Chemical and mineral compositions of bottom sediments from the East Pacific Rise near 13°N

During underwater photography and sampling of the rift valley bottom in the axial part of the East Pacific Rise, where water transparency is reduced due to hydrothermal input, ore manifestations have been found. The bottom is covered by them as by a jacket on both sides from the EPR axial zone. However, exposed pillow-lavas and clumpy blocks in rift ledges are covered by a thin metal-bearing film. It is supposed that sedimentation results mainly from hydrothermal input of dissolved chemical elements in seawater, their transformation on the geochemical barrier, and subsequent deposition as particulates. Contents of ore components in metalliferous sediments have been measured by atomic-absorption and X-ray radiometry methods. Sediment age has been determined as Middle Pleistocene - Holocene. Maximal hydrothermal activity was at the beginning of Early Holocene, about 10 Ka. A smoker has been found on the western slope of the rift valley.

Composition of minerals from peridotides at the Mid-Atlantic Ridge, 13°N

Peridotites (diopside-bearing harzburgites) found at 13°N of the Mid-Atlantic Ridge fall into two compositional groups. Peridotites P1 are plagioclase-free rocks with minerals of uniform composition and Ca-pyroxene strongly depleted in highly incompatible elements. Peridotites P2 bear evidence of interaction with basic melt: mafic veinlets; wide variations in mineral composition; enrichment of minerals in highly incompatible elements (Na, Zr, and LREE); enrichment of minerals in moderately incompatible elements (Ti, Y, and HREE) from P1 level to abundances 4-10 times higher toward the contacts with mafic aggregates; and exotic mineral assemblages Cr-spinel + rutile and Cr-spinel + ilmenite in peridotite and pentlandite + rutile in mafic veinlets. Anomalous incompatible-element enrichment of minerals from peridotites P2 occurred at the spinel-plagioclase facies boundary, which corresponds to pressure of about 0.8-0.9 GPa. Temperature and oxygen fugacity were estimated from spinel-orthopyroxene-olivine equilibria. Peridotites P1 with uniform mineral composition record temperature of the last complete recrystallization at 940-1050°C and FMQ buffer oxygen fugacity within the calculation error. In peridotites P2, local assemblages have different compositions of coexisting minerals, which reflects repeated partial recrystallization during heating to magmatic temperatures (above 1200°C) and subsequent reequilibration at temperatures decreasing to 910°C and oxygen fugacity significantly higher than FMQ buffer (delta log fO2 = 1.3-1.9). Mafic veins are considered to be a crystallization product from basic melt enriched in Mg and Ni via interaction with peridotite. The geochemical type of melt reconstructed by the equilibrium with Ca-pyroxene is defined as T-MORB: (La/Sm)_N~1.6 and (Ce/Yb) )_N~2.3 that is well consistent with compositional variations of modern basaltic lavas in this segment of the Mid-Atlantic Ridge, including new data on quenched basaltic glasses.