Geochemical, Mineralogical, and Textural Characterization of the Beacon Sill, McMurdo Dry Valleys, Antarctica

The Beacon sill, part of the Jurassic Ferrar Large Igneous Province, is a 150 meter thick, macroscopically uniform diabase sill intruding the Beacon sandstone at Beacon Heights. It is part of a complex that includes the underlying Basement and Peneplain sills, and the overlying Kirkpatrick basalts. Although these other units have been the subject of intense research, the Beacon sill, perhaps because of its apparent homogeneity, has not been examined in detail. In this study, variations in bulk-rock chemistry, modal mineralogy, and texture are measured in a complete section, sampled at 5 m intervals, through the sill. The textural variations, as quantified using CSDs, indicate that despite the generally homogenous nature of the diabase in the sill, this intrusion was inflated in a pulsatile fashion. In particular, small but distinguishable reversals in the overall coarsening-inwards trend mark the interfaces between partially solidified magma from an earlier injection pulse and fresh magma from a reinjection event. Further evidence for this model is provided by mineralogical and geochemical anomalies consistent with redistribution of interstitial liquid across the reinjection horizon. Thermal modeling will be used to constrain the frequency of reinjection events based on the spatial distribution of textural and compositional anomalies.