The behaviour of metamorphic apatite from mid-amphibolite to granulite facies metapelites and metapsammites: insights from the Stafford Member from the Arunta region, Australia

Abstract

The chemical behaviour of metamorphic apatite was investigated in a quasi-unchanging rock composition to gain insight into the stability of apatite and distribution of heat production in the continental crust. Samples of two rock types were collected from the Stafford Member: metapelites and metapsammites, that record calculated metamorphic conditions from mid-amphibolite facies (~ 2.1 kbar, 590 °C) to granulite facies (~ 3.9 kbar, 800 °C) conditions. The chemical compositions, petrographic relationships and abundances of accessory phosphate minerals were investigated as a function of metamorphic grade, with a focus on the relationship between apatite and monazite, both of which increase in abundance with metamorphic grade. At mid-amphibolite facies metamorphic conditions, apatite growth is driven by the partial consumption of accessory xenotime and complete consumption of low-grade allanite and detrital apatite (Reaction 4). At upper-amphibolite facies conditions, apatite growth is driven by the partial breakdown of xenotime and plagioclase (Reaction 5). Additionally, apatite was found to be intimately linked with the breakdown of biotite and muscovite micas at all metamorphic conditions (Fig. 3 & 5), as they supply the F, Cl and OH component required for apatite growth. Granulite facies monazite growth is facilitated by the dissolution of apatite (Fig. 9) and the breakdown of xenotime (Reaction 2). However, as the major reservoir for P2O5 in the samples, the stability and high abundance of apatite at granulite facies conditions (Figures 10, 5e, 5f, 3c, 3d) is enhanced by the enrichment of P2O5 in residual samples (Table 2). Furthermore, during partial melting, heat producing elements U + Th preferentially remain in monazite relative to apatite and the melt, therefore the melt and apatite is relatively depleted in HPEs. Consequently, melt loss of HPE depleted segregates could enrich HPEs in the residual, partially melted monazite bearing rocks of the Stafford Member.