivalents.

Spectral mapping of morphological features on the moon with MGM and SAM

Three types of morphological features observed in different lunar crustal terrains were studied and mapped using hyperspectral Moon mineralogy mapper (M3) data onboard Chandrayaan 1 mission in order to assess the utility of cascaded MGM-SAM spectral mixture modeling approach to characterize the surface materials, which may occur as mineral mixtures, at different topography of the lunar surface. Selected morphological features include: the impact melts in Orientale basin, sinuous rilles in Procellarum KREEP Terrane (PKT) and a rayed crater in Feldspathic Highland Terrane (FHT). Methodology involves extraction of spectrally pure pixels (endmembers) of the area using Pixel Purity Index (PPI), identification of mineralogy of the selected endmember spectrum using the Modified Gaussian Method (MGM) and mapping of mineralogically identified endmembers using the Spectral Angle Mapper (SAM) method.Mapping results demonstrate both the capabilities and the limitations of the MGM method of spectral deconvolution and the SAM method of spectral matching as effective tools for compositional characterizations of morphological features on the lunar surface. As a method of spectral deconvolution, MGM was able to identify and characterize both high- and low – Ca pyroxenes along with plagioclase feldspar. The Spectral Angle Mapper (SAM) was able to map identified mineral mixtures from MGM.

Characterization, testing, calibration, and validation of the Berlin emissivity database

The Berlin emissivity database is a spectral library providing a basis for the inter- pretation of thermal emission spectra of planetary regoliths. It contains spectra of plagioclase and potassium feldspars, low and high Ca pyroxenes, olivine, sulfur, Martian analogs, and a lunar highland sample in the wavelength range 3-50 μm. Four particle sizes with dimensions <25 ,2 5-63, 63-125, 125-250 μm are measured for each sample. An exhaustive suite of tests was implemented, to characterize the setup in the planetary emissivity laboratory and the emis- sivities collected in the spectral library. The tests improve the quality of measurements, opti- mizing the sample preparation, calibration algorithm, sample, blackbody, and chamber temperatures, and all the parameters entering in the measurements or calibration processes. The results of our tests are shown and discussed, together with the implications that these results have on the development of the measurement and calibration procedures.©TheAuthors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. (DOI: 10

Nepheline and sodalite in the matrix of the Ningqiang carbonaceous chondrite: Implications for formation through parent-body processes

Ningqiang is an ungrouped carbonaceous chondrite that chemically and petrologically resembles CV3 chondrites. The matrix of Ningqiang shows much higher abundances of Na, K, and Al by factors of 4.4, 2.7, and 1.6, respectively, than in CV3 chondrites. Our scanning and transmission electron microscope observations and synchrotron radiation X-ray diffraction measurements reveal that the major proportions of these elements can be attributed to the presence of nepheline and sodalite. Rietveld refinement of X-ray diffraction data shows that the feldspathoids constitute 7.7vol.% of all crystalline phases in the matrix. Nepheline and sodalite occur mostly as discrete, equidimensional grains 2–5μm in diameter that are dispersed homogeneously in the matrix. Most of the grains contain inclusions of Fe-rich olivine and minor Ca pyroxene, magnetite, troilite, and pentlandite.Despite the high abundances of Na, K, and Al in the matrix of Ningqiang, the bulk meteorite abundances of these elements are comparable to those of the CV group (e.g., Rubin et al., 1988). This means that the chondrules, which constitute a major proportion of the volume other than the matrix in Ningqiang, are depleted in Na, K, and Al. In fact, our analyses and observations show that the chondrules in Ningqiang overall contain very small amounts of these elements. Our interpretation of these findings suggests that nepheline and sodalite in the Ningqiang matrix were originally formed by Na-metasomatism of the chondrules and Ca–Al-rich inclusions in the meteorite parent body. Afterward, they were likely disaggregated and scattered into the matrix. However, it is difficult to envisage that the disaggregation and scattering occurred in situ in the present setting of the meteorite. Hence, we suggest that the Ningqiang meteorite underwent these processes before final lithification.

