TY - JOUR

T1 - Highly siderophile and chalcophile element behaviour in abyssal-type and supra-subduction zone mantle

T2 - New insights from the New Caledonia ophiolite

AU - Secchiari, Arianna

AU - Gleissner, Philipp

AU - Li, Chunhui

AU - Goncharov, Alexey

AU - Milke, Ralf

AU - Becker, Harry

AU - Bosch, Delphine

AU - Montanini, Alessandra

N1 - Funding Information: The authors are grateful to M. Feth and D. Cluzel for help and support during clean laboratory work and sampling in New Caledonia, respectively. A.G. wishes to thank Russian Science Foundation project 17-77-10103 for supporting iron oxidation state investigations at Freie Universität of Berlin. We are indebted to Nadia Malaspina and Tiziano Catelani (University of Milano Bicocca) for providing access to SEM-EDS analyses of sulphides. Funding Information: This study represents a continuation of the PhD project of A.S. on the New Caledonia ophiolite, that has been supported by DFG funding, Germany (SFB-TRR 170 Subproject B2 ) and Italian-PRIN prot. 2015C5LN35. This work has also benefited from the equipment and framework of the COMP-HUB Initiative, funded by the ‘Departments of Excellence’ program of the Italian Ministry for Education , University and Researc h (MIUR, 2018-2022). This is TRR 170 publication no. 86.

PY - 2020/2

Y1 - 2020/2

N2 - 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[sbnd]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[sbnd]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[sbnd]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 (Log ΔFMQ = 0.35 to 0.65). 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.

AB - 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[sbnd]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[sbnd]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[sbnd]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 (Log ΔFMQ = 0.35 to 0.65). 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.

UR - http://www.scopus.com/inward/record.url?scp=85077916306&partnerID=8YFLogxK

U2 - 10.1016/j.lithos.2019.105338

DO - 10.1016/j.lithos.2019.105338

M3 - Article

AN - SCOPUS:85077916306

VL - 354-355

JO - Lithos

JF - Lithos

SN - 0024-4937

M1 - 105338

ER -