Highly siderophile element geochemistry and Re–Os isotopic systematics of carbonatites: Insights from Tamil Nadu, India

0505893 - GLÚ 2020 RIV NL eng J - Journal Article
Ackerman, Lukáš - Polák, Ladislav - Magna, T. - Rapprich, V. - Ďurišová, Jana - Upadhyay, D.
Highly siderophile element geochemistry and Re–Os isotopic systematics of carbonatites: Insights from Tamil Nadu, India.
Earth and Planetary Science Letters. Roč. 520, 15 August 2019 (2019), s. 175-187. ISSN 0012-821X. E-ISSN 1385-013X
Institutional support: RVO:67985831
Keywords : carbonatite * enriched mantle * HSE * metasomatism * Re–Os isotopic systematics * silicate
OECD category: Geology
Impact factor: 4.824, year: 2019
Method of publishing: Limited access
https://www.sciencedirect.com/science/article/pii/S0012821X19303115?via%3Dihub

Carbonatite metasomatism has been widely implicated for worldwide mafic mantle suites but so far, no combined data have been available for highly siderophile element systematics (HSE – Os, Ir, Ru, Pt, Pd, Re) and Re–Os isotopic compositions in carbonatites themselves. We present the first systematic survey of the HSE and Re–Os isotopic compositions in a suite of well-characterized Neoproterozoic carbonatites, silicocarbonatites and associated silicate rocks (pyroxenites, monzogabbros, syenites) from south India in order to place constraints on the HSE systematics in carbonatite magmas, anchoring possible mantle sources of carbonatites and relationship to the ambient crustal lithologies as well as preliminary constraints on carbonatite metasomatism in Earth's mantle. The most plausible explanation for generally low HSE contents in calciocarbonatites from Tamil Nadu (∑HSE < 1.22 ppb) involves a low-degree (<1%) partial melting of the mantle source producing sulfur-saturated carbonatitic magmas leaving behind sulfide phases retaining HSE. The new data also indicate a strong FeO control on the distribution of Os and Pt during segregation of carbonatite melt from its enriched mantle source and/or melt differentiation. The combined ¹⁸⁷Re/¹⁸⁸Os values (from 0.10 to 217), ¹⁸⁷Os/¹⁸⁸Os ratios (0.186–10.4) and initial γOs values back-calculated to 800 Ma (from +0.1 to +6052) predict that most Tamil Nadu calciocarbonatites were plausibly derived from a carbonated peridotite source with <10% recycled component. This model would thus provide significant constraints on the origin/source of carbonatites, irrespective of their post-emplacement history. The unusual, volumetrically rare, Mg–Cr-rich silicocarbonatites (∑HSE = 14–41 ppb) display almost identical HSE patterns with those of host pyroxenites and predominantly high Pt (up to 38 ppb), the origin of which remains unknown. Positive co-variations between Pt, Pd and Re, and the well-developed positive correlation between Pt and MgO in these Mg–Cr-rich silicocarbonatites argue for a source coming predominantly from the upper mantle. The Re–Os isotopic systematics agree with direct incorporation of enriched mantle-derived material into parental melts but variable incorporation of potassium-rich crustal materials is evidenced by highly positive γ^Os_{800 Ma} values for a sub-suite of Mg–Cr-rich silicocarbonatites, indicating intense fenitization. The highly radiogenic Os isotopic compositions of monzogabbros and a syenite argue for their derivation from crustal lithologies with no or only negligible contribution of mantle material. Collectively, low Ir, Ru, Pt and Pd contents found in the Tamil Nadu carbonatites appear to indicate the incapability to significantly modify the total budget of these elements in the Earth's mantle during carbonatite metasomatism. In contrast, very high Re/Os ratios found in some of the analyzed carbonatites, paralleled by extremely radiogenic ¹⁸⁷Os/¹⁸⁸Os signature, can produce large modification of the Re–Os isotopic composition of mantle peridotites during carbonatite melt percolation when high melt/rock ratios are achieved.
Permanent Link: http://hdl.handle.net/11104/0297244