Quantifying 3D crystal populations, packing and layering in shallow intrusions: A case study from the Basement Sill, Dry Valleys, Antarctica

Abstract

In many shallow level intrusions, complex crystal layering structures can be preserved due to pulses of intrusion and magma batches that arrive with a significant crystal payload. In this study, we investigate crystal layering within the Dais intrusion of the Basement Sill, Dry Valley, Antarctica. A single three-dimensional (3D) Rhythmic layer is imaged using X-ray computed tomography (CT) to successfully measure the phase distributions of pyroxene and plagioclase through the rock, and a combination of textural quantification in 2D and the 3D high-resolution data allows the texture to be further broken down into the key components: orthopyroxene (OPX), cumulate OPX, clinopyroxene (CPX), and plagioclase (PLAG). The texture was formed by an initial OPX cumulate framework of 37%–47%. The OPX in the cumulate overgrew by an additional 7%–10%; with the concomitant growth of CPX and PLAG in the interstices, a mixture of fine plagioclase microcrystals and melt were expelled into the upper layer. Textural development in the upper layer was arrested by the rapid growth of OPX and CPX dendrites forming oikiocrysts containing the initial microcrystal population of existing PLAG nuclei, along with plagioclase overgrowths in the areas where the pyroxenes are absent. Calculations of liquid and solidification development at the distance the sample is from the sill contact suggests that the textures in this part of the sill developed over ∼140 yr or less.