Mineralogy of Neogene Mudrocks from ANDRILL AND-1B: A Combined Record of Long-Term Provenance Changes and Shorter-Term Weathering Effects

The goal of the ANDRILL McMurdo Ice Shelf (MIS) Project is to define the Late Neogene tectonic, volcanic, paleoclimatic, and paleoenvironmental histories of the NW sector of the Ross Ice Shelf. To achieve this goal, a 1284.87 m-long drillcore (AND-1B) was recovered from beneath the present-day MIS during October - December 2006. The core contains a range of lithologies, including diamictites, sandstones and mudstones (some clast-bearing), diatomites, volcanic ash/tuff, and one lava flow. The mineralogy of fine-grained terrigenous sediments in AND-1B was analyzed by x-ray diffractometry (XRD) of pressed powders to investigate sediment provenance and terrestrial weathering conditions. Data from 86 samples define both large-scale compositional patterns through AND-1B and more detailed compositional variations within obliquity-paced Pliocene glacial/interglacial cycles. In scans of unglycolated pressed powders, the most abundant phases generally are quartz and feldspar. The quartz/feldspar ratios are consistent with the relative importance of older mafic volcanic vs. ``basement’’ sources identified for each lithostratigraphic unit from sandstone and clast analyses. In scans of glycolated pressed powder samples, the relative abundance of illite generally follows the pattern of the quartz/feldspar ratio, whereas the relative abundance of expandable clays (``smectites’’) follows the abundance of the ``contemporaneous’’ volcanic component identified in smear slides. As a result, the long-term compositional pattern in AND-1B is dominated by sediment provenance - older volcanics vs. ``basement’’ vs. contemporaneous volcanics. The distribution of illite and chlorite at shorter timescales, however, also appears to carry a record of paleoweathering conditions. Chlorite generally is interpreted as the product of physical weathering processes, which are expected to dominate in cold glacial settings. In contrast, illite generally is interpreted as the product of weak-to-moderate chemical weathering processes, which are expected to dominate in more temperate settings. As a result, the relative abundances of illite and chlorite (i.e., the illite/chlorite ratio, or I/C) have been used to investigate the record of the relative importance of physical vs. chemical weathering processes in AND-1B. The I/C ratio in the matrix of AND-1B diamictite samples averages 2.14; the I/C ratio in samples of AND-1B “mudstones with clasts” averages 3.06; and the I/C ratio in samples of AND-1B mudstones averages 4.73. This relationship between sediment type and the average I/C ratio is consistent with the interpreted depositional conditions for each lithofacies; ice influence (or ice proximity) is interpreted to have decreased from the environment(s) of diamictite deposition to the environment(s) where mudstones with clasts were deposited, to the environment(s) of mudstone deposition. Decreased ice influence is expected to have been accompanied by decreased importance of physical weathering processes relative to chemical weathering processes, which would have produced more illite relative to chlorite and thereby increased the associated I/C ratio.