Formation of Highly Refractory Dunite by Focused Percolation of Pyroxenite-Derived Melt in the Balmuccia Peridotite Massif (Italy) MAURIZIO MAZZUCCHELLI 1 *, GIORGIO RIVALENTI 1 , DANIELE BRUNELLI 1 , 4 , ALBERTO ZANETTI 2 AND ELENA BOARI 3 1 DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITA ' DI MODENA E REGGIO EMILIA, P.LE S. EUFEMIA 19, I-41100 MODENA, ITALY 2 CNR^ISTITUTO DI GEOSCIENZE E GEORISORSE, SEZIONE DI PAVIA, VIA FERRATA 1, I-27100 PAVIA, ITALY 3 DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITA ' DEGLI STUDI DI FIRENZE, VIA LA PIRA 4, I-50121 FIRENZE, ITALY 4 CNR^ISTITUTO DI SCIENZE MARINE, SEZIONE DI BOLOGNA, VIA GOBETTI 101, I-40129 BOLOGNA, ITALY RECEIVED FEBRUARY 29, 2008; ACCEPTED OCTOBER 3, 2008 ADVANCE ACCESS PUBLICATION NOVEMBER 14, 2008 A 50 m thick and 150 m long dunite body occurs as a subconcordant, tabular structure in the Balmuccia Massif, an Alpine peridotite thought to represent part of the subcontinental mantle.The contacts with the host spinel-facies depleted lherzolite are sharp.The dunite body is composed of spinel-rich dunite containing centimetre-size lenses of relict Cr-diopside websterite, spinel-poor granoblastic dunite and virtually monomineralic Cr-spinel layers exhibiting flow structures. Orthopyroxene is a minor, relict phase in allthe litholo- gies; clinopyroxene is intergranular and amphibole is a minor acces- sory phase. Overall the dunite body is fairly refractory (Fo in olivine: 90 7^938). Strontium and neodymium isotope ratios of clinopyrox- ene separates from the dunitic body resemble those of a Cr-diopside websterite suite that forms a series of dykes cutting the main perido- tite host. It is proposed that the dunites weregenerated in a part of the mantle veined by early Cr-diopside websterites by a three-stage process involving partial melting of pyroxenite, infiltration of the pyroxenite-derived melt into the depleted lherzolite and its consequent open-system partial melting and focused flow of the resultant partial melts leading to the production of reactive dunite channels through both peridotite and pyroxenite. This process has been simulated using pMELTS assuming that the pyroxenite partially melts at 1 5 GPa and focused melt transport occurs at pressures greater than 0 7 GPa. The results show that, depending on the focusing factor assumed, dunite can form from peridotite at P 51 2 GPa and from pyroxenite at P 51 1 GPa, in both cases over a large pressure range. The model accounts for specific characteristics of the dunite, such as its refractory composition, the presence of orthopyroxene relics, the occurrence of relict websterite lenses in the spinel-rich dunites and the flow structures in the Cr-spinel layers.The proposed mechanism allows dunite formation to occur well within the spinel stability field, and therefore at greater depth than dunites in ophiolites, which generally formed within the plagioclase stability field. The aggregated model melts extracted from the segments where dunite forms are high-Mg alkali basalts resembling, after olivine fractionation, the compositions of enriched-type mid-ocean ridge basalt from slow- and ultraslow-spreading ocean ridges. KEY WORDS: Balmuccia; dunite; focused flow; pyroxenite melting; subcontinental mantle INTRODUCTION Tabular dunite bodies occurring in ophiolites are held to be the result of the reaction between the ambient mantle peridotite and melts flowing in high-porosity channels (Bodinier, 1988; Kelemen & Dick, 1995; Suhr, 1999; Morgan & Liang, 2005). In the mantle system the olivine primary field expands at the expense of pyroxenes at decreasing pressure (O’Hara, 1968; Stolper, 1980). *Corresponding author.Telephone: þ39-059-2055820. Fax: þ39-059-2055887. E-mail: maurizio.mazzucchelli@unimore.it ß The Author 2008. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@ oxfordjournals.org JOURNAL OF PETROLOGY VOLUME 50 NUMBER 7 PAGES 1205^1233 2009 doi:10.1093/petrology/egn053