Petrography, stable isotope compositions, microRaman spectroscopy, and presolar components of Roberts Massif 04133: A reduced CV3 carbonaceous chondrite

Here, we report the mineralogy, petrography, C-N-O-stable isotope compositions, degree of disorder of organic matter, and abundances of presolar components of the chondrite Roberts Massif (RBT) 04133 using a coordinated, multitechnique approach. The results of this study are inconsistent with its initial classification as a Renazzo-like carbonaceous chondrite, and strongly support RBT 04133 being a brecciated, reduced petrologic type >3.3 Vigarano-like carbonaceous (CV) chondrite. RBT 04133 shows no evidence for aqueous alteration. However, it is mildly thermally altered (up to approximately 440 °C); which is apparent in its whole-rock C and N isotopic compositions, the degree of disorder of C in insoluble organic matter, low presolar grain abundances, minor element compositions of Fe,Ni metal, chromite compositions and morphologies, and the presence of unequilibrated silicates. Sulfides within type I chondrules from RBT 04133 appear to be pre-accretionary (i.e., did not form via aqueous alteration), providing further evidence that some sulfide minerals formed prior to accretion of the CV chondrite parent body. The thin section studied contains two reduced CV3 lithologies, one of which appears to be more thermally metamorphosed, indicating that RBT 04133, like several other CV chondrites, is a breccia and thus experienced impact processing. Linear foliation of chondrules was not observed implying that RBT 04133 did not experience high velocity impacts that could lead to extensive thermal metamorphism. Presolar silicates are still present in RBT 04133, although presolar SiC grain abundances are very low, indicating that the progressive destruction or modification of presolar SiC grains begins before presolar silicate grains are completely unidentifiable.


INTRODUCTION
Roberts Massif (RBT) 04133 was found as a single stone weighing 459.4 g during the 2004 ANSMET field season at Roberts Massif, Antarctica. This Antarctic find was initially classified as a petrologic type 2 Renazzo-like carbonaceous (CR) chondrite with a B/C weathering grade . We report a multitechnique study of the petrography, stable isotope compositions, Raman spectral characteristics, and presolar components of RBT 04133. We present data that indicate RBT 04133 is a reduced Vigarano-like carbonaceous (CV3) chondrite.
In general, chondrites can be classified on the basis of their petrography and mineralogy (e.g., Weisberg et al. 2006), bulk element abundances (e.g., Kallemeyn et al. 1989), stable O-isotope compositions (e.g., Clayton and Mayeda 1999), and the C and N isotopic compositions of their insoluble organic matter (IOM; Alexander et al. 2007), although there is often some overlap between groups. The petrologic subtypes of type 3 (i.e., least altered) samples can be determined by various means, including: Cr 2 O 3 content of FeO-rich olivine (for types 3.00 to 3.2; Grossman and Brearley 2005), Ni and Co content of metal (e.g., Kimura et al. 2008), and the Raman spectral parameters of IOM (e.g., Quirico et al. 2003;Bonal et al. 2006;Busemann et al. 2007).
The CV chondrites are divided into three subgroups on the basis of their petrography and mineralogy: the reduced Vigarano-like (CV3 red ), the oxidized Allendelike (CV3 oxA ), and the oxidized Bali-like (CV3 oxB ) chondrites (e.g., McSween 1977; Kimura and Ikeda 1997;Weisberg et al. 1997;Krot et al. 1998Krot et al. , 2000. The modal mineralogies of the CV3 red and CV3 ox subgroups are somewhat distinct (Howard et al. 2010). For example, CV3 ox chondrites contain between 1.9 and 4.2 vol% phyllosilicates, which are typically absent in CV3 red chondrites (Howard et al. 2010). Some samples are intermediate between subgroups (e.g., Mokoia contains both CV3 oxA and CV3 oxB lithologies; Krot et al. 1998) or contain clasts of material from other subgroups (e.g., Vigarano contains clasts of oxidized CV material; Krot et al. 2000). As a result, the CV3 chondrites are considered to be regolith breccias from a single parent body (Krot et al. 2000).
Raman spectroscopy is useful for determining the relative degree of disorder (or maturity) of organic matter in meteorites (e.g., Quirico et al. 2003;Bonal et al. 2006;Busemann et al. 2007), IDPs (e.g., Wopenka 1988;Quirico et al. 2005;Busemann et al. 2009;Davidson et al. 2012), and cometary samples (e.g., Rotundi et al. 2008;Busemann et al. 2009;Davidson et al. 2012). The Raman spectra of samples containing disordered carbonaceous material are dominated by two bands: the D ("disordered"; breathing mode only active in disordered carbon; e.g., Ferrari and Robertson 2000) and G ("graphite") bands centered near 1360 and 1590 cm À1 , respectively (Rotundi et al. [2008] and references therein). With increasing disorder, these bands broaden significantly and the apparent G-band position moves toward higher wavenumbers. The Dand G-band peak parameters (relative intensities [I], full widths at half maximum [Γ], and peak positions [x]) detected in the organic matter are correlated with the alteration processes it experienced, such as parent body thermal metamorphism (e.g., Quirico et al. 2003;Bonal et al. 2006;Busemann et al. 2007). Raman spectroscopy can be used to estimate the peak metamorphic temperatures (PMTs) experienced by individual chondrites in their parent body (Busemann et al. 2007). The PMTs for the CV3 chondrites range from 260 to 590°C (Busemann et al. 2007), similar to those determined by X-ray absorption near edge structure (XANES) spectroscopy of IOM (415-554°C; Cody et al. 2008). Typically, IOM from the CV3 red chondrites experienced lower temperatures than that from the CV3 ox subgroups (Busemann et al. 2007;Cody et al. 2008). This demonstrates that the members of the CV chondrite group have experienced variable degrees of thermal metamorphism.
The types and abundances of presolar grains (microscopic dust grains formed in the outflows and cooling gases of stars prior to the formation of our solar system; Zinner 2014) present in a sample can also be used to qualitatively investigate its alteration history as they are typically found in high abundance in the least altered chondrites (e.g., Nguyen et al. 2010) and in lower abundance, in more aqueously or thermally altered samples (e.g., Huss and Lewis 1995;Davidson et al. 2014a). Presolar grain types include nanodiamonds, silicon carbide (SiC), graphite, oxides, and silicates; their resilience to parent body alteration depends upon grain type, with silicates generally being the most susceptible to destruction (Zinner 2014).

