Evolution of Crystallizing Interstitial Liquid in an Arc-Related Cumulate Determined by LA ICP-MS Mapping of a Large Amphibole Oikocryst

Abstract

High-resolution chemical mapping of a large, single, oikocrystic INTRODUCTION amphibole grain from the Eriksberg gabbro, by laser ablation The most commonly studied cumulus rocks are those inductively coupled plasma mass spectrometry (LA ICP-MS), formed by crystallization of relatively dry basaltic liquids documents the progressive evolution of interstitial liquid in a hydrous (e.g. Parsons, 1986; Cawthorn, 1996). These include most basaltic system. The amphibole has a nearly constant mg-number layered intrusions, ophiolites, oceanic cumulates, and and only minor variations in most major elements, in part because most cumulates recovered from the Moon and in metof the resorption of olivine buffering the liquid composition. However, eorite collections. Studies of these cumulates have innon-buffered trace elements including Sc, V, Nb, Zr, Th, U, and cluded efforts to understand how the crystals are the rare earth elements (REE) show at least an order of magnitude segregated from the liquid (cumulus processes), as well variation. For example, V varies from 1000 ppm, Zr as how the resulting pile of crystals plus liquid evolves from 4·3 to >1000 ppm, Nb from 0·14 to 12·3 ppm, and the during slow cooling (postcumulus processes). By comREE patterns range from bowed up with a negative Eu anomaly parison, very little effort has been directed toward underto bowed down with a positive Eu anomaly—all within a single standing these processes in the crystallization of relatively crystal. The distribution of amphibole compositions indicates that hydrous basaltic liquids such as those associated with arc the interstitial liquid was not uniformly distributed as crystallization magmatism. The presence of a significant amount of proceeded. Rather, the compositional variations reflect progressively water in the liquid is especially important for the postmore channelized flow of interstitial liquid during compaction. When cumulus evolution of a crystal pile, as it will stabilize the interstitial liquid evolved so that the crystallizing amphibole had hydrous minerals and cause vapor saturation while a >300 ppm V, vapor saturation was reached and the behavior of the significant amount of interstitial liquid remains. incompatible trace elements changed markedly. The final amphibole to The importance of fractional crystallization in procrystallize is enriched in soluble incompatible elements and depleted ducing compositional variability in crystallizing silicate in Zr and Nb—a relationship that is consistent with crystallization liquids has been well established since Bowen’s exfrom a fluid. Where compaction returns such evolved liquid or fluid perimental work (Bowen, 1928). During perfect fractional to an overlying magma reservoir, resulting fractionation trends will crystallization, the solids are isolated from the liquid before the liquid evolves appreciably and thus they proshow the relative depletions in Ti, Nb, and Zr seen in arc-related vide an instantaneous record of the liquid evolution. basalts. Incorporation of liquid in a crystal pile can change both the composition of the solids and the evolution of the supernatant liquid (initially the parental liquid). If the