Tectonic Implications of Illite/Smectite Diagenesis, Barbados Accretionary Prism

Schoonmaker, Jane; Mackenzie, Fred T.; Speed, R. C.

doi:10.1346/ccmn.1986.0340413

Cited by 27 publications

(16 citation statements)

References 27 publications

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“…The mineralogical interpretations outlined in this section are consistent with XRD data from the Barbados region (Schoonmaker, 1986;Schoonmaker et al, 1986), which generally indi cate the presence of smectite, illite, and kaolinite/chlorite.…”

Section: Mineralogical and Textural Observationssupporting

confidence: 81%

Backscatter SEM Imagery of Fine-Grained Sediments from Site 671, Leg 110—Preliminary Results

Prior¹,

Behrmann²

1990

Proceedings of the Ocean Drilling Program

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Backscatter imagery, on the scanning electron microscope (SEM), has two major advantages over secondary electron imagery. It gives compositional information, and the planar, polished surfaces required are suitable for accurate and easy two-dimensional quantification of grain sizes, shapes and orientations. Generally backscatter imagery has insuffi cient spatial resolution for examination of the fine-grained matrix of muddy sediments. Previous workers have mostly been limited to secondary electron imagery of broken surfaces and its inherent restrictions. Here we describe specimen preparation and SEM operation procedures that allow resolution of the fine-grained matrix of sediments.For high-resolution backscatterred electron imagery of any specimen a highly polished specimen surface is required. Careful preparation of polished surfaces of sediment specimens from Hole 671B of Leg 110 of the Ocean Drilling Pro gram, combined with an optimization of SEM operating conditions, has given a spatial resolution of 200 to 300 nm and, in some specimens, better than 100 nm. Best resolution is obtained by using a relatively high accelerating voltage (30 kV here) and short working distances (< 7 mm here). The resolution obtained is good enough to allow quantifica tion of clay and clast grain sizes and orientations, while backscatter intensities combined with semi-quantitative X-ray analysis is a powerful technique for identification of different clay phases.A study of the microfabric of the decollement zone of Hole 67IB indicates that there is no preferred alignment of clays or clasts. Thus, deformation must have occurred by independent particuate flow at high pore-fluid pressures. The "scaly fabric" of the decollement zone is defined by fractures that postdate and are unrelated to the microfabric.

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Section: Mineralogical and Textural Observationssupporting

confidence: 81%

Backscatter SEM Imagery of Fine-Grained Sediments from Site 671, Leg 110—Preliminary Results

Prior¹,

Behrmann²

1990

Proceedings of the Ocean Drilling Program

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“…On the Gulf Coast of the United States, for example, the temperature range for onset of illitization extends from 58 to 92 ~ and I/S ratios reach from 70 to 80% illite at temperatures of 88 to 142 ~ (Freed and Peacor 1989a). Tribble (1990) documented an intriguing example from the Barbados accretionary prism, in which illitization apparently begins at temperatures as low as 24 ~ (Schoonmaker et al 1986). Data from the Nankai accretionary prism are well within the anticipated window of burial temperature (60 to 90 ~ Consequently, we see no reason to invoke special hydrogeochemical circumstances to account for the diagenetic trend at Site 808.…”

Section: Conditions Promoting Smectite-to-illite Diagenesismentioning

confidence: 99%

Clay-Mineral Provenance, Sediment Dispersal Patterns, and Mudrock Diagenesis in the Nankai Accretionary Prism, Southwest Japan

Underwood

Pickering

1996

Clays and clay miner.

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Abstract--Offscraped strata within the toe of Nankai accretionary prism display an overall facies pattern of thickening and coarsening upward. Detrital clay minerals within the Quaternary trench-wedge facies are dominated by illite; chlorite is the second-most abundant clay mineral, followed by smectite. Relative mineral percentages change only modestly with depth. The hemipelagic clay-mineral population is virtually identical to clays washed from turbidite matrix, and different size fractions (<2 ixm and 2-6 ixm) show nominal amounts of mineral partitioning. Smectite content increases beneath the trench-wedge deposits, where the upper subunit of the Shikoku Basin stratigraphy (late Pliocene and early Pleistocene) includes abundance of volcanic ash. Syneruptive, subaerial chemical weathering of volcanic source rocks, together with in situ alteration of disseminated glass shards, caused the increase in smectite. Smectite begins a monotonic transformation to illite/smectite (US) mixed-layer clay at --555 mbsf and an estimated temperature of -65 ~ Ordered (R = 1) I/S interlayering first appears at --1220 mbsf (<2 ~m size fraction) and --1100 mbsf (<0.2 ~m size fraction). The illitization gradient coincides with a reduction in pore-water chlorinity, but depth-related changes in bulk mudstone geochemistry (K20, Rb) are subtle. The absolute abundances of discrete smectite and US appear to be insufficient to account for the magnitude of pore-water dilution via in situ dehydration reactions. Instead, pore water probably was transported to Site 808, either from sources located deeper in the accretionary prism, where bulk mudstone porosities are lower, or from strike-parallel sources where mudstones originally deposited in the Shikoku Basin might contain higher percentages of smectite.

