ALBERT

Description: Two types of serpentinized peridotites from Hole 670A of Leg 109 were studied in detail. A small piece of relatively unaltered sample, 109-670A-9R-1, #3 (22-24 cm), is olivine websterite characterized by aluminous chromian spinel with Cr/(Cr + Al) ratio of about 0.2. The other minerals have compositions essentially identical with those in more commonly observed serpentinized harzburgite like 109-670A-9R-01, #12 (94-97 cm). The occurrence of pyroxene-rich peridotite with normal harzburgite suggests that small scale heterogeneity in modal compositions exists in the upper mantle beneath the Mid-Atlantic Ridge. Low Cr/Al ratios of spinel and pyroxenes of those peridotites indicate that they are relatively less refractory among peridotites ever recovered from the oceanic region. Textures and the estimated equilibration temperatures indicate that peridotites recovered from Hole 670A are recrystallized and reequilibrated at subsolidus temperature. The occurrence of serpentinized peridotites from the rift valley of the active mid-oceanic ridge may suggest that they represent direct exposure of upwelling mantle materials rather than serpentine diapirs.

Description: The Ocean Drilling Program's Hole 670A is located ~45 km south of the Kane Fracture Zone. Non-constructive (amagmatic) axial divergence ~10 km east of Hole 670A exposes residual mantle peridotites at the seafloor. The average peridotite in the group of 18 studied has 68% of its original mineralogy replaced by serpentine and related minerals. Average reconstructed (preserpentinization) modal proportions are 81.6% OL, 14.7% OPX, 2.4% CPX, and 1.3% SPINEL. Average chemical parameters are OL Mg# = 90.5, OL NiO = 0.31 wt%, OPX Mg# = 90.5, OPX A12O3 = 4.21 wt%, CPX Mg# = 90.9, CPX Na2O = 0.19 wt%, and spinel Cr# = 24.7. Chemical and mineralogical compositions of most samples fall in a restricted range, suggesting the rocks record a similarly narrow range of melting percentages. Major and trace element modeling indicate 14%-16% melting of a partially depleted source like the Tinaquillo lherzolite can account for most of the sample compositions. One harzburgite collected 30 m away from the other samples contains minerals with more refractory compositions; calculated melting percentages for this harzburgite are 18%-20%. The difference in calculated melting percentages between the refractory harzburgite and the rest of the samples may indicate that the percent of melting (or the efficiency of melt extraction) in the sub-oceanic upper mantle melting varies on a scale of tens of meters. Comparison of the Hole 670A peridotite compositions with peridotites from the KFZ reveals no significant differences. Therefore, there is no support in this small sample set for a variation of residual peridotite compositions correlative with the transform fault effect documented from basalt compositional variations.

Description: The magnetic properties of 11 samples from Site 670 of Leg 109, 3 harzburgites and 8 highly serpentinized peridotites, have been studied. Reflected light microscopy and Curie temperatures confirm that magnetite is the dominant magnetic mineral in all samples. However, both rock types show different magnetic behavior. Susceptibility, saturation magnetization, and NRM are higher for the serpentinites, because of the higher magnetite content. The hysteresis parameters indicate magnetite particles with pseudosingle domain structure for both rock types. For the remarkable anisotropy of the magnetic susceptibility no definite explanation could be found, because of the complex texture of the samples. In both rock types the presence of maghemite, a product of low temperature oxidation of magnetite, has been indicated by reflected light microscopy and by thermomagnetic analysis. As the maghemite converts to hematite at temperatures above 350°C, the temperature during the serpentinization was below this value assuming that the maghemitization took place at the same time.

Description: In this paper we describe textural relationships in hydrated upper mantle peridotites emplaced at a nonconstructive ridge segment. Development of serpentinites and partially serpentinized peridotites takes place in four main stages: (1) pervasive serpentinization forming mainly lizardite, (2) a tensional stage forming chrysotile + talc + chlorite, (3) a deformational stage forming antigorite + tremolite, and (4) a late local tensional stage forming another generation of chrysotile veinlets. Mineral chemistry of serpentine pseudomorphs reflects in part primary mineral compositions. Olivine pseudomorphs are typically nickeliferous and depleted in aluminum and chromium. Orthopyroxene pseudomorphs have lower nickel contents and relatively high iron, aluminum, and chromium contents. Clinopyroxene pseudomorphs have very low nickel contents and relatively high aluminum and chromium contents. These chemical patterns in the serpentinites can be used to help discriminate between harzburgitic and lherzolitic protoliths. Oxygen isotopes and mineral parageneses suggest serpentine is derived from circulation of hydrothermal (200 infinity C) fluids through the peridotite body. Crystallization of tremolite, talc, and chlorite may have occurred at temperatures up to 525 infinity C if C02/H20 ratios were less than 0.25. Open fissures developing in aging upper mantle provide paths for important seawater circulation through a thin basaltic carapace down to shallow mantle rocks.

Description: Paleomagnetic studies on the serpentinized peridotites recovered from ODP Hole 670A were conducted in three laboratories. High NRM intensities and magnetic susceptibilities were observed in the serpentinized peridotites, which suggest that the remanent and the induced magnetizations of the peridotites cannot be neglected as a source of the magnetic anomalies observed at sea surface. The in situ low inclination of the magnetization indicated from the laboratory studies suggests that the peridotite body has been subjected to a large-scale deformation after the acquisition of the magnetization.