Nature of the continental lithosphere beneath NW Spitsbergen: A petrological and geochemical study mantle-derived peridotite xenoliths

Abstract

The youngest volcanic event known from Svalbard is represented by the Bockfjorden Volcanic Complex, located northwest on Spitsbergen at the passive continental margin. Mantle-derived xenoliths from Bockfjorden Volcanic Complex provide petrological insight to the mantle conditions beneath Svalbard, the northwestern extremity of the Eurasian lithospheric plate. New methods are available for pressure and temperature estimates of mantle rocks. These methods provide the possibility of revising the constructed lithosphere-asthenosphere boundary for northwestern Svalbard. Earlier studies have revealed evidence for metasomatic events, reflecting the lithospheric evolution and causing heterogeneity within the lithospheric mantle. The discovered pyroxenites are poorly studied and questions regarding their nature and origin remain unanswered. This integrated study is based on a new collection of mantle-derived xenoliths from Bockfjorden Volcanic Complex. Geochemical data is used for new pressure and temperature estimates and investigating the nature of mantle heterogeneity.

A large collection of mantle-derived xenoliths was obtained from all volcanic centers of the Bockfjorden Volcanic Complex during fieldwork in relation to this thesis. The samples have been petrographically investigated revealing the presence of spinel bearing lherzolites and harzburgites with crosscutting spinel bearing olivine websterite. Major and trace element abundances were obtained for Al-in-olivine geothermometry, Ca-in -spinel geobarometry and an additional two-pyroxene geothermometry. The pressure and temperature estimates by this study indicates heat flow conditions of 65 ± 5 mW/m2 and a petrological lithosphere-asthenosphere boundary at 80 ± 10 km depth. The findings are compared to the existing geophysical lithosphere-asthenosphere boundary which is slightly (20 km) shallower. All studies of the lithosphere-asthenosphere boundary and heat flow condition for northwestern Svalbard reflects a relatively thin lithosphere, comparable to a continental rift tectonic setting.

Whole rock highly siderophile elements systematics, including Re-Os isochron and Re-depletion ages, define Paleoproterozoic to Mesoproterozoic ages for both peridotites and pyroxenites. The websteritic veins are likely formed in relation to the initial melt depletion and separation of the lithospheric mantle from the convecting mantle. There is a difference in Re-depletion ages from the volcanic center of Sverrefjellet (2.2 Ga) to the volcanic centers of Halvdanpiggen and Sigurdfjellet (1.2 Ga). These are situated on either side of the Breibogen Fault Zone which may represent a boundary between two different blocks of lithospheric mantle. Osmium concentrations reveal that the lithospheric mantle below Svalbard is different than below the Greenlandic craton, Lena Trough and Gakkel Ridge.