Zoning patterns of Fe and V in spinel from a type B Ca‐Al‐rich inclusion: Constraints on subsolidus thermal history

Abstract

Abstract-We obtained two-dimensional concentration maps for the minor elements Fe and V in 21 spinel crystals in the Allende type B1 inclusion TS-34 with a 4-5 μm resolution. Locally high concentrations of Fe occur along at least one edge of the spinels and decrease toward the center of the grains. Enrichment in V can also occur along edges or at corners. In general, there is no overall correlation of the Fe and V distributions, but in local regions of two grains, the V and Fe distributions are correlated, strongly suggesting a local source for both elements. In these two grains, opaque assemblages are present that appear to locally control the V distributions. This, coupled with previous work, suggests that prior to alteration, TS-34 contained V-rich metal. Oxidation of this metal during alteration can account for the edge/corner V enrichments, but provide only minor FeO contributions, explaining the overall lack of correlation between Fe and V. Most of the FeO appears to have been externally introduced along spinel boundaries during alteration. These alteration phases served as sources for diffusion of FeO into spinel. FeO distributions in spinel lead to a mean attenuation length of ~8 μm and, using literature diffusion coefficients in isothermal and exponential cooling approximations for peak temperatures in the range 600-700 °C, this leads to a time scale for calciumaluminum-rich inclusion (CAI) alteration in the range of decades to centuries.

The formation of boundary clinopyroxenes and associated glass veins in type B1 CAIs

“…For example, thin rinds and blebs of clinopyroxene are commonly found along grain boundaries of spinel (MacPherson et al 1984;Kuehner et al 1989;Simon et al 1991). Paque et al (2007a) noted these boundary clinopyroxenes in TS-34, a type B1 inclusion from Allende that is also part of this study, but they were unable to undertake a detailed characterization because of the fine grain size. Similarly, Connolly and Burnett (1999) observed thin "halos" of undetermined mineralogical nature around spinel in anorthite from Leoville 3537-2, the other inclusion we studied, and in TS-23, another Allende type B1 inclusion.…”

“…Close examination of the interfacial region between spinel and melilite in TS-34 shows that ten of the eleven TS-34 spinel grains and clusters studied by Paque et al (2007a) have boundary clinopyroxenes. Of the eight spinels in mantle melilites they analyzed, six have clinopyroxene rinds, one is associated with blebs, and one has both a rind and blebs.…”

“…In the course of studying minor element concentrations in TS-34 spinels, Paque et al (2007a) noted the common occurrence of clinopyroxene on spinel inclusions in melilite. These grains were generally too thin to fully characterize and, therefore, the interface regions between spinel and melilite in five areas were chosen for more detailed FIB/ STEM examination.…”

“…Allende TS-34 is a large, spherical inclusion ~9 mm in diameter showing only a moderate degree of secondary alteration, which has been the subject of numerous studies (e.g., Clayton et al 1977;Paque et al 2007a;Simon et al 1991). Leoville USNM 3537-2 is a coarse-grained spherical type B1 CAI ~7 mm in diameter (Caillet et al 1993;Connolly and Burnett 1999).…”

“…These grains were generally too thin to fully characterize and, therefore, the interface regions between spinel and melilite in five areas were chosen for more detailed FIB/ STEM examination. Three of these grains, A2, E2 (mantle) and B2 (core) from TS-34, were previously studied by Paque et al (2007a). Two additional spinel grains, one each from the mantle (Leo1) and core (Leo2), were selected from Leoville 3537-2 based on SEM examination.…”