Ancient recycled crust beneath the Ontong Java Plateau: Isotopic evidence from the garnet clinopyroxenite xenoliths, Malaita, Solomon Islands
Introduction
Earth’s mantle heterogeneity documented by the geochemistry of oceanic basalts such as ocean island basalts (OIB) and mid-ocean ridge basalts (MORB) has been commonly attributed to recycling of crustal materials (e.g. Hofmann, 1997, Stracke et al., 2003) thought to be present as eclogitic/pyroxenitic bodies within the convecting peridotite-dominated mantle (e.g. Allégre and Turcotte, 1986). During the last decade, the role of eclogitic/pyroxenitic sources in the formation of mantle-derived magmas has received an increasing amount of attention in a number of detailed investigations (Hirschmann and Stolper, 1996, Hauri, 1996, Sobolev et al., 2005, Stracke and Sims, 1999). Their relatively low melting temperature, large garnet stability field, and enriched compositions have been considered attractive to explain variations in the physical and chemical characteristics of mantle-derived magmas as part of composite source models (eclogite/pyroxenite incorporated in peridotite). However, their nature, origin, recycling timescales and even their existence remain controversial because the identification of magma sources is not straightforward. For instance, Hauri (1996) presented correlations between major element and isotopic compositions in Hawaiian lavas as robust evidence for the presence of eclogite/pyroxenite in their source mantle. Subsequently, Stracke and Sims (1999) argued against this hypothesis, and attributed the observed isotope and trace element variations to compositionally distinct peridotite components. This contradiction of source identification is largely because the distinct chemical signatures of partial melts derived from different source materials are effectively diluted through magma mixing.
Alternatively, direct mantle samples such as mafic layers in orogenic peridotite massifs (Pearson and Nowell, 2004, Blichert-Toft et al., 1999a) or eclogite xenoliths in kimberlites (Jacob et al., 2005) have been investigated to reveal the isotopic characteristics of eclogitic/pyroxenitic mantle because they are thought to provide an analogue for the aging of old oceanic lithosphere. Moreover, such studies are able to document the spatial extent of isotopic variability and how this correlates with lithological variations. The isotopic compositions of these eclogite xenoliths and pyroxenite layers show extremely large variations relative to those observed in oceanic basalts, reflecting their derivation from ancient subcontinental lithosphere. This suggests that their involvement in basalt source regions in the convective mantle is largely questionable and underlines the importance of natural samples of pyroxenite/eclogite derived from known oceanic locations to better assess the origin of oceanic basalts and heterogeneities in the mantle.
Here we present radiogenic isotope compositions (Sr–Nd–Hf–Pb) of a suite of garnet pyroxenite xenoliths from Malaita, Solomon Islands, as a convincing example of recycled material from within Pacific convective mantle. This argument is founded on three main lines of evidence summarized as follows. First, Malaita represents the southwestern margin of the Ontong Java Plateau (Fig. 1A), which is the largest oceanic plateau, rapidly created on the western Pacific Plate at ca. 120 Ma (e.g. Tejada et al., 2002). The xenoliths were brought up to the surface by a 34-Ma alnöite magma (a silica undersaturated ultramafic magma with an affinity with kimberlite) intruded within the plateau basement, essentially in an oceanic setting. This geological evidence indicates that the xenoliths can be regarded as fragments of oceanic lithosphere influenced by the plateau emplacement but never affected by any known subducting slab. Second, thermobarometric analyses of extensive suites of xenoliths (Ishikawa et al., 2004) reveal that virtually the entire lithospheric mantle (Moho to ∼ 120 km in depth) is represented in the xenolith population (Fig. 1B). The suite of garnet pyroxenite xenoliths was sampled from the bottom of the lithosphere (110–120 km in depth) that had accreted to the pre-existing oceanic lithosphere which has a 160-Ma age (Ishikawa et al., 2005). This suggests that isolation of the garnet pyroxenites from the convective mantle occurred between 160 and 34 Ma, probably associated with the 120-Ma plateau emplacement. Third, the extent of petrochemical variation of the garnet pyroxenites is illustrated by the presence of quartz-garnet clinopyroxenite, which has not been recognized in other xenolith suites derived from suboceanic mantle. Typical mantle peridotite cannot melt to produce quartz-normative compositions under high-pressure conditions (> 3.0 GPa) and thus excludes derivation of the rock as a high-pressure cumulate or melt from normal peridotitic mantle. This is a clear indication that the rock originated from normative quartz-rich basaltic material, most likely ancient crust stored in the Pacific deep mantle. From these lines of evidence, it is expected that radiogenic isotope data of the garnet pyroxenites can give a unique opportunity to unravel not only the nature and timescale of the recycling process within the Pacific convective mantle, but also the unusual generation of the Ontong Java Plateau.
