Abstract
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 formed during the intrusion of hot sills into highly porous young (Pleistocene) cold sediments filled with seawater. Holes DSDP 477 and 477A were drilled in the high heat flow area and Holes 478 and 481/481A in the low heat flow area. Thick sills from Holes 477, 477A, and 478 (47 m, 30 m and 112 m, respectively) are fresh or very weakly altered mainly due to the fact that practically no water released from the host Pleistocene sediments during their heating penetrated into the sills. Olivine, whose content is small in these sills (a few percent) is unaltered. In mesostasis, the amount of which is also a few percent, there are clay minerals. The high heat flow in the area of Holes 477 and 477A (1250 mW/m2) did not change the unfavorable situation for changes in the mineral and chemical composition of the thick sills without water entering them. Thin sills with a thickness of 0.1 to 0.65 m and sills with a relatively small thickness of 1.2 to 4.5 m, which were intersected in Holes 478 and 481/481A are altered. They are fractured and therefore favorable for the penetration of water released from the heated sediments. In addition, in these sills a large volume is occupied by mesostasis (mainly from 18 to 45 %, in some samples from 53 to 70 %) replaced to varying degrees by clay minerals. Olivine is often completely replaced by clay minerals. In thin basalt sills, their alteration strongly decreases K contents. In some cases, the content of Mn, Mg, P, Na, and Ti decreases in them insignificantly. The low heat flow measured in the areas where Holes 478 and 481/481A (150 and 165 mW/m2) were drilled indicates that mineral and chemical changes in the thin sills occurred without heat and mass input from the convective long-lived hydrothermal systems.
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