Samples and Sample Preparation
A chip of RBT 04133 (split 5 from parent 3; 2.3 g) from the interior of the meteorite was obtained to avoid fusion crust and heavily terrestrially weathered portions of the whole stone. A thin section of the meteorite (RBT 04133,8) was also obtained. Crushed and homogenized whole-rock material (a 200 mg aliquot from 2 g of powdered sample) was used for O, C, and N stable isotope analyses. A sample was also leached with HCl to estimate the degree of terrestrial weathering (e.g., Bland et al. 2000;Greenwood et al. 2008) at the Open University (OU). An IOM residue (3 mg) was prepared using 1 g of material following the method of Cody and Alexander (2005) at the Carnegie Institution of Washington (CIW). The IOM residue was used for C and N isotopic analysis, Raman spectroscopy, and previously reported NanoSIMS ion imaging (Davidson et al. 2010(Davidson et al. , 2014a. Matrix material was extracted from crushed whole-rock material for NanoSIMS ion imaging; fragments were individually pressed into high-purity Au foils mounted on 10 mm Al stubs (see Davidson [2009] and Davidson et al. [2014a] for detailed methodology).

Light Element Stable Isotope Analysis
Whole-rock O-isotope analyses were undertaken at the OU on unwashed, homogenized samples (from a 200 mg split of 2 g of powdered whole-rock sample) and a leached subsample (see Greenwood et al. 2008) using an infrared laser fluorination system (Miller et al. 1999). Prior to laser fluorination, the system blank was reduced by flushing the chamber with aliquots of BrF 5 . The O 2 released by laser fluorination was purified by two cold traps on either side of a KBr bed (heated to 110°C). A Micromass Prism III dual inlet mass spectrometer analyzed the isotopic composition of the O 2 . Interference at m/z = 33 by NF + was monitored by performing scans for NF 2 + on the sample gas before analysis, and was below detection limits at all times. Analytical precision (1r) for homogeneous samples, based on replicate analyses of international (NBS-28 quartz, UWG-2 garnet) and internal standards, was approximately AE0.04& for d 17 O, AE0.08& for d 18 O, and AE0.024& for D 17 O (Miller et al. 1999). Isotopic compositions are reported relative to standard mean ocean water and D 17 O was calculated as: Carbon and N stable isotope analyses were undertaken at the OU on powdered and homogenized whole-rock subsamples (2.036 mg) and IOM residues (0.332 mg) using the Finesse mass spectrometer system (e.g., Verchovsky et al. 1997). The Finesse system consists of several mass spectrometers connected to a single extraction system to allow simultaneous isotope and element analysis of N and C. Stepped pyrolysis, combustion, or a combination of the two techniques, at temperatures up to 1450°C, was used to analyze microgram-sized samples. A high-sensitivity capacitance manometer allowed for the precise measurement of the amount of C in the form of CO 2 (Verchovsky et al. 1997).

