The smectite–chlorite transition in drillhole NJ‐15, Nesjavellir geothermal field, Iceland: XRD, BSE and electron microprobe investigations

Schiffman, P.; Friðleifsson, G. Ó.

doi:10.1111/j.1525-1314.1991.tb00558.x

Cited by 170 publications

(158 citation statements)

References 27 publications

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“…These charaeteristics are different than those of most electron microprobe analyses of low-grade chlorite, for which significant octahedral vacancies are frequently reported (Hillier and Velde 1991), a relation implying contamination by interlayered phases that occur commonly in metabasites (Shau et al 1990;Schiffman and Fridleifsson 1991). These relations and the lack of stable equilibrium as generally recognized (Velde and Medhioub 1988;Peacor 1992;de Caritat et al 1993) and demonstrated above for the Gasp6 chlorite suggest that use of chlorite composition as a geothermometer in low-grade regimes (Cathelineau and Nieva 1985;Cathelineau 1988) is not warranted.…”

Section: Compositional Variation Of Chloritementioning

confidence: 50%

“…The mixed-layer component is identified as C/S with ~ 60% chlorite and R 1 ordering based on the aforementioned features and peak positions. After heating at 375~ a significant decrease in the intensity of the ~ 14-~ reflection is associated with the addition of a high-20 shoulder, and the 7.51-,~ reflection like that shown in Figure 2 was shifted further toward the low 20 side in patterns of the diagenetic samples (Jiang 1993), typical of the characteristics of C/S (Bettison and Schiffman 1988;Schiffman and Fridleifsson 1991). In the pattern of the airdried specimen, the ~10-A peak of illite is slightly asymmetric and has a tail on the low 20 side.…”

Section: X-ray Powder Diffractionmentioning

confidence: 99%

“…The stepwise prograde evolution, the lack of long-range ordering in mixed-layer materials, and the similarity between XRD patterns of random C/S and chlorite/corrensite or corrensite/ smectite seem to imply that corrensite is a unique phase rather than a simple mixture of chlorite and smectite layers, and that random C/S (with a low or high percentage of expandable layers) may be a less stable and less common material than corrensite in prograde sequences (Reynolds 1988;Roberson 1988Roberson , 1989Shau et al 1990). Ordered interstratification of the smectite component in C/S and the proportion ofsmectite layers have been shown to be related to the grade ofdiagenesis and low-grade metamorphism, or to the compositions and thermal history of hydrothermal fluids (Hoffman and Hower 1979;Pollastro and Barker 1986;Kisch 1987;Schiffman and Fridleifsson 1991).…”

Section: Clays and Clay Mineralsmentioning

confidence: 99%

“…Mixed-layer chlorite/smectite (C/S) is the principal clay material that forms under subgreenschist facies conditions in regionally metamorphosed and hydrothermally altered mafic igneous rocks (Alt et al 1986(Alt et al , 1989Bettison and Schiffman 1988;Shau et al 1990;Bettison-Varga et al 199 l;Schiffman and Fridleifsson 1991). It is also common in clastic sedimentary rocks that have been altered by Mg2+-rich fluids or were composed of significant amounts of mafic volcano-…”

Section: Introductionmentioning

confidence: 99%

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Prograde Transitions of Corrensite and Chlorite in Low-Grade Pelitic Rocks from the Gaspé Peninsula, Quebec

Jiang

1994

Clays and clay miner.

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Abstract-A prograde sequence ofcorrensite and chlorite in pelitic rocks of the diagenetic zone, anchizone, and epizone (illite crystallinity indices = 0.17-0.58~ of the Gasp6 Peninsula, Quebec, was studied by analytical and transmission electron microscopy (AEM and TEM). The data collectively suggest that diagenesis/metamorphism of chlorite and corrensite follows a sequence of phase transitions, compositional homogenization, and recrystallization, approaching a state of equilibrium for which chlorite is the stable phase.Corrensite occurs as coalescing, wavy packets of layers intergrown with chlorite and illite in the diagenetic and low-grade anchizonal rocks. Intergrowths of discrete chlorite and corrensite crystals, interstratified packets of chlorite and corrensite layers, terminations of smectite-like layers by chlorite layers, and 2-3 repeats of R2-and R3-ordered chlorite-smectite mixed layers occur. These materials are alteration products of detrital biotite or other precursor phases like trioctahedral smectite. The crystal size and proportion of corrensite decrease significantly from the diagenetic zone to the anchizone. Deformed corrensite is crosscut by straight packets of chlorite and corrensite in the diagenetic sample. Some chlorite occurs as discrete, euhedral to subhedral crystals intergrown with or enclosed by other phases in the absence of corrensite. The crystal size of chlorite and definition of crystal boundaries increase whereas density of crystal imperfections and randomness in orientation decrease with increase in grade of diagenesis/metamorphism. Crystals that are kinked or bent, or display gliding along (001) form low-angle boundaries with relatively defect-free crystals, implying deformation during crystal growth. Abundant well-defined low-angle boundaries associated with dislocations are observed in the higher grade rocks, consistent with a stage of readjustment of crystal boundaries during crystal growth. The AEM analyses show that the corrensite has lower Fe/(Mg + Fe) and A1/(Si + A1) than the coexisting chlorite in the diagenetic sample, and that the ranges of composition of chlorite of different grades overlap and become smaller with increasing grade, implying prograde homogenization.The data imply that corrensite is a unique phase that is metastable relative to chlorite: its conversion to chlorite occurred at a grade as low as that of the high-grade diagenetic zone. The textural relations suggest that the metamorphic crystallization and recrystallization were coeval with deformation processes due to tectonism, partially modified by subsequent contact metamorphism. The data, combined with those of previous reports, suggest that the Gasp60rdovician rocks constitute a part of a regional distribution of trioctahedral phyllosilicate-rich rocks in the northern Appalachians. The regional occurrence of abundant chloritic minerals is thus directly related to a specific tectonic re#me with precursor sediments largely derived from an andesitic arc system(s).

