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The Almahata Sitta (AhS) meteorite consists of disaggregated clasts from the impact of the polymict asteroid 2008 TC3, including ureilitic (70%–80%) and diverse non‐ureilitic materials. We determined the 40Ar/39Ar release patterns for 16 AhS samples (3–1500 μg) taken from three chondritic clasts, AhS 100 (L4), AhS 25 (H5), and MS‐D (EL6), as well as a clast of ureilitic trachyandesite MS‐MU‐011, also known as ALM‐A, which is probably a sample of the crust of the ureilite parent body (UPB). Based on our analyses, best estimates of the 40Ar/39Ar ages (Ma) of the chondritic clasts are 4535 ± 10 (L4), 4537–4555 with a younger age preferred (H5), and 4513 ± 17 (EL6). The ages for the L4 and the H5 clasts are older than the most published 40Ar/39Ar ages for L4 and H5 meteorites, respectively. The age for the EL6 clast is typical of older EL6 chondrites. These ages indicate times of argon closure ranging up to 50 Ma after the main constituents of the host breccia, that is, the ureilitic components of AhS, reached the >800°C blocking temperatures of pyroxene and olivine thermometers. We suggest that these ages record the times at which the clasts cooled to the Ar closure temperatures on their respective parent bodies. This interpretation is consistent with the recent proposal that the majority of xenolithic materials in polymict ureilites were implanted into regolith 40–60 Ma after calcium–aluminum‐rich inclusion and is consistent with the interpretation that 2008 TC3 was a polymict ureilite. With allowance for its 10‐Ma uncertainty, the 4549‐Ma 40Ar/39Ar age of ALM‐A is consistent with closure within a few Ma of the time recorded by its Pb/Pb age either on the UPB or as part of a rapidly cooling fragment. Plots of age versus cumulative 39Ar release for 10 of 15 samples with ≥5 heating steps indicate minor losses of 40Ar over the last 4.5 Ga. The other five such samples lost some 40Ar at estimated times no earlier than 3800–4500 Ma bp. Clustering of ages in the low‐temperature data for these five samples suggests that an impact caused localized heating of the AhS progenitor ~2.7 Ga ago. In agreement with the published work, 10 estimates of cosmic‐ray exposure ages based on 38Ar concentrations average 17 ± 5 Ma but may include some early irradiation. [ABSTRACT FROM AUTHOR]

Oxygen fugacity (f[o.sub.2]) is an important parameter because it influences the crystallization sequences of magmas, as well as the composition of the resultant minerals. Traditionally f[o.sub.] has been estimated using coexisting Fe-Ti oxide compositions or [Fe.sup.3+]/[Fe.sup.2+] ratios in silicate melts, for which the relationship between f[o.sub.2] and [Fe.sup.3+] has been calibrated. Problems arise in systems where oxides and/or glasses (melts) are absent, or when oxides are suspected to have been re-equilibrated. Therefore, it is important to develop alternate ways to look at f[o.sub.2] in volcanic rocks. In this work. we describe a set of experiments to establish the relationship between the [Fe.sup.3+] content of clinopyroxene and a basaltic melt as a function of oxygen fugacity. The starting bulk compositions were chosen to be representative of martian parental melts, with pyroxene (either augite or pigeonite) on their liquidi, in order for the results to be applicable to the martian meteorite magma source regions. Experimental conditions ranged from f[o.sub.2] = IW to ~MW+8.6 at 1165 [degrees]C for pigconite/melt pairs and at 1235 [degrees]C for melts alone, and f[o.sub.2] = IW-1 to IW+4 at ~1190 [degrees]C for augite/melt pairs. Results show that [Fe.sup.3+] varies from 0-65% of the total Fe over this range of experimental conditions, and mass D (wt%, [Fe.sup.3+.sub.Py]/wt%[Fe.sup.3+.sub.melt]) ranged from 0.75-1.44 and from 0.00-0.77, for pigeonite and augite respectively. XANES and Mossbauer analytical methods give similar results within known error bars of the two techniques, with the best agreement in compositions with >10% [Fe.sup.3+]. Our data show that only at high values of f[o.sub.2] (>IW + 3.5. or [approximately equal to] QFM) can [Fe.sup.3+] distribution in pyroxene be used to estimate crystallization oxygen fugacity.

