Petrology and relict mineralogy of serpentinites at DSDP Leg 84 Holes

Eleven serpentine samples from DSDP Leg 84 and four serpentinized ultramafic samples from Costa Rica and Guatemala were described and their relict mineral compositions measured by electron microprobe to try to determine the origin of the Leg 84 serpentinites and their relationship to the ultramafic rocks of the onshore ophiolites. The Leg 84 samples comprise more than 90% secondary minerals, principally serpentine, with hematitic and opaque oxides, and minor talc and smectites. Four distinct textural types can be identified according to the distribution of opaque phases and smectite. Remnants of spinel, olivine, orthopyroxene, and clinopyroxene occur variously in the samples; spinal occurs in all the samples. Textural evidence suggests that the serpentinites were originally clinopyroxene-bearing harzburgites. Relict mineral compositions are refractory and relatively uniform: olivine, Fo90.6-90.9; orthopyroxene, En90-91; clinopyroxene, Wo47 En50 Fs3; spinels, Cr/Cr + Al = 0.4-0.6. 567A-29-2, 30-35 cm has slightly more magnesian olivines (Fo92) and orthopyroxene, and more aluminous spinels (Cr/Cr + Al = 0.3). These compositions are similar to those inferred for refractory upper-mantle materials and also fall within the range of compositions for relict minerals in abyssal peridotites. They could be of oceanic origin. The onshore samples include serpentinites, a clinopyroxene-bearing harzburgite, and a clinopyroxenite. They too have magnesium-rich silicate assemblages, but relative to the drilled samples have more iron-rich olivines (Fogo) and more aluminous and sodic pyroxenes; spinels which are clearly relicts are very aluminum-rich (Cr/Cr + Al = 0.1-0.25). These samples are most likely mantle materials, but significantly less depleted. Their relationship to the drilled samples is unclear. Serpentinites were the most common basement materials recovered during Leg 84, and there appears to be a bimodal assemblage (basalt/diabase and serpentine) of igneous rocks sampled from the trench slope. Diapirism of serpentine throughout the trench slope and forearc is suggested as an explanation for this distribution of samples.

Geochemistry, mineralogy, and 40K-40Ar radiometric dating of DSDP Leg 84 basalts - Guatemala Trench

Geochemical (atomic absorption, neutron activation analyses), mineralogical (microprobe), and radiometric (40K - 40Ar) data are presented for five basalts from the Guatemala Trench area (Deep Sea Drilling Project, Leg 84). Strong geochemical and mineralogical differences distinguish two types among these basalts: (1) One basalt (Sample 567A-19,CC), recovered below Upper Cretaceous limestone has the following characteristics: it is quartz normative and has low TiO2, content, as well as low amounts of Cr, Ni and other transition metals, an LREE depleted pattern, and affinities of clinopyroxene phenocryst plotted into the field of tholeiitic and calc-alkalic pyroxenes. (2) Four alkaline basalts, recovered from the mafic and ultramafic acoustic basement, are nepheline normative and show high TiO2 content, high amounts of Cr, Ni and so on, an LREE enriched pattern and compositions of clinopyroxene phenocryst plotted close to or within the field of alkali basalt pyroxenes. These basalts are comparable to those recognized in the lower part of the Santa Elena complex and are clearly different from the oceanic basalts of the Cocos Plate. The radiometric age of the orogenic basalt seems to be close to 80 Ma. The alkaline basalts are clearly older, even if a discrepancy appears between the results of different analyses because of the secondary effects of alteration.