Section snippets
Methods
The samples presented here include one quartz-garnet clinopyroxenite (QGC) and four bimineralic garnet clinopyroxenites (BGC), whose petrographic and major element characteristics, investigated by electron probe microanalysis (EPMA), have already been described (Ishikawa et al., 2004). All preparation and analyses were performed at the Pheasant Memorial Laboratory, Okayama University at Misasa (Nakamura et al., 2003). To avoid the influence of secondary processes that alter the whole-rock
Mineral equilibria
An apparent lack of compositional gradient in either major or trace element concentrations of minerals within individual xenoliths, revealed by the results of multiple spot analyses with EPMA and SIMS, is thought to reflect their high-temperature equilibration (1300–1350 °C) (Ishikawa et al., 2004). This is further demonstrated by the uniform trace element partitioning behaviors between clinopyroxene and garnet (Table 1, Fig. 2). Our data for clinopyroxene/garnet partition coefficients are
Isotopic variability in garnet clinopyroxenites
In contrast to homogeneous features at the scale of individual xenolith specimens, the reconstructed whole-rock data display significant variations as shown in Sr–Nd–Hf–Pb isotope compositions (Fig. 4, Fig. 5, Fig. 6). It is clear that the QGC has distinctive compositions relative to other BGC. While Sr, Nd, Hf and Pb isotopic variations of BGC are within the OIB-MORB array, the QGC is characterized by remarkably lower 206Pb/204Pb–143Nd/144Nd and higher 87Sr/86Sr–207Pb/204Pb ratios, showing
Origin of quartz-garnet clinopyroxenite
Isotopic characteristics of QGC reflect a long time-integrated evolution with high Rb/Sr–Lu/Hf and low Sm/Nd–U/Pb ratios without recent interaction with ambient peridotite. This is indicated by the preservation of the normative quartz-rich composition of this pyroxenite. When the Hf–Nd data are viewed at 120 Ma, the data somewhat approaches the terrestrial array because of its subchondritic Sm/Nd and superchondritic Lu/Hf ratios (Fig. 5B). This illustrates that present-day parent–daughter
Recycling history of the garnet pyroxenite
The recycling timescale and material documented by the chemical and isotopic compositions of the QGC are consistent with the presence of Neoproterozoic lower crust-like material within the Cretaceous Pacific mantle. This raises the question of how such lower crust material may have been recycled in the context of the geodynamic history of the Pacific region. Based on available geologic records and paleomagnetic data, it has been suggested that a supercontinent Rodinia formed at ca. 1.1 Ga and
Implications for the Ontong Java Plateau magmatism
Despite the uncertainty in recycling history of the pyroxenites, our data provide the first direct evidence for the existence of ancient crustal material beneath the Ontong Java Plateau. An important question is the role of such ancient material in the generation of the Ontong Java Plateau. Traditionally, Cretaceous oceanic plateaus (Fig. 8B) that are widespread in the present-day western Pacific have been attributed to large-scale mantle plume activity during this period (Larson, 1991a,
Acknowledgements
We are grateful to S. Maruyama, T. Komiya and the Solomon Islands Geological Survey for field assistance and all members of the Pheasant Memorial Laboratory for their technical support. D.G. Pearson, B.N. Nath, A. Utsunomiya, T. Kogiso and J.J. Mahoney are thanked for their constructive comments on an early version of the manuscript. Reviews by two anonymous referees and editorial handling by R.W. Carlson are greatly appreciated. We also thank C.W. Dale for a final check of the English. This
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