Changes in the Mineral and Chemical Composition of Sills during their Intrusion Emplacement into the Sedimentary Cover, Guaymas Basin, Gulf of California (DSDP Holes 477, 477A, 478, 481/481A)

In the Guaymas Basin of the Gulf of California, changes in the mineral and chemical composition of sills of different thicknesses were studied in hydrothermal systems, which originated during the intrusion of the hot sills into young (Pleistocene) cold seawater-saturated sediments. Holes DSDP 477 and 477A were drilled in a high heat flow area, and holes 478 and 481/481A, in a low heat flow area. Thick sills from holes 477, 477A, and 478 (47, 30, and 112 m, respectively) remained fresh or very weakly altered because water released from the host Pleistocene sediments upon heating practically did not penetrate them. Olivine (a few wt %) in the sills remains unaltered, whereas mesostasis also amounting to a few percent is replaced by clay minerals. The high heat flow alone, without water input, could not provide favorable conditions for the changes in the mineral and chemical composition of thick sills in holes 477 and 477A (1250 mW/m 2 ). The thin (from 0.1 to 0.65 m thick) and relatively thin (from 1.2 to 4.5 m) sills recovered in holes 478 and 481/481A were altered. They are fractured and therefore favorable for the penetration of water released from heated host sediments. In addition, a large volume of these sills is occupied by mesostasis (mainly from 18 to 45%, reaching in some samples from 53 to 70%) variably replaced by clay minerals. Olivine is frequently completely replaced by clay minerals. The alteration of thin basalt sills is accompanied by a sharp decrease in the K content, which in some cases is accompanied by an insignificant decrease in Mn, Mg, P, Na, and Ti. The low heat flow measured in the areas of holes 478 and 481/481A (150 and 165 mW/m 2 ) indicates that the established mineral and chemical changes in the thin sills were caused by auto-hydrothermal processes, without heat and mass impact of convective long-lived hydrothermal systems.