Chondrule fragments from Comet Wild2: Evidence for high temperature processing in the outer Solar System

Terminal grains from C2063,1,154,1,0 (Track 154) and C2061,1,113,5 (Track 113) from the Stardust collection of Comet Wild2's coma have been studied by TEM and NanoSIMS. Terminal grain 2 of C2063,1,154,1,0 consists of an Al-rich diopside (En 97–99%, Al2O3 9–11wt%), pigeonite (En 85% Wo 15% with TiO2 and Al2O3 contents of 0.5 and 5.2wt%) and minor forsterite and enstatite. The mineral assemblage and Al-rich, Ti-poor composition of the grain are consistent with being a fragment of an Al-rich chondrule, similar to those present in carbonaceous chondrites. The oxygen isotopic composition of the C2063,1,154,1,0 grain was determined by NanoSIMS analyses and found to be δ17O −10.6±5.7‰, δ18O −7.5±2.5‰ and δ17O +1.4±4.3‰, δ18O −6.5±1.6‰ (1σ errors) for the two sections. These figures are distinct from CAIs and consistent with an origin as Al-rich chondrule fragments. Terminal grain 5 of C2061,1,113,5 consists of low Ca pyroxene En 86–87% Fs 10–11% Wo 3–4% and ≤2wt% Al2O3 and in one section 5–10% of a Na-rich silicate phase. This assemblage may be a fragment of a low-Ca pyroxene-bearing chondrule and mesostasis. The original chondrule diameter for the C2063,1,154,1,0 and C2061,1,113,5 samples, by analogy with carbonaceous chondrite chondrules, might have been in the range 0.2–1.0mm. If they were of that size, then the presence of large grains of high temperature material (e.g. ≥1500K for such refractory assemblages) could be explained through commonly invoked models of radial drift from inner to outer Solar System, but only if the chondrules were first fragmented to dust within the inner Solar System. An alternative scenario is that some chondrule formation was associated with high temperature processing and planetesimals in the outer Solar System.

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.

Characterization of the 1.2 μm M1 pyroxene band: Extracting cooling history from near‐IR spectra of pyroxenes and pyroxene‐dominated rocks

Abstract— The 1.2 μm band in near‐infrared spectra of pyroxenes results from Fe2+in the M1 crystallographic site. The distribution of Fe and Mg between the M1 and M2 sites is in part a function of the cooling rate and thermal history of a pyroxene. Combining near‐infrared and Mössbauer spectra for a series of compositionally controlled synthetic Mg, Fe, Ca pyroxenes, we quantify the strength of the 1.2 μm band as a function of Fe2+in the M1 site. Near‐infrared spectra are deconvolved into component absorptions that can be assigned to the M1 and M2 sites using the modified Gaussian model. The relative strength of the 1.2 μm band is shown to be directly related to the amount of Fe2+in the M1 site measured by Mössbauer spectroscopy. The strength of the 1.2 μm band relative to the combined strengths of the 1.2 and 2 μm bands, or the M1 intensity ratio, is calculated for 51 howardite, eucrite, and diogenite (HED) meteorites. Diogenites and cumulate eucrites exhibit the lowest M1 intensity ratios, consistent with their formation as slowly cooled cumulates. Basaltic eucrites exhibit a large range of M1 intensity ratios, all of which are consistently higher than the diogenites and cumulate eucrites. This example illustrates how the M1 intensity ratio can be a used as a tool for characterizing the cooling history of remotely detected pyroxene‐dominated rocks.

Mafic xenoliths in Proterozoic kimberlites from Eastern Dharwar Craton, India: Mineralogy and P– T regime

