The formation conditions of enstatite chondrites: Insights from trace element geochemistry of olivine‐bearing chondrules in Sahara 97096 (EH3)

Abstract

International audienceWe report in situ LA-ICP-MS trace element analyses of silicate phases in olivine-bearing chondrules in the Sahara 97096 (EH3) enstatite chondrite. Most olivine and enstatite present rare earth element (REE) patterns comparable to their counterparts in type I chondrules in ordinary chondrites. They thus likely share a similar igneous origin, likely under similar redox conditions. The mesostasis however frequently shows negative Eu and/or Yb (and more rarely Sm) anomalies, evidently out of equilibrium with olivine and enstatite. We suggest that this reflects crystallization of oldhamite during a sulfidation event, already inferred by others, during which the mesostasis was molten, where the complementary positive Eu and Yb anomalies exhibited by oldhamite would have possibly arisen due to a divalent state of these elements. Much of this igneous oldhamite would have been expelled from the chondrules, presumably by inertial acceleration or surface tension effects, and would have contributed to the high abundance of opaque nodules found outside them in EH chondrites. In two chondrules, olivine and enstatite exhibit negatively sloped REE patterns, which may be an extreme manifestation of a general phenomenon (possibly linked to near-liquidus partitioning) underlying the overabundance of light REE observed in most chondrule silicates relative to equilibrium predictions. The silicate phases in one of these two chondrules show complementary Eu, Yb, and Sm anomalies providing direct evidence for the postulated occurrence of the divalent state for these elements at some stage in the formation reservoir of enstatite chondrites. Our work supports the idea that the peculiarities of enstatite chondrites may not require a condensation sequence at high C/O ratios as has long been believed

Chemistry and oxygen isotopic composition of cluster chondrite clasts and their components in LL3 chondrites

“…Similar REE patterns and even stronger Zr/Hf fractionations (88, 123) were observed in pyroxene crystals from two porous (ferroan) pyroxene chondrules in an EH3 chondrite (Sahara 97096; Jacquet et al. ). These authors discuss an experimental work by Saito et al.…”

“…On the other hand we cannot strictly rule out that cluster chondrites formed by unknown secondary processes, which led to reworking and reheating of already existing chondrule populations, as advocated by Krot (personal communication) and Jacquet et al. (). In any case cluster chondrite formation seems to represent an early process in solar system history, since it preceded the thermal metamorphic phase of ordinary chondrite asteroids.…”

“…() and Jacquet et al. (). Since these patterns are partly complementary to REE patterns of oldhamite from EH3 chondrites (see Lehner et al.…”

“…), these authors explain the bulk chondrule REE signatures as the result of oldhamite crystallization, followed by oldhamite expulsion due to inertial acceleration or surface tension effects (Jacquet et al. [] and references therein). The REE patterns of our chondrules shown in Fig.…”

Discovery of a meteoritic ejecta layer containing unmelted impactor fragments at the base of Paleocene lavas, Isle of Skye, Scotland

“…The meteoritic ejecta layer deposits at both sites suggest that the impactor was very reduced, with mineralogy representative of that found in enstatite chondrites (Jacquet et al, 2015). It is tempting to speculate on the extent of the Skye ejecta layer and location of the possible source crater.…”