Refinement of Measurement of Crystal-Mush Compaction in the Palisades Sill, New Jersey

University Of Connecticut, Storrs CT

Investigators

Abstract

Philpotts/Gray EAR-0106538 The goal of this research is to determine directly the amount of compaction that occurred during the solidification of the Palisades Sill, New Jersey, one of the simplest and best studied intrusive igneous bodies. Previous studies revealed that a possible explanation for the chemical variation through this sill involved compaction of crystal mush with upward expulsion of the residual liquid. The evidence, however, is entirely chemical and open to other interpretations. Recent studies of a thick flood-basalt flow have shown that plagioclase crystals link together to form a three-dimensional network of chains early in the crystallization of the magma. If compaction of crystal mush occurs during solidification, the network of plagioclase crystals is deformed, and its degree of anisotropy is a direct measure of the amount of compaction. In the case of the thick lava flow, the compaction amounts independently determined from the anisotropy of the plagioclase network and from the chemical variation through the flow agree remarkably well. Initial measurements of the fabric of rocks in the Palisades Sill document that they are indeed anisotropic and that the compaction inferred from the chemical evidence did occur. Three new, independent methods have been developed for measuring the anisotropy of a rock's fabric. The methods involve measuring various aspects of the plagioclase network in oriented thin sections. One is based on the average intercept length between plagioclase chains along mutually perpendicular traverses; another considers links in the chains as vectors whose distribution can be analyzed for anisotropy; and the third determines the average moment ellipse that can be fitted to the interstitial patches between the chains. Each of the methods is amenable to a thorough statistical analysis of the standard errors for the degree of compaction and orientation estimates. These techniques will be used on sets of three mutually perpendicular thin sections cut from a series of oriented samples through the sill. These measurements will allow us to determine, for each sample, the 3-D ellipsoid of the anisotropy of the rock1s fabric, from which will be calculated the amount and direction of compaction. Differences between the three independent methods of measuring compaction can be used to shed light on the mechanism of the compaction process and its relation to layering in the sill. The anisotropism of magnetic susceptibility, which has commonly been used as an indirect measure of the anisotropy of a rock1s fabric, will be measured in each sample to determine how it relates to the anisotropy of the plagioclase-chain network. The direct measurement of compaction in a crystal mush is of importance, because of the insight it will provide into the chemical and physical processes involved in the differentiation of magma bodies and in the extraction of magma from partly melted mantle sources.