ALBERT

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: We present results of a microprobe investigation of fresh and least-deformed and metamorphosed gabbroic rocks from Leg 118, Hole 735B, drilled on the east side of the Atlantis II Fracture Zone, Southwest Indian Ridge. This rock collection comprises cumulates ranging from troctolites to olivine-gabbro and olivine-gabbronorite to ilmenite-rich ferrogabbros and ferrogabbronorites. As expected, the mineral chemistry is variable and considerably expands the usual oceanic reference spectrum. Olivine, plagioclase, and clinopyroxene are present in all the studied samples. Orthopyroxene and ilmenite, although not rare, are not ubiquitous. Olivine compositions range from Fo85 to Fo30, while plagioclase compositions vary from An70 to An27. Mg/(Mg + Fe2+) of clinopyroxene (mostly diopside to augite) varies from 0.88 to 0.54. Mg/(Mg + Fe2+) of orthopyroxene varies from 0.84 to 0.50. These minerals are not significantly zoned. All mineralogical data indicate that fractional crystallization is an important factor for the formation of cumulates. However, sharp contacts, interpreted as layering boundaries or intrusion margins, suggest polycyclic fractionation of several magma batches of limited volumes. Calculated compositions of magmas in equilibrium with the most magnesian mineral samples at the bottom of the hole represent fractionated liquids through separation of olivine, plagioclase, and clinopyroxene at moderate to low pressures (less than 9 kb). Crystallization of orthopyroxene and ilmenite occurs in the most differentiated liquids. Mixing of magmas having various compositions before entering the cumulate zone is another mechanism necessary to explain extremely differentiated iron-rich gabbros formed in this slow-spreading ridge environment.

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.