Search for the proverbial mantle osmium sources to the oceans: Hydrothermal alteration of mid-ocean ridge basalt

摘要

We present Os and Sr isotopes and Mg, Os, and Sr concentrations for ridge-crest high-temperature and diffuse hydrothermal fluids, plume fluids and ridge-flank warm spring fluids from the Juan de Fuca Ridge. The data are used to evaluate the extent to which (1) the high- and low-temperature hydrothermal alteration of mid-ocean ridge basalts (MORBs) provides Os to the deep oceans, and (2) hydrothermal contributions of non-radiogenic Os and Sr to the oceans are coupled. The Os and Sr isotopic ratios of the high-temperature fluids (265-353 degrees C) are dominated by basalts (Os-187/Os-188 = 0.2; Sr-87/(86) Sr = 0.704) but the concentrations of these elements are buffered approximately at their seawater values. The Os-187/Os-188 of the hydrothermal plume fluids collected similar to 1 m above the orifice of Hulk vent is close to the seawater value (= 1.05). The low-temperature diffuse fluids (10-40 degrees C associated with ridge-crest high-temperature hydrothermal systems on average have [Os] = 31 fmol kg(-1), (OS)-O-187/(OS)-O-188 = 0.9 and [Sr] = 86 mu mol kg(-1), Sr-87/(86) Sr = 0.709. They appear to result from mixing of a high-temperature fluid I and a seawater component. The ridge-flank warm spring fluids (10-62 degrees C) on average yield [Os] = 22 fmol kg(-1 187)Os/Os-188 = 0. 8 and [Sr] = 115 mu mol kg(-1), Sr-87/(86) Sr = 0.708. The data are consistent with isotopic exchange of Os and Sr between basalt and circulating seawater during low-temperature hydrothermal alteration. The average Sr concentration in these fluids appears to be similar to seawater and consistent with previous studies. In comparison, the average Os concentration is less than seawater by more than a factor of two. If these data are representative they indicate that low-temperature alteration of MORB does not provide adequate non-radiogenic Os and that another source of mantle Os to the oceans must be investigated. At present, the magnitude of non-radiogenic Sr contribution via low-temperature seawater alteration is not well constrained. If non-radiogenic Sr to the oceans is predominantly from the alteration of MORB, our data suggest that there must be a different source of non-radiogenic Os and that the Os and Sr isotope systems in the oceans are decoupled. (C) 2007 Elsevier Ltd. All rights reserved.