Lithophile Trace Element Mass Balance in Spinel Lherzolites from Borée, Massif Central, France: the Effects of Melt Impregnation on Variably Depleted Peridotites

Abstract:

Isotopic and elemental investigations of peridotite xenoliths can be used to model the timing and degree of melt extraction in the mantle. However, the primary melt depletion signature preserved in these xenoliths can be overprinted by subsequent metasomatism and melt-rock interactions, which tend to obscure the degree of original melt depletion either through the crystallisation of secondary phases or through the pervasive addition of lithophile element-rich basalt or its derivatives.

Anhydrous spinel lherzolite xenoliths from the volcanic Maar de Borée in the French Massif Central have textures that vary from equigranular to protogranular. There is clear evidence of melt infiltration of up to several modal per cent in an anastomosing network of veins and stringers. Bulk-rock major elements and compatible trace element concentrations suggest that the Borée peridotites are genetically related by differing degrees of melt extraction, but the influence of melt infiltration, clearly visible at the thin section scale, and likely comprising several modal percent of the rock, makes this melt depletion difficult to quantify accurately. Some incompatible trace element concentrations in clinopyroxenes are consistent with melt depletion, while elevated (La/Yb)_Nratios in others are clearly indicative of cryptic metasomatism and suggest yet another style of metasomatism. Furthermore, infiltrated melt along grain boundaries and in interstitial pockets disturbs the incompatible element mass balance, further obscuring the true nature of the prior melt depletion event(s).

Quantitative lithophile element mass balance calculations will allow the effects of melt infiltration to be determined and stripped away from bulk-rock xenolith compositions, permitting a more realistic appraisal of the original melt depletion event.