MicroRaman Spectroscopy
Laser Raman analyses of IOM were conducted with a Horiba Jobin Yvon Labram HR Raman system at the OU (see also Rotundi et al. 2008). Excitation was delivered by an argon ion laser (514.5 nm) and the spectra were acquired with a spectral resolution of 3 Dcm À1 using a 600 g mm À1 grating. The laser delivered a power of 0.07 mW at the sample surface. The beam was focused with a 9100 long working distance objective giving a spatial resolution of approximately 1.2 lm. Spectra were recorded across the whole of each particle with a 1.2 lm step in both x and y directions. Spectra were accumulated as 5 sets of 30 s integrations for a total analysis time of 150 s per spot. A total of 150 spectra were accumulated over three large (each approximately 50 9 50 lm in diameter) IOM fragments (84, 34, 32 spectra each). The carbonaceous D-and G-band features (intensity, position, and full width at half maximum) were fitted in the spectral range of 850-2100 cm À1 to Lorentzian profiles. The steep sloping fluorescence baseline was subtracted with a freefloating linear background. Following the method of Busemann et al. (2007) only spectra that fit strict criteria were used for further data reduction. Spectra were excluded if they exhibited a large fluorescence background; large relative fitting errors (>100%) for band areas, widths, or positions; or band widths/ positions that were more than 3r from the average. The reported parameters are averages of all spectra that meet the selection criteria (145 of 150 spectra; see Busemann et al. [2007] for detailed selection criteria and data reduction procedures).

NanoSIMS Ion Imaging
Raster ion images of 15 matrix fragments (15-25 lm in diameter; total area = 2700 lm 2 ) were collected using the Cameca NanoSIMS 50L at the OU. The fragments were presputtered with a 16 keV Cs + primary ion beam and currents of typically 100 pA for 5-10 min (depending on fragment size) until sputter equilibrium was reached. All analyses were undertaken with currents of 1 pA and raster sizes of 512 9 512 pixels (for areas of 15 9 15 to 25 9 25 lm 2 ), providing a typical spatial resolution of better than 150 nm.
Two sets of measurements were performed on the same areas: (1) to locate C-anomalous phases such as presolar SiC and other C-/N-anomalies ( 12 C À , 13 C À , 12 C 14 N À , 12 C 15 N À , 16 O À , 28 Si À , and 24 Mg 16 O À ; 70 min total analysis time), and (2) to locate O anomalous phases such as presolar oxides and silicates ( 16 O À , 17 O À , 18 O À , 28 Si À , 29 Si À , 30 Si À , and 24 Mg 16 O À ; 240 min total analysis time; Davidson et al. 2010). A mass resolution of m/Dm = 9000 (Cameca definition) was used to resolve interferences from neighboring peaks (e.g., 10 B 16 O À from 12 C 14 N À and 11 B 16 O from 12 C 15 N À for the first set of measurements; 17 O À from 16 OH À in the second set of measurements). The isotopes 28 Si and 24 Mg 16 O were measured to distinguish between silicate and oxide grains.
Ion images were processed and quantitatively analyzed with the L'IMAGE software (L. R. Nittler, CIW). Prior to data extraction, individual image planes were aligned with each other to correct for stage and/or beam drift during the measurement. See Davidson (2009) for complete measurement conditions and data reduction procedures. The presolar SiC abundance determined by NanoSIMS raster ion imaging of RBT 04133 IOM was previously reported by Davidson et al. (2014a).

Mineralogy and Petrography Analysis
An optical microscope was used to initially characterize the thin section. Backscattered electron (BSE) and X-ray element maps were obtained (operating conditions: 15.0 kV and 40.0 nA) with the Cameca SX-50 electron probe microanalyzer (EPMA) at The University of Arizona's Lunar and Planetary Laboratory (LPL). These maps show the elemental and mineralogical distributions within the sample, and were used to identify mineral phases for study. Highresolution BSE and secondary electron images were obtained for each chondrule selected for study using a dual beam FEI Quanta 3-D scanning electron microscope (SEM) at the OU. Modal abundances of different phases (Table 1) were determined from BSE images of the full thin section and of individual chondrules using the IQmaterials â program and pixel (i.e., point) counting in Adobe Photoshop â (e.g., Schrader et al. 2011;N.B., area% determined by point counting is equivalent to vol%; Eisenhour [1996] and references therein). Apparent chondrule diameters were determined by measuring the major and minor axes of chondrules in BSE images using Adobe Photoshop â .
Major and minor element abundances (Na, Si, Mg, Al, P, Ca, K, Mn, Ti, Fe, Cr, Ni, and Zn for silicate phases; Na, Si, Mg, Al, P, S, Ca, Cr, Mn, Ti, Fe, Ni, Co, Cu, and Zn for opaque phases) were determined quantitatively with the Cameca SX-50 EPMA at the LPL. The polished thin section was carbon-coated and analyzed with a 1 lm beam (operating conditions: 15 keV and 20 nA), a PAP correction method (a Phi-Rho-Z correction technique; Armstrong 1988), and counting times of 20 s on the peak and 10 s on each background for a total of 40 s per element. Standards (with detection limits in wt%) are listed in Table 2 for silicate analyses and Table 3 for metal and sulfide analyses. Only metal and sulfide, and stoichiometric olivine and pyroxene analyses with totals between 98.5 and 101.5 wt% were retained and are presented here.

