Characterization of weathering and heterogeneous mineral phase distribution in brachinite Northwest Africa 4872

Abstract

Abstract-Terrestrial weathering of hot desert achondrite meteorite finds and heterogeneous phase distributions in meteorites can complicate interpretation of petrological and geochemical information regarding parent-body processes. For example, understanding the effects of weathering is important for establishing chalcophile and siderophile element distributions within sulfide and metal phases in meteorites. Heterogeneous mineral phase distribution in relatively coarsely grained meteorites can also lead to uncertainties relating to compositional representativeness. Here, we investigate the weathering and high-density (e.g., sulfide, spinel, Fe-oxide) phase distribution in sections of ultramafic achondrite meteorite Northwest Africa (NWA) 4872. NWA 4872 is an olivine-rich brachinite (Fo 63.6 AE 0.5 ) with subsidiary pyroxene (Fs 9.7 AE 0.1 Wo 46.3 AE 0.2 ), Cr-spinel (Cr# = 70.3 AE 1.1), and weathered sulfide and metal. Raman mapping confirms that weathering has redistributed sulfur from primary troilite, resulting in the formation of Fe-oxide (-hydroxide) and marcasite (FeS 2 ). From Raman mapping, NWA 4872 is composed of olivine (89%), Ca-rich pyroxene (0.4%), and Cr-spinel (1.1%), with approximately 7% oxidized metal and sulfide and 2.3% marcasite-dominated sulfide. Microcomputed tomography (micro-CT) observations reveal high-density regions, demonstrating heterogeneities in mineral distribution. Precision cutting of the largest high-density region revealed a single 2 mm Cr-spinel grain. Despite the weathering in NWA 4872, rare earth element (REE) abundances of pyroxene determined by laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) indicate negligible modification of these elements in this mineral phase. The REE abundances of mineral grains in NWA 4872 are consistent with formation of the meteorite as the residuum of the partial melting process that occurred on its parent body. LA-ICP-MS analyses of sulfide and alteration products demonstrate the mobility of Re and/or Os; however, highly siderophile element (HSE) abundance patterns remain faithful recorders of processes acting on the brachinite parent body(ies). Detailed study of weathering and phase distribution offers a powerful tool for assessing the effects of low-temperature alteration and for identifying robust evidence for parent-body processes.

Differentiation processes in FeO‐rich asteroids revealed by the achondrite Lewis Cliff 88763

“…The susceptibility of sulfides and metals-the primary hosts of the HSE in the studied samples-to terrestrial aqueous alteration is well documented in chondrites (e.g., Horan et al 2010) and sulfide-and metal-rich achondrites (e.g., Shearer et al 2010;Day et al 2012;Hyde et al 2014). However Os, Ir, Ru, Pt, and Pd do not appear to have been modified within the meteorite.…”

“…; Hyde et al. ). Rhenium, rather than Os mobility with the meteorites, is indicated by the variability in Re concentrations and Re/Os measured among different splits of achondrite meteorites (e.g., Zag [b]), from the alteration of sulfide and metal.…”

“…; Hyde et al. ). However Os, Ir, Ru, Pt, and Pd do not appear to have been modified within the meteorite.…”

“…This non-isochronous behavior is similar to that observed for some ureilites (Rankenburg et al 2007), brachinites, and brachinitelike achondrites (Day et al 2012). In the case of brachinites and brachinite-like achondrites, it has been demonstrated that low Re/Os reflects mobility of Re within the meteorites, due to terrestrial alteration (Day et al 2012;Hyde et al 2014). Rhenium, rather than Os mobility with the meteorites, is indicated by the variability in Re concentrations and Re/Os measured among different splits of achondrite meteorites (e.g., Zag [b]), from the alteration of sulfide and metal.…”

Meteorites from the Lut Desert (Iran)

“…() and Hyde et al. () also mention the alteration of troilite to pyrite and marcasite for Omani OCs and NWA 4872 brachinite, respectively.…”