Basalts from DSDP Leg 37 and the FAMOUS area: compositional and petrogenetic comparisons

Abstract

Basalts from the DSDP Leg 37 sites can reasonably be supposed to have originated at or near the present-day spreading center (FAMOUS area) investigated and sampled in detail by surface vessels and submersible vehicles in the median valley. Nevertheless, the Leg 37 samples show a more restricted range of compositions than those presently observed in the median valley. Basalt glasses from DSDP Leg 37 contain 0.6 to 1.3 wt.% TiO2, with FeO*/MgO ranging between 0.8 to 1.5. In contrast, basalt glasses from the median valley range from 0.5 to 2.0 wt.% TiO2, and FeO*/MgO varies from 0.7 to 1.8.The low-TiO2 basalts at site 332B can be compared to similar very young basalts that characterize the central volcanic extrusive hills such as Mt. Pluto and Mt. Venus in the median valley. The more iron-enriched basalts can be compared with certain FAMOUS basalts dredged from the rift mountains south of fracture zone A. The major- and minor-element variation trends among Leg 37 basalt glasses parallel trends within the FAMOUS data set, which are compatible with high level fractional crystallization. Overall, the best parent for site 332 basalts is a 'low-olivine' candidate parent magma type, which has been found in fracture zone B and in the median valley south of fracture zone B, and which resembles basalt at site 334. Compared to some parent–residual pairs tested in the FAMOUS area, the fractionation calculations for site 332 variations show good fits for TiO2 and K2O, and show little excess LIL-element enrichment. In all calculations, pyroxene dominates olivine as the most important mafic phase. This is consistent with the observed slight relative enrichment in light rare-earth elements in the 332A glasses compared to their inferred parents, but cannot be reconciled with light rare-earth depletion at site 335.A possible interpretation of the lava sequence at site 332B is that it represents a rapid accumulation of basalts extruded over a relatively short time span in the center of the median valley. Fractionation may have taken place in a stagnating chamber isolated by collapse of the valley walls. In this chamber there was not sufficient time or space to develop the thermal gradients and marginal enrichment in LIL-elements and volatile components which may have generated the high TiO2 basalts in the FAMOUS area. Geochemically unique basalt at site 335 shows depleted light rare earths combined with LIL-element and TiO2 enrichment. This basalt could have been a flank eruption unrelated to processes in the median valley.