Metals and isotopes in Juan de Fuca Ridge hydrothermal fluids and their associated solid materials

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Abstract
The 87Sr/86Sr ratio of the hydrothermal solution (HTS) (0.7034) is larger than that of basalt (0.7025) at the southern vent field of the Juan de Fuca Ridge (SJFR). Both the Sr isotopic ratio for HTS and the water/rock interaction ratio lie between those at two sites farther south on the East Pacific Rise, 13°N and 21°N. These parameters may be closely related to subsurface temperatures and rates of magma ascent and to extent of faulting and surface areas of the frameworks of the hydrothermal systems. For these three Pacific Ocean sites there is no steady geographical progression of these measured parameters, nor of reported spreading rate, with increasing latitude northward. Pb and Nd isotopic measurements are uniform for all samples from the SJFR, ranging only from 18.43 to 18.58 for 206Pb/204Pb (fluids and associated solids) and centering near 0.5131 for 143Nd/144Nd (only fluids measured). Values for basalts and sulfides from the site have similar values. Relatively high 206Pb/204Pb values at the SJFR suggest the potential for the existence of an anomalous radiogenic heat source in the underlying mantle material. A Nd/Sm mass ratio of about 5 suggests that the system of the SJFR, like those at 13°N and 21°N, may be light rare earth element enriched. The availability of three types of solid material associated with each fluid sample has provided an opportunity to study chemical distribution between fluids and solids. Sr isotopic compositions provide strong evidence that at least two of the types of solids are precipitates from their individually associated fluids. For most metals measured, the masses in the fluids were greater than the masses in solid materials. However, some metals or the solids that contain them may not have been conserved, fractions possibly being lost during sampling or earlier. A Sr isotopic method of determining extent of seawater dilution of the fluid samples yields different results from the traditionally used method based on Mg concentrations, perhaps due to differing degrees of precipitation of the two metals.