Formation of feldspathic and metallic melts by shock in enstatite chondrite Reckling Peak A80259

Abstract

Abstract-The enstatite chondrite Reckling Peak (RKP) A80259 contains feldspathic glass, kamacite, troilite, and unusual sets of parallel fine-grained enstatite prisms that formed by rapid cooling of shock melts. Metallic Fe,Ni and troilite occur as spherical inclusions in feldspathic glass, reflecting the immiscible Fe-Ni-S and feldspathic melts generated during the impact. The Fe-Ni-S and feldspathic liquids were injected into fractures in coarse-grained enstatite and cooled rapidly, resulting in thin (110 pm) semicontinuous to discontinuous veins and inclusion trails in host enstatite. Whole-rock melt veins characteristic of heavily shocked ordinary chondrites are conspicuously absent. Raman spectroscopy shows that the feldspathic material is a glass. Elevated MgO and Si02 contents of the glass indicate that some enstatite and silica were incorporated in the feldspathic melt. Metallic Fe,Ni globules are enclosed by sulfide and exhibit Nienrichment along their margins characteristic of rapid crystallization from a Fe-Ni-S liquid. Metal enclosed by sulfide is higher in Si and P than metal in feldspathic glass and enstatite, possibly indicating lower 0 fugacities in metal/sulfide than in silicate domains. Fine-grained, elongate enstatite prisms in troilite or feldspathic glass crystallized from local pyroxene melts that formed along precursor grain boundaries, but most of the enstatite in the target rock remained solid during the impact and occurs as deformed, coarsegrained crystals with lower CaO, A1203, and FeO than the fine-grained enstatite. Reckling Peak A80259 represents an intermediate stage of shock melting between unmelted E chondrites and whole-rock shock melts and melt breccias documented by previous workers. The shock petrogenesis of RKPA80259 reflects the extensive impact processing of the enstatite chondrite parent bodies relative to those of other chondrite types.

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

“…The initial (nanosecond-scale) peak pressure in the shock front and the resulting shock temperature can have grain-to-grain variations of an order of magnitude (Sharp and DeCarli, 2006). Because many EH and EL chondrites have been altered by impacts (e.g., Kimura and Lin, 1999;Burbine et al, 2000;Fagan et al, 2000;Patzer et al, 2004;Keil, 2007) and because impact processes can produce changes in bulk composition (e.g., ) and mineralogy (e.g., ) on a scale of $10 mm (e.g., , the standard taxonomic parameters may yield different results for different samples. Caution must therefore be taken in weighting these parameters.…”

“…Many EH chondrites have been described as impactmelt breccias or impact-melt rocks (e.g., Rubin, 1997a;Lin and Kimura, 1998;Kimura and Lin, 1999;Burbine et al, 2000;Fagan et al, 2000;Keil, 2007). The heat generated by the associated impacts could potentially cause thermal metamorphism in these materials, a process involving recrystallization, mineral equilibration, and partial loss of highly volatile elements.…”

A comparative study of opaque phases in Qingzhen (EH3) and MacAlpine Hills 88136 (EL3): Representatives of EH and EL parent bodies

“…of production of the schreibersite microspherules together with their host sulfides. Shock-induced melt inclusions of metallic Fe-Ni and troilite were reported in RPKA80259 (Fagan et al, 2000), but they are usually enclosed in silicates and not in sulfides. Many immiscible sulfide melt inclusions have been found in oldhamite from the Pefia Blanca Springsaubrite.…”

Melt inclusions in augite of the Nakhla martian meteorite: Evidence for basaltic parental melt

“…The major and minor elemental (Na, K, Mg, Al, Cr, Si, P, K, Ca, Ti, Mn, and Fe) compositions of the melt inclusions and their hosts were measured on a Cameca SX-50 electron microprobe at Virginia Tech (Table 1). Previous work by Fagan et al (2000) on glass analyses showed that using an accelerating voltage of 15 kV, a current of 2 nA and a defocused beam (5 to 10 µm) minimizes loss of Na, and so these instrumental conditions were employed in this study. In addition, Na was always analyzed first to minimize its loss during exposure to the beam.…”