Shock effects in “EH6” enstatite chondrites and implications for collisional heating of the EH and EL parent asteroids

Among the many ungrouped meteorites, Acfer 370, NWA 7135, and El Médano 301—probably along with the chondritic inclusion in Cumberland Falls and ALHA 78113—represent a homogeneous grouplet of strongly reduced forsterite‐rich chondrites characterized by common textural, chemical, mineralogical, and isotopic features. All of these meteorites are much more reduced than OCs, with a low iron content in olivine and low‐Ca pyroxene. In particular, Acfer 370 is a type 4 chondrite that has olivine and low‐Ca pyroxene compositional ranges of Fa 5.2–5.8 and Fs 9.4–33.4, respectively. The dominant phase is low‐Ca pyroxene (36.3 vol%), followed by Fe‐Ni metal (16.3 vol%) and olivine (15.5 vol%); nevertheless, considering the Fe‐oxyhydroxide (due to terrestrial weathering), the original metal content was around 29.6 vol%. Finally, the mean oxygen isotopic composition Δ17O = +0.68‰ along with the occurrence of a silica phase, troilite, Ni‐rich phosphides, chromite, and oldhamite confirms that these ungrouped meteorites have been affected by strong reduction and are different from any other group recognized so far.

Section: Resultssupporting

confidence: 56%

A detailed mineralogical, petrographic, and geochemical study of the highly reduced chondrite, Acfer 370

Pratesi

Caporali

Greenwood

et al. 2019

“…Rubin and Wasson () concluded that Y‐793225 has an anomalous EH composition, based on the EH‐like Si‐content of kamacite. Their bulk rock siderophile and chalcophile element data, however, demonstrate an EL‐like composition.…”

Section: Discussionmentioning

confidence: 99%

Petrogenesis of anomalous Queen Alexandra Range enstatite meteorites and their relation to enstatite chondrites, primitive enstatite achondrites, and aubrites

Niekerk

Keil

Humayun

2014

Abstract-Queen Alexandra Range (QUE) meteorite 94204 is an anomalous enstatite meteorite whose petrogenesis has been ascribed to either partial melting or impact melting. We studied the meteorite pairs QUE 94204, 97289/97348, 99059/99122/99157/99158/99387, and Yamato (Y)-793225; these were previously suggested to represent a new grouplet. We present new data for mineral abundances, mineral chemistries, and siderophile trace element compositions (of Fe,Ni metal) in these meteorites. We find that the texture and composition of Y-793225 are related to EL6, and that this meteorite is unrelated to the QUEs. The mineralogy and siderophile element compositions of the QUEs are consistent with petrogenesis from an enstatite chondrite precursor. We caution that potential reequilibration during melting and recrystallization of enstatite chondrite melt-rocks make it unreliable to use mineral chemistries to assign a specific parent body affinity (i.e., EH or EL). The QUEs have similar mineral chemistries among themselves, while slight variations in texture and modal abundances exist between them. They are dominated by inclusion-bearing millimeter-sized enstatite (average En 99.1-99.5 ) with interstitial spaces filled predominantly by oligoclase feldspar (sometimes zoned), kamacite (Si approximately 2.4 wt%), troilite (≤2.4 wt% Ti), and cristobalite. Siderophile elements that partition compatibly between solid metal and liquid metal are not enriched like in partial melt residues Itqiy and Northwest Africa (NWA) 2526. We find that the modal compositions of the QUEs are broadly unfractionated with respect to enstatite chondrites. We conclude that a petrogenesis by impact melting, not partial melting, is most consistent with our observations.

“…Many of the higher thermal metamorphic grade E chondrites show evidence of annealing from higher shock stages, such as the preservation of relict exsolution lamellae, fractures, and twin boundaries, injected by metal or sulfide; and trains of metal or sulfide inclusions along annealed fracture planes. It is possible that the higher petrologic types of E chondrites are the result of intense shock heating rather than metamorphism due to endogenic heating (e.g., Rubin 2009). Alternatively, evidence for multiple, overlapping episodes of shock metamorphism and thermal annealing could indicate that shock and endogenic thermal metamorphic processes were penecontemporaneous on the parent asteroids of the E chondrites.…”

Section: Discussionmentioning

confidence: 99%

“…The thermal metamorphism of E chondrites may therefore reflect a combination of exogenic (impact) and endogenic ( 26 Al and other radionuclides, gravitational compression) heat sources (Rubin et al. 1997; Rubin 2009).…”

Section: Introductionmentioning

confidence: 99%

Micro‐X‐ray diffraction assessment of shock stage in enstatite chondrites

Izawa¹,

Flemming

Banerjee

et al. 2011

Abstract-A new method for assessing the shock stage of enstatite chondrites has been developed, using in situ micro-X-ray diffraction (lXRD) to measure the full width at half maximum (FWHM v ) of peak intensity distributed along the direction of the Debye rings, or chi angle (v), corresponding to individual lattice reflections in two-dimensional XRD patterns. This lXRD technique differs from previous XRD shock characterization methods: it does not require single crystals or powders. In situ lXRD has been applied to polished thin sections and whole-rock meteorite samples. Three frequently observed orthoenstatite reflections were measured: (020), (610), and (131); these were selected as they did not overlap with diffraction lines from other phases. Enstatite chondrites are commonly fine grained, stained or darkened by weathering, shock-induced oxidation, and metal ⁄ sulfide inclusions; furthermore, most E chondrites have little olivine or plagioclase. These characteristics inhibit transmitted-light petrography, nevertheless, shock stages have been assigned MacAlpine Hills (MAC) 02837 (EL3) S3, Pecora Escarpment (PCA) 91020 (EL3) S5, MAC 02747 (EL4) S4, Thiel Mountains (TIL) 91714 (EL5) S2, Allan Hills (ALHA) 81021 (EL6) S2, Elephant Moraine (EET) 87746 (EH3) S3, Meteorite Hills (MET) 00783 (EH4) S4, EET 96135 (EH4-5) S2, Lewis Cliff (LEW) 88180 (EH5) S2, Queen Alexandra Range (QUE) 94204 (EH7) S2, LaPaz Icefield (LAP) 02225 (EH impact melt) S1; for the six with published shock stages, there is agreement with the published classification. FWHM v plotted against petrographic shock stage demonstrates positive linear correlation. FWHM v ranges corresponding to shock stages were assigned as follows: S1 < 0.7°, S2 = 0.7-1.2°, S3 = 1.2-2.3°, S4 = 2.3-3.5°, S5 > 3.5°, S6-not measured. Slabs of Abee (EH impactmelt breccia), and Northwest Africa (NWA) 2212 (EL6) were examined using lXRD alone; FWHM v values place both in the S2 range, consistent with literature values. Micro-XRD analysis may be applicable to other shocked orthopyroxene-bearing rocks.