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

High-resolution chemical mapping of a large, single, oikocrystic amphibole grain from the Eriksberg gabbro, by laser ablation inductively coupled plasma mass spectrometry (LA ICP-MS), documents the progressive evolution of interstitial liquid in a hydrous basaltic system. The amphibole has a nearly constant mg-number and only minor variations in most major elements, in part because of the resorption of olivine buffering the liquid composition. However, non-buffered trace elements including Sc, V, Nb, Zr, Th, U, and the rare earth elements (REE) show at least an order of magnitude variation. For example, V varies from 1000 ppm, Zr from 4·3 to >1000 ppm, Nb from 0·14 to 12·3 ppm, and the REE patterns range from bowed up with a negative Eu anomaly to bowed down with a positive Eu anomaly—all within a single crystal. The distribution of amphibole compositions indicates that the interstitial liquid was not uniformly distributed as crystallization proceeded. Rather, the compositional variations reflect progressively more channelized flow of interstitial liquid during compaction. When the interstitial liquid evolved so that the crystallizing amphibole had ∼300 ppm V, vapor saturation was reached and the behavior of the incompatible trace elements changed markedly. The final amphibole to crystallize is enriched in soluble incompatible elements and depleted in Zr and Nb—a relationship that is consistent with crystallization from a fluid. Where compaction returns such evolved liquid or fluid to an overlying magma reservoir, resulting fractionation trends will show the relative depletions in Ti, Nb, and Zr seen in arc-related basalts.