Light Element Isotopic Compositions
RBT 04133 has a whole-rock O-isotopic composition of d 17 O = À3.32&, d 18 O = 0.21&, and plots close to the carbonaceous chondrite anhydrous mineral (CCAM) line (e.g., Clayton et al. 1977) in the area occupied by analyses of CV, CK, and CO chondrites on a three-isotope plot (Fig. 1a). As RBT 04133 is moderately weathered, an aliquot of powder was leached with HCl to remove weathering products (Greenwood et al. 2008) yielding a significantly lighter O-isotopic composition (d 17 O = À7.59&, d 18 O = À4.09&; Fig. 1a).

MicroRaman Spectroscopy
Raman spectroscopy of RBT 04133 IOM generated both D-and G-band features (Fig. 1c); x D = 1346.6 AE 1.0 cm À1 , Г D = 124.2 AE 4.7, x G = 1596.0 AE 1.3 cm À1 , Г G = 62.4 AE 2.1 (uncertainties are 1 standard deviation of the mean). These parameters indicate that the organic matter is moderately disordered and are comparable to previous studies of meteoritic IOM that has experienced moderate metamorphism in their parent bodies (Quirico et al. 2003;Bonal et al. 2006;Busemann et al. 2007). The D-and G-spectral bands yield a peak height ratio I D /I G of 1.143 AE 0.028 that was used to infer a PMT of approximately 440°C for RBT 04133 IOM (following the method of Busemann et al. 2007). The petrographic type of RBT 04133 lies between CV3 red Efremovka (3.1-3.4) and CV3 ox Grosnaja (approximately 3.6) on the basis of Г D , and between the CV3 ox chondrites Bali (>3.6) and Axtell (>3.6) on the basis of I D /I G following the method of Bonal et al. (2006).

Presolar Components
Analysis of RBT 04133 matrix revealed the presence of one presolar SiC (200 nm in diameter), one other C-anomalous grain (potentially presolar graphite or interstellar carbonaceous material; 250 nm in diameter), three presolar silicates (440-650 nm in diameter), and one presolar oxide (340 nm in diameter; Davidson et al. 2010). On the basis of its C and N isotopic compositions (d 13 C = À711 AE 115&, d 15 N = À95 AE 70&), the SiC grain is most likely a Y grain but could potentially be an X grain within error (Davidson et al. 2010). The Si isotopic composition of the SiC grain (determined during subsequent analyses) is solar within 3r error (Davidson et al. 2010). The nature of the other C-anomalous grain (d 13 C = À719 AE 132&, d 15 N below detection limit) could not be determined as it was sputtered away during analysis. However, as its isotopic composition falls outside the range of compositions previously reported for isotopically  Clayton et al. 1977). RBT 04133 plots with the CV3, CK, and CO chondrites, and away from the CR chondrites (which typically lie along their own mixing line above the CCAM line). All other data are from Weisberg et al. (1993), Clayton and Mayeda (1999), Martins et al. (2007), Choi et al. (2009), Greenwood et al. (2010. b) Carbon and N isotopic compositions of RBT 04133 IOM compared to those previously determined for IOM from other carbonaceous chondrites ). RBT 04133 is clearly not a CR chondrite (which exhibit isotopically heavier N and lighter C), but agrees with both CO and CV3 red chondrites. c) Raman D-band parameters (x D = position of D band, and Γ D = full width at half maximum) for RBT 04133 IOM compared to IOM from other carbonaceous chondrites (Busemann et al. 2007 (Nittler 1997). The matrix-normalized SiC abundance determined here from the matrix fragments (12 þ28 À10 ppm) agrees with that determined by raster ion imaging of IOM within error (5 þ4 À2 ppm; Davidson et al. 2014a). Errors associated with silicate (240 þ234 À131 ppm) and oxide (33 þ76 À27 ppm) abundances determined here (based on counting statistics; Gehrels 1986; see Davidson et al. 2014a) are very large as a result of the low number of grains identified. As such, the apparently high abundances determined here are unreliable and would benefit from improved counting statistics by expanding the total amount of area analyzed. However, for this study, the important parameter is the presence of presolar silicates, which are typically only located in primitive chondrites (e.g., Nguyen et al. 2010).

General Description, Mineralogy, and Petrography
Remnant fusion crust and several very large calcium-aluminum-rich inclusions (CAIs) are visible on the exterior of the whole RBT 04133 stone (approximately 1 cm in diameter; Fig. S1). It has experienced some weathering (B/C grade; Weisberg et al. 2008) that is evident from the Fe staining in the thin section (Fig. S2a).

Clast
The texturally distinct clast contains smaller chondrules and coarser-grained matrix than the host (Fig. 3). The clast appears much lighter than the host in plane-polarized light (Fig. S2a) as a result of the optical transparency of its coarser grained matrix. A total of 30 whole chondrules in the clast have apparent diameters ranging from 0.05 to 1.08 mm with an average of 0.31 AE 0.23 mm (mean AE 1r standard deviation). Three type I chondrules in the clast were analyzed in detail, and as they were found to be compositionally indistinguishable from type I chondrules from the host (with the exception of the presence of Fe-rich rims around individual chondrule grains) their silicate compositions are not discussed separately here (Fig. 4). There is no apparent evidence that the clast is a more terrestrially weathered part of the stone; opaque minerals in the clast's matrix do not appear to be more weathered than opaque minerals in the host matrix and the clast does not exhibit the Fe staining that is apparent in the more weathered areas of the section (Fig. S2a).

