Nicole A. Stroncik

• The geochemical composition of oceanic basalts provides us with a window into the distribution of geochemical elements within the Earth’s mantle in space and time. In conjunction with a throughout knowledge on how the different elements behave e.g. during melt formation and evolution or on their partition behaviour between e.g. minerals and melts this information has been transformed into various models on how oceanic crust is formed along plume influenced or normal mid-ocean ridge segments, how oceanic crust evolves in response to seawater, on subduction recycling of oceanic crust and so forth. The work presented in this habilitation was aimed at refining existing models, putting further constraints on some of the major open questions in this field of research while at the same time trying to increase our knowledge on the behaviour of noble gases as a tracer for melt formation and evolution processes. In the line of this work the author and her co-workers were able to answer one of the major questions concerning the formation ofThe geochemical composition of oceanic basalts provides us with a window into the distribution of geochemical elements within the Earth’s mantle in space and time. In conjunction with a throughout knowledge on how the different elements behave e.g. during melt formation and evolution or on their partition behaviour between e.g. minerals and melts this information has been transformed into various models on how oceanic crust is formed along plume influenced or normal mid-ocean ridge segments, how oceanic crust evolves in response to seawater, on subduction recycling of oceanic crust and so forth. The work presented in this habilitation was aimed at refining existing models, putting further constraints on some of the major open questions in this field of research while at the same time trying to increase our knowledge on the behaviour of noble gases as a tracer for melt formation and evolution processes. In the line of this work the author and her co-workers were able to answer one of the major questions concerning the formation of oceanic crust along plume-influenced ridges – in which physical state does the plume material enter the ridge? Based on submarine volcanic glass He, Ne and Ar data, the author and her co-workers have shown that the interaction of mantle plumes with mid-ocean ridges occurs in the physical form of melts. In addition, the author and her co-workers have also put further constraints on one of the major questions concerning the formation of oceanic crust along normal mid-ocean ridges – namely how is the mid-ocean ridge system effectively cooled to form the lower oceanic crust? Based on Ne and Ar data in combination with Cl/K ratios of basaltic glass from the Mid-Atlantic ridge and estimates of crystallisation pressures they have shown, that seawater penetration reaches lower crustal levels close to the Moho, indicating that hydrothermal circulation might be an effective cooling mechanism even for the deep parts of the oceanic crust. Considering subduction recycling, the heterogeneity of the Earth’s mantle and mantle dynamic processes the key question is on which temporal and spatial scales is the Earth’s mantle geochemically heterogeneous? In the line of this work the author along with her co-workers have shown based on Cl/K ratios in conjunction with the Sr, Nd, and Pb isotopes of the OIBs representing the type localities for the different mantle endmembers that the quantity of Cl recycled into the mantle via subduction is not uniform and that neither the HIMU nor the EM1 and EM2 mantle components can be considered as distinct mantle endmembers. In addition, we have shown, based on He, Ne and Ar isotope and trace-element data from the Foundation hotspot that the near ridge seamounts of the Foundation seamount chain formed by the Foundation hotspot erupt lavas with a trace-element signature clearly characteristic of oceanic gabbro which indicates the existence of recycled, virtually unchanged lower oceanic crust in the plume source. This is a clear sign of the inefficiency of the stirring mechanism existing at mantle depth. Similar features are seen in other near-axis hotspot magmas around the world. Based on He, Sr, Nd, Pb and O isotopes and trace elements in primitive mafic dykes from the Etendeka flood basalts, NW Namibia the author along with her co-workers have shown that deep, less degassed mantle material carried up by a mantle plume contributed significantly to the flood basalt magmatism. The Etendeka flood basalts are part of the South Atlantic LIP, which is associated with the breakup of Gondwana, the formation of the Paraná-Etendeka flood basalts and the Walvis Ridge - Tristan da Cunha hotspot track. Thus reinforcing the lately often-challenged concept of mantle plumes and the role of mantle plumes in the formation of large igneous provinces. Studying the behaviour of noble gases during melt formation and evolution the author along with her co-workers has shown that He can be considerable more susceptible to changes during melt formation and evolution resulting not only in a complete decoupling of He isotopes from e.g. Ne or Pb isotopes but also in a complete loss of the primary mantle isotope signal. They have also shown that this decoupling occurs mainly during the melt formation processes requiring He to be more compatible during mantle melting than Ne. In addition, the author along with her co workers were able to show that incorporation of atmospheric noble gases into igneous rocks is in general a two-step process: (1) magma contamination by assimilation of altered oceanic crust results in the entrainment of air-equilibrated seawater noble gases; (2) atmospheric noble gases are adsorbed onto grain surfaces during sample preparation. This implies, considering the ubiquitous presence of the contamination signal, that magma contamination by assimilation of a seawater-sourced component is an integral part of mid-ocean ridge basalt evolution.…