A Petrophysical Predictor for Degree of Serpentinzation

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The Atlantis Massif comprises heterogeneous mafic and ultramafic rocks that characterise slow spreading ridges. The rocks now exposed along this part of the Mid-Atlantic Ridge form deep in the Earth's mantle and therefore have very different chemistry to the lavas typically found at the sea floor. The reactions of these mantle rocks with seawater is known as serpentinization, produce methane, hydrogen and generate heat and these are key reactions that could fuel the initiation of life without light and photosynthesis. The International Ocean Discovery Program will core these rocks during the mission-specific platform Expedition 357 in October-December this year.In the proposed project using data generated as part of this expedition, we seek to investigate and characterise the changes in the rock physical properties following serpentization. Previous studies in the peer-reviewed literature have identified that, during serpentinization, magnetic susceptibility increases as new minerals are generated. The rock volume also increases by up to 30% and hence the rock density will decrease. Our project will test the published hypothesis that magnetic susceptibility is inversely correlated to density and determine whether this can be applied to application to rocks to be sampled during Expedition 357.We will use primary data sources, including magnetic susceptibility and density data measured from whole-round core using a multi-sensor core logger as part of the expedition. The density data from core will be supplemented by moisture and density (MAD) data measured on discrete samples also during the expedition. These physical property (petrophysical) data will be evaluated in tandem with petrographic analyses of thin sections to identify minerals contributing to the magnetic susceptibility signal and their modal proportions. The petrographic analysis will examine the mineralogy using the shipboard thin sections first. A subset used to guide sampling for generating polished, uncovered thin sections to analyse using the electron microprobe. The microprobe will be key for documenting changing Fe-Ni oxide phase relations with changing degree of serpentinization where grain size is small.By integrating physical property measurements with lithological and mineralogical information, we aim to develop a 'degree of serpentinization' predictive tool based on physical properties inputs that could be applied in situations where only physical property data is available.