HYDROTHERMAL ALTERATION OF MAGNETIC MINERALS IN THE VALLES CALDERA CORES

First order reversal curve (FORC) diagrams are a sensitive, new technique for probing the grain-size distributions and interactions of magnetic minerals. Because of this, they represent an excellent, non-destructive tool for investigating the effect of hydrothermal alteration on magnetic mineral assemblages. We used FORC diagrams to study core samples of Paleozoic sedimentary formations obtained from the VC-1 and VC-2B cores drilled in the 1.2 Myr Valles caldera in New Mexico. These formations are a logical starting point for studies of the effects of thermal gradients on magnetic minerals because previous work has shown that corresponding stratigraphic units in the two cores have experienced very different temperature regimes since development of the caldera. Various magnetic measurements and Mössbauer spectroscopy indicate that the only magnetic mineral present in the samples is hematite so that the main differences in the FORC diagrams can be attributed to changes in grain size distribution. Our initial results suggest that grain sizes increase with increasing temperature, with the largest changes occurring mainly around 240-250°C. This may represent a threshold temperature for a grain-coarsening reaction such as Ostwald ripening. Once the effects of thermal alteration are better understood in the Valles caldera system, FORC diagrams can be used to look at the effects on magnetic minerals of other hydrothermal systems, such as the Salton Sea.