Ponencia
Platinum-group elements and Os isotopic evidence for a depleted mesoproterozoic lithospheric mantle beneath the Deseado Massif, southern Patagonia
Autor
Schilling-Danyau, Manuel Enrique
Carlson, Richard
Conceição, Rommulo
Bertotto, Gustavo
Jalowitzki, Tiago
Gervasoni, Fernanda
Morata, Diego
Institución
Resumen
The Re-Os isotopic system is utilized in mantle rocks to constrain the time of major mantle melting episodes, which are commonly related to significant periods of crust formation [1]. A previous study based on this geochemical approach was performed on 29 mantle xenoliths from a wide area of southern South America, and coherent model ages for the lithospheric mantle and known cristal formation ages where observed in most cases [2]. However, in the southwestern edge of the Deseado Massif (DM), Patagonia,
Proterozoic Os depletion ages (1.34 to 2.11 Ga) were obtained that are considerably older than radiogenic formation ages known for the few basements rocks of the DM (~0.6 Ga). We report here new Os isotopic compositions and platinum-group elements (PGEs) contents of 18 mantle xenoliths from the DM and surrounding areas. The 187Os/188Os isotopic ratios of seven mantle xenoliths from the DM range from 0.1139 to 0.1165 (Re-depletion ages range from 1.59 Ga to 2.13 Ga), but one sample has a ratio of 0.1245, rising the average to 0.1168. In contrast, the Os isotopic composition of ten mantle xenoliths from the surrounding areas have a wider range from
0.1145 to 0.1239 (Re depletion ages range from 0.78 to 2.04 Ga), and a higher average of 0.12137. The PGEs systematics for the DM xenoliths show them to be residues of high degrees of partial melting, with progressive depletion of Pt, Pd, and Re (PtN/IrN: 0.5 - 1.0;
106
PdN/IrN 0.1 - 0.7; and ReN/IrN: 0.03 - 0.7; with averages of 0.7, 0.3, and 0.3, respectively). In the surrounding areas, similar depleted
PGE patterns are observed together with patterns having fertile mantle to PPGE enrichment (PtN/IrN: 0.6 - 2.7; PdN/IrN: 0.1 - 1.5, and
ReN/Ir: 0.1 - 1.4, with averages of 1.3, 0.8, and 0.7, respectively).
Thus, our data support previous results, and confirm the presence of an old and depleted lithospheric mantle beneath the DM, which is probably related to the Malvinas/Falkland Islands and plateau, where Mesoproterozoic ages of crustal rocks have been previously reported.
[1] Carlson et al. (2005) Rev. Geophys 29, 31–50.
[2] Schilling et al. (2008) Earth Planet. Sci. Lett. 268, 89-101.