Chondrule Textural Types and Opaque Mineral Assemblages
The most dominant chondrule textural types are porphyritic-olivine-pyroxene (POP) and porphyriticolivine (PO) chondrules (Fig. 4). Type I chondrules Fig. 2. BSE image of the RBT 04133,8 thin section. Type I chondrules (e.g., Ch3, 5, 6, and 7) appear darker than type II chondrules (e.g., Ch1 and 4) as a result of their lower FeOcontents. Matrix appears bright, like type II chondrules, consistent with the fayalitic compositions of matrix olivine. The brightest regions are opaque mineral assemblages consisting primarily of sulfide and metal. The boundary between the clast and host material is indicated by a dashed line at the bottom right of the thin section. The clast is shown in higher resolution in Fig. 3. Labels correspond to chondrules in Fig. 4, opaque assemblages in Fig. 5, and the CAI in Fig. 8. (90 vol%) are more abundant than type II chondrules (6 vol%) and Al-rich chondrules (4 vol%). Of the 45 complete chondrules measured in the host material, five (11%) are also compound chondrules (e.g., chondrule 7; Fig. 4d).
Type II (FeO-rich; Fa >10 ) chondrules consist primarily of olivine phenocrysts, but also contain FeOpoor relict-grains (Figs. 4e and 4f). Olivine phenocrysts in these chondrules have Fa 11.7-54.6 (mean Fa 38.2AE12.2 ). Relict grains have Fa 3.7-9.3 (mean Fa 6.0AE2.4 ). Type II chondrules typically contain fewer opaque minerals than    (Table 3; Fig. 6). Metal in type II chondrules generally contains more Co (up to 1.54 wt%) than metal in type I chondrules (up to 1.08 wt%) from the host material, but less than the Co-contents of metal in the clast (up to 2.8 wt% Co). Nickel and Co are not positively correlated in Ni-rich and Ni-poor metal from individual type I (Ch5, Ch8, Ch10, and Ch23) and type II (Ch1) chondrules, i.e., the Ni/Co relationship is not solar (Fig. 6). Sulfide minerals are either troilite (with atomic ratios of Fe/S = 1.00) or pyrrhotite (Fe/S from 0.98 to 1.02), with Ni contents below the detection limits in all but a few rare cases where Ni was present at up to 0.7 wt%. No pentlandite, (Fe,Ni) 9 S 8 , was seen.

Matrix
The host matrix consists of fine-grained silicates, metals, and sulfides. The host and clast matrices both contain abundant FeO-rich olivine grains with fayalite values of Fa 59 and Fa 60 , respectively (Table 2). FeO-rich olivine grains are generally smaller in the host matrix (mostly <10 lm diameter; Fig. 7a) than the clast matrix (many >10-20 lm diameter; Fig. 7d).   (Kimura et al. 2008). The solar Ni:Co ratio line is shown for reference (data from Lodders et al. 2009).
The largest opaque mineral phases (up to approximately 500 lm in diameter) within both matrices appear to be sulfides (e.g., Fig. 5g). Fe,Ni metal in the clast matrix is Ni-poor and contains 90.2-93.0 wt% Fe, 3.2-6.7 wt% Ni, and 1.01-1.65 wt% Co (Table 3; Fig. 6). Although they are compositionally similar, matrix textures and chondrule characteristics (e.g., average size) differ significantly between the clast and host material. The clast matrix is coarser grained than the host matrix (Fig. 7) and contains abundant FeO-rich olivine grains and fewer chondrule fragments. All chondrules within the clast (all type I) possess thin, Fe-rich rims around individual grains (Figs. 4g and  4h). The contact between the host and clast matrix is sharp (Fig. 7).

