Highly siderophile element geochemistry of mantle pyroxenites
Final Report Abstract
The purpose of this study was to evaluate the contribution of mantle pyroxenites to the HSE inventory of the mantle, to constrain the processes of distribution of the HSE during magma-peridotite interaction and to evaluate the impact pyroxenites might have as lithologies in the source regions of plume derived melts. Websterites from the Lanzo and Lherz ultramafic bodies display relatively unfractionated HSE patterns similar to fertile lherzolites. Neither simple sulfid/silicate nor mss/sulfide melt partitioning seem to be able to explain the observed HSE abundances in Lanzo and Lherz pyroxenites. Therefore lowered, more empirical sulfide/silicate HSE partition coefficients under disequilibrium conditions were assumed for the facilitated sulfide/silicate partitioning model. Modelling silicate melt/sulfide melt HSE partitioning in websterites implies a reaction of 20-40% melt (derived by 10% melting of peridotite) with a fertile peridotitic wall rock. In contrast, clinopyroxenites show MORB like HSE compositions with an enrichment of Pt, Pd, Au, Re over Os, Ir, Ru, Rh. Modelling silicate melt/ sulfide melt HSE partitioning resulted in a reaction of 60- 90% partial melt with peridotite. Hence, compared to the websterites, the HSE composition of clinopyroxenites suggests less interaction and contamination of their parent melts with wall rock peridotite. The concentrations of Pd, Au and Re in pyroxenites from Lanzo and Lherz are only rarely higher than in the primitive mantle and peridotites. Thus the significance of mantle pyroxenites for the HSE budget of the mantle may be that they provide localized reservoirs of enhanced Re and radiogenic Os, and therefore complement mantle domains comprised of depleted peridotites. Out of 39 samples only two have the required Pt/Re of 88-100 to make them viable sources for the radiogenic 186-Os-187-Os end member observed in plume related picrites and komatiites. Because of their low volumetric abundance in the mantle (<10%) and the small numbers of pyroxenites displaying the required Pt and Re abundances it appears unlikely that pyroxenites are the cause of the suprachondritic 186-Os-187-Os in plume-related picrites and komatiites. Most sulfides from pyroxenites from Lanzo and Lherz show variable depletions of Pt and Au (at Lanzo also Pd) relative to other HSE, likely reflecting sub- solidus formation of secondary sulfides, Te-Bi phases, alloys and native Au. Some sulfides in pyroxenites have similar HSE ratios as sulfides in peridotites whereas others display a more MORB like HSE signature. Varying Pd/Ir, Re/Os and Ru/Ir in sulfides on the thin section scale reflect entrainment of sulfide melt from peridotite mixed with sulfides from reacted melt. The range of HSE in sulfides indicates multistage melt infiltration and disequilibrium between different populations of sulfides in pyroxenites.
Publications
- (2011): Highly siderophile element abundances and Re-Os model ages of pyroxenite layers from the Lanzo peridotite (northern Italy). Abstract V31D-2555 presented at 2011 Fall Meeting, AGU, San Francisco, Calif., 5-9 Dec
Gawronski T. and Becker, H.
- (2011): Highly siderophile element and Os isotope systematics of pyroxenite layers from the Lanzo peridotite body (northern Italy). Mineralogical Magazine, 75(3) 900
Gawronski T. and Becker, H.
- (2012): HSE and Os isotope systematics of mantle pyroxenites from the Lherz and Lanzo ultramafic massifs. Mineralogical Magazine, 76(6) 1744
Gawronski T. and Becker, H.
- (2013): Peridotite- derived sulfides in pyroxenites from the Lanzo and the Lherz ultramafic massifs? Mineralogical Magazine, 77(5) 1149
Gawronski T. and Becker, H.
- 2013. Partial re-equilibration of highly siderophile elements and the chalcogens in the mantle: A case study on the Baldissero and Balmuccia peridotite massifs (Ivrea Zone, Italian Alps). Geochimica et Cosmochimica Acta, 108: 21-44
Wang, Z., Becker, H., Gawronski, T.
(See online at https://doi.org/10.1016/j.gca.2013.01.021)
