ROOSEVELT COUNTY 027: A LOW‐SHOCK UREILITE WITH INTERSTITIAL SILICATES AND HIGH NOBLE GAS CONCENTRATIONS

Abstract

The lightly‐shocked ureilite RC027 was found in Roosevelt County, New Mexico in 1984. In terms of petrography, texture, mineral compositions, bulk chemical composition, and oxygen isotopic composition it is a typical ureilite. It contains ∼75% olivine (Fo 79.4) and 25% pigeonite (mg 81.3, Wo 8.0), with intergranular graphite and (Fe, Ni) metal. It also contains less than 1% of fine‐grained, interstitial silicate material, which had not previously been recognized in any ureilite. This material is an assemblage of low‐Ca pyroxene (Wo 3.5–9, mg 87–93), augite (Wo 24–36, mg 90–98), glass (typically ∼95% SiO2, 4% Al2O3, 0.5% Na2O), and crystalline SiO2. This material has an igneous texture, indicating that it crystallized from an interstitial liquid. Low‐Ca pyroxene compositions indicate that the interstitial liquid was not in equilibrium with core pigeonite and olivine and cannot have been either an evolved intercumulus liquid or a low‐degree partial melt. It may contain a component of shock‐melted olivine and pigeonite, although petrographic evidence indicates that it could not have been an in situ shock melt. One sample of RC027 has a V‐shaped rare earth element pattern, typical of ureilites. Another is depleted in light rare earth elements (LREE), similar to acid‐treated samples of ureilites, which suggests that LREE in ureilites are contained in an inhomogeneously‐distributed phase. RC027 shows the strongest olivine preferred‐orientation yet observed in a ureilite. Its fabric is characteristic of fabrics formed by tabular minerals in a fluid laminar flow regime and is unlike those formed by syntectonic recrystallization and plastic flow.The elemental and isotopic compositions of noble gases in RC027 are typical of previously analyzed ureilites. This result indicates that there is no correlation of noble gas content with degree of shock in ureilites, and thus suggests that the gases were present in the ureilite material before shock. Cosmogenic He and Ne contents indicate cosmic ray exposure ages of 1.7 and 1.9 Myr, respectively. Thus, RC027 is not paired with Kenna (a ureilite also found in Roosevelt County), which has an exposure age of ∼33 Myr.