The Interplay between Melting, Refertilization and Carbonatite Metasomatism in Off-Cratonic Lithospheric Mantle under Zealandia: an Integrated Major, Trace and Platinum Group Element Study

Abstract

It is widely accepted that stabilization of the continental crust requires the presence of sub-continental lithospheric mantle. However, the degree of melt depletion required to stabilize the lithosphere and whether widespread refertilization is a significant process remain unresolved. Here, major and trace element, including platinum group elements (PGE), characterization of 40 mantle xenoliths from 13 localities is used to constrain the melt depletion, refertilization and metasomatic history of lithospheric mantle underneath the micro-continent Zealandia. Our previously published Re-Os isotopic data for a subset of these xenoliths indicate Phanerozoic to Paleoproterozoic ages and, reinterpreted with the new major and trace element data presented here, demonstrate that a large volume (>2 million km 3) of lithospheric mantle with an age of 1Á99 6 0Á21 Ga is present below the much younger crust of Zealandia. A peritectic melting model using moderately incompatible trace elements (e.g. Yb) in bulk-rocks demonstrates that these peridotites experienced a significant range of degrees of partial melting, between 3 and 28%. During subsolidus equilibration clinopyroxene gains significant rare earth elements (REE), which then leads to the underestimation of the degree of partial melting by 12% in fertile xenoliths. A new approach taking into account the effects of subsolidus reequilibration on clinopyroxene composition effectively removes discrepancies in the calculated degree of melting and provides consistent estimates of between 4 and 29%. The estimated amount of melting is independent of the Re-Os model ages of the samples. The PGE patterns record simple melt depletion histories and the retention of primary base metal sulfides in the majority of the xenoliths. A rapid decrease in Pt/Ir N observed at c. 1Á0 wt % Al 2 O 3 is a direct result of the exhaustion of sulfide in the mantle residue at c. 20-25% partial melting and the inability of Pt to form a stable alloy phase. Major elements preserve evidence for refertilization by a basaltic component that resulted in the formation of secondary clinopyroxene and low-forsterite olivine. The majority of xenoliths show the effects of cryptic metasomatic overprinting, ranging from minor to strong light REE enrichments in bulk-rocks (La/Yb N ¼ 0Á16-15Á9). Metasomatism is heterogeneous, with samples varying from those with weak REE enrichment and notable positive Sr and U-Th anomalies and negative Nb-Ta anomalies in clinopyroxene to those that have extremely high concentrations of REE, Th-U and Nb. Chemical compositions are consistent with a carbonatitic component contributing to the metasomatism of the lithosphere under Zealandia. Notably, the intense metasomatism of the samples did not affect the PGE budget of the peridotites as this was controlled by residual sulfides.

Rapid Cenozoic ingrowth of isotopic signatures simulating “HIMU” in ancient lithospheric mantle: Distinguishing source from process

“…DPP-3, WFP-2 and WFP-8), consistent with the 244 regional Paleoproterozoic melting age based on Re-Os systematics (McCoy-West et al, 2015; 245 2013). Additionally, the steepest array in Sm-Nd isotopic space provides a minimum estimate 246 of the timing of this ancient melting event at >1525 Ma ( Fig.…”

“…The remaining model ages between 0.5 and 1. (Ionov et al, 2005; Pearson et al, 2004; Stosch et al, 1986 southern Zealandia has been variably modified by interaction with a carbonatitic component 297 (McCoy-West et al, 2015; Scott et al, 2014a; 2014b). A strong correlation is observed 298 between the strength of this metasomatism, as for example reflected in LREE patterns and the 299 level of isotopic heterogeneity (Fig.…”

Os isotopic constraints on crustal contamination in Auckland Volcanic Field basalts, New Zealand

“…Alternative models to explain the radiogenic Os/ 188 Os geochemical composition involve the presence of eclogitic or pyroxenitic or carbonated peridotite domains in the asthenospheric or lithospheric mantle (e.g., Hoernle et al, 2006;Sprung et al, 2007;Timm et al, 2009;McCoy-West et al, 2010;McGee et al, 2013McGee et al, , 2015 Scott et al, 2014;McGee et al, 2015) or pyroxenite (e.g., elevated Al2O3 coupled with radiogenic 187 Os/ 188 Os; Marchesi et al, 2014). This is in agreement with a recent study by McCoy-West et al (2015) concluding that because the PGE budget is primarily controlled by residual sulphides, carbonatitic metasomatism beneath Zealandia (the wider micro-continent on which New Zealand sits) does not affect the PGE budget of the subcontinental peridotites.…”

“…However, Platinum Group Elements (PGE), including Os, in addition to being siderophile are also chalcophile, and thus are relatively concentrated in sulphide minerals or PGE-rich metal alloys (e.g., DOs ~ 10 4 between sulphides and silicate melt; Roy- Barman et al, 1998;Jamais et al, 2008;Park et al, 2013;McCoy-West et al, 2015). Very minimal amounts of sulphide fractionation will therefore preferentially remove Os from a melt, without a strong co-variance with other elements.…”

Sulfide-scale insights into platinum-group element behavior during carbonate mantle metasomatism and evolution of Spitsbergen lithospheric mantle

“…7a) and unradiogenic Os isotopic composition (Fig. 6a, c) of the carbonate pocket-bearing whole-rock sample SVF36 indicates that carbonate melt metasomatism did not obliterate ancient Os isotope or melt-depletion signatures on the whole-rock scale, in keeping with recent finding for off-cratonic xenoliths from Nambia (Aulbach et al, 2014) and Zealandia (McCoy-West et al, 2015). The five Os-rich sulfide grains (SVF41-S4, S8, S16, S17, and SVF48-S1) yield T RD model ages of ca.…”