JOHN SHERVAIS’ CONTRIBUTIONS TO OUR UNDERSTANDING OF UPPER MANTLE STRUCTURE AND CHEMICAL HETEROGENEITIES

Dating back to his PhD dissertation in 1979, John Shervais was a pioneer in deciphering the structure, and intrinsic petrologic and geochemical heterogeneities of the upper mantle through careful study of Alpine peridotite massifs, particularly at Balmuccia, Italy. The senior author was privileged to work with Shervais during second-phase studies which focused on the chemical and isotopic heterogeneities of this upper-mantle body. The Balmuccia Massif consists largely of lherzolite, with subordinate harzburgite and dunite, and dike rocks of various mafic lithologies formed in the mantle before crustal emplacement. Chondrite-normalized rare earth element (REE) data by secondary ionization mass spectrometry (SIMS) on clinopyroxene (cpx) and amphibole (amph) show that Cr-diopside suite dikes have strong light REE depletions (La at ~0.5 times chondrite) and flat middle and heavy REE at ~7-10 times chondrite. This feature is more likely the result of repeated melt extraction from this dike suite than a direct reflection of its original source. Cpx from the Al-augite suite is generally less light REE depleted compared to cpx from the Cr-diopside suite dikes, and exhibits higher REE concentrations overall.

Cr-diopside suite dikes have Sr and Nd isotopic compositions similar to those of the host lherzolite and within the range of compositions defined by ocean-island basalts. The Al-augite dikes and the hornblendite veins have Sr and Nd isotopic compositions similar to those of Cr-diopside suite lherzolite and websterite. The late gabbro dikes have isotopic compositions similar to MORB asthenosphere, reflecting a change in magma source region during uplift. These data show that tectonic thinning of subcontinental lithosphere during extension caused a change in the source regions of mantle-derived magmas from an OIB-like lithosphere to the underlying MORB asthenosphere. Thus, isotopic and trace element signatures in orogenic peridotite massifs preserve the record of magmatic activity both within the mantle and at lower crustal levels.