Sr, Nd, Hf isotopic analyses of separated clinopyroxenes and in situ Re-Os isotopic analysis of sulphides in mantle-peridotite xenoliths from Baker Rocks (BR) and Greene Point (GP), less than 100 km apart in Northern Victoria Land (NVL), Antarctica, provide further constraints on the evolution of the subcontinental lithospheric mantle beneath NVL and point out the role that eclogitic reservoirs may have played in the metasomatism and magmatism of the area. Most of the BR sulphides have radiogenic 187Os/188Os (0.1318 – 0.379 with 187Re/18824 Os ratios between 0.46 and 3.3), while unradiogenic 187Os/18825 Os characterizes the GP suite (0.1068 – 0.1279 with 187Re/188Os ratios from 0.0002 to 0.045). In BR silicates 8726 Sr/86Sr varies between 0.70296 and 0.70488, 143Nd/144Nd lies within a narrow range (0.51271 – 0.51296), and 176Hf/17727 Hf ranges from 0.28300 to 0.28337. Cpx from GP have similar 87Sr/86Sr (0.70277 to 0.70434), 143Nd/14428 Nd between 0.51261 and 0.51347, and 176Hf/17729 Hf between 0.28332 and 0.28519. Notwithstanding the rather limited number of Os-rich sulphides in the GP suite, Os model ages for BR and GP largely overlap. A histogram of TRD (Time of Rhenium Depletion) model ages shows peaks at 3.0 – 3.3 Ga, 2.3 Ga, 1.3 – 1.4 Ga, 0.9 – 1.1 Ga, 580 – 620 Ma and 120 Ma (the youngest being recorded only at BR). The highly radiogenic Os found in the BR xenoliths cannot be explained by contamination from the host basalts, unless they were derived from a source which was enriched in Re far in the past. Alternatively they can be explained through mixing with an eclogitic lithotype. Eclogite may have been introduced into the sublithospheric NVL mantle during the Ross Orogeny (550 Ma) and 38 reactivated during the opening of the West Antarctic Rift System. In this case up to 60% of recycled material would be required to explain the most radiogenic Os-isotope values. Alternatively if older (up to Archean) rocks are envisaged the percentage of mixed material can be reduced to 15%. Based on Hf systematics the most radiogenic GP cpx could also be explained by adding an eclogite percentage varying between 35 and 20%. However in this locality the possible presence of garnet in the source and successive equilibration in the spinel stability field, could also account for the highly radiogenic cpx values.
The role of eclogite in the rift-related metasomatism and Cenozoic magmatism of Northern Victoria Land, Antarctica
MELCHIORRE, Massimiliano;COLTORTI, Massimo;BONADIMAN, Costanza;FACCINI, Barbara;
2011
Abstract
Sr, Nd, Hf isotopic analyses of separated clinopyroxenes and in situ Re-Os isotopic analysis of sulphides in mantle-peridotite xenoliths from Baker Rocks (BR) and Greene Point (GP), less than 100 km apart in Northern Victoria Land (NVL), Antarctica, provide further constraints on the evolution of the subcontinental lithospheric mantle beneath NVL and point out the role that eclogitic reservoirs may have played in the metasomatism and magmatism of the area. Most of the BR sulphides have radiogenic 187Os/188Os (0.1318 – 0.379 with 187Re/18824 Os ratios between 0.46 and 3.3), while unradiogenic 187Os/18825 Os characterizes the GP suite (0.1068 – 0.1279 with 187Re/188Os ratios from 0.0002 to 0.045). In BR silicates 8726 Sr/86Sr varies between 0.70296 and 0.70488, 143Nd/144Nd lies within a narrow range (0.51271 – 0.51296), and 176Hf/17727 Hf ranges from 0.28300 to 0.28337. Cpx from GP have similar 87Sr/86Sr (0.70277 to 0.70434), 143Nd/14428 Nd between 0.51261 and 0.51347, and 176Hf/17729 Hf between 0.28332 and 0.28519. Notwithstanding the rather limited number of Os-rich sulphides in the GP suite, Os model ages for BR and GP largely overlap. A histogram of TRD (Time of Rhenium Depletion) model ages shows peaks at 3.0 – 3.3 Ga, 2.3 Ga, 1.3 – 1.4 Ga, 0.9 – 1.1 Ga, 580 – 620 Ma and 120 Ma (the youngest being recorded only at BR). The highly radiogenic Os found in the BR xenoliths cannot be explained by contamination from the host basalts, unless they were derived from a source which was enriched in Re far in the past. Alternatively they can be explained through mixing with an eclogitic lithotype. Eclogite may have been introduced into the sublithospheric NVL mantle during the Ross Orogeny (550 Ma) and 38 reactivated during the opening of the West Antarctic Rift System. In this case up to 60% of recycled material would be required to explain the most radiogenic Os-isotope values. Alternatively if older (up to Archean) rocks are envisaged the percentage of mixed material can be reduced to 15%. Based on Hf systematics the most radiogenic GP cpx could also be explained by adding an eclogite percentage varying between 35 and 20%. However in this locality the possible presence of garnet in the source and successive equilibration in the spinel stability field, could also account for the highly radiogenic cpx values.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