A new method has been developed to orient 50-100 [micro]m size single crystals to obtain XANES or EXAFS spectra along any crystallographic orientation relative to a polarized synchrotron X-ray beam. The method uses a modified spindle stage and an X-ray goniometer head to hold a crystal and the glass fiber on which it is mounted. The orientation of the crystallographic axes within the crystal can be determined by X-ray diffraction in all cases or with the use of the polarizing light microscope if the minerals are biaxial. Once an optical orientation direction has been located, it can be oriented parallel to the polarization direction of the synchrotron source for spectral acquisition. Mineral samples representing the three optical classes (i.e., isotropic, uniaxial, and biaxial) were oriented along their a, b, and c crystallographic axes, and Fe-XANES spectra were obtained. In all cases the interactions of polarized X-rays with the crystal structures are analogous to the interactions of visible light (e.g., as represented by refractive index). For the isotropic almandine sample the XANES spectra were similar at all orientations. For the uniaxial buergerite and scapolite samples the spectra were similar along the a and b axes and different along c. A detailed set of spectra acquired at multiple (known) optical orientations in the scapolite crystal demonstrates that spectral features vary in intensity as a function of orientation, with maxima and minima when the X-rays are polarized along optical orientations. For the biaxial fayalite sample, the XANES spectra differed along all three crystallographic axes, as expected by analogy to optical properties.

Synchrotron micro-XANES (X-ray absorption near-edge structure) spectroscopy (SmX) was used to measure [Fe.sup.3+] and [Fe.sup.2+] distribution among minerals in standard thin sections with an X-ray beam size of 10 x 15 [micro] m. Measurements were made at beamline X26a, National Synchrotron Light Source, Brookhaven National Lab. Samples studied included metapelites from garnet to upper sillimanite zone rocks that coexist with graphite, graphite/ilmenite, or varying combinations of ilmenite, magnetite, and hematite. SmX results are compared with Mossbauer spectroscopic measurements of [Fe.sup.3+] on mineral separates from the same rocks. Results show excellent agreement (within [+ or -] 5-10%) between Mossbauer and SmX for the Fe-rich phases biotite, chlorite, staurolite, and garnet. Mossbauer spectra of muscovite typically show lower values than SmX for [Fe.sup.3+]/[SIGMA]Fe (especially at low grades), suggesting contamination of the muscovite separates by fine-grained chlorite. However, heterogeneity of [Fe.sup.3+] and [Fe.sup.2+] is probably the chief source of discrepancy between the bulk and micro-scale measurements. The % [Fe.sup.3+] (relative to total Fe) in these samples ranges from a high of 90% in muscovite to a low of 0-2% in garnet, with Ms > St = Bt = Chl> Tur > Grt in graphite/ilmenite-bearing rocks. SmX measurements suggest that the averaged [Fe.sup.3+]/[SIGMA]Fe in each mineral species does not change as a function of grade, but varies as a function of buffering assemblage, especially on a very localized scale. This effect shows that the oxygen buffering capacity of mineral assemblages is very large compared to the oxidizing/reducing potential of metamorphic fluids. However, distribution of Fe atoms among phases at each grade also reflects crystal chemistry. Across all grades, the [Fe.sup.2+]/Mg ratio is such that Grt >> St > Bt > Chl> Tur > Ms. [Fe.sup.3+]/Al is highest in biotite in garnet and sillimanite-zone rocks. For [Fe.sup.3+]/Al, Bt > Chl> Tur > St > Ms, and for [Fe.sup.3+]/[Fe.sup.2+], partitioning behaves as Ms >> Bt [approximately equal to] St [approximately equal to] Chl> Tur > Grt in graphite/ilmenite-bearing rocks. Overall, partitioning of [Fe.sup.3+] in any of the minerals of interest reflects both oxide assemblage and crystal-chemical constraints on the amount of [Fe.sup.3+] that can be substituted into the structure.

To understand compositional variation in igneous biotite, full analyses of a suite of biotites of variable composition from volcanic and xenolith parageneses have been completed. Major and minor elements were determined by electron microprobe analysis, water was determined by manometry and SIMS analysis, and [Fe.sup.3+]/[Fe.sup.2+] was determined by microXANES and Mossbauer spectroscopy. Our new data, together with previous biotite analyses (total of 52 analyses), reveal correlations between [O.sup.2-] (2-F-Cl-OH) and the sum of the octahedral cations Al + Ti + [Fe.sup.3+] + Cr. This correlation allows estimation of either OH-or [Fe.sup.3+]/[Fe.sup.2+] as long as one or the other has been determined. The hydroxyl site in most mantle micas contains at least 1.0 [O.sup.2-] atoms per formula unit (apfu), indicating that the oxy-component cannot be ignored. The large oxy-component in melt inclusion micas from the martian meteorite Chassigny does not necessarily indicate oxidized or hydrous magmatic conditions because dehydrogenation may have occurred and/or because the oxy-component may be stable at low oxygen fugacity. The large variation in Ti, Al, and [Fe.sup.3+] in natural igneous micas is most likely dependent upon bulk compositional differences in each specific system such as variation of [a.sub.Ti[O.sub.2]] and [a.sub.[Al.sub.2][O.sub.3]] in silicate melts.

