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Description:
Secondary minerals found in fracture fillings and in fragments of altered basalt from Holes 417A and 417D were studied by both X-ray diffraction and chemical techniques. Minerals found in fracture fillings from Hole 417A are dominated by montmorillonite, "protoceládonite," analcite, and lesser saponite; celadonite and ferrosaponite are the characteristic secondary minerals in Hole 417D fracture fillings. Assuming that minerals found in such fracture fillings reflect the composition of the secondary fluids that produced them, it is apparent that those from Hole 417A were dominantly Al-rich, while those from Hole 417D were more enriched in Fe, Mg, and K. X-ray diffraction study of bulk samples support such fundamental differences in secondary mineralogy. In addition, the X-ray data on bulk samples suggest that primary plagioclase is the feldspar in Hole 417D rocks, and secondary potassium feldspar is the feldspar in Hole 417A altered rocks. Using available published data on secondary miner?1" found in other altered oceanic crust, it is possible to interpret the differences in secondary mineralogy that exist between the two sites. Secondary minerals present in Hole 417D rocks are believed to have formed under hydrothermally influenced, low temperature, nonoxidative diagenesis; whereas, those present in Hole 417A were produced under similarly low temperatures, but much more highly oxidizing conditions. The fundamental differences in secondary mineralogy between the two sites can be best explained by the accompanying remobilization of elements that involved plagioclase alteration in Hole 417A rocks. A comparison of the composition of Hole 417A and 417D secondary minerals with those found in younger crust suggests that the age of crust, influenced by the changing conditions of alteration, control the chemistry of secondary minerals found in available pore spaces in altered rocks. Minerals found in young crust (<15 m.y.B.P.) are highly Mg-rich; minerals found in crust of intermediate age (—15-50 m.y.B.P.) are ...