Hidden early clinopyroxene relicts record reactive porous flow in oceanic plutonic series

Almost two-thirds of the Earth’s magmatic budget is concentrated at mid-ocean ridges, with 85% of this being emplaced as intrusive rocks. In these systems, now understood to consist mostly of a crystal-dominated igneous medium (mush), melt differentiation at depth is predominantly governed by melt-mush reactions. These reactions have been well described for primitive lithologies (ranging from troctolite to gabbro) and are characterized by disequilibrium processes between a reactive melt and the crystal framework through which it percolates. This leads to mineral assimilation and crystallization reactions. Although thermodynamic and geochemical models (assimilation and fractional crystallization) consistently indicate the need for clinopyroxene assimilation in the case of a gabbroic mush, petrographic observations have only revealed poikilitic and late interstitial clinopyroxene interpreted as a late crystallizing phase. Here, we present chromium and titanium chemical maps of an apparently interstitial poikilitic clinopyroxene from ODP Hole 735B, Southwest Indian Ridge. We present key evidence for Cr-rich amoeboidal clinopyroxene relicts preserved within the larger poikilitic grains. In this reactive context, these primitive relicts are remnants of earlier clinopyroxene assimilation. Accordingly, clinopyroxene may be present early in oceanic plutonic crystallization, at least at slow- and ultraslow-spreading ridges, and it must participate in assimilation reactions occurring during the reactive porous flow differentiation process.