Rock Magnetic and Mineralogical Analysis of IODP Expeditions 390 and 393 Basement Cores and their Implications for Fluid-Rock Interaction along the Mid-Atlantic Ridge Flank

During the International Ocean Drilling Program (IODP) expeditions 390 and 393 - also referred to as the South Atlantic Transect (SAT), basement cores have been drilled from a total of 6 holes which penetrates around 150 – 300 meters of the uppermost South Atlantic seafloor. The cores mainly consist of basalts of varying age (~7 to 61 Ma) and alteration states. Some intervals of sedimentary breccia were found in older cores as well. Analyzing how these rocks have been altered and characterizing it based on age and depth are crucial to understanding how the oceanic crust along the South Atlantic has evolved throughout spreading and how fluid-rock interaction has influenced the process. Magnetic minerals can be a useful proxy of such alteration as its effects can be observed in both rock magnetism and mineralogy.   

In this study, we observed the composition and microstructure of magnetic minerals within basaltic samples of varying alteration degrees using a scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS). These observations were then compared with shipboard rock magnetic data to determine which property shows correlation with the mineralogical characteristics. Magnetic minerals within the SAT basalt samples are mostly titanomaghemites with Fe content of 20 – 36 at% and Ti content of 5 – 13 at%. Ti content is generally lower in more altered samples, with some highly oxidized samples showing very low percentage (< 3 at%). In addition, heavily altered samples show smaller (< 3 μm) and elongated magnetic mineral grains. Such mineralogical properties show correlation with key rock magnetic properties such as magnetic susceptibility and coercivity of remanence (B_cr). It is also notable that samples with high alteration degree also show reversals in remanence directions caused by strong secondary magnetization that persists after 20 mT demagnetization. This finding implies that production of secondary magnetic minerals may have occurred along with the oxidation of existing grains during the alteration of basalts.