Mantle xenoliths from Tenerife (Canary Islands): Evidence for reactions between mantle peridotites and silicic carbonatite melts inducing Ca metasomatism

Mantle xenoliths from Tenerife show evidence of metasomatism and recrystallization overprinting the effects of extensive partial melting. The evidence includes: recrystallization of exsolved orthopyroxene porphyroclasts highly depleted in incompatible trace elements into poikilitic orthopyrokene with no visible exsolution lamellae: formation of olivine and REE-Cr-rich, strongly Zr-Hf-Ti-depleted clinopyroxene at the expense of orthopyroxene: the presence of phlogopite: whole-rock CaO/Al2O3 >>1 (Ca metasomatism) in recrystallized rocks and enrichment in incompatible elements in recrystallized rocks, relative to rocks showing little evidence of recrystallization. The 'higher-than-normal' degree of partial melting that preceded the metasomatism probably results from plume activity during the opening of the Central Atlantic Ocean. Sr-Nd isotopic compositions are closely similar to those of Tenerife basalts. indicating resetting from the expected original mid-ocean ridge basalt composition by the metasomatizing fluids. Metasomatism was caused by silicic carbonatite melts, and involved open-si-stem processes. such a trapping of element compatible with newly formed acceptor minerals, leaving residual fluids moving to shallower levels. The compositions of the metasomatizing fluids changed with time, probably as a result of changing compositions of the melts produced in the Canary, Islands plume. Spinel dunites and wehrlites represent rocks where all. or most, orthopyroxene has been consumed through the metasomatic reactions.