Oxygen isotope geochemistry of back-arc lavas

Oxygen isotope ratios of arc-related magmas can constrain the amount and nature of slab-derived components contributing to their mantle sources because: (i) most geological solids and fluids contain similar concentration of oxygen; and (ii) possible slab components (fluids, melts or rocks from sedimentary, mafic or ultramafic parts of the lithosphere) differ from one another in δ18O values and these distinctive oxygen-isotope compositions can be preserved in deeply subducted materials. Previous laser fluorination oxygen isotope data on back-arc basin basalts (BABB) show slightly but consistently elevated δ18O values (from 5.5 to 6.1 ‰ [1-3]) with respect to mid-ocean ridge basalts or MORB (δ18O = 5.5 ± 0.2 ‰ [4]). The subtle elevations in δ18O characteristic of BABB are generally associated with 'enriched' radiogenic isotope compositions (e.g., high 87Sr/86Sr) and high apparent degrees of melting of their peridotitic sources that may reflect fluxed melting of the mantle by fluids and/or hydrous melts released from subducted oceanic lithosphere [1-3]. These trends could be used to precisely define the sources and amounts of slab- derived components delivered to the back arc mantle. However, two issues demand that this problem be better constrained through further studies. First, the subtle difference in δ18O between MORB and BABB involves comparison of data generated in different laboratories that may not share common standardization; this difference must be demonstrated within a single laboratory using a common set of standards. Second, the trends of δ18O vs. other geochemical indices observed in BABB lavas are based on samples from several relatively small and unrelated suites of lavas. These trends must be documented through study of one or more relatively large and diverse suites of closely related lavas. We are performing new laser-fluorination oxygen isotope measurements of a comprehensive suite of fresh back-arc glasses collected along the Eastern Lau Spreading Center (ELSC). This suite displays a continuous trend of increasing subduction-component influence with decreasing distance from the Tonga arc (from north to south). So far, new δ18O values of ELSC lavas are within the range previously described for back-arc samples and are consistent with the global trend of 18O enrichment with increasing indices of slab-derived components. In particular, when compared to the MORB dataset obtained in the same laboratory, ELSC lavas appear to define a continuous "mixing" trend between low-δ18O depleted peridotite and high- δ18O subducted component. We anticipate that further expansion of the ELSC data set will permit detailed tests of hypotheses for the origins of geochemical variability of the Lau basin lavas and help constraining the properties and amounts of slab-derived fluids and/or melts sampled by these lavas and the parts of the slab from which they were derived. [1] Macpherson and Mattey (1998), Chem. Geol., 144, 177-194. [2] Macpherson et al. (2000), EPSL, 176, 171-183. [3] Eiler et al., J. Petr. (2000), 41, 229-256. [4] Eiler (2001), Rev. Min. Geochem., 43, 319-362.