Synchrotron micro-XANES spectroscopy (SmX) is used to examine the amount and distribution of [Fe.sup.2+] and [Fe.sup.3+] in five samples of fayalite previously studied by Mossbauer spectroscopy. Rockport fayalite is homogeneous and the Fe in it is completely reduced. Olivine from Qianan is almost completely oxidized, and probably contains finely intergrown silica, laihunite, and hematite with hematite predominating. Pantelleria olivine has an oxidized rim around a reduced core, suggesting either a post-crystallization reaction with fayalite and oxygen going to laihunite and hematite or a change in prevailing oxidation during growth. The texture of olivine from the Mourne Mountains suggests exsolution from a fayalite-laihunite solid solution, based on the substitution of three [Fe.sup.3+] atoms for two [Fe.sup.2+] and one vacancy, that was stable at high temperature. Laihunite from the type locality (Lai-He village) shows 1-3 [[micro]meter] mottling between light and dark areas in backscatter images, but these areas cannot correspond to pure fayalite and laihunite exsolution because the bulk [Fe.sup.3+]/[Sigma]Fe of such a combination would not correspond to the SmX value of 67% [Fe.sup.3+]. Rather, the texture observed in backscatter is interpreted to represent alternating areas of magnetite-rich and laihunite-rich compositions with a similar (i.e., 67% [Fe.sup.3+]/[Sigma]Fe) extent of oxidation.

In situ measurements of the oxidation state of iron in common minerals have been made by using the synchrotron microXANES (SmX) technique. The results compare very well with wet-chemical and Mossbauer spectroscopic analyses of the same samples. Areas of 10 x 20 [[micro]meter] have been measured successfully, and the results demonstrate that both zoning of [Fe.sup.3+] and the effect of oxide inclusions on bulk analyses can be quantified. Such sample heterogeneity cannot be detected by conventional bulk analytical techniques. The ability to measure [Fe.sup.3+]/[Sigma]Fe with spatial resolution comparable to that of the electron probe further enhances the importance of microbeam techniques in the earth and planetary sciences. Direct measurements of elemental oxidation states that can constrain oxidation-reduction processes are now possible with close to the spatial resolution of traditional compositional analyses derived from electron, or ion, beam and optical microscopic techniques. New, more intense, synchrotron sources such as the Advanced Photon Source at Argonne National Laboratory, are becoming available for analyses of 1-10 [[micro]meter] areas.

We present a method for 40Ar/39Ar step‐heating measurements on anhydrous K‐bearing minerals. Samples placed in dimples on Ta foil platforms are heated indirectly with a laser while temperatures are measured with a two‐color pyrometer. The system provides a simple, inexpensive, and reliable means for the heating of samples as small as a few to tens of micrograms. Results for plagioclase separates from the Rustenburg Layered Suite of the circa 2.06 Ga Bushveld Complex closely match previously published data. Key Points: Tantalum platforms provide a simple and reliable support for the indirect, reproducible, stepwise laser heating of microgram samplesPlagioclase diffusion parameters measured from the Bushveld Complex closely match published data from the complex, validating the method [ABSTRACT FROM AUTHOR]

Freshwater limestones are uncommon in the sedimentary record of the East African Rift System. Recent research in the Olduvai Basin, Tanzania has revealed an extensive carbonate unit ca 1.8 Ma in age that varies spatially in thickness, petrography, trace element and rare earth geochemistry, C and O isotope values and freshwater microfossils. Five distinct lithologies are recognized. Four reflect deposition in carbonate‐rich environments that occurred at the same time in a catena‐like pattern over a heterogeneous landscape: spring, wetland, freshwater lake/pond and saline playa lake. The fifth is carbonate nodular soil that formed by paedogenesis on areas of topographically higher terrane. The δ18O of the carbonate, as negative as −6.5‰, indicates that the groundwater is meteoritic derived. The origin of the carbonate comprising this thick, extensive, isochronostratigraphic deposit has not been determined, but it is believed that carbonatite volcanism may have supplied elevated levels of carbonate to the basin which then entered the groundwater system. The carbonate was deposited just prior to a major trachyte‐phonolite eruption of Mount Olmoti, and the deposition of a basin wide tuff, Tuff IF. The region is seismically active today and seismic events were likely associated with Olmoti volcanism. The widespread carbonate unit may have preceded the Olmoti eruption and formed from a recurring seismically induced increase in aquifer porosity and permeability. The carbonate‐enriched groundwater episodically discharged onto the land surface, interacting locally with both fresh and saline ponded water. [ABSTRACT FROM AUTHOR]

