摘要：

Mid-ocean ridge basalts (MORB) from the Arctic Ocean have been significantly less studied than those from other oceans. The Arctic ridges (Gakkel Ridge and Lena Trough) are ultraslow-spreading ridges with low melt productivity and are thus the best locations to investigate mantle heterogeneity. We report the major and trace elem-ent and Sr^Nd^Pb^Hf isotope compositions of basalts generated along the Lena Trough and the westernmost part of the Gakkel Ridge in the Arctic Ocean. Basalts from the northern Lena Trough and westernmost Gakkel Ridge (NLT^WGR) have compositions close to normal MORB. The geochemical composition of the NLT^ WGR lavas confirms a binary mixing model involving melts from a depleted MORB mantle source and a Spitsbergen amphibole-bearing subcontinental lithospheric mantle (SCLM) source. In contrast, in the central part of the Lena Trough (CLT), the basalts are alkalic with relatively high Mg-number (60^65), high SiO 2 (51·0^51·6 wt %), Al 2 O 3 (18·1^18·4 wt %), Na 2 O (4·0^4·2 wt %), K 2 O (1·0^1·6 wt %), K 2 O/TiO 2 (0·6^0·9) and (La/Sm) PM (1·4^1·8), and low FeO (6·5^6·8 wt %) contents. These basalts display isotope variations with 87 Sr/ 86 Sr ranging from 0·70361 to 0·70390, 143 Nd/ 144 Nd from 0·51283 to 0·51290 (e Nd þ 3·7 to þ5·2), 176 Hf/ 177 Hf from 0·28313 to 0·28322 (e Hf þ 11·6 to þ14·9) and 206 Pb/ 204 Pb from 17·752 to 17·884, 207 Pb/ 204 Pb from 15·410 to 15·423 and 208 Pb/ 204 Pb from 37·544 to 37·670. These isotope compositions clearly distinguish the CLT lavas from those generated along the Gakkel Ridge. For the CLT lavas, involvement of a phlogopite-or amphibole-and (possibly garnet)-bearing SCLM source component is proposed. Owing to SCLM contamination along the entire length of the Lena Trough, we classify the LenaTrough as an ocean^continent transition bound-ary. Magmatism similar to that observed in the Lena Trough would be expected to occur wherever ocean spreading initiates. KEY WORDS: major and trace elements; Sr^Nd^Pb^Hf isotopes; ultraslow-spreading ridge; low-degree melt; Arctic Ocean; MORB; ocean^continent transition boundary I N T RO D UC T I O N The distribution of incompatible-element-enriched compo-nents in the mantle has been under discussion for over 30 years (e.g. Wood et al., 1979). Many researchers have argued for a relatively homogeneous upper mantle that is in chemical equilibrium over a large area (Hofmann & Hart, 1978). However, basalts from ocean islands, and in some cases from mid-ocean ridges, show trends that indi-cate mixing between melts of distinct mantle sources that are sampled over a wide area (e.g. le Roex et al., 1981). The mixing generally occurs between an early melt fraction and melts derived from upper mantle peridotite at a rela-tively high level in the mantle (le Roex et al., 1992). Studies of peridotites from the SW Indian Ridge have documented clinopyroxene compositions indicative of disequilibrium between the peridotites and melts sampled nearby (Salters & Dick, 2002; Snow et al., 1994), which provides some evidence in favor of such a mixing model. The data

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