Significance of highly siderophile element and Re–Os isotope systematics in global carbonatites

Abstract

The systematics of highly siderophile elements (HSE) in carbonatites have rarely been investigated although carbonatite melts are among key metasomatic agents in the Earth's mantle. Thus, establishing the Os isotope systematics of carbonatites is of prime importance because carbonatite metasomatism might play a determining role in driving ¹⁸⁷Os/¹⁸⁸Os signature of the Earth's mantle as a consequence of extreme mobility of carbonatite liquids. In this study, a suite of carbonatites from continental settings was analyzed for HSE abundances and Re–Os isotope systematics, combined with detailed petrography of sulfide phases, to provide constraints on the HSE behavior during the formation of carbonatite melts, the role of sulfides in distribution of HSE and the reliability of Re–Os isotope compositions. All investigated carbonatites display low ΣHSE <1 ppb whereby calciocarbonatites exhibit supra-chondritic Os_N/Ir_N ratios from ∼1.4 to 15, in contrast to magnesiocarbonatites and Mg/Fe-rich carbonatites showing exclusively sub-chondritic Os_N/Ir_N. We posit that preferential removal of Os over Ir during the segregation of calcite-normative melts from the co-existing S-rich silicate melts is the main driver of Os/Ir elemental fractionation. The present-day ¹⁸⁷Os/¹⁸⁸Os ratios in carbonatites range from mildly unradiogenic (∼0.124) to extremely radiogenic values (∼115–152), with ¹⁸⁷Os/¹⁸⁸Os_(i) ratios spanning from negative to highly radiogenic (∼17) values, paralleled by highly variable ¹⁸⁷Re/¹⁸⁸Os ratios between 0.096 and ∼ 26,000, and without any clear relationship to the carbonatite type or emplacement age. The Re–Os systematics of carbonatites appear to be significantly modified by hydrothermal processes resulting in either removal of both Re and Os, or Re addition and, consequently, commonly negative calculated γ_Os values, which may indicate a rather limited potential of the Re–Os systematics to evaluate the nature of carbonatite melts. Conversely, limited Re–Os data for the samples that were likely not affected by late-stage perturbation indicate a largely radiogenic ¹⁸⁷Os/¹⁸⁸Os nature of carbonatite melts. Nevertheless, carbonatite melts may, in part, be responsible for the radiogenic γOs values commonly observed in mantle-derived silicate melts influenced by carbonatite-like metasomatism.