Lithium isotopic systematics of peridotite xenoliths from Hannuoba, North China Craton: Implications for melt–rock interaction in the considerably thinned lithospheric mantle
Introduction
Li is a light alkali metal element. The large mass difference (∼16%) between its two stable isotopes (6Li ∼ 7.5% and 7Li ∼ 92.5%) leads to strong isotopic fractionation during many geological processes, producing overall isotopic variation of >50‰ (Tomascak, 2004). As a mobile element, Li tends to partition preferentially into the fluid/melt phase during fluid/melt–rock interaction (Brenan et al., 1998b), leading to Li enrichment in differentiated crust relative to the primitive mantle. It is believed that Li+ and Na+ can substitute for each other in the M2 site in cpx (Ottolini et al., 2004). Furthermore, Li+ has a similar ionic radius to that of Mg2+ or Fe2+, allowing the substitution of Li for these elements in olivine and pyroxene, potentially charge-balanced by trivalent cations such as Al3+, Fe3+, Cr3+, Sc3+, V3+, and REE3+ (Seitz and Woodland, 2000). These characteristics make Li a potential tracer for a number of important mantle processes.
With rapid improvement in analytical techniques and growing interest in light element geochemistry, Li and its isotopes have received great attention in recent years as a potential geochemical tracer for geological processes (Tang et al., 2007) such as the recycling of subducted crustal material (Moriguti and Nakamura, 1998a, Tomascak et al., 2000, Chan et al., 2002a, Elliott et al., 2004, Wunder et al., 2006), mantle partial melting and crystal fractionation (Chan et al., 1992, Chan et al., 1993, Chan et al., 1999, Seitz and Woodland, 2000, Tomascak, 2004, Lundstrom et al., 2005, Teng et al., 2006b), mantle metasomatism or peridotite–melt interaction (Chan et al., 1992, Chan et al., 1993, Chan et al., 1999, Seitz and Woodland, 2000, Woodland et al., 2004, Jeffcoate et al., 2007, Rudnick and Ionov, 2007), and low-temperature alteration (Chan et al., 1992, Chan et al., 2002a, Decitre et al., 2002, Teng et al., 2004, Woodland et al., 2004).
The Li isotopic compositions of fluids, melts, and rocks from subduction-zone settings have been used to probe crust–mantle mass transfer processes. Because seawater has high δ7Li values (∼+31‰), low-temperature seafloor alteration produces seafloor basalts rich in heavy Li relative to fresh MORBs (Chan et al., 1992, Chan et al., 1996, Chan et al., 2002a). Fluids released from subducted-slab metamorphism are variable in composition, depending on the prior history of dehydration, but isotopically heavier than the rocks from which they were released (Tomascak et al., 2002, Elliott et al., 2004). Thus, progressive dehydration of pelagic sediments and altered oceanic crust should produce rocks depleted in the heavy Li isotope relative to seafloor basalt. It has been proposed that subducted eclogite with extremely low δ7Li values (−11‰) can result from the dehydration of subducted altered oceanic crust (Zack et al., 2003). Recycling of slab residues into the deeper mantle could thus deliver a light Li isotopic component (δ7Li < 0‰) to the deep mantle. However, this low-δ7Li signature has not yet been observed in either arc or ocean-island volcanic rocks (+1.4–+11‰; Chan et al., 1992, Chan et al., 2002b, Moriguti and Nakamura, 1998a, Tomascak et al., 1999, Tomascak et al., 2000, Tomascak et al., 2002, James and Palmer, 2000, Chan and Frey, 2003, Nishio et al., 2004) except for glass inclusions in olivine from Hawaiian basalts (Kobayashi et al., 2004).
Li isotope geochemistry is a relatively young area of research, and most of the work has been conducted during the last decade. Although previous studies have shed new light on the fluid/melt–mineral interaction at the Earth’s surface and into the mantle, many questions remain regarding the Li isotopic compositions of common materials, and the nature and mechanism of Li isotopic fractionation. Data bearing on the Li isotopic systematics in mantle peridotite xenoliths are even more scarce. Therefore, data from these ultramafic rocks and minerals are crucial to our understanding of Li behavior in the mantle.