RBT 04133: A Reduced CV3 Carbonaceous Chondrite
This study has employed a range of different analytical methods in order to determine the true classification of RBT 04133. All methods indicate that RBT 04133 is a CV3 red carbonaceous chondrite as is discussed further below.
The whole-rock O-isotopic composition of RBT 04133 indicates that it is not a CR chondrite, but is consistent with CO and CV3 chondrites (Fig. 1a here; Clayton and Mayeda 1999;Greenwood et al. 2010). Terrestrial weathering is known to affect the O-isotopic composition of meteoritic samples (Greenwood and Franchi 2004). Acid-washing techniques have been used to "clean" meteorites, but they cannot readily discriminate between nebular/asteroidal and terrestrial alteration products and thus may also remove nonterrestrial phases. The O-isotopic compositions of such washed samples tend to shift away from the terrestrial fractionation (TF) line, with a co-variation in d 18 O, the direction of which varies between Antarctic Fig. 7. BSE images of a) fine-grained host matrix, b) abundant chondrules and chondrule fragments in fine-grained host matrix, c) the boundary between host and clast matrix (marked by white arrows), and d) coarse-grained clast matrix, which appears to contain abundant FeO-rich olivine grains and fewer chondrule fragments than the host matrix. All scale bars are 100 lm. and hot desert weathering (Greenwood et al. 2012). However, the acid-leached RBT 04133 sample is much more 16 O-rich than expected, yielding an O-isotopic composition more 16 O-rich than any whole-rock CV3 reported by Greenwood et al. (2010). It is very unlikely that this can be explained by removal of terrestrial contamination alone and likely that a native 16 O-poor component was lost too. Many CV3 chondrites contain magnetite (e.g., Howard et al. 2010), which is relatively 16 O-poor (e.g., Davidson et al. 2014b). However, it seems unlikely that the leached component included magnetite as none was observed in the thin section and it should be more resistant to acid etching. While not conclusive, the highly 16 O-rich composition indicates a stronger affinity with the CV rather than CR chondrites as the latter are inferred to have a relatively 16 O-poor anhydrous silicate component (e.g., Clayton and Mayeda 1999;Schrader et al. 2011).
The C and N isotopic compositions of IOM from different meteorites cluster with others of the same class ( Fig. 1b)  ), they also typically contain similar abundances of C in their IOM within a group. The C and N isotopic compositions of RBT 04133 IOM are consistent with data from both the CV3 red and the CO chondrites, and are significantly different from those of the CR chondrites and CV3 ox chondrites. The abundance of C in the IOM of RBT 04133 (69 wt%) is similar to those of the CRs, COs, and CV3 red . The insoluble C content of the bulk meteorite (0.6 wt%) also agrees with other CV3 red chondrites ).
The IOM residue was also analyzed by Raman spectroscopy, which is useful for determining the relative degree of disorder (or maturity) of IOM ( Fig. 1c; e.g., Bonal et al. 2006;Busemann et al. 2007). Busemann et al. (2007) showed that, in terms of their spectral parameters, different meteorites cluster with those of the same class (Fig. 1c). Considering this relationship, RBT 04133 does not appear to be a CR chondrite as its organics are not as primitive as those from known CR chondrites (Busemann et al. 2007). However, organic material in RBT 04133 is consistent with that in CV and CO chondrites.
Presolar grains were incorporated into all chondrite groups; SiC grains can be found in similar abundances in the most primitive members of each (e.g., Davidson et al. 2014a). Lower SiC abundances indicate destruction of grains/loss of their isotopic signatures as a result of parent body processing and thus indicate higher petrologic types. RBT 04133 was recently found to have a presolar SiC abundance (5 þ4 À2 ppm; Davidson et al. 2014a) that is much lower than is expected for a CR chondrite (average 32 AE 9 ppm; Davidson et al. 2014a), but which agrees with the low abundance of approximately 0.48 ppm reported for the CV3 red chondrite Vigarano (Huss and Lewis 1995). This not only suggests that RBT 04133 is not a CR chondrite, it also indicates that this meteorite has experienced sufficient parent body processing to destroy the majority of the initial presolar grain inventory in its parent body.
The CV3 chondrites contain abundant matrix (40 vol%), porphyritic ferromagnesian chondrules (45 vol%), refractory inclusions (10 vol%), and opaque mineral assemblages (0-5 vol%) (e.g., Weisberg et al. 2006;Rubin 2010;Jones 2012). These values are consistent with RBT 04133 being a member of the CV3 chondrite group. The average apparent diameter of full chondrules in RBT 04133 (0.85 AE 0.69 mm from 45 chondrules in the host) is consistent with the average chondrule diameter of approximately 1 mm for CV and CK chondrites, and larger than 0.15 mm for CO chondrites (Weisberg et al. 2006). The dominance of porphyritic chondrules over other chondrule types and the abundance of type I chondrules compared to type II chondrules are also consistent with CV chondrites (e.g., Weisberg et al. 2006;Rubin 2010;Jones 2012). Neither the abundance of refractory inclusions (Table 1)  They are, however, similar to those commonly found in the CV and CK chondrites (Weisberg et al. 2006). It is unlikely that RBT 04133 is a CK chondrite as it lacks the characteristic abundant magnetite (e.g., Geiger and Bischoff 1995;Weisberg et al. 2006), and has matrix olivine compositions of Fa 57-60 that are far more Fe-rich than those in CK chondrites (Fa 30 ; Weisberg et al. 2006). On the basis of all petrographical and compositional data, RBT 04133 is a CV3 red chondrite. Furthermore, microstructural analysis of matrix from RBT 04133 has been found to be similar to other CV3 red chondrites .

