Apatite recrystallisation during prograde metamorphism, Cooma, southeast Australia: implications for using an apatite – graphite association as a biotracer in ancient metasedimentary rocks

Abstract

Early Silurian low-pressure (<500 MPa) greenschist to granulite facies metamorphism in Ordovician sedimentary rocks of the Cooma Complex culminating in the production of the Cooma Granodiorite, Lachlan Fold Belt (southeast Australia) provides an opportunity to determine the physical and chemical response of apatite (Ca phosphate) to prograde metamorphism. Electron probe elemental mapping (P, Ca, Ce, Si) of metapelite polished sections shows that apatite grains become less abundant but larger with increasing metamorphic grade (<20 μm at chlorite grade to >200 μm in enclaves in migmatites). Many apatite grains are composite, with clear overgrowths and cloudy cores, which in rare cases contain microscopic inclusions interpreted to delineate microfolds. Therefore, during progressive metamorphism there was dissolution and regrowth of apatite. Recrystallisation and growth of apatite crystals with increasing metamorphic grade was accompanied by the progressive dissolution then regrowth of monazite (Ce – Th phosphate), as previously reported from the Cooma metasedimentary sequence. The Cooma apatites mostly have similar REE patterns at all grades. But upgrade, there is a confluence in REE abundances of apatite grains, indicating an increasing volume of chemical equilibration. Infrared absorbance spectrometry indicates that carbon is present as carbonate substituting for phosphate and hydroxyl ions in the lattice of apatites at all metamorphic grades. Carbonaceous material (graphite?) is only a rare inclusion phase found in the younger clear apatite in the higher-grade rocks. The Cooma apatite study shows that, contrary to previous assertions for earliest (>3700 Ma, amphibolite – granulite facies) metasediments from Greenland, apatite is unlikely to be a stable ‘capsule’ which will reliably protect any biogenic carbon (graphite at high metamorphic grade) from reactions and isotopic exchange during prograde metamorphism.