Secondary‐volatiles linked metallic iron in eucrites: The dual‐origin metals of Camel Donga

Abstract

The unique occurrence of abundant (~1 vol%) near‐pure‐Fe metal in the Camel Donga eucrite is more complicated than previously believed. In addition to that component of groundmass metal, scattered within the meteorite are discrete nodules of much higher kamacite abundance. We have studied the petrology and composition of two of these nodules in the form of samples we call CD2 and CD3. The nodules are ovoids 11 (CD2) to 15 (CD3) mm across, with metal, or inferred preweathering metal, abundances of 12–17 vol% (CD2 is unfortunately quite weathered). The CD3 nodule also includes at its center a 5 mm ovoid clumping (6 vol%) of F‐apatite. Both nodules are fine‐grained, so the high Fe metal and apatite contents are clearly not flukes of inadequate sampling. The metals within the nodules are distinctly Ni‐rich (0.3–0.6 wt%) compared to the pure‐Fe (Ni generally 0.01 wt%) groundmass metals. Bulk analyses of three pieces of the CD2 nodule show that trace siderophile elements Ir, Os, and Co are commensurately enriched; Au is enriched to a lesser degree. The siderophile evidence shows the nodules did not form by in situ reduction of pyroxene FeO. Moreover, the nodules do not show features such as silica‐phase enrichment or pyroxene with reduced FeO (as constrained by FeO/MgO and especially FeO/MnO) predicted by the in situ reduction model. The oxide minerals, even in groundmass samples well away from the nodules, also show little evidence of reduction. Although the nodule boundaries are generally sharp, groundmass‐metal Ni content is anti‐correlated with distance from the CD3 nodule. We infer that the nodules represent materials that originated within impactors into the Camel Donga portion of the eucrite crust, but probably were profoundly altered during later metamorphism/metasomatism. Origin of the pure‐Fe groundmass metal remains enigmatic. In situ reduction probably played an important role, and association in the same meteorite of the Fe‐nodules is probably significant. But the fluid during alteration was probably not (as previously modeled) purely S and O, of simple heat‐driven internal derivation. We conjecture a two‐stage metasomatism, as fluids passed through Camel Donga after impact heating of volatile‐rich chondritic masses (survivors of gentle accretionary impacts) within the nearby crust. First, reduction to form troilite may have been triggered by fluids rich in S2 and CO (derived from the protonodules?), and then in a distinct later stage, fluids were (comparatively) H2O‐rich, and thus reacted with troilite to form pure‐Fe metal along with H2S and SO2. The early eucrite crust was in places a dynamic fluid‐bearing environment that hosted complex chemical processes, including some that engendered significant diversity among metal+sulfide alterations.

Eucrite‐type achondrites: Petrology and oxygen isotope compositions†

“…Non-polymict eucrites typically contain ≤0.3 vol% metal (Delaney et al 1984), which is usually in grains of a few to a few tens of microns in size; most metal grains contain ≤1 wt% Ni (Duke 1965;Mayne et al 2009;this study). Exceptional basaltic eucrite Camel Donga contains about 1 vol% metal containing up to ~0.6 wt% Ni (Cleverly et al 1986;Palme et al 1988;Warren et al 2017). EET 92023 contains ~0.5 wt% metal that is up to ~300 µm across (Kaneda and Warren 1998) and forms composite grains of kamacite, taenite, and probable tetrataenite (Mayne et al 2009;Yamaguchi et al 2017;this study).…”

How do secondary iron enrichments form within basaltic eucrites? An experimental approach

“…2013, 2017; Warren et al. 2014, 2017; Pang et al. 2017; Patzer and McSween 2018; Roszjar 2019; Vollmer et al.…”

“…2013; Roszjar 2019); (3) secondary Fe metal, in, e.g., Camel Donga and NWA 5738 (Warren et al. 2014, 2017); and (4) Fe‐enrichments and/or veinlet‐like textures mainly composed of fayalitic olivine in, e.g., Pasamonte, NWA 049/1000/1109/2061/5073/5738, Yamato (Y)‐75011/82202 basaltic eucrites (Takeda et al. 1983, 1994; Warren 2002; Barrat et al.…”

Association of silica phases as geothermobarometer for eucrites: Implication for two‐stage thermal metamorphism in the eucritic crust

“…In particular, PO tridymite is present in the clast A, and MC tridymite was in the clast B, indicating that each clast has experienced distinct cooling rates before brecciation. These supported that Camel Donga should be classified as a polymict eucrite as Warren et al (2017) suggested by pyroxene composition and Fe-nodule, while there is a possibility that the clasts in Camel Donga reflect a large variety of the cooling rate in the same mass. Therefore, silica minerals can distinguish whether brecciated eucrites are monomict or polymict by combining with pyroxene chemical compositions.…”