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This paper examines the factors controlling the petrogenesis and chemical alteration of oceanic layer 2 basalts recovered from the margins and interior of the Nazca plate in the southeast Pacific. Specifically, the extent and nature of fractional crystallization, spreading-rate variations, proximity to mantle plumes and prominent fracture zones, mantle source-rock heterogeneities, and secondary alteration are evaluated as some of the more salient factors influencing the composition of upper oceanic lithsophere of the plate. A total of 274 analyses from 88 locations are available for this purpose, and many analyses are of crust generated at the fastest spreading part of the world’s mid-ocean ridge system, the East Pacific Rise.

We find that oxidative, low-temperature sea-water alteration, extensive shallow-level fractional crystallization, and mantle source-rock inhomogeneities are the most important factors. The most pronounced effects of oxidative alteration (high Fe2O3/FeO ratios and K2O contents; low MgO abundances; preponderance of a celadonite-iron oxide secondary mineral assemblage) are observed in older basalts (<10 m.y.) dredged from topographic highs on the plate interior, whereas basalts dredged from the crestal portions of the East Pacific Rise and the Galapagos spreading center show only minimal chemical effects of alteration. Only where drilling or recent tectonism has made possible the recovery of basalts from lower in the crustal section are the effects of nonoxidative (hydrothermal) alteration preserved (low Fe2O3/FeO ratios and K2O contents; high MgO abundances; smectite-dominated secondary mineral assemblage). Extensive shallow-level fractional crystallization, involving plagioclase, clinopyroxene, and olivine, is clearly the most important process controlling the range in composition of basalts observed in many areas; Fe-Ti basalts are the end products of this process and are common on the plate. Clinopyroxene fractionation is required for the formation of many of these highly evolved basalts, and this is indicated by observed phenocryst assemblages, normative mineralogy, standard variation diagrams and petrogenetic modeling. First-order regional differences in the composition of mantle source rocks are required to explain the pronounced light-rare-earth-element enrichment and high 87Sr/86Sr ratios of basalts associated with suspected mantle-plume activity of Easter Island and the Galapagos Isalnds. Less pronounced mantle source-rock heterogeneity may be responsible for the occurrence of mid-ocean ridge basalts with similar major-element abundances but markedly different rare-earth-element patterns. Except for fracture-zone offsets near 85° and 95°W longitude along the Galapagos spreading center, we find little evidence supporting the concept that such prominent offsets may be compositional interfaces between opposing rise-crest segments of the East Pacific Rise. Continuous compositional variation exists along the East Pacific Rise, although basalts tend to become somewhat more primitive as the Nazca-Pacific-Cocos triple junction is approached. Compared to basalts from the slow-spreading Mid-Atlantic Ridge, basalts analyzed from the fast-spreading East Pacific Rise and the Galapagos Rise are significantly more evolved (higher FeO*/MgO ratios and TiCh contents; lower CaO and AI2O3 abundances). Such systematic differences in major-element abundances appear related to a near order-of-magnitude difference in spreading rates of the divergent plate margins and to the size and continuity of subaxial magma chambers that can be physically maintained beneath them. It is proposed that an enhanced thermal regime beneath a fast-spreading center favors the existence of a large, steady-state magma chamber. Primitive magma entering such a reservoir will mix with much larger volumes of highly evolved magma, thereby reducing the probability that primitive magma can be erupted on the sea floor. The observed extensive fractionation required to account for the highly evolved character of Nazca plate basalts would require a thicker layer of oceanic layer 3 cumulate; available seismic refraction data support the existence of such a layer for older crust of the Nazca plate.

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