Petrologic Variations Within Submarine Basalt Pillows of the South Pacific-Antarctic Ocean

摘要

The chemistry of submarine basalts has been used in upper mantle petrogenesis models but criteria for selecting fresh, representative samples and an understanding of within-specimen variations are vague. In an effort to define alteration criteria, variations in the mineralogical, chemical and magnetic parameters of one alkalic and eight tholeiitic basalt pillows from abyssal hills in eight widely scattered localities of the Southern Ocean have been studied from glass rim to aphanitic interior. These variations are related to primary cooling (quenching ane deuteric alteration) and secondary alteration (hydration and recrystallization). Four texturally gradational zones are defined from glass rim inward: hydrated glass, unhydrated glass, 'variolitic' zone, and aphanitic zone. This transition occurs through a minimum thickness of three centimeters. These zones are also characterized by low intensity of magnetization (J) and low susceptibility (c.g.s. units) in the glass (no opaques), high J and susceptibility in the variolitic zone (very minute opaques), and medium J and susceptibility in the aphanitic basalt (opagues larger than single domain size). Deuteric alteration affects the intensity and coercivity of the basalts. Vesicularity in the eight tholeiites does not exceed 3% at depths greater than 3200 meters which is consistent with physico-chemical data. Three types of alteration are common: (1) glass hydration (>2.0% total H2O) resulting in loss of magnesium and calcium and increase of total iron ferric/ferrous ratio, and potassium; (2) serpentinization adjacent to joints and fractures; and (3) higher temperature deuteric alteration in the aphanitic zone causing depletion of magnesium and total iron as FeO (up to 2.0 wt. percent of rock). The iron migrates to joints where it is oxidized and made available in significant quantities for redeposition in iron-rich sediments and ferro-manganese concretions.