The Retention of the Biogenic Oxygen Isotope Imprint in Marine Phosphates During Metamorphism

Analyses were made of the oxygen isotope ratios of phosphates within a sequence of metamorphosed black shales from the greenschist to lower granulite facies. This study was done to test if the oxygen isotope signature of marine biogenic phosphate could survive metamorphism of the host rock. Biogenic phosphates formed in low temperature marine environments are characterized by high oxygen isotope values relative to non-biogenic detrital igneous apatites. If the same environment existed in the past a similar fractionation should be observed in biogenic deposits. Metamorphism in the presence of water can result in the equilibration of the oxygen isotopes in the phosphate with the matrix oxygen of the rock and thereby compromise the original isotopic signature. Oxygen isotope ratios of phosphate of 8 samples from the Smalls Falls metamorphic sequence of Silurian age (western Maine to Connecticut), ranging from lower greenschist (250 degrees C) to lower granulite (650 degrees C) facies rocks, were determined. Of the 8 samples, 5 indicate that the phosphate has equilibrated with the whole rock oxygen characterized by a sedimentary silica and clay minerals matrix. However, 3 samples with metamorphic temperatures of 325, 550 and 625 degrees C had phosphate δ ¹⁸O values of 17.5, 16.4, and 18.6 permil respectively, significantly higher than the putative matrix δ ¹⁸O values and similar to contemporary marine biogenic phosphates. If indeed the original phosphate oxygen isotope ratio is preserved, then it must be due to conditions that inhibited equilibrium such as the absence of water.