Hydrous metasomatism of oceanic sub-arc mantle, Lihir, Papua New Guinea: Part 2. Trace element characteristics of slab-derived fluids
Introduction
Mantle xenoliths brought to the surface in basalts provide a direct insight into the composition and evolution of the deep part of the lithosphere in both continental and oceanic settings (e.g., Frey and Prinz, 1978, Griffin et al., 1984, Sen, 1987, Grégoire et al., 1997), but are relatively rare in volcanic arcs (e.g., Takahashi, 1986, Maury et al., 1992, Kepezhinskas et al., 1995). Chemical characteristics such as high field strength element (HFSE) depletion (for Nb and Ta and sometimes for Ti, Zr and Hf) relative to other incompatible elements are well-known but poorly understood features of arc-related volcanism. Geochemical models proposed to explain HFSE depletions involve fluids generated by dehydration of subducted slabs and precipitation of amphibole, mica or rutile in the overlying mantle wedge (e.g., Hofmann, 1988, Ionov et al., 1995, Bodinier et al., 1996). Other workers (e.g., Kelemen et al., 1990) have argued that HFSE depletions are produced because olivine, orthopyroxene and spinel in mantle harzburgites have higher crystal/liquid distribution coefficients for these elements than for other incompatible trace elements. HFSE have also been suggested to be retained in the upper mantle during partial melting by Ti-bearing minerals such as rutile or ilmenite Saunders et al., 1980, Ryerson and Watson, 1987, Ayers and Watson, 1993. Most models point out that HFSE depletions are probably related to their low solubilities in metasomatic fluids that transfer incompatible trace elements from subducted slab to the overlying mantle wedge, where they are ultimately incorporated by partial melts that give rise to arc volcanic rocks (e.g., Tatsumi et al., 1986, McCulloch and Gamble, 1991, Saunders et al., 1991, Maury et al., 1992, Hawkesworth et al., 1993, You et al., 1996). Because mantle wedge xenoliths giving direct insight into the trace element features of subduction zone processes are rare (Maury et al., 1992), many of these studies have drawn heavily on indirect evidence derived from geochemical studies of arc lavas.
Here we report trace element data for mantle wedge xenoliths from the Tabar–Lihir–Tanga–Feni island arc in Papua New Guinea. These samples provide a direct record of the role of slab-derived fluids in the modification of the chemical characteristics of the oceanic lithospheric mantle in an intraoceanic arc setting. We demonstrate that metasomatic minerals (orthopyroxene, clinopyroxene, amphibole and phlogopite) precipitated from a hydrous melt have low HFSE contents. We suggest that trace element characteristics of arc magmas originate by partial melting of mantle wedge regions metasomatised by HFSE-poor, slab-derived hydrous melts.
Section snippets
Geological setting, sampling and analytical methods
The Tubaf and Edison submarine cinder cones (McInnes et al., 2001) located SE of Lihir island are some of the most recent volcanic products of the Tabar–Lihir–Tanga–Feni (TLTF) high-K calc–alkaline island arc (Fig. 1). Arc volcanism in the TLTF occurred in a post-collisional setting following the collision of the Ontong–Java plateau with the Australia–Pacific convergent plate boundary (McInnes et al., 1999). Peridotite xenoliths described in McInnes et al. (2001) investigated in this study
Petrography and modal compositions
Fifteen mantle xenoliths (13 clinopyroxene-bearing spinel harzburgites, one clinopyroxene-bearing spinel dunite and one orthopyroxenite) were selected for study from the suite of xenoliths described in McInnes et al. (2001). The peridotites mainly have porphyroclastic microstructures in which a fine-grained mosaic of olivine and orthopyroxene neoblasts (<1 mm) surround larger porphyroclasts (2–4 mm). Poikilitic microstructures locally grade into granoblastic textures depending on the
Trace element compositions
The major element compositions of whole-rocks and minerals have been described in detail in McInnes et al. (2001). Trace element compositions for whole-rocks and minerals are provided in Table 1, Table 2, Table 3. The geochemical data in Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6 are presented with elements in sequence of their decreasing incompatibility during mantle partial melting processes (Sun and McDonough, 1989), and normalised to primitive mantle abundances (McDonough and Sun, 1995). In the
Partial melting
The Lihir harzburgites show many characteristics of solid residues resulting from high degrees of mantle partial melting, including similarities of modal compositions with the Atlantic abyssal peridotites and Kerguelen harzburgite mantle xenoliths Michael and Bonatti, 1985, Bonatti and Michael, 1989, Grégoire et al., 1997, Grégoire et al., 2000; and bulk rock compositions depleted in basaltic major elements (Al2O3, CaO, Na2O), Sc and V, and high in Ni and Co. These compositional depletions and
Conclusions
This study demonstrates that the mantle wedge beneath the New Ireland fore-arc region is mainly refractory in composition (mostly harzburgitic), and was depleted in basaltic elements by high degrees of partial melting, probably during mid-ocean ridge spreading (McInnes et al., 2001). This refractory mantle was later modified by subduction-related mantle metasomatism processes after plate movement away from the ridge to its current island arc-setting. The occurrence of the orthopyroxene-rich
Acknowledgments
This work has been made possible by the generous assistance and technical expertise of N.J. Pearson, A. Sharma and C. Lawson (GEMOC Geochemical Analysis Unit). This work has been supported by Macquarie University Research Fellowship and Grant Schemes (M.G.), Australian Research Council Large and Small Grants (S.Y. O'R.) and the ARC International Fellowship (M.G.) Scheme. Samples used in this study were collected during Cruise SO-94 of RV Sonne (EDISON project) which was organized by Freiberg
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