2019 MSA AWARD LECTURE: THE EFFECT OF THE CRUST ON THE DIFFERENTIATION OF BASALTIC MAGMA

Basalts represent the most common magma type that erupts at the surface of the Earth and are produced by low-to-high degrees of wet or dry partial melting in the upper mantle. Chemical variability of basalts is partly attributed to melting of different mantle sources under differing pressure-temperature conditions. However, crustal processing also plays a major role in controlling the compositions of erupted basalts. This talk will address the effect of melt migration and emplacement through the crust on the composition of melts that ultimately erupt at the Earth’s surface. Two different examples will be used to illustrate what constraints on crustal processes can be gained from the study of natural rocks and using experiments in the laboratory. The first example is an experimental study of the crystallization products of two near-primary basalts (one enriched and one depleted) from Iceland under crustal pressure conditions. Phase equilibria show drops in melt fraction with decreasing temperature that are significantly higher for depleted melts than for enriched ones. As a consequence, for mantle melts ascending through the crust, the likeliness of reaching the surface before being fully solid is much higher for the enriched melt. This has important consequences for the major and trace element contents of erupted basalts, for estimates of the relative proportions of enriched and depleted melts produced in the mantle and is also important for constraining the mantle sources of continental and oceanic basalts. The effect of crustal differentiation of basaltic magmas can also be observed in the crystal record of erupted lavas. Examples of the Shatsky Rise oceanic plateau and the Calbuco volcano (Andes) will be used to illustrate that basaltic melts progressively crystallize in various storage regions before being erupted. Mass transport across the different magma chambers has important effects on crystal compositions and textures but also on the compositions of magmas that finally erupt that can be far from representing true liquid compositions.