Impact of volatiles and variations in local bulk composition on deformation and magma emplacement processes in the deep crust and upper mantle parts of continental rift systems

The coupling of CO2 emissions and tectonic activity in active plate margins is becoming increasingly prominent, as remote sensing techniques make this relationship readily observable on a global scale. However, direct observations of the processes that link emissions and seismicity are lacking. This study documents observations from the deep part of an ancient continental rift system, now exposed at the Earth’s surface. We demonstrate how volatiles and preexisting magma chamber structures affect the influx of new magma and how magma induced deformation plays a key role during the shift from initial plume related magmatism to rifting, by altering the rock rheology and facilitating strain localization. The outcrops are comprised of ultramafic cumulates, intersected by mafic dykes. The ultramafic cumulates consist of three units: the central series, upper layered series and the lower layered series, with the central series being the youngest and partly replacing the upper and lower layered series. The dykes intersecting the upper layered series are partially remolten and replaced by the influx of the central series cumulates. This is especially evident in mafic dykes in wherlitic cumulates of the upper layered series. Younger melts of the central series used the contact between the dykes and host wehrlite as a pathway. The heating caused partial melting of the mafic dyke, which acted as a lubricant during deformation. In addition to the lubrication effect of the melt, volatiles within the mafic dykes, including CO2 react with the mafic minerals within the host ultramafic rocks, leading to fracturing and brecciation, and locally followed by diffusion creep in the finer-grained material. PT-estimates indicate that this brecciation took place under lower crustal/upper mantle conditions. Hence, conditions of deformation can shift from low strain rate plastic creep to ultrafast localized seismic creep in a short time due to local structural and compositional inhomogeneities.