Geochemical variability in MORB controlled by concurrent mixing and crystallisation

- Mixing and crystallisation occur concurrently in MORB.
- Coherent mixing relationships in MORB are apparent up to the ∼300 km scale.
- Ridge segmentation may represent a length scale over which melts are cogenetic.
- Magma mixing means information on mantle chemistry is lost in differentiated basalts.

The isotopic and elemental diversity in mid-ocean ridge basalts (MORB) traces the history of mantle differentiation, recycling and convective stirring. However, to interpret this record it is critical to account for the magma transport and storage processes modifying the primary geochemical variability of mantle derived melts. Magma mixing during low pressure differentiation is a key petrological process that controls the chemical variability of basalts throughout the global mid-ocean ridge system. Mixing occurs concurrent with crystallisation and must in general be dominant over any assimilatory processes in controlling the chemical evolution of basalts with MgO concentrations >5 wt% MgO. The effect of this mixing is to collapse the diversity of melt compositions leaving the mantle into the narrow range expressed in most mid-ocean ridge settings. In this context magma mixing can be viewed as contaminating the variance structure of primitive mantle melts, which leads to irreversible information loss on the sources and processes involved in melt generation unless primitive, unmixed, liquids and crystal phases are erupted. However, where we can track magma mixing, the homogenisation itself offers the potential to be an important petrological tool, which constrains the storage and transport processes magma experiences during its ascent through the mantle and crust. In the global dataset interrogated here systematic mixing trends are visible up to the length scales of first order ridge segmentation (∼300 km), indicating the possible links between surface tectonics and the record of mantle heterogeneity in basalts. The importance of magma mixing at mid-ocean ridges hints at the need to reevaluate the MORB-ocean island basalt chemical dichotomy, given the poorly understood mixing processes operating during intraplate magma transport from mantle to surface.