Mafic xenoliths of garnet pyroxenite and eclogite from the Wajrakarur, Narayanpet and Raichur kimberlite fields in the Archaean Eastern Dharwar Craton (EDC) of southern India have been studied. The composition of clinopyroxene shows transition from omphacite (3–6 wt% Na 2O) in eclogites to Ca pyroxene (<3 wt% Na 2O) in garnet pyroxenites. Some of the xenoliths have additional phases such as kyanite, enstatite, chromian spinel or rutile as discrete grains. Clinopyroxene in a rutile eclogite has an X Mg value of 0.70, which is unusually low compared to the X Mg range of 0.91–0.97 for all other samples. Garnet in the rutile eclogite is also highly iron-rich with an end member composition of Prp 26.5Alm 52.5Grs 14.7Adr 5.1TiAdr 0.3Sps 1.0Uv 0.1. Garnets in several xenoliths are Cr-rich with up to 8 mol% knorringite component. Geothermobarometric calculations in Cr-rich xenoliths yield different P– T ranges for eclogites and garnet pyroxenites with average P– T conditions of 36 kbar and 1080 °C, and 27 kbar and 830 °C, respectively. The calculated P– T ranges approximate to a 45 mW m −2 model geotherm, which is on the higher side of the typical range of xenolith/xenocryst geotherms (35–45 mW m −2) for several Archaean cratons in the world. This indicates that the EDC was hotter than many other shield regions of the world in the mid-Proterozoic period when kimberlites intruded the craton. Textural and mineral chemical characteristics of the mafic xenoliths favour a magmatic cumulate process for their origin as opposed to subducted and metamorphosed oceanic crust.

The Berlin emissivity database (BED)

Remote-sensing infrared spectroscopy is the principal field of investigation for planetary surfaces composition. Past, present and future missions to the solar system bodies include in their payload, instruments measuring the emerging radiation in the infrared range. Apart from measuring the reflected radiance, more and more spacecrafts are equipped with instruments measuring directly the emitted radiation from the planetary surface. The emitted radiation is not only a function of the composition of the material but also of its texture and especially the grain size distribution. For the interpretation of the measured data an emissivity spectral library of planetary analogue materials in grain size fractions appropriate for planetary surfaces is needed. The Berlin emissivity database (BED) presented here is focused on relatively fine-grained size separates, providing thereby a realistic basis for the interpretation of thermal emission spectra of planetary regoliths. The BED is therefore complimentary to existing thermal emission libraries, like the ASU library for example. BED currently contains emissivity spectra of plagioclase and potassium feldspars, low Ca and high Ca pyroxenes, olivine, elemental sulfur, Martian analogue minerals and volcanic soils, and a lunar highland soil sample measured in the wavelength range from 7 to 22 μm as a function of particle size. For each sample we measured the spectra of four particle size separates ranging from <25 to 250 μm. The device we used is built at DLR (Berlin) and is coupled to a Fourier-transform infrared spectrometer Bruker IFS 88 purged with dry air and equipped with a nitrogen-cooled MCT detector. All spectra were acquired with a spectral resolution of 4 cm −1. We are currently working on upgrading our emissivity facility. A new spectrometer (Bruker VERTEX 80 V) and new detectors will allow us to measure the emissivity of samples in the wavelength range from 1 to 50 μm in a vacuum environment. This will be particularly important for the interpretation of the MERcury Thermal Infrared Imaging Spectrometer (MERTIS) data.

A set of laboratory analogue materials for the MERTIS instrument on the ESA BepiColombo mission to Mercury

The MERTIS (Mercury Thermal Imaging Spectrometer) instrument on the ESA mission BepiColombo will study the surface of Mercury in the wavelength range from 7 to 14 μm. The analysis of the MERTIS data will be posing a number of significant challenges. To facilitate the development of analytic tools, support planned ground based observations and facilitate a cross calibration with other instruments on BepiColombo and MESSENGER, a list of Mercury analogue materials was compiled. This set of analogue materials is based on our current knowledge of the surface composition of Mercury and includes plagioclase and potassium feldspars, low and high Ca pyroxenes, olivine, elemental sulfur, and as an extraterrestrial analogue a lunar highland soil sample. The samples are prepared in four different grain size separates ranging from 0 to 250 μm to cover the expected grain size distribution on the surface of Mercury.

A critical evaluation of oxidation versus reduction during metamorphism of L and LL group chondrites, and implications for asteroid spectroscopy