Pre-Accretionary Formation Conditions
The compositions of mineral phases within chondrules provide information about the conditions under which they formed in the protoplanetary disk. The O fugacity (fO 2 ) was calculated for each chondrule with the quartz-iron-fayalite buffer reaction using the average Fa content of olivine and the average Fe content of Fe,Ni metal, and was referenced to the iron-w€ ustite (IW) buffer (Table 4). We assume a silica activity of 0.9 for the three-component system metalolivine-low-Ca pyroxene (e.g., Benedix et al. 2005). The online MELTS calculator (http://melts.ofm-research. org/CalcForms/index.html) of Sack and Ghiorso (1989) was used to obtain the activity of olivine. The necessary equilibrium constants were obtained from the HSC 7.0 Chemistry Reaction Equation module. The corresponding H 2 O/H 2 ratios for all the chondrules were calculated assuming a temperature of 1600°C, which is near the average of the temperature range in which chondrules likely crystallized (Hewins and Radomksy 1990 (Zanda et al. 1994) (Table 4). Both type I and type II chondrules formed under H 2 O/H 2 ratios enhanced relative to solar; which is consistent with those in other chondrite groups (e.g., Zanda et al. 1994;Lauretta et al. 2001;Schrader et al. 2013), but lower than those predicted by thermodynamic calculations for the hightemperature formation of FeO-rich olivine in the early solar system (e.g., Palme and Fegley 1990;Wood and Hashimoto 1993;Ebel and Grossman 2000).
The oxidizing conditions present during chondrule formation may also be recorded in the relationships between Ni versus Cr and Ni versus P in the metal of RBT 04133 (Table 3). The depletion of Cr and P in metal is similar to the results of high-temperature oxidation and sulfidation experiments of Fe,Ni alloys (Schrader and Lauretta 2010). Depletion of Cr and P in metal in unequilibrated chondrites (including the CV3 red Leoville) has been attributed to the presence of micronsized chromite and phosphate inclusions within the metal that form by oxidation during chondrule cooling and also thermal metamorphism (Zanda et al. 1994). However, unlike chondrules from the CR chondrites (Schrader et al. 2013), the abundance of sulfides in chondrules in RBT 04133 does not increase with increasing oxidation state (i.e., sulfides are abundant in type I chondrules while they are uncommon in type II chondrules). This suggests either that (1) the sulfides are secondary (i.e., parent body), or (2) they are preaccretionary and the oxidation state and sulfidation state of chondrules in the CV chondrites were decoupled (i.e., H 2 O/H 2 was decoupled from H 2 S/H 2 during chondrule formation). If the sulfides are secondary, they should be present in similar relative abundances within both type I and type II chondrules; however, this is not the case (Figs. 4a-d versus 4e and 4f). In addition, if the sulfides formed via parent body aqueous alteration, the presence of additional secondary minerals, such as phyllosilicates and magnetite, would be expected. Marrocchi and Libourel (2013) reported the presence of abundant sulfides within type I chondrules in the CV3 red chondrite Vigarano, and based on the apparent co-crystallization of troilite and low-Ca pyroxene during high-temperature events they suggest that sulfides formed during chondrule formation. Furthermore, Hezel et al. (2010) argue that the bulk chondrule Fe isotope compositions of chondrules from oxidized CV chondrites Mokoia, Allende, and Grosnaja are indicative of evaporation and re-condensation during chondrule formation; implying that metal and sulfides in these chondrules are pre-accretionary. We suggest that sulfides within type I chondrules from RBT 04133 are also pre-accretionary, providing further evidence that some sulfide minerals formed prior to accretion of the CV chondrite parent body. This contradicts the general belief that CV chondrite sulfides formed by parent body aqueous alteration (e.g., see Brearley (2006) review and discussion therein).

