The New Caledonia Ophiolite hosts one of the largest obducted mantle sections worldwide, offering a unique opportunity to investigate key mantle processes. The ophiolite comprises refractory harzburgites, locally overlain by mafic-ultramafic cumulates, and minor lherzolites. Previous geochemical studies indicated that the lherzolites are akin to abyssal-type peridotites, while the harzburgites underwent multiple melting episodes in MOR and supra-subduction zone environments, followed by late stage metasomatism. In this work, Os isotopes, highly siderophile (HSE) and chalcophile element data are reported for the New Caledonia peridotites, in order to constrain the behaviour of these elements in abyssal-type and fore-arc mantle. The variably serpentinised lherzolites (LOI = 6.4 - 10.7 %) yield slightly subchondritic to suprachondritic initial Os isotopic compositions (187Os/188Osi = 0.1273-0.1329) and subchondritic to chondritic Re/Os ratios (0.04-0.11). The gently sloping HSE patterns with increasing depletion towards Au show concentrations in the range of other lherzolites from MOR or continental setting. Sulphur contents are high and variable (202-1268 ppm), and were likely increased during serpentinisation. By contrast, Se/Te ratios and concentrations are within the range of primitive mantle (PM) values, meaning that these elements were not significantly mobilised during serpentinisation. Although displaying homogenous petrographic and geochemical features, the harzburgites are characterised by extremely heterogeneous Re-Os and HSE compositions. Type-A harzburgites exhibit subchondritic 187Os/188Osi (0.1203-0.1266) and low Re/Os ratios (0.01-0.04). The strong IPGE-PPGE fractionations (PdN/IrN = 0.21-0.56), coupled with positive Pt anomalies and S-Se-Te abundances often below the detection limit, suggest high melt extraction rates, resulting in sulphide consumption and Os-Ru metal alloy stabilisation. Type-B harzburgites possess strongly fractionated, Os-Ir-Pt poor (Os = 0.003-0.072 ng/g, Ir = 0.0015-0.079 ng/g) and Pd-Re enriched patterns, associated with chondritic to suprachondritic measured 187Os/188Os (0.127-0.153). These characters are uncommon for highly depleted mantle residues. Interaction with an oxidised component does not appear as a viable mechanism to account for the IPGE-depleted patterns of type-B harzburgites, as calculated oxygen fugacities are close to the FMQ buffer . The strikingly uniform mineralogical and geochemical features displayed by both harzburgite sub-types suggest that the different HSE patterns are not linked to their recent evolution, implying that subduction-related processes were superimposed on geochemical heterogeneous mantle domains, which exerted an important control on HSE behaviour during melt extraction and post melting metasomatism. We propose that the HSE characters of the studied peridotites reflect the presence of a highly heterogeneous mantle source with a long term ( > 1 Ga) evolution, possibly linked to the Zealandia formation.

Highly siderophile and chalcophile element behaviour in abyssal-type and supra-subduction zone mantle: new insights from the New Caledonia ophiolite / A. Secchiari, P. Gleissner, C. Li, A. Goncharov, H. Becker, D. Bosch, A. Montanini. - In: LITHOS. - ISSN 0024-4937. - 354-355:(2020 Feb), pp. 105338.1-105338.17. [10.1016/j.lithos.2019.105338]

Highly siderophile and chalcophile element behaviour in abyssal-type and supra-subduction zone mantle: new insights from the New Caledonia ophiolite

A. Secchiari
Primo
;
2020

Abstract

The New Caledonia Ophiolite hosts one of the largest obducted mantle sections worldwide, offering a unique opportunity to investigate key mantle processes. The ophiolite comprises refractory harzburgites, locally overlain by mafic-ultramafic cumulates, and minor lherzolites. Previous geochemical studies indicated that the lherzolites are akin to abyssal-type peridotites, while the harzburgites underwent multiple melting episodes in MOR and supra-subduction zone environments, followed by late stage metasomatism. In this work, Os isotopes, highly siderophile (HSE) and chalcophile element data are reported for the New Caledonia peridotites, in order to constrain the behaviour of these elements in abyssal-type and fore-arc mantle. The variably serpentinised lherzolites (LOI = 6.4 - 10.7 %) yield slightly subchondritic to suprachondritic initial Os isotopic compositions (187Os/188Osi = 0.1273-0.1329) and subchondritic to chondritic Re/Os ratios (0.04-0.11). The gently sloping HSE patterns with increasing depletion towards Au show concentrations in the range of other lherzolites from MOR or continental setting. Sulphur contents are high and variable (202-1268 ppm), and were likely increased during serpentinisation. By contrast, Se/Te ratios and concentrations are within the range of primitive mantle (PM) values, meaning that these elements were not significantly mobilised during serpentinisation. Although displaying homogenous petrographic and geochemical features, the harzburgites are characterised by extremely heterogeneous Re-Os and HSE compositions. Type-A harzburgites exhibit subchondritic 187Os/188Osi (0.1203-0.1266) and low Re/Os ratios (0.01-0.04). The strong IPGE-PPGE fractionations (PdN/IrN = 0.21-0.56), coupled with positive Pt anomalies and S-Se-Te abundances often below the detection limit, suggest high melt extraction rates, resulting in sulphide consumption and Os-Ru metal alloy stabilisation. Type-B harzburgites possess strongly fractionated, Os-Ir-Pt poor (Os = 0.003-0.072 ng/g, Ir = 0.0015-0.079 ng/g) and Pd-Re enriched patterns, associated with chondritic to suprachondritic measured 187Os/188Os (0.127-0.153). These characters are uncommon for highly depleted mantle residues. Interaction with an oxidised component does not appear as a viable mechanism to account for the IPGE-depleted patterns of type-B harzburgites, as calculated oxygen fugacities are close to the FMQ buffer . The strikingly uniform mineralogical and geochemical features displayed by both harzburgite sub-types suggest that the different HSE patterns are not linked to their recent evolution, implying that subduction-related processes were superimposed on geochemical heterogeneous mantle domains, which exerted an important control on HSE behaviour during melt extraction and post melting metasomatism. We propose that the HSE characters of the studied peridotites reflect the presence of a highly heterogeneous mantle source with a long term ( > 1 Ga) evolution, possibly linked to the Zealandia formation.
Settore GEO/07 - Petrologia e Petrografia
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