Rhenium Loss During Outgassing of Hawaiian Lavas

Ocean island basalts have on average lower Re contents than MORB, which has been thought to reflect primary compositional variations in the plume source, or the presence of residual sulfide and/or garnet. We have measured the concentrations of Re, Pt, Cu, Bi, Cd, and Sb by laser ablation ICPMS, and S and major elements by electron microprobe in a suite of moderately evolved (6-7% MgO) tholeiitic glasses from Koolau volcano, Hawaii, to test our previous suggestions that Re can be lost from subaerial basaltic lavas during magmatic outgassing, and that the lower Re contents of OIB vs. MORB may be a geological artifact acquired during eruption rather than a primary magmatic feature (Bennett, Norman, and Garcia, 2000, EPSL 183, 513). Our new data on the Koolau glasses show clear correlations between Re, Bi, Cd and S contents that are consistent with loss of these elements as volatile species during outgassing of the magmas. Glasses with primary S contents (800-900 ppm) have 1.3-1.5 ppb Re, with Cu/Re (100-110 ppm/ppb) and Re/Yb (0.65-0.75 ppb/ppm) ratios close to primitive mantle values (107 and 0.63, respectively). In contrast, outgassed melts with low S (100-200 ppm) have about a factor of two lower Re, with Cu/Re (200-250) and Re/Yb (0.2-0.4) ratios significantly fractionated from mantle values. Pt/Re ratios in the outgassed melts are also higher than those with primary S contents (6-11 vs. 3-6, respectively). Copper and Pt contents of these glasses show no correlation with S, ruling out removal of an immiscible magmatic sulfide phase as a cause of the Re variations. However, Cu and Pt are well correlated with each other, with decreasing Cu/Pt ratios at lower Cu contents, consistent with a variably depleted mantle source. Antimony behaved as a non-volatile, incompatible lithophile element in these magmas. This study provides direct evidence for Re-loss associated with outgassing of S from basaltic lavas, and supports interpretations that relate the lower Re contents of OIB relative to MORB to subaerial vs. submarine eruptions. Global budgets for Re and other volatile elements need to be revised to account for these effects.