187Re- 187Os Nuclear Geochronometry of Diamond Sulphide Inclusions: Constraining the Chemical Evolution of the SCLM

English Abstract:

Nuclear geochronometry [1-2] is a new research field constrained by other scientific fields like cosmology, cosmochemistry and nuclear theory. It connects geochronology with nuclear astrophysics. Based upon terrestrial signatures from at least two rapid (r) neutron-capture processes [3], so-called nucleogeochronometric TPI ages are calculated by means of two-point-isochrones (TPI). Because of identified Re/Os nuclear production ratios ≈ 1 and ultra-subchondritic initial ¹⁸⁷Os/¹⁸⁸Os ratios, it is argued that Earth’s inner core (IC) contains isotopic signatures of a 13.78 Ga old component, as it can be found within the komatiitic basalt [5085 BasKom] (Onverwacht Group, South Africa) [4]. The other signatures are assigned to the Earth’s outer core (OC), due to at least one gravitational collapse of the old component ≈ 3.48 Ga [2]. ¹⁸⁷Os/¹⁸⁸Os_i ratios of sulphide inclusions from eclogitic diamonds usually plot above an r-process chronometer evolution line in an ¹⁸⁷Os/¹⁸⁸Os evolution diagram, the eclogite field [5] showing a pronounced bulge contemporaneous with the Great Oxidation Event ≈ 2.22 Ga – 2.46 Ga. This coincides with an ¹⁸⁷Re/¹⁸⁸Os fractionation event ≈ 2.3 ± 0.3 Ga, constrained by TPI ages for the Ellendale (Australia) peridotitic diamond sulphide inclusions EL50 and EL23 reported in the literature [6]. Contrary to eclogitic sulphide inclusions, ¹⁸⁷Os/¹⁸⁸Os ratios of most peridotitic sulphide inclusions [6] plot below the r-process chronometer evolution line, pointing to significant Re losses of the r-process reservoir. From the emerging pattern it may be concluded that ¹⁸⁷Os/¹⁸⁸Os_i ratios of eclogitic diamond sulphide inclusions between 3 Ga and 1 Ga are due to episodic mixing, mingling and fractionation of IC/OC isotopic signatures and redistribution of Re during the evolution of the SCLM. This could explain the difficulty to obtain reliable conventional model or isochrone ages for these inclusions, a problem which may be solved by means of nuclear geochronometry.

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