Mantle Heterogeneity in the Southern Mariana Trough Indicated Through B and Sr Isotopic Systematics.

We have studied glasses from the Southernmost Mariana Trough (13.4-14.3° N and 144-144.6° E) recovered during the Cook 7 expedition (2001). Due to their differing proximities to the volcanic front of the Mariana island arc (33 to 106 km), these samples are critical for evaluating across arc elemental and isotope transport and possible linkages between back-arc spreading ridges and the adjacent volcanic arc chains. Petrographic examination shows all of the samples to be unaltered dark colored volcanic glasses with rare crystals of olivine, pyroxene and plagioclase. Their Mg #s range between 33 and 60, with most Mg# >50. Although samples were recovered from water depths of 3.2 to 4.2 km, there is lack of major element and Cl/K concentrations that indicate any seawater alterations (e.g. LOI and Na contents). The lack of seawater signature is also shown by low (0.70275 to 0.70302) 87Sr/86Sr, which is depleted MORB range. The glasses also have low H2O (~1 wt%). This lack of secondary glass alteration allows study of the behavior of volatile and low- temperature alteration sensitive tracers (e.g. B, Cl and B and Sr isotopes). There is no correlation between depth of recovery and B, Cl or d11B systematics, indicating that the degree of vesiculation did not control the d11B. The B content of the glasses is low (generally MORB- like) and the Cl contents show progressive increase with distance from the volcanic front and range between 150 and 1300 ppm. The d11B ratios vary widely and are between +5.3 to -5.6 per mil. As expected, the d11B values become progressively heavier closer to the volcanic front, where they are indistinguishable from the Mariana arc lavas. Interestingly, the heaviest d11B samples are also associated with elevated ratios of Ba/La and U/Nb, and to some extent also with elevated Th/Yb. In arcs, U and Ba are fluid mobile, while Th is a sediment melt sensitive element. From the preliminary dataset it appears that the samples from the S. Mariana Trough are transitional between typical island arc and back-arc magmas. Our new data confirms the proposed highly variable mantle sources along the strike of the Izu-Bonin-Mariana back-arc basalts (Pearce et al., 2005) and stress the fact that distance from the volcanic front may be an important constraint to consider when evaluating high resolution tomographic imaging and modeling mantle flow in the southern Marianas.