Origin and Metamorphic Redistribution of Silicon, Chromium, and Phosphorus in the Metal of Chondrites

Abstract

Chromium, silicon, and phosphorus concentrations of 0.1 to 1 percent by weight are common in metal grains in the least metamorphosed ordinary and carbonaceous chondrites. These concentrations are fairly uniform within single chondrules (but different from chondrule to chondrule) and are inversely correlated with the fayalite concentrations of the chondrule olivines. This relation shows that these chromium, silicon, and phosphorus concentrations could not have been established by condensation or equilibration in the solar nebula but are the result of metal-silicate equilibration within chondrules. Two generations of inclusions made by the exsolution of those elements have been identified: One formed during chondrule cooling and the other formed during metamorphism. The distribution and composition of the latter in type 3 to type 5 chondrites are consistent with increasing metamorphism relative to type 2 and type 3.0 material.

Fe‐Ni metal in primitive chondrites: Indicators of classification and metamorphic conditions for ordinary and CO chondrites

“…Tiny inclusions of chromite and Fe-or Ca-phosphate, smaller than a few micrometers in size, also occur in metal spherules of many chondrules (Fig. 1b), as previously reported (e.g., Zanda et al 1994). Fe-Ni metal grains on chondrule surfaces and occurring as isolated minerals in the matrix of Semarkona do not show plessitic intergrowth textures.…”

“…This metal may contain minor elements such as Cr and Si (Rambaldi et al 1980), and tiny inclusions of phosphate, chromite and other phases (Zanda et al 1994). Some Fe metal formed by reduction of silicates, and is marked by a strong depletion in Ni (Rambaldi and Wasson 1984).…”

Experimental study and TEM characterization of dusty olivines in chondrites: Evidence for formation by in situ reduction

“…silicates from highly unequilibrated chondrites (Hanon, Robert, and Chaussidon 1998), in olivines or glass inclusions in olivines of CV3 carbonaceous chondrites , or in metal as graphite inclusions in a number of type 3 ordinary chondrites, including Bishunpur (Mostefaoui et al 2000;Zanda et al 1994). In the latter case, graphite is interpreted as residual carbon that did not undergo reduction processes.…”

Shape, metal abundance, chemistry, and origin of chondrules in the Renazzo (CR) chondrite

“…14). It is not at all clear how these compositional gradients would be predicted or explained by evaporation-recondensation models for bimodal metal compositions (e.g., Zanda et al 1994;Campbell et al 2005b). Instead, these gradients suggest that the core region of chondrule 3 equilibrated with vapor at high temperatures, and that successive layers of silicate and metal were subsequently accreted and annealed, with only partial re-equilibration of the inner core minerals.…”