Re-Os Chronology and Systematics of Graphite

Abstract

Graphite-forming events occurred throughout Earth history and extend up to solar and pre-solar contexts. However, a comprehensive understanding of the timeline of these events has been hindered by a lack of chronological information for graphite. Here we explore the geochronological potential of natural graphite using the Re-Os decay system and investigate its application thereafter in terrestrial systems. Graphite Re and Os concentrations and isotopic ratios are shown here to be highly variable (Re = 0.2-1520 ppb; Os = 89-19,577 ppt; 187Re/188O = 10.8-4101; 187Os/188Os = 0.57- 42.18) but analogous to those observed in terrestrial sulfides, organic-rich sedimentary rocks, and hydrocarbons. The variability in graphite Re contents is ostensibly controlled by graphite crystallinity (d002 and Lc(002)) with Re contents exhibiting an inverse relationship with graphite crystallinities. High-precision (<1%) graphite Re-Os isochron dating is established here using hydrothermal graphite formed in mid-crustal shear zones (1731 ± 7 Ma [2σ; MSWD = 1]; Saskatchewan, Canada) and tanzanite-tsavorite gemstone deposits (587 ± 2 Ma [2σ; MSWD = 1]; Merelani Hills, Tanzania) and metamorphic graphite-pyrite formed in the Franciscan subduction zone (161 ± 2 Ma [2σ; MSWD = 0.2]; California, USA). Graphite Re-Os dating is then coupled with additional geochronological (pyrite Re-Os dating and monazite/zircon U-Pb dating), isotopic tracer (187Os/188Os and 13C/12C) and XRD/Raman thermometry data to constrain the timing of large-scale carbon cycling in Paleoproterozoic graphitic shear zones associated with Nuna assembly, tanzanite-tsavorite gemstone mineralization in the Mozambique belt, and Os cycling associated with subduction- zone metamorphism in the Franciscan subduction system.