Geochemical characteristics of off-axis lavas from the Chile Rise

Abstract

The Chile Rise, a divergent boundary between the Nazca and Antarctic plates, is unique because it is being subducted beneath the South American plate. Even though the ridge is located on the subducting slab, previous studies (e.g. Klein and Karsten, Nature,V374, 1995) of axial lavas have proposed that the easternmost part of ridge (Segment 1) might be contaminated by a young subduction component. The most plausible explanation for this posited subduction component is transport of the subduction components through a "slab window" from the mantle wedge. For this study, off-axis lavas from Segment 1 were obtained on board R/V Mirai, JAMSTEC in 2009. In contrast to axial lavas, which are sediment covered due to the high sedimentation rate of the near shore area, many of the off-axis lava flows were directly exposed on the seafloor, suggesting recent off-axis volcanic activity. These off-axis lavas were analyzed for major and trace element abundances to evaluate both the melting processes involved in this off-axis magmatism and the extent of mantle heterogeneity beneath this ridge-subduction environment. The off-axis lavas are primitive (MgO is 7.5~ 8.4) and similar to N-MORB in their major element compositions. Compared to published on-axis data for the Chile Rise basalts (Klein and Karsten, Nature,V374, 1995), they are slightly higher in TiO2 for a given MgO content, but have similar Na2O. H2O/Ce contents of the lavas are close to the Pacific MORB/OIB maximum (~220), but one sample is higher (>300). These off-axis lavas are also slightly enriched in fluid mobile elements such as K2O, Ba and Pb compared to N-MORB, suggesting their lava source was similarly influenced by subduction components. For example, a diagram of K2O/TiO2 vs. K2O/H2O shows a trend that is oblique to the Pacific MORB trend, but may be going towards a subduction component. Trends towards a subduction component are less pronounced in off-axis lavas than those seen among the axial lavas

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