Universität zu Köln

Luguet, Ambre, Behrens, Melanie, Pearson, D. Graham, Koenig, Stephan and Herwartz, Daniel ORCID: 0000-0003-3604-8295 (2015). Significance of the whole rock Re-Os ages in cryptically and modally metasomatised cratonic peridotites: Constraints from HSE-Se-Te systematics. Geochim. Cosmochim. Acta, 164. S. 441 - 464. OXFORD: PERGAMON-ELSEVIER SCIENCE LTD. ISSN 1872-9533

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Abstract

The Re-Os isotopic system is the geochronometer of choice to constrain the timing of lithospheric mantle root formation and reconstruct the evolution of Earth's dynamics from the mantle perspective. In order to constrain the effects of metasomatic processes on the Re-Os isotopic system, eleven peridotites from the Letlhakane kimberlite pipe were investigated for whole rock major and trace elements, highly siderophile elements (HSE), Se, Te and Os-187/Os-188 signatures. These spinel peridotites (SP), garnet peridotites (GP), garnet-phlogopite peridotites (GPP) and phlogopite peridotites (PP) experienced cryptic metasomatism and the GP-GPP-PP additionally constitute a sequence of increasing modal metasomatism. The cryptically metasomatised SP appear devoid of base metal sulphides (BMS) and show suprachondritic Se/Te ratios (15-40) and extremely Pd- and Pt-depleted HSE patterns. These features are characteristic of high-degree partial melting residues. Their Os-187/Os-188 signatures are thus considered to be inherited from the partial melting event. This implies a Neoarchean (2.5-2.8 Ga, T-RD eruption) stabilisation of the Letlhakane mantle root and supports the Letlhakane mantle root being a westerly extension of the Zimbabwe cratonic root. The modally metasomatised peridotites contain BMS whose abundance significantly increases from the GPP to the GP and PP. The BMS-poor GPP are only slightly richer in Pt and Pd than the BMS-free SP but have similarly high Se/Te ratios. The BMS-rich GP and PP exhibit significant enrichments in Pt, Pd, Se, Te resulting in HSE-Se-Te signatures similar to that of the Primitive Upper Mantle (PUM). Addition of 0.001-0.05 wt.% metasomatic BMS +/- PGM (platinum group minerals, i.e., Pt-tellurides) to highly refractory residues, such as the Letlhakane SP, reproduce well the HSE-Se-Te systematics observed in the BMS-poor and BMS-rich modally metasomatised peridotites. In the GPP, the negligible addition of metasomatic BMS +/- PGM did not disturb the Os-187/Os-188 signatures, indicating T-RD eruption ages identical with those of the SP. By contrast, the PP and GP, which suffered significant BMS +/- PGM addition, have Os-187/Os-188 considerably shifted toward more radiogenic values. As a result, unrealistically young T-RD eruption ages are obtained that cannot be used to constrain the time frame of the stabilisation of the cratonic roots and the mechanisms of craton formation. The Se-Te and incompatible HSE (i.e., Pt, Pd) are powerful geochemical tools to assess the robustness of the Re-Os isotopic system in mantle peridotites. Specifically, Se/Te and Pd/Ir ratios provide complementary insights into the complex metasomatic history of mantle peridotites. The Se/Te ratio is very sensitive to Os-free PGM metasomatism producing highly variable Se/Te >15 at low, relatively constant Pd/Ir< 0.4 but does not affect the Os-187/Os-188 systematics. This contrasts with BMS metasomatism, where Pd/Ir becomes more variable (>0.4), at constant but low Se/Te <10 and which is accompanied by modification of the Os isotopic composition of the host peridotite. As they are sensitive indicators of BMS +/- PGM metasomatism, HSE, Se and Te should be systematically considered when investigating the timing of stabilisation of lithospheric mantle and its petrogenetic history. This would allow a more robust assessment of the Re-Os ages obtained and provide firmer constraints on the evolution and formation of Archean cratons and early Earth dynamics. (C) 2015 Elsevier Ltd. All rights reserved.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Luguet, Ambre UNSPECIFIED UNSPECIFIED UNSPECIFIED Behrens, Melanie UNSPECIFIED UNSPECIFIED UNSPECIFIED Pearson, D. Graham UNSPECIFIED UNSPECIFIED UNSPECIFIED Koenig, Stephan UNSPECIFIED UNSPECIFIED UNSPECIFIED Herwartz, Daniel UNSPECIFIED orcid.org/0000-0003-3604-8295 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-396017
DOI:	10.1016/j.gca.2015.06.016
Journal or Publication Title:	Geochim. Cosmochim. Acta
Volume:	164
Page Range:	S. 441 - 464
Date:	2015
Publisher:	PERGAMON-ELSEVIER SCIENCE LTD
Place of Publication:	OXFORD
ISSN:	1872-9533
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language HIGHLY SIDEROPHILE ELEMENTS; PLATINUM-GROUP ELEMENTS; EAST-AFRICAN RIFT; MANTLE XENOLITHS; LITHOSPHERIC MANTLE; ISOTOPE SYSTEMATICS; LETLHAKANE KIMBERLITES; TELLURIUM SYSTEMATICS; ABYSSAL PERIDOTITES; GARNET PERIDOTITES Multiple languages Geochemistry & Geophysics Multiple languages
URI:	http://kups.ub.uni-koeln.de/id/eprint/39601