Textures in Partially Solidified Crystalline Nodules: a Window into the Pore Structure of Slowly Cooled Mafic Intrusions

Abstract

Abundant glass is present along grain boundaries in coarse-grained, glass-bearing, crystalline gabbroic and peridotitic nodules entrained and erupted in lavas from Iceland, Santorini and Mauna Loa (Hawaii), even when the total porosity is less than a few volume per cent. The glass films vary from a few microns to a few tens of microns thick, and are associated with strings of small lensoid grain boundary pockets formed by impingement during crystal growth. Additional porosity occurs as extensive liquid-filled pockets adjacent to included grains within oikocrysts and as large triangular pockets formed by impingement of planar-sided grains. Interstitial material within glass films, and the irregularity of film thickness along a single grain boundary, suggest that the present pore structure is representative of the pore structure before entrainment and eruption. Pore geometry is consistent with a dominant control by crystal growth during solidification, with little or no evidence for control by minimization of internal energies driven by textural equilibration. Similarities between liquid distribution in the crystalline nodules and that of late-stage, interstitial phases in fully solidified mafic cumulates from the Rum and Skaergaard intrusions demonstrate that the crystalline nodules provide information about the latest stages of solidification in slowly cooled mafic plutons. The highly permeable network of intersecting liquid films, lenses and pockets may promote in situ crystallization in the solidifying mush, explaining the common presence of adcumulates in such intrusions.