The Magnetic and Opaque Petrological Response of Basalts to Regional Hydrothermal Alteration

TLDR

Basalt lavas are altered by both deuteric oxidation and regional hydrothermal processes, the latter occurring in zeolite facies up to 300 °C and producing rapid spatial variation that yields a wide range of lava properties. The first detectable response to regional hydrothermal alteration is a steady rise in the strong‑field Curie point of titanomagnetite, followed by pseudomorph titanomagnetite grains, distinctive ilmenite changes, and eventual magnetic alteration of even highly oxidized basalts when prehnite forms.

Abstract

The alteration state of basalt lavas is shown to be largely the result of the combined action of two alteration processes, deuteric oxidation and regional hydrothermal alteration. The former process is well described in the literature while the latter is shown here to be another widespread source of lava alteration. The environment of regional hydrothermal alteration is that of the zeolite metamorphic facies, with temperatures of up to 300 °C, and abundant groundwater being the major elements. Independent and rapid spatial variation of the two alteration processes results in the wide range of observed lava properties. Microscopically undetectable titanomagnetite alteration, indicated by steady rise in strong field Curie point, is the first response of a non-deuterically oxidized basalt to regional hydrothermal alteration. With more extreme alteration, titanohematite etc. is seen to pseudomorph titanomagnetite grains. Ilmenite also shows very distinctive mineralogical changes. Initially, high deuteric oxidation specimens appear to be immune magnetically to moderate alteration but these too eventually succumb when conditions are extreme enough for the formation of prehnite in the rock.