Highly Refractory Peridotites on Macquarie Island and the Case for Anciently Depleted Domains in the Earth’s Mantle

Abstract

Macquarie Island (Southern Ocean) is a fragment of Miocene ocean crust and upper mantle formed at a slow-spreading ridge system, uplifted and currently exposed above sea-level. The crustal rocks on the island have unusually enriched compositions and the strong signature of an enriched source requires low overall degrees of melt depletion in the underlying mantle. Peridotites on the island, however, are highly refractory harzburgites that can be modeled as residues of420^25% of near-fractional melting from which all the free clinopyroxene was melted out. The peridotites have some of the highest spinel Cr-numbers (0•40^0•49) and lowest orthopyroxenecore Al 2 O 3 concentrations (2•7^3•0 wt %) reported so far for oceanic peridotites. The peridotites were subsequently modified by melt^rock reactions underneath the Miocene ridge system. The refractory character of the peridotites is inconsistent with the slow-spreading ridge setting as well as with the enriched character of the overlying crust, and must indicate a previous depletion event; the peridotites are not the source residue of the overlying ocean crust on Macquarie Island. Osmium isotopic compositions of peridotite samples are very unradiogenic (187 Os/ 188 Os ¼ 0•1194^0•1229) compared with normal abyssal peridotites and indicate a long-lived rhenium depletion. Proterozoic rhenium-depletion ages indicate that these rocks have preserved a memory of an old mantle melting event. We argue that the Macquarie Island harzburgites are samples from an anciently depleted refractory mantle reservoir that may be globally important, but that is generally overlooked because of its sterility; that is, its inability to produce basalts. This reservoir may preserve key information about the history of the Earth' s mantle as a whole.

Orogenic, Ophiolitic, and Abyssal Peridotites

“…This scenario is consistent with Os data indicating ages > 1 billion years for abyssal peridotites (Alard et al, 2005;Brandon et al, 2000;Harvey et al, 2006;Liu et al, 2008) and Hf isotopes pointing to extreme and ancient mantle depletion in the oceanic mantle (Stracke et al, 2011). The role and importance in volume of anciently depleted refractory mantle might have been overlooked until now because of its 'sterility' (Dijkstra et al, 2010).…”

“…Harzburgites in Macquarie Island, southwest Pacific, occur at the base of a complete section of exhumed oceanic crust. Hence, the Macquarie section is commonly considered as an ophiolite, although it was not obducted (Dijkstra and Cawood, 2004;Dijkstra et al, 2010;Goscombe and Everard, 2001;Varne, 1978, 1980;Varne et al, 1969).…”

“…These peridotites plot at the depleted end-member of the abyssal peridotites in terms of forsterite content in olivine (cationic ratio Mg/(Mg þ Fe) ¼ 0.91-0.92), chromium component in spinel (cationic ratio Cr/(Cr þ Al) ¼ 0.43-0.51), and trace element content in whole rocks and cpx (Godard et al, 2008;Seyler et al, 2007). Similarly, the harzburgites sampled in Macquarie Island, which are considered as being formed at a slow-spreading ridge, have highly refractory compositions inconsistent with the enriched character of the overlying crust (Dijkstra et al, 2010). A possible explanation for the occurrence of refractory perdidotites at slow-spreading ridges is efficient melt extraction associated with secondary convection cells, which would represent a more actively convecting mantle than generally supposed below slow-spreading centers (Godard et al, 2008).…”

Multiscale chemical heterogeneities beneath the eastern Southwest Indian Ridge (52°E-68°E): Trace element compositions of along-axis dredged peridotites

“…Although the clearest and freshest domains of pyroxene grains were selected, submicron sized cracks and alteration veinlets following the crystallographic directions could not be avoided. In agreement with Dijkstra et al [2010], anomalously high counts of Pb, U, Ba, and to a lesser extent Sr, are attributed to microheterogeneities. Opx is particularly sensitive to alteration effects due to its extremely low trace element content and to the fact that it is more alterable than Cpx.…”

Mineralogical and geochemical constraints on the shallow origin, ancient veining, and multi-stage modification of the Lherz peridotite

“…Den Tex, 1969;Wyllie, 1969), including the Lherz peridotite, has been the subject of much debate and it has been suggested that these layered assemblages of peridotite and pyroxenite may represent; (1) exhumed sub-continental lithospheric mantle (e.g., Menzies and Dupuy, 1991;Reisberg and Lorand, 1995;Burnham et al, 1998;Downes, 2001) from the mechanical boundary layer (MBL; White, 1988), or (2) inherited heterogeneities from asthenospheric upper-mantle (c.f. Peate et al, 1997;Parkinson and Pearce, 1998;Brandon et al, 2000;Bizimis et al, 2005;Harvey et al, 2006;Mü ntener and Manatschal, 2006;Bizimis et al, 2007;Liu et al, 2008;Simon et al, 2008;Liu et al, 2009;Warren et al, 2009;Dijkstra et al, 2010;Ishikawa et al, 2011 for examples of isotopic and mineralogical heterogeneity present in Earth's upper-mantle) that may even be linked to upwelling diapirs (e.g., Bodinier et al, 1988;Fabriès et al, 1991). Several authors have used the chemistry of mantle materials to provide estimates of the composition of primitive upper mantle (PUM) and bulk-silicate Earth (BSE; e.g., Palme and Nickel, 1985;Zindler and Hart, 1986;McDonough and Sun, 1995;Becker et al, 2006), and some authors have integrated information from ultramafic bodies into models of the spatial distribution of chemical diversity in Earth's upper mantle (e.g., Allègre and Turcotte, 1986;Morgan and Morgan, 1999).…”