Nickel abundance in stony cosmic spherules: Constraining precursor material and formation mechanisms

Abstract

Abstract-We report bulk and olivine compositions in 66 stony cosmic spherules (Na 2 O < 0.76 wt%), 200-800 lm in size, from the Transantarctic Mountains, Antarctica. In porphyritic cosmic spherules, relict olivines that survived atmospheric entry heating are always Ni-poor and similar in composition to the olivines in carbonaceous or unequilibrated ordinary chondrites (18 spherules), and equilibrated ordinary chondrites (one spherule). This is consistent with selective survival of high temperature, Mg-rich olivines during atmospheric entry. Olivines that crystallized from the melts produced during atmospheric entry have NiO contents that increase with increasing NiO in the bulk spherule, and that range from values similar to those observed in chondritic olivines (NiO generally <0.5 wt%) to values characteristic of olivines in meteoritic ablation spheres (NiO > 2 wt%). Thus, NiO content in olivine cannot be used alone to distinguish meteoritic ablation spheres from cosmic spherules, and the volatile element contents have to be considered. We propose that the variation in NiO contents in cosmic spherules and their olivines is the result of variable content of Fe, Ni metal in the precursor. NiO contents in olivines and in cosmic spherules can thus be used to discuss their parent body. Ni-poor spherules can be derived from C-rich and ⁄ or metal-poor precursors, either related to CM, CI, CR chondrites or to chondritic fragments dominated by silicates, regardless of the parent body. Ni-rich spherules (NiO > 0.7 wt%) that represent 55% of the 47 barred-olivine spherules we studied, were derived from the melting of C-poor, metal-rich precursors, compatible with ordinary chondrite or CO, CV, CK carbonaceous chondrite parentages.

HED-like cosmic spherules from the Transantarctic Mountains, Antarctica: Major and trace element abundances and oxygen isotopic compositions

“…below detection limit) and low SiO 2 contents (<52 wt.%). Thus, they differ from the terrestrial glass particles found in the micrometeorite traps, volcanic glass shards (Na 2 O >3 wt.%; Curzio et al, 2008) or microtektites (SiO 2 >59 wt.%; Folco et al, 2009), and from ablation debris spherules (Na 2 O $2 wt.%; Genge and Grady, 1999;van Ginneken et al, 2010;Cordier et al, 2011c). Therefore, all the studied particles can be classified as cosmic spherules.…”

Dendritic magnetite crystals in rapid quenched fine spherules produced by falling experiments through the high temperature furnace with controlled gas flow

“…In the chondrule formation processes, crystallographic constrain at crystal growth occur in barred olivine and radial pyroxene chondrules. Barred olivine texture is also reported in cosmic spherules, which are melted micrometeorite (Cordier et al, 2011). The fine particle falling apparatus developed in this study has expanded cooling rate in laboratory experiments up to 2 × 10 6 K/h.…”

Vesicle dynamics during the atmospheric entry heating of cosmic spherules

“…Study of the composition of such spherules, in particular the Ni‐contents of their olivines, as shown by Cordier et al. (), together with vesicle contents should allow these two possibilities to be discriminated.…”

“…Approximately 50% of large cosmic spherules (>200 μm) have been shown to have ordinary chondrite‐like precursors (Cordier et al. ), while the remainder are probably derived mainly from precursors similar to carbonaceous chondrites (Genge et al. ).…”

“…Evaluating the origins of cosmic spherules, however, is difficult since their extensive melting has removed most of the pre-existing minerals. Studies of the compositions of cosmic spherules have, however, suggested the majority are derived from primitive materials similar to carbonaceous and ordinary chondrites, with a rare contribution from differentiated, basaltic, parent bodies (Kurat et al 1994;Genge et al 1997;Taylor et al 2007;Cordier et al 2011aCordier et al , 2011bSuavet et al 2011b). Micrometeorites that survive atmospheric entry without significant heating may also represent the direct precursor materials of many cosmic spherules since a proportion of particles in any population incident on the Earth undergoes minimal heating at low entry angles.…”