Parent Body Processing
There is no apparent evidence for significant aqueous alteration within the RBT 04133 host or clast, attested to by the lack of abundant alteration minerals (e.g., phyllosilicates and magnetite). However, there do appear to be several indicators of thermal metamorphism. Nickel-rich metal in RBT 04133 is depleted in Co relative to Ni-poor metal (Fig. 6); this is similar to the composition of metal in the CV3 red chondrites Efremovka (Nazarov et al. 2000) and Vigarano (Krot et al. 2000) that both have estimated subtypes of 3.1-3.4 (Bonal et al. 2006;Busemann et al. 2007). Similar observations have been made for the thermally metamorphosed ordinary and CO chondrites (Kimura et al. 2008) and the shock heated CR chondrite GRA 06100 . More pristine (i.e., less-altered) chondrites, such as the majority of the CR chondrites (e.g., Weisberg et al. 1993), exhibit a positive relationship between Ni and Co indicating that Co becomes mobile during thermal metamorphism. The Ni and Co content of metal in RBT 04133 is most similar to that in type 3.5-3.9 LL chondrites and the CO chondrite Y-791717, which is considered to be of either petrologic type 3.3 (Busemann et al. 2007) or 3.6 ( Kimura et al. 2008). This suggests that RBT 04133 is likely of metamorphic type 3.3 or greater. Furthermore, the Co content of Ni-poor metal in the clast is significantly greater than that of Ni-poor metal in the host material indicating that clast metal has experienced higher degrees of thermal metamorphism than the host metal. However, as the chondrule silicates are unequilibrated (Fa 0.9-54.6 ), this thermal metamorphism was likely mild. As RBT 04133 is of petrologic type >3.2, it is not possible to use the Cr 2 O 3 content of FeO-rich olivine to determine its subtype (Grossman and Brearley 2005). Furthermore, this method has not been calibrated for CV3 chondrites. Thermal metamorphism has also been shown to alter chromite textures in type II chondrules from euhedral to subhedral (Johnson and Prinz 1991). The composition (high Cr 2 O 3 ; 46.6-52.4 wt%) and morphology (euhedral/subhedral) of chromite within type II chondrules of RBT 04133 are also consistent with mild thermal metamorphism (Figs. 5b and 5d and Table 3) (Davidson et al. 2011).
Lower presolar SiC abundances indicate that presolar grains have been destroyed by parent body processing and thus indicate higher petrologic types (e.g., Davidson et al. 2014a). RBT 04133 was found to have presolar SiC abundances (12 þ28 À10 in matrix, this work, and 5 þ4 À2 ppm in IOM; Davidson et al. 2014a) that are lower than is expected for a CR chondrite (average 32 AE 9 ppm; Davidson et al. 2014a), suggesting that RBT 04133 is not a CR chondrite and that it has experienced sufficient parent body processing to destroy the majority of its initial presolar SiC grain inventory in its parent body. Presolar SiC grains are typically more resistant to low temperature parent body aqueous alteration than presolar silicates (Zinner 2014). For example, presolar SiC grains in the CR chondrites are present in similar abundance in even the most aqueously altered CR1 chondrite GRO 95577 (Davidson et al. 2014a). However, presolar silicates have been mostly destroyed in the heavily aqueously altered CR2 chondrite Renazzo (no presolar silicate grains were found in one NanoSIMS ion imaging study (Floss and Stadermann 2005), and only two were found in another (Leitner et al. 2012)). Although highly speculative given the low number of grains identified, a rough presolar silicate/oxide ratio of 3 indicates that this sample has undergone significant parent body alteration (e.g., Floss and Stadermann 2009b). It is curious that presolar silicates are still present in RBT 04133 when many SiC grains appear to have either been destroyed (unlikely at a PMT of 440°C) or have lost their isotopic signatures via diffusion. Another study identified only a single presolar silicate grain and no SiC grains in the thermally metamorphosed CO3 chondrite QUE 97416 (Bose et al. 2014). These observations indicate that the progressive destruction or modification of presolar SiC grains begins before presolar silicate grains are completely unidentifiable.
The source of heat for thermal metamorphism of RBT 04133 on the CV parent body is likely to be either radiogenic or low-velocity impact heating. Linear foliation of chondrules has been observed in Leoville (CV3 red ) and attributed to impact heating (Scott et al. 1992;Rubin 2012). However, foliation has been experimentally shown to only occur in CV chondrites at impact pressures >10 GPa (e.g., Nakamura et al. 1995Nakamura et al. , 2000. Linear foliation of chondrules was not observed in RBT 04133. While the presence of a distinct CV3 red clast (Figs. 2 and 3) shows that RBT 04133 is brecciated and thus experienced impact processing, RBT 04133 did not experience high velocity impacts that could lead to extensive thermal metamorphism; consistent with compositional and textural indications for mild thermal alteration.
Clasts of oxidized CV material (CV3 oxA and CV3 oxB ) have been identified in Vigarano (CV3 red ; Krot et al. 2000), and Mokoia (CV3 ox ) consists of both oxidized lithologies (Krot et al. 1998). Although the clast present in RBT 04133 consists of CV3 red material like the host rock, the coarser-grained matrix, Fe-rich rims on olivine, and compositions of metal suggest that it is more thermally metamorphosed. This further demonstrates the brecciated nature of the CV parent body.

SUMMARY AND IMPLICATIONS
We have performed a multitechnique characterization of the Antarctic meteorite RBT 04133. Data presented here conflict with its initial classification as a CR2 . Petrographically, RBT 04133 appears to be a CV3 red , based on the presence of large CAIs and chondrules, the apparent lack of magnetite, and a matrix composition of Fa 59-60 . This is in agreement with its whole-rock C, N, and O-isotope compositions, and the Raman spectral characteristics of its IOM.
Thermodynamic calculations indicate that type I and type II chondrules in RBT 04133 formed under different, but relatively oxidizing conditions (H 2 O/H 2 ratios = 40-420 times solar). This is also reflected in the Ni, Cr, and P compositions of their metal, which are similar to those of other CV3 red chondrites. Sulfide minerals are abundant in type I chondrules of RBT 04133, but are uncommon in type II chondrules. We suggest that they formed prior to accretion of the CV chondrite parent body.
There is no evidence that suggests RBT 04133 experienced significant aqueous alteration. However, its low presolar grain abundances, the PMT of 440°C estimated from Raman spectral data, the metal compositions, the chromite compositions and morphologies, and the presence of unequilibrated silicates indicate that RBT 04133 is mildly thermally altered in nature and appears to be of petrologic type ≥CV3.3 red . The presence of a more thermally metamorphosed clast of reduced CV3 material within RBT 04133 indicates that it, like several other CV chondrites, is a breccia.