Pre-subduction metasomatic enrichment of the oceanic lithosphere induced by plate flexure

Oceanic lithospheric mantle is generally interpreted as depleted mantle residue after mid-ocean ridge basalt extraction. Several models have suggested that metasomatic processes can refertilize portions of the lithospheric mantle before subduction. Here, we report mantle xenocrysts and xenoliths in petit-spot lavas that provide direct evidence that the lower oceanic lithosphere is affected by metasomatic processes. We find a chemical similarity between clinopyroxene observed in petit-spot mantle xenoliths and clinopyroxene from melt-metasomatized garnet or spinel peridotites, which are sampled by kimberlites and intracontinental basalts respectively. We suggest that extensional stresses in oceanic lithosphere, such as plate bending in front of subduction zones, allow low-degree melts from the seismic low-velocity zone to percolate, interact and weaken the oceanic lithospheric mantle. Thus, metasomatism is not limited to mantle upwelling zones such as mid-ocean ridges or mantle plumes, but could be initiated by tectonic processes. Since plate flexure is a global mechanism in subduction zones, a significant portion of oceanic lithospheric mantle is likely to be metasomatized. Recycling of metasomatic domains into the convecting mantle is fundamental to understanding the generation of small-scale mantle isotopic and volatile heterogeneities sampled by oceanic island and mid-ocean ridge basalts

Mantle metasomatism

Mantle metasomatism is a relatively recent concept introduced in the early 1970s when detailed studies of lithospheric mantle rock fragments (xenoliths), brought to the surface of in basaltic to kimberlitic magmas, became widespread. Two main types of metasomatism were defined: modal (or patent) metasomatism describes the introduction of new minerals; cryptic metasomatism describes changes in composition of pre-existing minerals without formation of new phases. A new type of metasomatism is introduced here, stealth metasomatism; this process involves the addition of new phases (e.g. garnet and/or clinopyroxene), but is a “deceptive” metasomatic process that adds phases indistinguishable mineralogically from common mantle peridotite phases. The recognition of stealth metasomatism reflects the increasing awareness of the importance of refertilisation by metasomatic fluid fronts in determining the composition of mantle domains. Tectonically exposed peridotite massifs provide an opportunity to study spatial relationships of metasomatic processes on a metre to kilometre scale. The nature of mantle fluids can be determined from the nature of fluid inclusions in mantle minerals and indirectly from changes in the chemical (especially trace-element) compositions of mantle minerals. Metasomatic fluids in off-craton regions cover a vast spectrum from silicate to carbonate magmas containing varying types and abundances of dissolved fluids and solutes including brines, C-O-H species and sulfur-bearing components. Fluid inclusions in diamond and deep xenoliths reveal the presence of high-density fluids with carbonatitic and hydro-silicic and/or saline-brine end-members. The deep cratonic xenolith data also reinforce the importance of highly mobile melts spanning the kimberlite-carbonatite spectrum and that may become immiscible with changing conditions. A critical conceptual advance in understanding Earth’s geodynamic behaviour is emerging from understanding the linkage between mantle metasomatism and the physical properties of mantle domains recorded by geophysical data. For example, metasomatic refertilisation of cratonic lithospheric mantle increases its density, lowers its seismic velocity and strongly affects its rheology. Introduction of heat-producing elements (U, Th, K) increases heat production, and the key to understanding electromagnetic signals from mantle domains may be closely related to fluid distribution and type (e.g. carbonatitic) and its residence in or between grains. The lithospheric mantle is a palimpsest recording the multiple fluid events that have affected each domain since it formed. These events, involving different fluids and compositions, have repeatedly overprinted variably depleted original mantle wall-rocks. This produces a complex, essentially ubiquitously metasomatised lithospheric mantle, heterogeneous on scales of microns to terranes and perhaps leaving little or no “primary” mantle wall-rock. Decoding this complex record by identifying significant episodes and processes is a key to reconstructing lithosphere evolution and the nature and origin of the volatile flux from the deep Earth through time.63 page(s