This paper reports major and trace element and Li isotopic compositions of the well-studied spinel peridotite xenoliths from Hannuoba Tertiary basalts, North China Craton, by means of both multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) for bulk mineral separates and in situ secondary ionization mass spectrometry (SIMS) analyses, with aims to clarify the distribution of Li in mantle minerals, the inter- and intra-mineral fractionation of Li isotopes during melt–rock interaction, and evaluate the possible geochemical implications of this information.
Section snippets
Geologic setting
The North China Craton is one of the world’s oldest Archean cratons, preserving crustal remnants as old as 3.8 Ga (Liu et al., 1992). The Late Paleozoic-Early Mesozoic Central Asian orogenic belt bounds the craton to the north, and in the south the Qinling-Dabie and Su-Lu high- to ultrahigh-pressure metamorphic belts amalgamated the craton with the Yangtze Craton (Fig. 1). The North China Craton consists largely of Archean to Paleoproterozoic basement (Zhao et al., 2001). Based on differences in
Sample descriptions
The Hannuoba basalts (10–22 Ma) occur as a highland of over 1700 km2 in the northern margin of the North China Craton (Fig. 1). Two series, tholeiitic and alkaline basalts, are intercalated with alkalic basalt dominantly occurring at the base of each sequence. Alkalic basalts host abundant mantle and lower crust xenoliths whereas xenoliths have been found only rarely in the tholeiites (Chen et al., 2001). These xenoliths are mafic to felsic granulites (Chen et al., 2001), spinel and garnet
Analytical methods
All of the clean room procedures, including the wet chemical treatments and trace element and Li isotope analyses, were undertaken at the Pheasant Memorial Laboratory (PML) for Geochemistry and Cosmochemistry, Institute for Study of the Earth’s Interior, Okayama University at Misasa, Japan (Nakamura et al., 2003).
Fresh xenoliths were first cut, crushed and then sieved with stainless steel sieves (180–250 μm). Olivine, opx and cpx were separated by handpicking under a binocular microscope,
Elemental geochemistry
Comparisons of core and rim analyses by EPMA and SEM-EDX demonstrated that the minerals in these peridotites are homogeneous in major oxides. As summarized by Rudnick et al. (2004), the minerals in the Hannuoba xenoliths are similar in composition to those in off-craton spinel peridotite xenoliths worldwide but differ significantly from those in cratonic peridotites (Fig. 2). The differences are primarily in bulk rock and mineral compositions, rather than pressure (garnet vs. spinel) or
Bulk Li enrichment and peridotite–melt interaction
Depletions in LREE (Fig. 3a) and highly incompatible elements, in particular the mobile elements Rb, U, and Ba (Fig. 3b), in most of the peridotites suggest that these rocks have experienced partial melting. The negative Li anomaly in cpx (Fig. 3b) could reflect the preference of olivine to incorporate Li relative to REE.
Seitz and Woodland (2000) reported that the compositional ranges of Li in olivine and pyroxenes in fertile to moderately depleted mantle are 1–1.8 ppm and 0.5–1.3 ppm,
Conclusion
Large Li elemental and isotopic disequilibria within and between mantle minerals from the Hannuoba spinel lherzolites have been observed in this study. These observations suggest that the Hannuoba xenoliths have experienced two-stage metasomatism involving mafic silicate melts. The low δ7Li values and high Li concentrations of bulk and individual minerals suggest that the early-stage metasomatic agents had low δ7Li and high Li concentrations, due to the derivation from subducted, altered
Acknowledgments
We are very grateful to A. Makishima for his help at setting up Li isotope analysis by MC-ICP-MS, to T. Moriyama and C. Sakaguchi for their technical support in PML and to A. Ishikawa for his supply of SIMS standard materials from Solomon island. We thank Bjorn Mysen for the editorial encouragement and patience. The manuscript was considerably improved by the thoughtful comments and suggestions of Bjorn Mysen, James Brenan, Paul Tomascak and an anonymous reviewer. Gray Bebout and Bence Paul are
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