ALBERT

Description: Recent siliceous sediments of the Guaymas Basin, a rapid spreading center in the Gulf of California, are intruded by basaltic sills, which induced significant diagenetic and metamorphic changes in the sediments. The transformation from opal-A to opal-CT, opal-CT to quartz, and opal-A directly to quartz in these sediments, cored on DSDP Leg 64, can be used to infer the temperature history and order of emplacement of intrusives. At or near contacts with sills of 30 m or greater average thickness, opal-A inverts directly to authigenic quartz, but there is less quartz than would be expected from the amount of opal-A dissolution. Opal-CT forms in sediments sandwiched between adjacent sills. Based on high rates of quartz nucleation and growth at high temperatures (〉 150°C), and on considerations of convective solution transfer, opal-CT is thought to form only where temperatures were lower or at positions between sills, an environment which prevents rapid convective dissipation of silica in solution. Where temperatures were higher or convection uninhibited, solutions remained at silica concentrations too low to permit opal-CT crystallization, and opalA inverted directly to quartz by dissolution and reprecipitation. These arguments allow inference of the order of sill emplacement and estimates of thermal history in a high heat flow regime perturbed by local sill intrusions. These are the youngest oceanic siliceous sediments known to have been converted to opal-CT and quartz; in the Guaymas Basin this evolution took only thousands of years, whereas in deep sea oceanic environments far from igneous activity it would have taken millions to tens of millions of years.