Metasomatic Control Of Water Distribution In The Lithospheric Mantle Of Spitsbergen,northern Norway

The lithospheric mantle is an important reservoir of water on Earth,which nominally anhydrous minerals?NAMs?,such as olivine,pyroxene and garnet,could contain a small amount of water as hydroxyl in their crystal lattice defects.Such water plays an important role in mantle geodynamics and is an essential part in the global water cycle.The distribution of water in lithospheric mantle can be affected by partial melting and metasomatism.Generally,partial melting decreases water contents in minerals,and metasomatism could alter the water contents as well.The change of mineral water content can significantly affect the physical and chemical properties of mantle rocks,leading to affect the mantle processes.Spitsbergen is the largest island of Svalbard archipelago,which is located in the north of Norway.Mantle metasomatism was prevailing processes in its lithospheric mantle.And it provides a unique opportunity to understand the influence of water on geological processes and mantle evolution.In this study,13 mantle xenoliths from three quaternary volcanoes in the northwest of Spitsbergen have been analyzed for water.All the samples are spinel lherzolite and harzburgite except that one is wehrlite.In order to study the effect of geological process on the water content of mantle minerals,Fourier transform infrared spectroscopy?FTIR?was used to quantitatively analyze the water contents in olivine,orthopyroxene and clinopyroxene.The trace element compositions of clinopyroxene were measured by LA-ICP-MS.Water loss occurred at mineral grain boundaries due to degassing during magma emplacement.However,at the center of grain,water is uniformly distributed.The water contents at the centers of olivine,orthopyroxene and clinopyroxene are 1-10 ppm,130-290 ppm and 350-560 ppm respectively.The water contents are linearly correlated,suggesting that the minerals are in equilibrium and preserved the mantle value of Spitsbergen.No obvious correlation between the water content of Spitsbergen minerals and the indices of mantle partial melting?forsterite content of olivine,spinel Cr#,etc.?has been observed,which suggests that partial melting cannot be the main factor of controlling water content.The mantle metasomatism of Spitsbergen which controls the water content in mantle rocks is related to the distance.Positive correlations between water contents and strongly incompatible trace elements?La,Th?have been observed in the samples far away from the metasomatic melt conduits.This suggests that metasomatism dominates the distribution of water.The samples near the metasomatic melt conduits could have reached chemical equilibrium with the melt.The water distribution may be affected by the fractionation of amphibole,evidenced by the correlation between clinopyroxene Nb and NAMs water contents.In summary,during the same metasomatic event,Spitsbergen lithospheric mantle has experienced different degrees of hydration.The source of metasomatic melt can be inferred by H₂O/Ce ratio.The H₂O/Ce ratios of the melt in equilibrium with clinopyroxene?near metasomatic melt?are 93-218,which is in the range of oceanic island basalt.This is consistent with Sr-Nd isotopic results,indicating that the source of metasomatic melt of Spitsbergen may be similar to that of oceanic island basalt.The lithospheric mantle of Spitsbergen has been believed to have been thinned.The mantle viscosity calculated using olivine water contents are 5.1×10²²?5.2×10²⁵,which is much higher than that of asthenosphere.This indicates that the strength of Spitsbergen lithospheric mantle can not be weakened by water.Thus,water is not but temperature is the major factor leading to the thinning of Spitsbergen lithospheric mantle.