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

Abstract

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

Multiple and Fast: The Accretion of Ordinary Chondrite Parent Bodies

“…We previously showed (Vernazza et al 2010) that the application of our model to the shorter near-IR range (0.8-2.5 μm instead of 0.4-2.5 μm) only increases the error in the ol/(ol+opx) ratio by 2%. Our model results are in agreement with measurements by independent techniques (Menzies et al 2005;Dunn et al 2010aDunn et al , 2010b; Mössbauer spectra, XRD). Our new spectral data ( Figure 1) indicate that the ol/(ol+low-Ca px) ratio is the same for type 3.0-3.4 OCs…”

“…In particular, the correlation may indicate that smaller H-like bodies formed with a more pyroxene-rich interior than larger H-like bodies (the data for families are in agreement with this hypothesis with Koronis members being on average more olivine-rich than Agnia or Merxia ones). In the context of the H4-H6 sequence (H4 < H5 < H6 in terms of ol/(ol+low-Ca px) ration; see Dunn et al 2010aDunn et al , 2010b this may imply that smaller bodies were formed with a higher fraction of H4 material while larger bodies formed with more H5 and/or H6 material. Thermal models (e.g., Henke et al 2012aHenke et al , 2012bHenke et al 2013) could directly test this hypothesis.…”

“…We found that for the lowest petrologic types (<3.5), H, L, and LL meteorites have a similar ol/(ol+low-Ca px) ratio (>0.65). For petrologic types greater than 3.6, the compositional trend evolves gradually toward the more familiar compositional spread seen/known for EOCs (Dunn et al 2010a(Dunn et al , 2010bJarosewich 1990): on average 58.8 ± 4.5% for Hs; 64.2 ± 6.8% for Ls; and 75.1 ± 4.5% for LLs (Vernazza et al 2008). …”

“…2. Textural variations and corresponding mineral and chemical trends indicate that differing degrees of thermal metamorphism (heating) took place within each chondrite group (Van Schmus & Wood 1967;Dunn et al 2010b). Based on these variations, a petrologic classification scheme (Van Schmus & Wood 1967) for ordinary chondrites was developed (H, L, and LL groups are further subdivided into four groups-from three to six), which consisted in distinguishing the less metamorphosed chondrites (type 3, called UOCs) from chondrites that have undergone higher degrees of thermal metamorphism (types 4-6, called equilibrated ordinary chondrites: EOCs; see Huss et al 2006 for a review).…”

A mutli-technique search for the most primitive CO chondrites

“…A subsequent presolar grain search showed that it has the highest matrix-normalized presolar silicate abundance of any chondrite (Nittler et al, 2013). Position sensitive detector X-ray diffraction (PSD-XRD) has proven to be a very useful tool both for determining bulk mineral abundances and for classification Menzies et al, 2005;Dunn et al, 2010;Howard et al, 2010Howard et al, , 2011Howard et al, 2015). Lastly, bulk O isotopes are an indispensable tool for classifying meteorites and for providing constraints on the influence of fluids during parent body processing (e.g., Clayton and Mayeda, 1999;Young et al, 1999;Greenwood and Franchi, 2004;Schrader et al, 2011).…”