Abstract
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 temper...
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