Dynamic evolution of flow structures and viscosity during basaltic magma emplacement and crystallization in an upper-crustal sill

Abstract:

An upper-crustal intrusive network in the 201.5 Ma, rift-related Central Atlantic Magmatic Province is exposed in the western Newark basin (PA, USA). Alpha-MELTS modeling was used to track magma evolution starting with initial pyroxene crystallization at depth (1000-500 MPa); plagioclase crystallized in the upper crust or during ascent. For magma emplaced at 5-6 km (170 MPa), six MELTS models were generated to bracket different composition, H2O (1-3 wt.%), and crystallinity (28-49 vol.%). Corresponding magma viscosity evolved from 3 to 1624 Pa-sec (predicted using Giordano et. al, 2008; Moitra and Gonnermann, 2014).

Detailed crystal mush structures in a diabase sill are revealed in a dimension stone quarry. Ubiquitous asymmetric modal layers a few mm thick comprising plag-rich layers (PRL, 75% modal plag) overlying more pyx-rich layers outline the tops of hundreds of dm-m scale flow lobes in the quarry. Tabular plag in PRL show shape-preferred orientations, tiling, and pressure shadows around larger pyx that resemble analog experiments on particle slurries and indicate flow with limited mechanical compaction. During emplacement, magmatic flow self-organized by recursive interactions of propagation, sorting, and crystallization into flow lobes with plag entrained and aligned along lobe tops. Our calculations show plag separation can reduce bimodal suspension viscosity; a positive feedback likely enhanced by shear thinning and crystal alignments.

EDS analyses and X-ray maps show that plag has oscillatory-zoned cores (An82-67) with patchy-zoned mantles (An67) filled in by An66-63. In PRL, plag are cemented together by An62-55; Na-rich rims occur next to qtz-Kspar pockets. By the end of cementation, PRL liquid volume was significantly reduced to 11-18% compared with 28-45% in overall magma based on MELTS models for An62-55 plag. Diabase suspension viscosity increased to 10⁴ - >10⁶ Pa-sec; PRL viscosity cannot be modeled by equations based on random packing. PRL with aligned interlocking crystals were more rigid and less permeable than surrounding diabase. Upward flow of magma after modal layer development was channelized into pipes truncated and deflected by PRL. Thus, lateral flow during emplacement developed sub-vertical heterogeneities that exemplify complex mush rheology over m-scale distances.