Sedimentological and Geochemical Characteristics of Leg 129 Siliceous Deposits

Abstract

Siliceous deposits drilled on Ocean Drilling Program Leg 129 accumulated within a few degrees of the equator during the Jurassic through early Tertiary, as constrained by paleomagnetic data. During the Jurassic and Early Cretaceous, radiolarian ooze, mixed with a minor amount of pelagic clay, was deposited near the equator, and overall accumulation rates were moderate to low. At a smaller scale, in more detail, periods of relatively higher accumulation rates alternated with periods of very low accumulation rates. Higher rates are represented by radiolarite and limestone; lower rates are represented by radiolarian claystone. Our limited data from Leg 129 suggests that accumulation of biogenic deposits was not symmetrical about the equator or consistent over time. In the Jurassic, sedimentation was siliceous; in the Cretaceous there was significant calcareous deposition; in the Tertiary claystone indicates significantly lower accumulation rates at least the northern part of the equatorial zone. Accumulation rates for Leg 129 deposits in the Cretaceous were higher in the southern part of the equatorial zone than in the northern part, and the southern side of this high productivity zone extended to approximately 15°S, while the northern side extended only to about 5°N.Accumulation rates are influenced by relative contributions from various sediment sources. Several elements and element ratios are useful for discriminating sedimentary sources for the equatorial depositional environments. Silica partitioning calculations indicate that silica is dominantly of biogenic origin, with a detrital component in the volcaniclastic turbidite units, and a small hydrothermal component in the basal sediments on spreading ridge basement of Jurassic age at Site 801. Iron in Leg 129 sediments is dominantly of detrital origin, highest in the volcaniclastic units, with a minor hydrothermal component in the basal sediments at Site 801. Manganese concentrations are highest in the units with the lowest accumulation rates. Fe/Mn ratios are >3 in all units, indicating negligible hydrothermal influence. Magnesium and aluminum concentrations are highest in the volcaniclastic units and in the basal sediments at Site 801. Phosphorous is very low in abundance and may be detrital, derived from fish parts. Boron is virtually absent, as is typical of deep-water deposits. Rare earth element concentrations are slightly higher in the volcaniclastic deposits, suggesting a detrital source, and lower in the rest of the lithologic units. Rare earth element abundances are also low relative to "average shale." Rare earth element patterns indicate all samples are light rare earth element enriched. Siliceous deposits in the volcaniclastic units have patterns which lack a cerium anomaly, suggesting some input of rare earth elements from a detrital source; most other units have a distinct negative Ce anomaly similar to seawater, suggesting a seawater source, through adsorption either onto biogenic tests or incorporation into authigenic minerals for Ce i...

Protoliths and phase petrology of whiteschists

“…Although palygorskite-bearing clays can also be part of deep-sea sediments (Bonatti and Joensuu 1968;Couture 1977;Church and Velde 1979;Karpoff 1992;Karl et al 1992), these rocks have too high iron, calcium, and alkalielement contents to be potential protoliths for whiteschists.…”

“…The largest amount of the palygorskitesepiolite group minerals forms in epicontinental and inland seas and lakes as chemical sediments, and in calcareous soils by direct crystallization (Callen 1984;Jones and Conko 2011;Murray et al 2011). These deposits often contain m-wide layers of pure clay minerals without additions of alkali elements, calcium or iron (e.g., Karl et al 1992) and, therefore, are highly suited potential protoliths for whiteschists. Palygorskite-and sepiolite-rich clays generated in soils, lakes and shallow marine environment were mostly associated with a Mediterranean or warm, semi-arid climate, that is, in low latitudes (Moine et al 1981).…”

Element transport from slab to volcanic front at the Mariana arc

“…and a large number of existing major and trace element analyses of samples from throughout the core [Karl et al, 1992;Karpoff, 1992]. The budgets of most incompatible elements considered in this study are dominated by the pelagic clay and volcaniclastics, and so representative samples from these units were the focus of further analysis by ICP-MS.…”

Izu-Mariana Margin

“…4). The sediment input is fairly well constrained by previous coring during Leg 129 (Sites 800-802), and by the extensive chemical analyses of the recovered material (Karl et al, 1992;Lees et al, 1992;France-Lanord et al, 1992) as well as the geochemical logs for the different holes (Pratson et al, 1992;Fisher et al, 1992 Peacock (1990), alteration studies of DSDP Hole 504B (Leg 69; Alt et al, 1986) and DSDP Sites 417/418 (Legs 51, 52, and 53; Staudigel et al, 1995), and assuming 10% interpillow material in Hole 801C (Castillo et al, 1992b). These estimates show that the alteration fluxes may be large, but are poorly known.…”

“…Thus, Izu sediments should be much poorer in K and Ba than Marianas sediments. One way to explain this would be to replace the volcaniclastic sections in the Marianas sediment columns with cherts, which are barren of K and may be very poor in Ba (Karl et al, 1992). This makes some sense given what we know about the history of sedimentation in this part of the ocean-the Cretaceous overprint east of the Marianas may be absent to the north, east of Izu ( Fig.…”