Lithium isotopic systematics of peridotite xenoliths from Hannuoba, North China Craton: Implications for melt–rock interaction in the considerably thinned lithospheric mantle

Abstract

Li concentrations and isotopic compositions of coexisting minerals (ol, opx, and cpx) from peridotite xenoliths entrained in the Hannuoba Tertiary basalts, North China Craton, provide insight into Li isotopic fractionation between mantle minerals during melt–rock interaction in the considerably thinned lithospheric mantle. Bulk analyses of mineral separates show significant enrichment of Li in cpx (2.4–3.6 ppm) relative to olivine (1.2–1.8 ppm), indicating that these peridotites have been affected by mantle metasomatism with mafic silicate melts. Bulk olivine separates (δ⁷Li ∼ +3.3‰ to +6.4‰) are isotopically heavier than coexisting pyroxenes (δ⁷Li ∼ −3.3‰ to −8.2‰ in cpx, and −4.0‰ to −6.7‰ in opx). Such large variation suggests Li elemental and isotopic disequilibrium. This conclusion is supported by results from in situ SIMS analyses of mineral grains where significant Li elemental and isotopic zonations exist. The olivine and opx have lower Li concentrations and heavier Li isotopes in the rims than in the cores. This reverse correlation of δ⁷Li with Li concentrations indicates diffusive fractionation of Li isotopes. However, the zoning patterns in coexisting cpx show isotopically heavier rims with higher Li abundances. This positive correlation between δ⁷Li and Li concentrations suggests a melt mixing trend. We attribute Li concentration and isotope zonation in minerals to the effects of two-stage diffusive fractionation coupled with melt–rock interaction. The earliest melts may have been derived from the subducted oceanic slab with low δ⁷Li values produced by isotopic fractionation during the dehydration of the seawater-altered slab. Melts at later stages were derived from the asthenosphere and interacted with the peridotites, producing the Li elemental and isotopic zoning in mineral grains. These data thus provide evidence for multiple-stage peridotite–melt interaction in the lithospheric mantle beneath the northern North China Craton.

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.