Abstract— Modal mineralogies of individual, equilibrated (petrologic type 4–6 L and LL chondrites have been measured using an electron microprobe mapping technique, and the chemical compositions of coexisting silicate minerals have been analyzed. Progressive changes in the relative abundances and in the molar Fe/Mn and Fe/Mg ratios of olivine, low‐Ca pyroxene, and diopside occur with increasing metamorphic grade. Variations in olivine/low‐Ca pyroxene ratios (Ol/Px) and in metal abundances and compositions with petrologic type support the hypothesis that oxidation of metallic iron accompanied thermal metamorphism in ordinary chondrites. Modal Ol/Px ratios are systematically lower than normative Ol/Px ratios for the same meteorites, suggesting that the commonly used C.I.P.W. norm calculation procedure may not adequately estimate silicate mineral abundances in reduced chondrites. Ol/Px ratios calculated from visible and near‐infrared (VISNIR) reflectance spectra of the same meteorites are not in agreement with other Ol/Px determinations, possibly because of spectral complexities arising from other minerals in chondrites. Characteristic features in VISNIR spectra are sensitive to the proportions and compositions of olivine and pyroxenes, the minerals most affected by oxidative metamorphism. This work may allow spectral calibration for the determination of mineralogy and petrologic type, and thus may be useful for spectroscopic studies of asteroids.

OXYGEN AND CARBON ISOTOPIC COMPOSITION OF GRENVILLE MARBLE, AND AN APPRAISAL OF EQUILIBRIUM IN THE DISTRIBUTION OF ISOTOPES BETWEEN CALCITE AND ASSOCIATED MINERALS, OTTER LAKE AREA, QUEBEC, CANADA

Marble occurs abundantly in a portion of the Grenville Province of the Canadian Shield extending from the Adirondack Mountains, through Ontario, and into the Laurentian Highlands of Quebec, a distance of 300 km. In the Otter Lake area of western Quebec, major marble units, which are obviously metamorphosed limestone, consist of combinations of calcite, dolomite, olivine, humite-group minerals, Ca pyroxene, Ca amphibole, biotite, graphite, and retrograde serpentine and brucite, whereas minor marble units, which are evidently metasomatic rocks, consist of combinations of (pink) calcite, Ca pyroxene, Ca amphibole, biotite, garnet, K-feldspar, scapolite, and titanite. The oxygen and carbon isotopic composition of calcite in the major marble units (δ18OVSMOW in the range 28 to 17‰; δ13CPDB–1 in the range +6 to −2‰) is the same as in the Adirondacks and in unmetamorphosed Proterozoic limestone; only slight shifts (~1‰) can be attributed directly to metamorphic reactions. In bodies of minor marble, mean concentrations of 18O and 13C are slightly lower (+14.5 and +0.23‰, respectively). Mean fractionation-factors (α, expressed as 1000 ln α, are as follows: 18O, calcite/dolomite, +0.295 ( n = 11); 13C, calcite/dolomite, −0.346 ( n = 12); 18O, calcite/biotite, +3.49 ( n = 5); 13C, calcite/graphite, +3.74 ( n = 5). Additional (exploratory) results ( n = 1) on the distribution of 18O are as follows: calcite/olivine, +3.5; calcite/clinohumite, +5.1; calcite/garnet, +2.4; calcite/titanite, +3.1; calcite/Ca pyroxene, +2.0; calcite/Ca amphibole, +2.4, calcite/K-feldspar +1.8; calcite/scapolite, +2.0; calcite/serpentine, +13.9; calcite/brucite, +19.7; a trend of increasing α is related to increasing OH/(O + OH) in the non-carbonates and the preference of OH for 16O. For the first group of mineral pairs ( n ≥ a small variation in α across the study area (relative standard deviation ~0.05%) is viewed as an expression of a close approach to isotopic exchange equilibrium, and for certain pairs from the second group ( n = 1), a temperature estimate close to 700° (the biotite–garnet temperature of associated gneisses) is also viewed as evidence for equilibrium. Attainment of isotopic exchange equilibrium was facilitated by dynamic recrystallization, which occurred during the peak of metamorphism.

