MAGNETIC MINERALOGY OF THE BB2 SERIES SEDIMENT CORES FROM SUMMER LAKE, OR

Three oriented piston sediment cores were retrieved in the Fall of 2009 with the purpose of resolving the question surrounding the existence of the Mono Lake Excursion. Prior to paleomagnetic analysis the sediments were analyzed to determine the magnetic mineralogy of the Summer Lake, OR sediments, including the possible presence of greigite, a magnetic mineral that would likely be responsible for an overprinted signal. This research builds on the original B&B core taken from the same location in 1992, whose mineralogy was tentatively determined by reflective light microscopy to be magnetite (Fe₃O₄), titanomagnetite (Fe_3-xTi_xO₄). and greigite (Fe₃S₄).

Samples were taken from areas of low, medium, and high susceptibility and the magnetic minerals were separated from the rest of the sediment using a shaped glass tube and neodymium-based magnets. The magnetic separates were then subjected to both XRD and SEM/EDX analysis. The XRD analysis strongly suggests that the primary magnetic mineral is a magnetite-series spinel of the magnesioferrite-magnetite solid solution series. Diffraction peaks attributed to greigite were not present in the x-ray scan. The SEM analysis first focused on detecting sulfur using an EDS element-mapping program. Rare areas of significant sulfur content were subjected to further investigation leading to the discovery of several spinel group shaped iron sulfides that may be greigite. These particles were found to be sub-micron in size and extremely rare. Subsequent analyses focused on identifying elemental constituents of the most common magnetic minerals associated with a volcanic provenance and grain-sizes that ranged from 0.1 to 20 µm. Based on semiquantitative elemental weight percents in the mineral grains large enough to analyze (>5 µm), the dominant magnetic mineralogy of the Summer Lake sediments was found to be consistent with (titano)magnetite containing an appreciable amount of magnesium substitution. Magnetic grains large enough to observe (>0.1 µm), but too small to analyze, (<5 µm) had similar morphologies to the (titano)magnetites.