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“…On the Gulf Coast, for example, the temperature range extends from 58° to 92°C for onset of illitization, and I/S ratios reach 70% to 80% illite at temperatures of 88° to 142°C (Freed and Peacor, 1989b). In contrast, Tribble (1990) an intriguing example from the Barbados accretionary prism, in which illitization apparently has been accelerated, such that smectite alteration began at temperatures as low as 24°C (see also Schoonmaker et al, 1986). Data from the Nankai accretionary prism are well within the anticipated window of burial temperature (-60° to 90°C).…”

Section: Smectite-illite Diagenesismentioning

confidence: 99%

Sediment Geochemistry, Clay Mineralogy, and Diagenesis: A Synthesis of Data from Leg 131, Nankai Trough

Underwood¹,

Pickering²,

Gieskes³

et al. 1993

Proceedings of the Ocean Drilling Program

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This paper presents a synthesis of data from X-ray diffraction analyses of clay minerals and X-ray fluorescence analyses of bulk mudstones from Ocean Drilling Program Site 808. The samples come from three closely spaced holes drilled through the toe of the Nankai accretionary prism offshore Shikoku, Japan. Detrital assemblages of clay minerals are unusually uniform throughout the Nankai trench-wedge facies. Illite is the most abundant detrital clay mineral, followed by chlorite, smectite, and traces of kaolinite. Relative percentages of smectite increase within the upper subunit of the Shikoku Basin stratigraphy. This subunit contains abundant layers of volcanic ash, and the corresponding change in clay mineralogy probably was caused by synvolcanic weathering in source areas during the Pliocene, together with in-situ alteration of disseminated glass shards within the Shikoku Basin.At a depth of ~555 mbsf, the detrital and/or authigenic smectite component begins its transformation to illite/smectite mixed-layer clay. With increasing depth below this horizon there is a monotonic increase in illite interlayers. The onset of illitization occurs at an estimated temperature of ~65°C. Ordered (R = 1) illite/smectite interlayering first appears at depths of -1220 mbsf (<2-µm size fraction) and -1100 mbsf (<0.2-µm size fraction). The depth interval of the smectite-to-illite transformation coincides with a reduction in pore-water chlorinity; however, the absolute abundance of smectite appears to be insufficient to account for the changes in aqueous chemistry via in-situ dehydration reactions. Instead, significant volumes of diluted pore water probably were transported to Site 808, either from sources located deeper in the accretionary prism (where bulk mudstone porosities are lower) or from lateral sources where mudstones originally deposited in the Shikoku Basin may contain higher percentages of smectite. Significantly, we did not detect any anomalies in clay mineralogy or clay-mineral diagenesis within or near the décollement zone (945-964 mbsf).X-ray fluorescence analyses show that hemipelagic muds and mudstones at Site 808 are chemically uniform throughout most of the section. There are no geochemical perturbations, for example, within the décollement zone. Data from interbeds of volcanic ash demonstrate that the chemical effects of mud/ash dissemination and/or in-situ alteration of pyroclastic material are limited. In addition, ash layers are chemically heterogeneous within Unit III and Subunit IVa, which indicates that tephra was transported from a variety of andesitic to rhyolitic sources on the Japanese Islands during the Pliocene. In contrast, Miocene rocks of Unit V display a clear chemical divergence (little mixing) between rhyolitic tuffs and interbeds of multicolored mudstone.The most significant geochemical anomaly at Site 808 occurs well below the décollement zone between 1087 and 1111 mbsf. Variegated mudstones in this interval contain unusually high ratios of MgO/Al 2 O 3 , Fe total /Al 2 O 3 , Mn...

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Tectonic Implications of Illite/Smectite Diagenesis, Barbados Accretionary Prism

Cited by 27 publications

References 27 publications

Backscatter SEM Imagery of Fine-Grained Sediments from Site 671, Leg 110—Preliminary Results

Backscatter SEM Imagery of Fine-Grained Sediments from Site 671, Leg 110—Preliminary Results

Clay-Mineral Provenance, Sediment Dispersal Patterns, and Mudrock Diagenesis in the Nankai Accretionary Prism, Southwest Japan

Sediment Geochemistry, Clay Mineralogy, and Diagenesis: A Synthesis of Data from Leg 131, Nankai Trough

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