Mineralogical controls on the oxygen isotopic compositions of UOCs

We report the results of oxygen isotopic analyses made on mineral separates prepared from chondrules and whole meteorites (UOCs). Feldspar/glass (mainly chondrule mesostasis) is more 16O-poor than the olivine/low Ca pyroxene from which it was separated. This relationship is true both for chondrules and whole meteorites from the three UOC groups. Feldspar and glass have exchanged to a higher degree with 16O-poor gases than ferromagnesian minerals because of the susceptibility of framework silicates and glass to gas–solid diffusion. Olivine and low Ca pyroxene have preserved primitive isotopic compositions. In this way, mineralogy is the dominant control on the oxygen isotopic compositions of UOCs. The similarity of chondrule and whole meteorite isotope compositions suggests that for many UOCs, matrix and chondrules have undergone exchange with the same isotope reservoirs. This exchange postdates chondrule formation and is believed to have occurred after consolidation with the surrounding chondrite matrix in a parent body regolith. After this gas–solid exchange, some isotopic equilibration took place between feldspar and the other relatively 16O-rich solids, as a result of metamorphism. A progressive decrease in the values of δ 18O feldspar/glass–δ 18O olivine/pyroxene for bulk meteorites occurs with increasing petrologic subtype, from 8 0 00 for a type 3.2 to 2.5% for a type 3.9. A chemical origin for the initial 16O-poor gas reservoirs in the H, L, and LL parent bodies by processes of mass independent fractionation is considered more likely than nucleosynthesis. Feldspar/glass separated from Adrar 003 (L/LL3.2) may be in, or close to having, isotopic equilibrium with the 16O-poor gases for L or LL chondrites. On this basis and assuming that the gas was dominated by H 2O and its isotopic composition lay along a line of slope 1 (corresponding to what has been called the equilibrated chondrite line), a tentative upper limit for gas–solid isotopic exchange in UOCs can be calculated, which gives 370°C.

Experimental studies of group A1 chondrules

Dynamic crystallization and annealing experiments have been conducted using a group Al chondrule bulk composition to examine the formation and alteration of mesostases that emit a bright yellow cathodoluminescence (CL). This type of mesostasis has only been observed in group Al and A2 chondrules in the least altered chondritic meteorites. The dynamic crystallization experiments produce yellow luminescing mesostases only in experiments cooled to their final quench temperatures at the slowest cooling rates. The yellow CL is present only where low Ca pyroxene grows from the melt so the remaining liquid is enriched in normative anorthite and diopside. This composition is strongly associated with yellow luminescence in group Al chondrule mesostases. Low temperature hydrothermal annealing experiments demonstrate that the yellow luminescence can only be destroyed and not enhanced or altered to blue luminescence by aqueous alteration. Annealing under dry conditions increased both the amount and intensity of the yellow luminescent material. Annealing experiments designed to further devitrify the mesostasis converted both yellow luminescing and nonluminescent mesostases to blue luminescing mesostases. These results confirm that yellow luminescing mesostases are the result of the thermal history group Al chondrues experienced prior to accretion, and their alteration to blue luminescing mesostases is a direct result of their metamorphic history.

Petrogenesis and geochemical composition of biotites in rare-element granitic pegmatites in the southwestern Grenville Province, Canada

Field, mineralogical, and chemical determinations of biotite from late-tectonic rare-element (U, Th, Mo, Nb, REE) Grenville pegmatites are used to characterize and evaluate their petrogenesis in part of the southwestern Grenville Province. These pegmatites occur within middle to upper amphibolite facies rocks along and adjacent to shear zones and have hybridized margins because of interaction with their host rocks. Endo- and exomorphic biotite forms by the mechanical incorporation or hydrothermal replacement of pre-existing biotite, hornblende, Ca pyroxene and/or feldspar; accompanied by chemical re-equilibration, an increase in grain size, and inherit some of the chemical characteristics of the pegmatite. In general, the Fe/(Fe + Mg) ratio ranges between 0.22 and 0.86. The most highly fractionated biotites have high Fe/(Fe + Mg), Al, Mn, Rb, Nb, and Zn and low Ba. The chemical compositions of biotite from unzoned, partially-zoned, and zoned pegmatites indicate a trend of increasing chemical fractionation based on LIL enrichment. Overlap in calculated log $$(f_{H_2 O} /f_{HF} )$$ (3.2 to 4.7) and log $$(f_{H_2 O} /f_{HCl} )$$ (1.3 to 2.8) for biotite (@ 600°C) among the different pegmatites is extensive. Commonly, magnetite and microcline coexist with biotites having an Fe/(Fe + Mg) between 0.54 to 0.65. Volatile enrichment and vapor-phase saturation are probably responsible for the development of zonation in the pegmatites. The diffusive loss of H2 at or near H2O vapor saturation at high H2O/Fe2+ may be responsible for the oxidized nature of some pegmatites. Rare-element enrichment due to pegmatite fractionation combined with partitioning of rare-elements from the pegmatite melt into the volatile phase and subsequent interaction with the host rocks is key to the formation of these rare-element mineral deposits.

