Thermodynamic Modelling of Mantle–Melt Interaction Evidenced by Veined Wehrlite Xenoliths from the Rockeskyllerkopf Volcanic Complex, West Eifel Volcanic Field, Germany
The oldest volcanic center of the Rockeskyllerkopf Volcanic Complex (RVC) in the West Eifel volcanic field hosts three distinct compositional groups of mantle xenolith: two groups of lherzolite and harzburgite and a group of wehrlite xenoliths that are cross-cut by phlogopite–clinopyroxene veins and variably impregnated by these same minerals. The lherzolite and harzburgite xenoliths represent mantle that was affected by metasomatism prior to the Quaternary magmatic activity below the RVC. We interpret the wehrlites to be the result of infiltration of orthopyroxene-undersaturated alkaline melt into the orthopyroxene-bearing lithospheric mantle. Reaction between mantle and melt consumed orthopyroxene and precipitated olivine and clinopyroxene as well as phlogopite. Models of the equilibration of peridotite and infiltrated melt created with alphaMELTS produce wehrlite with olivine, phlogopite and spinel compositions that are similar to those observed in the natural samples for melt:rock ratios between 0 11 and 1 11. Model clinopyroxene compositions do not match the observed range of compositions in wehrlite. The intra-sample range in clinopyroxene composition in wehrlite xenoliths suggests that clinopyroxene did not reach equilibrium, whereas olivine compositions in the same samples show much less variation, suggesting that they attained equilibrium. We calculate the time required for olivine homogenization to be around 3300 years, whereas the time needed for clinopyroxene to be homogenized is 100 times longer. We suggest that the range of clinopyroxene compositions within and between the wehrlite xenoliths is the result of varying degrees of equilibration of the initial lherzolite or harzburgite clinopyroxene with clinopyroxene formed by reaction with the melt. Melt compositions from the alphaMELTS models suggest that the differences in lava composition observed in the RVC can be produced by mixing between melts from a garnet peridotite source with secondary melts produced by peridotite–melt reaction in the spinel-facies mantle. We suggest that mixing occurred during melt transport through the lithospheric mantle as secondary melts produced during wehrlite formation were drawn into fractures that were exploited by several generations of magma rising from the garnet peridotite source.