A Rosetta stone linking melt trajectories in the mantle to the stress field and lithological heterogeneities (Trinity ophiolite, California)

Infiltration triggered by selective dissolution of pyroxenes is a major mode of melt migration in the mantle. A common view, supported by experiments and numerical models, is that the geometry of the melt plumbing system is governed by the stress field induced by solid-state flow of the host peridotite. Yet, salient melt migration structures frozen at an early stage of development in the mantle section of the Trinity ophiolite reveal that lithological heterogeneities drastically impact melt trajectories. Where melts reach a pyroxenite layer, dissolution-induced permeability abruptly increases, initiating a feedback loop confining melt migration to that layer regardless of its orientation relative to the stress field. This process results in the development of a network of interweaved dunitic channels evolving to thick tabular dunites where the melt reacts with closely spaced pyroxenite layers. This reacting melt was rich in alkali elements and water, as evidenced by the minerals (mostly amphibole and micas) encapsulated in the Cr-spinel grains that crystallized during the reaction. This “pioneer melt” differs from the volumetrically dominant depleted andesite that fed the crustal section. In fact, the migration of andesite benefited from the enhanced permeability provided by the dunites formed by the pioneer melt. As a result, dunites are palimpsests, the compositions of which record successive percolation events. The geometry of the melt pathways is extremely challenging to model because the abundance, spacing, and orientation of lithological heterogeneities cannot be predicted, being inherited from a long geological history.