Data report: clay mineral assemblages within trench-floor and accreted trench-floor deposits, IODP Expedition 372B/375 Sites U1518 and U1520, offshore New Zealand

Abstract

The frontal accretionary prism of the Hikurangi subduction margin is composed mostly of interbeds of hemipelagic mud and silty turbidites that were deposited on the floor of Hikurangi Trough during the Pleistocene. Expedition 375 of the International Ocean Discovery Program (IODP) included coring those deposits at Sites U1518 and U1520, which are located on the frontal accretionary prism and the trench wedge, respectively. This report provides the results of 208 X-ray diffraction analyses of the clay-sized fraction (<2 mm spherical settling equivalent). Sampling focused on the background lithology of hemipelagic mud. Normalized weight percent values for common clay-sized minerals (where smectite + illite + undifferentiated [chlorite + kaolinite] + quartz = 100%) exhibit unusual amounts of scatter in all of the lithostratigraphic units. Furthermore, the results reveal neither depth-dependent trends nor excursions at unit boundaries, and compositional differences among sites, lithologic units, and subunits are insignificant. At Site U1520, the mean (µ) and standard deviation (σ) values for Units I–III are smectite = 31.4 wt% (σ = 11.1), illite = 41.5 wt% (σ = 6.9), chlorite + kaolinite = 11.6 wt% (σ = 3.6), and quartz = 15.5 wt% (σ = 6.1). At Site U1518, the mean and standard deviation values for Units I–III are smectite = 38.4 wt% (σ = 9.8), illite = 41.5 wt% (σ = 6.9), chlorite + kaolinite = 11.8 wt% (σ = 4.1), and quartz = 8.3 wt% (σ = 2.3). Indicators of clay diagenesis are relatively monotonous throughout the cored intervals. The average value of the illite crystallinity index is 0.485Δ°2θ (σ = 0.036) at Site U1520 and 0.517Δ°2θ (σ = 0.020) at Site U1518. Smectite expandability averages 70.4% (σ = 7.2) at Site U1520 and 75.2% (σ = 6.0) at Site U1518. At Site U1520, the average proportion of illite in illite/smectite mixed-layer clay is 12.6% (σ = 6.5); the comparable values at Site U1518 are µ = 10.3% and σ = 5.0.