187RE- 187OS NUCLEAR GEOCHRONOMETRY: CONSTRAINING OXYGEN AS A LIGHT ELEMENT WITHIN THE EARTH’S CORE FROM RE/OS, IR/OS AND PD/PT FRACTIONATION TRENDS

Nuclear geochronometry is a new dating method, which combines principles of geochronology with nuclear astrophysics [1]. It is based upon identified terrestrial Re/Os element ratios ≈ 1 (¹⁸⁷Re/¹⁸⁸Os = 5.873), interpreted as the nuclear production ratio, and ultra-subchondritic initial ¹⁸⁷Os/¹⁸⁸Os ratios, suggesting that Earth’s core still contains element ratios and isotopic signatures of at least two rapid (r) neutron-capture process events [2]. Terrestrial Re/Os ratios observed within lherzolitic diamond sulphide inclusions suddenly drop from ≈ 0.8 to 0.2 – 0.05 for nucleogeochronometric ages between 2.3 Ga and 1.4 Ga [3]. It has been argued that the ¹⁸⁷Re/¹⁸⁸Os fractionation pattern of the r-process signature is due to a change in oxygen fugacity [4], the oxygenation process being related to the physics and chemistry of the Earth’s core [5]. A similar fractionation pattern and trend for Pd/Pt has recently been identified by means of nucleogeochronometric age calculations, applying the BARBERTON nuclear geochronometer [2] to 24 H chondrite components from the literature [6]. While Ir/Os r-process ratios remain fairly constant, Pd/Pt ratios ≈ 0.8 – 0.6 (13.78 Ga – 13.6 Ga) suddenly drop to 0.4 – 0.06 (13.5 Ga – 13.1 Ga), merging with the Re/Os ratios (0.1 – 0.04, from 13.78 Ga to 13.1 Ga) in a striking parallel trend. Comparing these H chondrite Re/Os and Pd/Pt fractionation trends with the Re/Os fractionation pattern of the diamond sulphide inclusions, and considering that Re is readily oxidized even at ultra-low oxygen fugacity, it may be concluded that (i) extremely reducing conditions without any significant amount of Oxygen still dominate Earth’s inner and outer core, respectively, thus preserving the r-process Re/Os ratios; and (ii) the physics and chemistry behind the observed fractionation patterns are the same in stellar and planetary objects, as suggested by nuclear planetology [7].

[1] Roller (2015), Geophys. Res. Abstr. 17, EUG2015-35. [2] Roller (2015), Geophys. Res. Abstr. 17, EUG2015-17. [3] Roller (2015), Abstract VGP34C-0421, Spring Meeting, AGU 2015. [4] Roller (2015), Geophys. Res. Abstr. 17, EUG2015-2399. [5] Roller (2015), Abstract PG34A-0283, Spring Meeting, AGU 2015. [6] Horan et al. (2009), GCA 73, 6984-6997. [7] Roller (2015), Abstract T34B-0407, Spring Meeting, AGU 2015.