Intrinsic Nd, Pb, and Sr Isotopic Heterogeneities Exhibited by the Lherz Alpine Peridotite Massif, French Pyrenees

• Published in: Journal of petrology 1991, Vol.Special_Volume (2), p.117-134
• Main Authors: Mukasa, S. B., Shervais, J. W., Wilshire, H. G., Nielson, J. E.
• Format: Article
• Language: English
• ISSN: 0022-3530; 1460-2415
• DOI: 10.1093/petrology/Special_Volume.2.117

Optically clear, acid-leached Cr-diopside, Al-augite, pargasitic amphibole, and phlogopite from peridotite and ultramafic dikes of the Lherz massif (French Pyrenees) exhibit chemical and isotopic heterogeneities indicative of a complex history. The data cover a wide range on the 143Nd/144Nd vs. 87Sr/86Sr covariation diagram, encompassing much of the mantle array and a large area below the array. Pb-isotopic signatures cover a wide range on Pb-Pb correlation diagrams, with several samples falling in the mid-ocean ridge basalt (MORB) field, but with others defining trends towards the enriched mantle II (EM II) and high-mu (HIMU) endmembers of Zindler & Hart (1986). Peridotites display the largest range in isotopic compositions, which formed in response to variable degrees of partial melting and subsequent enrichment during the intrusion of the ultramafic dikes. The ultra-mafic dikes represent at least four generations. Dike isotopic compositions range from ‘depleted’ to ‘enriched’, relative to Bulk Earth, reflecting the average composition of their source regions and subsequent melt-wall rock interactions. No simple isochronous relationships are observed on the 143Nd/144Nd vs. 147Sm/144Nd diagram for any lithologic group, demonstrating that the rock assemblages were not created by simple mixing of two homogeneous endmembers. Some ultramafic dikes, which represent low-temperature melt fractions, are enriched in the heavy isotopes of Sr, Nd, and Pb; any magmas derived from partial melting of these dikes are likely to have enriched isotopic signatures, particularly when the degrees of partial melting are only a few per cent.