Chemical Diversity of Abyssal Volcanic Glass Erupted Along Pacific, Atlantic, and Indian Ocean Sea-Floor Spreading Centers

The abundant glassy rinds and chilled contacts of submarine extrusions and intrusions provide the most reliable indicators of primary magmatic compositions of deep-sea igneous rocks. They are homogeneous at the precision of electron microprobe analyses, so that even small sub-samples are reliable indicators of the composition of larger eruptive units. Analyses of such sea-floor glasses by electron microprobe techniques for Si, Al, Fe, Mg, Ca, Na, K, Ti, P reveal an extremely diverse compositional spread, ranging form highly differentiated basalt to varieties extremely depleated in large ion lithophile elements. A preliminary look at glasses from active spreading centers suggests that the FETI basalt group (high FeO and TiO2, low MgO) is more common at rapidly spreading ridges (East Pacific rise system) than at slower spreading ridges (mid-Atlantic ridge system). The data presented here appear for the most part inadequate for meaningful assessment of chemical gradients along spreading centers. There is strong evidence, though, that some segments of spreadings centers have erupted similar lava compositions over tens of millions of years. These similar chemical groups are symmetrically distributed on each side of spreading segments as is expected from sea-floor spreading models. There is very little overlap in the compositions of the basaltic chemical groups at the level precision of the electron probe analyses. However, there are a few Atlantic chemical groups that occur both in the North and South Atlantic. Even these intra-ocean matches of chemical groups from one locality to another are, however remarkably few. This survey shows the great predominance of various basalt types at spreading centers, but other rock types, including soda rhyolite from the Galapagos rise, occur rarely.