Radioelement distribution in U, Th, Mo, and rare-earth-element pegmatites, skarns, and veins in a portion of the Grenville Province, Ontario and Quebec

Gamma-ray spectrometer measurements were obtained at and in the vicinity of 104 of the 124 U, Th, Mo, and rare-earth-element (REE) occurrences examined in the Central Metasedimentary Belt of the Grenville Province. Spatial, temporal, mineralogical, and geochemical relationships among granitic pegmatites, phlogopite – scapolite – Ca pyroxene skarns, and fluorite – apatite – calcite veins hosting U, Th, Mo, and REE minerals indicate a common magmatic–hydrothermal origin. Quartz–feldspar gneisses in the Central Metasedimentary Belt (n = 54) have low abundances of uranium (1–7 ppm) and thorium (4–27 ppm) suggesting that partial melting, fractional crystallization, and volatile phase separation were responsible for the enrichment of uranium (2–37 ppm) and thorium (5–102 ppm) in uncontaminated granitic pegmatites (n = 163) derived during ultrametamorphism. The U/Th ratio is probably inherited from the source quartz–feldspar gneiss protolith and enhanced during fractionation.Average U and Th concentrations and U/Th ratios at numerous localities show significant positive correlations among pegmatites, skarns, and veins, providing further evidence for a related origin. The interaction of the pegmatite-derived hydrothermal fluids with host rocks produced a spectrum of types and styles of alteration, which include (i) hybridization and (or) endoskarnification along pegmatite margins; (ii) marble- and clinopyroxenite-hosted exoskarn; and (iii) fluorite–apatite–calcite veins. The deposition of U, Th, Mo, and REE from the evolving hydrothermal fluid is responsible for the heterogeneous distribution of U, Th, and REE minerals and molybdenite within pegmatites, skarns, and veins at each locality. Secondary enrichment of uranium in association with hematitized sheared pegmatites and veins may be responsible for the observed large variation in U/Th ratios at some sites.

Deformation and thermal histories of chondrules in the Chainpur (LL3.4) chondrite

Abstract— Transmission‐electron‐microscopy (TEM) and optical data suggest that chondrules in the Chainpur (LL3.4) chondrite experienced varied thermal and deformation histories prior to the final agglomeration of the meteorite. Chainpur may be regarded as an agglomerate or breccia that experienced little deformation or heating during and after the final accumulation and compaction of its constituents.One chondrule in Chainpur was impact‐shocked to high pressures (∼ 20–50 GPa), almost certainly prior to final agglomeration, either while it was an independent entity in space or while it was in the regolith of a parent body. However, most (&gt;85%) of the chondrules in Chainpur were evidently not significantly shock‐metamorphosed subsequent to their formation. The dearth of shock effects implies that most chondrules in Chainpur did not form by shock melting, although some chondrules may have formed by this process.Dusty‐metal‐bearing olivine grains, which are widely interpreted to have escaped melting during chondrule formation, contain moderate densities of dislocations (∼ 108 cm−2). The dislocations in these grains were introduced before or during the last episode of melting in at least one chondrule. This observation can be explained if olivine was impact‐deformed before or during chondrule formation, or if olivine was strained by reduction or thermally‐induced processes during chondrule formation.Low‐Ca pyroxene grains in chondrules are often strained. In most cases this strain probably arose as a by‐product of polytype transformations (protoenstatite → clinoenstatite/orthoenstatite and clinoenstatite → orthoenstatite) that occurred during the igneous crystallization and static annealing of chondrules.Droplet chondrules with glassy mesostases were minimally annealed, consistent with an origin as relatively rapidly cooled objects in an unconfined, cold environment. Some irregular chondrules and at least one droplet chondrule were thermally metamorphosed prior to final agglomeration, either as a result of moderately slow cooling (∼ 100 °C/hr) from melt temperatures (during autometamorphism) or as a result of reheating episodes. Two of the most annealed chondrules contain relatively abundant plagioclase feldspar, and one of these has a uniform olivine composition appropriate to that of an LL4 chondrite.