Microanalysis of Fe3+/ΣFe in geological samples using synchrotron-based X-ray absorption spectroscopy has become routine since the introduction of standards and model compounds. Existing calibrations commonly use least-squares linear combinations of pre-edge data from standard reference spectra with known coordination number and valence state acquired on powdered samples to avoid preferred orientation. However, application of these methods to single mineral grains is appropriate only for isometric minerals and limits their application to analysis of in situ grains in thin sections. In this work, a calibration suite developed by acquiring X-ray absorption near-edge spectroscopy (XANES) data from amphibole single crystals with the beam polarized along the major optical directions (X, Y, and Z) is employed. Seven different methods for predicting %Fe3+ were employed based on (1) areanormalized pre-edge peak centroid, (2) the energy of the main absorption edge at the location where the normalized edge intensity has the highest R2 correlation with Fe3+/ΣFe, (3) the ratio of spectral intensities at two energies determined by highest R2 correlation with Fe3+/ΣFe, (4) use of the slope (first derivative) at every channel to select the best predictor channel, (5 and 6) partial least-squares models with variable and constant numbers of components, and (7) least absolute shrinkage and selection operator models. The latter three sophisticated multivariate analysis techniques for predicting Fe3+/ΣFe show significant improvements in accuracy over the former four types of univariate models. Fe3+/ΣFe can be measured in randomly oriented amphibole single crystals with an accuracy of ±5.5-6.2% absolute. Multivariate approaches demonstrate that for amphiboles main edge and EXAFS regions contain important features for predicting valence state. This suggests that in this mineral group, local structural changes accommodating site occupancy by Fe3+ vs. Fe2+ have a pronounced (and diagnostic) effect on the XAS spectra that can be reliably used to precisely constrain Fe3+/ΣFe. [ABSTRACT FROM AUTHOR]

The Hayabusa mission to asteroid 25143, Itokawa, brought back 2000 small particles, which most closely resemble material found in LL4-6 chondrites. We report an 40Ar/39Ar age of 1.3 ± 0.3 Ga for a sample of Itokawa consisting of three grains with a total mass of ~2 µg. This age is lower than the >4.0 Ga ages measured for 75% of LL chondrites but close to one for Y-790964 and its pairs. The flat 40Ar/39Ar release spectrum of the sample suggests complete degassing 1.3 Ga ago. Recent solar heating in Itokawa's current orbit does not appear likely to have reset that age. Solar or impact heating 1.3 Ga ago could have done so. If impact heating was responsible, then the 1.3 Ga age sets an upper bound on the time at which the Itokawa rubble pile was assembled and suggests that rubble pile creation was an ongoing process in the inner solar system for at least the first 3 billion years of solar system history. [ABSTRACT FROM AUTHOR]

Measurements of Fe3+/ΣFe in geological materials have been intractable because of lack of access to appropriate facilities, the time-consuming nature of most analyses, and the lack of precision and reproducibility in most techniques. Accurate use of bulk Mössbauer spectroscopy is limited by largely unconstrained recoilless fraction (f), which is used to convert spectral peak area ratios into valid estimates of species concentrations and is unique to different mineral groups and compositions. Use of petrographic-scale synchrotron micro-XANES has been handicapped by the lack of a consistent model to relate spectral features to Fe3+/ΣFe. This paper addresses these two deficiencies, focusing specifically on a set of garnet group minerals. Variable-temperature Mössbauer spectra of the Fe2+-bearing almandine and Fe3+-bearing andradite end-members are used to characterize f in garnets, allowing Fe3+/ΣFe to be measured accurately. Mössbauer spectra of 19 garnets with varying composition were acquired and fit, producing a set of garnet-specific standards for Fe3+ analyses. High-resolution XANES data were then acquired from these and 15 additional previously studied samples to create a calibration suite representing a broad range of Fe3+ and garnet composition. Several previously proposed techniques for using simple linear regression methods to predict Fe3+/ΣFe were evaluated, along with the multivariate analysis technique of partial least-squares regression (PLS). Results show that PLS analysis of the entire XANES spectral region yields the most accurate predictions of Fe3+ in garnets with both robustness and generalizability. Together, these two techniques present reliable choices for bulk and microanalysis of garnet group minerals. [ABSTRACT FROM AUTHOR]

Deals with a study which measured iron distribution among minerals in metapelites. Background to the study; Theoretical framework for sample selection; Results and discussion.

