ESTIMATING SUB-SURFACE OXIDATION PROCESSES THROUGH THE ABUNDANCE OF SUPERPARAMAGNETIC GRAINS IN A CONTINENTAL OPHIOLITE COMPLEX: A MULTISCALE CORRELATIVE STUDY

摘要

The hydration of mantle rocks provides one of essential mechanisms for allowing habitability inthe deep biosphere. In particular, the low temperature serpentinisation process results in a seriesof mineralogical transformations, which produce magnetite and hydrogen at each step of thebreakdown sequence. Microorganisms in the deep subsurface influence many of these stepsand can be observed in seasonal fluxes in biological, and outcrop scale electrical conductivityvariations at the Cost Range Ophiolite Microbial Observatory . This paper will focus onconnecting seasonal observations with serpentinite breakdown products observed in a correlativestudy of thin section samples taken from drill cores at the observation site. We conductedBackscattered electron (BSE) mapping (Figs. 1a and 1c), quantitative evaluation of minerals byscanning electron microscopy (QEMSCAN) (Fig. 1b), multiscale EDS phase mapping,FIB-nanotomography (Fig. 1d), and STEM-EELS to identify and quantify the superparamagneticminerals that contribute to the measured magnetic susceptibility signals in our rock samples. Largearea BSE and iron maps generated through the QEMSCAN technique identified several regions ofiron enrichment not associated with olivine (Figs. 1a and 1b). Using the FIB, cross-sectioningthrough the region highlighted in Fig. 1c, reveals a complex breakdown microstructure as theserpentine continues to breakdown into chlorite and talc (Fig. 1d). On-going investigations areunderway to better localise regions in this breakdown texture, which are rich in superparamagneticmagnetite grains (<100 nm major axis). This correlative approach will improve the estimation ofbiogenic hydrogen production based on the abundance of sub-100 nm magnetite particles.Measuring magnetite production in this layer develops a new quantitative tool for the accurateestimation of bulk deep-biomass hosted by in situ serpentinisation processes.