Rheology of subliquidus magmas. 1. Picritic compositions

Abstract

The relationship between the rheology and phase equilibria of a picritic basalt from Kilauea Iki has been investigated at 1 atm along the QFM buffer. Between 1270° and 1180°C olivine and minor spinel are the only liquidus phases, and the melt volume decreases from 85 to 74 vol %. At the ol-sp-pc-cpx cotectic, melt is consumed more rapidly dropping to 47 vol % at 1139°. The rheology of the magma is non-Newtonian, is characterized by time-dependent, pseudoplastic behavior, and is consistent with power law flow or Bingham pseudoplastic behavior. The non-Newtonian behavior in these magmas is due to reorientation of solids in the flow field and solid-solid interactions. We develop a model in which the time-dependent pseudoplasticity and yield strengths are produced by the relationship between solid phase contiguity (the fraction of solid internal surface area shared with other solids) and grain dispersive pressure due to shearing. Increased contiguity is related to increased apparent viscosities and yield strengths, while increased grain dispersive pressure due to shearing acts to diminish contiguity. -from Authors