Crystallization and Degassing in the Basement Sill, McMurdo Dry Valleys, Antarctica

Abstract

The Basement Sill is part of the Ferrar Large Igneous Province exposed in the McMurdo Dry Valleys, Antarctica. The sill is � 330 m thick in the Bull Pass area and 450 þ m thick in the Dais area, � 12 km to the west, and is characterized by phenocryst-free lower and upper margins and an orthopyroxene-rich central ‘tongue’ (opx 1^5 mm in size). Halogen variations in apatite from a suite of samples collected along vertical transects through the sill were examined to evaluate the process of crystallization-induced degassing (i.e. second boiling) and its effects on magma chemistry. Apatite grains from any given sample are generally unzoned with respect to Cl and F concentrations, but may vary by 20^30 mol% in the halogen site between grains. Overall average Cl/F mass ratios increase with height from the lower margin to the center of the sill, and then decrease to near zero towards the top margin where the rocks are relatively oxide-rich. The Cl/F trend parallels those of bulk MgO and grain size. The upper margin contains abundant mafic pegmatoids and the apatite in these segregations has lower Cl/F ratios compared with that in the host-rocks, although REE show no measurable difference. Numerical modeling illustrates that a cooling and crystallizing sill initially develops two separate vapor-saturated zones at the lower and upper margins owing to the irreversible heat loss to the cooler country rock. Vapor separating from the lower zone migrates upward into hotter silicate liquid, where it is resorbed, thus increasing the Cl/F mass ratio of the liquid. This process leads to saturation and precipitation of apatite from the liquid with a higher Cl/F ratio than would otherwise occur. Volatile enrichment can also aid compaction and grain growth in the central part of the sill. In contrast, the relatively Fe-rich, Cl-poor nature of the upper zone rocks suggests that these rocks may have crystallized from more evolved, degassed silicate liquid, possibly compacted out of the underlying crystal mush. In addition, as vapor sourced from the lower and central parts of the sill ascends into the cooler upper zone of the sill, the vapor may be localized (along with late interstitial silicate liquid) to form pegmatoids at temperatures at which Cl is less favored in apatite and can be leached from existing apatite by the ascending vapor, the latter causing the observed decrease in the Cl/F mass ratio of apatite in the (evolved) pegmatoids.