Hydrothermal alteration of upper oceanic crust formed at fast spreading rates

Abstract

Hydrothermal circulation plays a fundamental role in the chemical transfer from deep in the Earth’s interior to the ocean crust, the oceans and the atmosphere. It is also on of the principal mechanisms for heat transfer from the mantle to the oceans, atmosphere and ultimately, outer space. This process fundamentally influences the composition of the ocean crust during formation and aging as it spreads away from the ridge axis. However, despite much research into hydrothermal alteration of oceanic crust questions still remain including: the thermal and chemical evolution of hydrothermal fluids, the geometry of hydrothermal fluid flow, and the factors that control the nature and extent of hydrothermal alteration of oceanic crust.

In this study, whole rock and secondary mineral characteristics of drilled-in situ ocean crust are used to (i) Characterise hydrothermal alteration for a range of drilled, in-situ fast spread ocean crust sites (ii) assess the factors that control hydrothermal alteration within fast spread ocean crust and (iii) assess the evolution and architecture of hydrothermal fluid.

Deep Sea Drilling Project, Ocean Drilling Program, and Integrated Ocean Drilling Program Sites 504, 896, 843, 1179, 1149, 1224, 1243 and 1256 represent some the most significant penetrations into the upper portion of intermediate and fast spread crust to date. Analyses of whole rock chemical changes, Sr, O. C and S isotope systematics, petrographic observations and analysis of secondary minerals indicate that all sites underwent variable degrees of cold seawater dominated hydrothermal alteration. All these sites represent variations in the composition of the upper crust, basement topography, sedimentation rates, spreading rates, capping rocks, and age. Comparisons between these factors and style and intensity of alteration for each site indicate that spreading rate and age exherts the strongest influence on hydrothermal activity.

Sites 1256 and 504 are the only sites in which both low temperature and high temperature alteration are recovered, both sites now have complete chemical and isotopic records which trace the evolution of hydrothermal fluid through the crust. Chemical and isotopic analyses of anhydrite within the ocean crust and consideration of the sulfur budget at these sites imply that the majority of hydrothermal fluid is heated to moderate temperatures (~250oC) and returns to the oceans as warm diffuse fluids at unaccounted for venting sites.

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