PETROLOGICAL SYSTEMATICS OF THE MID-ATLANTIC RIDGE SOUTH OF KANE - IMPLICATIONS FOR OCEAN CRUST FORMATION

Models of ridge segmentation, mantle flow and melt focusing predict ho w the chemical compositions of mantle melts should vary along a mid-oc ean ridge axis. The compositions of basaltic lavas can be compared to these predictions to test the models. Such tests have been carried out using basalts from the neovolcanic zone south of the Kane fracture zo ne (the MARK area), where there are both a large transform and nontran sform offsets. Before evaluating mantle models, the effects of differe ntiation must be accounted for. Fractional crystallization at low pres sures (constrained by new melting experiments on these samples) does n ot account for the data. High pressure or in situ crystallization bett er account for the differentiation trends; however, these two processe s imply different relationships between magmatic differentiation and p osition within a segment. Irrespective of the differentiation model, s ignificant differences exist among parental magmas. Magmas near the tr ansform have much lower levels of highly incompatible trace elements b ut higher levels of moderately incompatible trace elements, suggesting both lower extents of melting and a more depleted source. These two c haracteristics may be natural consequences of the truncation of a melt ing regime by a large-offset transform: depleted mantle from across th e transform may contribute to the melting regime, while the cooler the rmal environment produces less melt. Quantitative modeling of these ge ochemical characteristics produces thin crust near the transform, cons istent with seismic and gravity studies. In contrast, thin crust adjac ent to nontransform offsets is associated with no reduction in extent of mantle melting. These results, along with data from other regions, suggest that nontransform offsets overlie a continuous melting regime, and melt focusing creates the variations in crustal thickness. Focuse d flow may also lead to incompatible element enrichment at segment cen ters, and relative depletion at segment margins. Only offsets that tru ncate the melting regime, such as large transforms, are associated wit h diminished extents of melting within the mantle. Petrological eviden ce obtained thus far is not consistent with active upwelling to explai n crustal thickness variations along nontransform offset bounded segme nts.