U-Pb systematics and trace element characteristics in titanite from a high-pressure mafic granulite

Abstract

Simultaneous acquisition of U-Pb isotope ratios and trace element abundances across titanite crystals formed in an anatectic, high pressure granulite using LA-ICP-MS split-stream analysis has enabled evaluation of titanite compositional systematics and intracrystalline variability during growth and residence in high-temperature, melt-present environments. Although the titanite studied here have a comparatively low initial Pb (Pb0) component (Pb0/Pb*), the Pb0 is highly radiogenic relative to model crustal values, indicating inheritance from U-bearing accessory minerals consumed in the melt/titanite-forming reactions. Additionally, titanite crystals typically exhibit core-rim decreases in Pb0/Pb*, as defined by 204Pb/206Pb, calculated 206Pb0/206PbT, and uncorrected 206Pb/238U spot date profiles. Near the margins this is clearly dominated by local U-enrichment, but in the uniformly low-U interiors outwardly decreasing Pb0/Pb* appears to reflect decreasing Pb0 concentrations during growth. The positive correlation among Pb0 and Sr concentrations in crystal interiors over length scales of hundreds of micrometers is consistent with each having experienced similarly small degrees of diffusional relaxation, Given the high crystallization temperatures (>800°C) and likely slow cooling rates (~5°C), our data support slow Pb diffusivity in titanite, even at high temperature conditions, as has been proposed in a number of recent studies.Along the outer ~50–100μm, U, Th, Zr, and REE concentrations are variably elevated relative to the crystal interiors, with profiles taking one of two forms: 1) sharply increasing to highest concentrations inboard from the crystal edge and decreasing again to lower values near the crystal edge, or 2) gradually increasing to highest concentrations nearest the crystal edge. High-contrast BSE imaging shows that the former profiles are associated with ~1–2μm wide bright bands surrounding polygonal subgrains that tend to be developed where titanite is (or formerly was) in contact with matrix feldspar (i.e. crystallized melt), and are inferred to represent trace-element-enriched dissolution-precipitation reaction fronts. The latter profiles are associated with diffuse, locally wispy, brightness gradients adjacent to resorbed crystal boundaries, subgrain boundaries, or thicker bright bands formed in contact with matrix hornblende crystals, and are interpreted as regions of enhanced element mobility potentially resulting from the development of local micro-porosity pathways at some point in the recrystallization process.