Early Thermal Histories of IAB Main Group Irons: Insights From Ca‐Phosphates in Campo del Cielo and Nantan

Abstract

AbstractCa‐phosphates in Campo del Cielo (CdC) and Nantan were comprehensively studied to provide insights into the thermal histories of the IAB main group (MG) and related irons. In CdC, apatite grains are characterized by (a) close intergrowth with troilite/graphite in border area between silicate and metal in most cases and (b) near‐flat rare earth elemental patterns (LaN/YbN = 0.6–0.7). This indicates they were formed during a metal‐silicate mixing event at a relatively high temperature. Combining with petrographic textures, we suggest that the replacement of high‐Ca pyroxene by low‐Ca pyroxene at ∼950–1,000°C could release Ca and facilitate the formation of apatite grains. In the Nantan nodule, Ca‐phosphates do not share a similar origin with those in CdC, as indicated by their different mineral chemistries and mineral associations. Ca‐phosphates and associated silicates could crystallize from a P‐C‐S‐rich metallic melt with the oxidation of lithophile elements. Combining all analyses from CdC and Nantan yielded a SIMS Pb‐Pb isochron age of 4,558 ± 56 Ma. Considering that all the IAB‐MG irons experienced a rapid high‐temperature cooling process, the age of 4,558 ± 56 Ma provides another line of evidence that the parent body of IAB‐MG and related irons experienced metal‐silicate mixing in first 50 Myr of solar system. The previously reported Ar‐Ar ages of ≤4.47 Ga could be related to the late reheating process(es). The degrees of late shock heating may vary from specimen to specimen.