High abundances of presolar grains and 15N-rich organic matter in CO3.0 chondrite Dominion Range 08006

Abstract

NanoSIMS C-, N-, and O-isotopic mapping of matrix in CO3.0 chondrite Dominion Range (DOM) 08006 revealed it to have in its matrix the highest abundance of presolar O-rich grains (257 +76/-96 ppm, 2σ) of any meteorite. It also has a matrix abundance of presolar SiC of 35 (+25/-17, 2σ) ppm, similar to that seen across primitive chondrite classes. This provides additional support to bulk isotopic and petrologic evidence that DOM 08006 is the most primitive known CO meteorite. Transmission electron microscopy of five presolar silicate grains revealed one to have a composite mineralogy similar to larger amoeboid olivine aggregates and consistent with equilibrium condensation, two non-stoichiometric amorphous grains and two olivine grains, though one is identified as such solely based on its composition. We also found insoluble organic matter (IOM) to be present primarily as sub-micron inclusions with ranges of C- and N-isotopic anomalies similar to those seen in primitive CR chondrites and interplanetary dust particles. In contrast to other primitive extraterrestrial materials, H isotopic imaging showed normal and homogeneous D/H. Most likely, DOM 08006 and other CO chondrites accreted a similar complement of primitive and isotopically anomalous organic matter to that found in other chondrite classes and IDPs, but the very limited amount of thermal metamorphism experienced by DOM 08006 has caused loss of D-rich organic moieties, while not substantially affecting either the molecular carriers of C and N anomalies or most inorganic phases in the meteorite. One C-rich grain that was highly depleted in C andN was identified; we propose it originated in the Sun's parental molecular cloud.

A mutli-technique search for the most primitive CO chondrites

“…That DOM 08006 is more primitive than ALH 77307 is also consistent with the higher presolar silicate abundance of DOM 08006 matrix (Nittler et al, 2013) and its lower petrologic type based on the Cr contents of its olivine (Davidson et al, 2014a). The higher H/C ratio of its IOM (Table 2) is also indicative of the more primitive nature of DOM 08006 compared to ALH 77307.…”

“…Bulk C and H abundances and isotopes in a suite of COs were used to identify DOM 08006 as a potentially very primitive CO3 (Alexander et al, 2014). 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).…”

Oxygen and Al‐Mg isotopic compositions of grossite‐bearing refractory inclusions from CO3 chondrites

“…Using a standard deviation versus mean Cr 2 O 3 content in ferroan olivine phenocrysts of type II (Fe/(Fe + Mg) ≥ 0.10) chondrules, the leastmetamorphosed CO chondrites can be further subdivided from 3.00 to 3.2 (Grossman and Brearley 2005). Using this criterion, as well as bulk carbon content, bulk carbon and hydrogen isotopic compositions, and the abundance of presolar grains, DOM 08006 and 08004 were classified as CO3.00 and CO3.1 chondrites, respectively (Davidson et al 2014;Howard et al 2014;Alexander et al 2018;Nittler et al 2018). In addition, Tenner et al (2018) examined the coarse-grained silicates, metal grains, and matrix of DOM 08006 and suggested 3.01 as an upper limit on petrologic type, consistent with the features observed.…”

“…; Nittler et al. ; Tenner et al. ), in which >10% of the refractory inclusions contain grossite (Simon ), provides an opportunity to fill this gap in the database and to see if grossite‐bearing inclusions formed with canonical contents of 26 Al, undetectable levels of 26 Al, or some other value or values, to improve our understanding of the distribution of 26 Al in the solar nebula.…”

“…Much of the available data on ( 26 Al/ 27 Al) 0 in CAIs are from secondary ion mass spectrometry (SIMS) analyses of hibonite, anorthite, and melilite-the most common phases with high Al/Mg ratios (Krot et al 2012;Kita et al 2013). The recognition of pristine (i.e., unmetamorphosed) and nearly pristine CO3 chondrites such as Dominion Range (DOM) 08006 and DOM 08004, respectively (Krot et al 2017b;Alexander et al 2018;Nittler et al 2018;Tenner et al 2018), in which >10% of the refractory inclusions contain grossite (Simon 2016), provides an opportunity to fill this gap in the database and to see if grossite-bearing inclusions formed with canonical contents of 26 Al, undetectable levels of 26 Al, or some other value or values, to improve our understanding of the distribution of 26 Al in the solar nebula.…”