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Section: Compositional Variation Of Chloritementioning

confidence: 50%

Section: X-ray Powder Diffractionmentioning

confidence: 99%

Section: Clays and Clay Mineralsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Prograde Transitions of Corrensite and Chlorite in Low-Grade Pelitic Rocks from the Gaspé Peninsula, Quebec

Jiang

1994

Clays and clay miner.

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“…We used the empirical calibration of Cathelineau and Nieva (1985), as revised by Cathelineau (1988), to estimate the temperature of equilibration of chlorite in the metasedimentary rocks of Hole 917A. As Schiffman and Fridleifsson (1991) cautioned, this thermometer should only be applied to nearly pure chlorites with very low amounts of interlayered smectite. Bettison and Schiffman (1988) showed that chlorite containing more than 6.25 Si cations per formula unit (based on 28 oxygens) include interlayered smectite.…”

Section: Metamorphic Processes and P/t Conditionsmentioning

confidence: 99%

Metamorphosed sedimentary (volcaniclastic?) rocks beneath Paleocene basalt in Hole 917A, East Greenland Margin

Vallier¹,

Calk²,

Stax³

et al. 1998

Proceedings of the Ocean Drilling Program

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Ocean Drilling Program Leg 152 drilled Hole 917A first through Quaternary and Eocene sediments, then through about 779 m of Paleocene basalt and interlayered dacite that comprise the dipping seismic reflectors along the southeast Greenland margin, and thence into about 53.7 m of steeply dipping and metamorphosed sandstone and siltstone. Ten centimeters of quartz sandstone, probably of fluvial origin, was recovered between the basalt and metamorphosed sedimentary rocks. The sedimentary rocks, believed to be in part volcaniclastic, are mostly well-bedded turbidites and laminites and, in places, are intensely burrowed. Minerals are characteristic of the greenschist facies and consist of albite, chlorite, quartz, graphite, white mica, and epidote with lesser amounts of biotite, sphene/leucoxene, and pyrite. Normalized SiO 2 contents range from about 50 to 60.6 wt% and average about 54.4 wt%. Na 2 O is highly variable with albite-rich specimens having as much as 7.6 wt% and albitepoor specimens as little as 1.5 wt%. K 2 O contents range from a low of 0.04 wt% to a high of 2.32 wt%; variation is related to white mica content. Organic carbon contents range from about 0.3 wt% to 3.28 wt%, with an average of about 1.0 wt%. Mineralogy and chemistry suggest a volcanic (basaltic) source. Temperature of metamorphism was greater than 350°C, and possibly as high as 450°C, as shown by the crystallinity of graphite, chlorite geothermometry, and the presence of metamorphic biotite. The characteristics of similar onshore rocks from East Greenland, within the Late Cretaceous (Maastrichtian)-early Paleocene Ryberg Formation, may suggest a correlation with the metamorphosed sedimentary rocks recovered near the base of Hole 917A. We presume that the sediments were deposited in a basin during early rifting of the continents that led to the formation of the North Atlantic Ocean basin, possibly during the Late Cretaceous-earliest Paleocene interval. The original sediments were tilted, raised above sea level and eroded, and then buried beneath a thick section of basalt and dacite. Metamorphism at high temperatures and low pressure (2-4 kg?) probably took place prior to burial by the volcanic cover. The entire sequence was faulted and tilted during subsequent opening and spreading apart of the North Atlantic Ocean basin.

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Grain size‐dependent strength of phyllosilicate‐rich gouges in the shallow crust: Insights from the SAFOD site

Phillips

White

2017

JGR Solid Earth

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The San Andreas Fault Observatory at Depth (SAFOD) drilling project directly sampled a transitional (between creeping and locked) segment of the San Andreas Fault at 2.7 km depth. At the site, changes in strain rate occur between periods of coseismic slip (>10−7 s−1) and interseismic creep (10−10 s−1) over decadal scales (~30 years). Microstructural observations of core retrieved from the SAFOD site show throughgoing fractures and gouge‐rich cores within the fractures, evidence of predominantly brittle deformation mechanisms. Within the gouge‐rich cores, strong phases show evidence of deformation by pressure solution once the grain size is reduced to a critical effective grain size. Models of pressure solution‐accommodated creep for quartz‐phyllosilicate mixtures indicate that viscous weakening of quartz occurs during the interseismic period once a critical effective grain size of 1 μm is achieved, consistent with microstructural observations. This causes pronounced weakening, as the strength of the mixture is then controlled by the frictional properties of the phyllosilicate phases. These results have pronounced implications for the internal deformation of fault zones in the shallow crust, where at low strain rates, deformation is accommodated by both viscous and brittle deformation mechanisms. As strain rates increase, the critical effective grain size for weakening decreases, localizing deformation into the finest‐grained gouges until deformation can no longer be accommodated by viscous processes and purely brittle failure occurs.

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The smectite–chlorite transition in drillhole NJ‐15, Nesjavellir geothermal field, Iceland: XRD, BSE and electron microprobe investigations

Cited by 170 publications

References 27 publications

Prograde Transitions of Corrensite and Chlorite in Low-Grade Pelitic Rocks from the Gaspé Peninsula, Quebec

Prograde Transitions of Corrensite and Chlorite in Low-Grade Pelitic Rocks from the Gaspé Peninsula, Quebec

Metamorphosed sedimentary (volcaniclastic?) rocks beneath Paleocene basalt in Hole 917A, East Greenland Margin

Grain size‐dependent strength of phyllosilicate‐rich gouges in the shallow crust: Insights from the SAFOD site

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