Examines the compositional variation in igneous biotite through analyses of a suite of biotites of variable composition from volcanic and xenolith parageneses. Effect of dehydrogenation on the original composition of biotites; Examination of biotite data from mantle samples; Analysis of mantle rocks from Mars.

The Almahata Sitta (AhS) meteorite consists of disaggregated clasts from the impact of the polymict asteroid 2008 TC3, including ureilitic (70%–80%) and diverse non‐ureilitic materials. We determined the 40Ar/39Ar release patterns for 16 AhS samples (3–1500 μg) taken from three chondritic clasts, AhS 100 (L4), AhS 25 (H5), and MS‐D (EL6), as well as a clast of ureilitic trachyandesite MS‐MU‐011, also known as ALM‐A, which is probably a sample of the crust of the ureilite parent body (UPB). Based on our analyses, best estimates of the 40Ar/39Ar ages (Ma) of the chondritic clasts are 4535 ± 10 (L4), 4537–4555 with a younger age preferred (H5), and 4513 ± 17 (EL6). The ages for the L4 and the H5 clasts are older than the most published 40Ar/39Ar ages for L4 and H5 meteorites, respectively. The age for the EL6 clast is typical of older EL6 chondrites. These ages indicate times of argon closure ranging up to 50 Ma after the main constituents of the host breccia, that is, the ureilitic components of AhS, reached the >800°C blocking temperatures of pyroxene and olivine thermometers. We suggest that these ages record the times at which the clasts cooled to the Ar closure temperatures on their respective parent bodies. This interpretation is consistent with the recent proposal that the majority of xenolithic materials in polymict ureilites were implanted into regolith 40–60 Ma after calcium–aluminum‐rich inclusion and is consistent with the interpretation that 2008 TC3 was a polymict ureilite. With allowance for its 10‐Ma uncertainty, the 4549‐Ma 40Ar/39Ar age of ALM‐A is consistent with closure within a few Ma of the time recorded by its Pb/Pb age either on the UPB or as part of a rapidly cooling fragment. Plots of age versus cumulative 39Ar release for 10 of 15 samples with ≥5 heating steps indicate minor losses of 40Ar over the last 4.5 Ga. The other five such samples lost some 40Ar at estimated times no earlier than 3800–4500 Ma bp. Clustering of ages in the low‐temperature data for these five samples suggests that an impact caused localized heating of the AhS progenitor ~2.7 Ga ago. In agreement with the published work, 10 estimates of cosmic‐ray exposure ages based on 38Ar concentrations average 17 ± 5 Ma but may include some early irradiation. [ABSTRACT FROM AUTHOR]

Provides information on a study which examined the amount and distribution of iron, Fe2+ and Fe3+, in five samples of fayalite previously studied by Mossbauer spectroscopy. Review of related literature; Experimental methods; Results and discussion.

40Ar/39Ar ages of single feldspar grains from the paired meteorites Graves Nunatak 06128 (GRA8; 8 grains) and 06129 (GRA9; 26 grains) are presented. Plateau ages (⩾70% of the 39Ar released) ranged from 4000 to 4600Ma with an average 1-σ uncertainty of ±90Ma. The most precise ages obtained were 4267±17Ma for a grain from GRA8 and 4437±19Ma and 4321±18Ma for two grains from GRA9. Isotope correlation diagrams yield less precise ages ranging from 3800 to 5200Ma with an average 1-σ uncertainty of 250Ma; they indicate a negligible trapped component. Plateau ages, integrated total fusion ages, and isochron ages are internally concordant at the 95% confidence level. The distribution of the plateau ages for GRA9 is bimodal with peaks at 4400 and 4300Ma. In contrast, the plateau age distribution for GRA8 peaks at about 4260Ma with broad wings extending toward younger and older ages. To explain the distributions of grain ages we prefer a scenario that includes a major post-formation event about 4400Ma ago and a later melt intrusion event that heated GRA8 more than some parts of GRA9. [ABSTRACT FROM AUTHOR]