Sr and Nd isotopic evidence for punctuated clay diagenesis, Texas Gulf Coast

Abstract--To characterize the evolution of dioctahedral interstratified clay minerals in the Golden Cross epithermal deposit, New Zealand, hydrothermally altered volcanic rocks containing the sequence smectite through illite-smectite (I-S) to muscovite were examined by optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission and analytical electron microscopies (TEM/AEM).XRD analyses of 30 oriented clay samples show a broad deposit-wide trend of increasing illite content in I-S with increasing depth and proximity to the central vein system. Six representative samples were selected for SEMFFEM study on the basis of petrographic observations and XRD estimates of I-S interstratification. Ca and Na are the dominant interlayer cations in smectite, but as the proportion of illite layers in I-S increases, so does the K content and (WAl + VlAl)/Si ratio. Layers and packets tend to flatten and form larger arrays, reducing the amount of pore space. Smectite coexists with (R = l) I-S, rather than being (R = 0) I-S where R is the Reichweite parameter. The highest alteration rank samples contain discrete packets of mica to --300 A thick, but a limited chemical and structural gap exists between illite, which is intermediate in composition between common illite and muscovite, and illite-rich I-S. Selectedarea electron diffraction (SAED) patterns of mica show that the 1M polytype dominates, rather than the common 2M~ polytype.Petrographic, SEM, and TEM data imply that all phyllosilicates formed via neoformation directly from fluids. Relatively mature I-S and micas form simultaneously, without progressing through the series of transformations that are commonly assumed to characterize diagenetic sequences during burial metamorphism in mud-dominated basins. Although the overall distribution of clay minerals is consistent with temperature as a controlling variable, local heterogeneities in the distribution of clay minerals were controlled by water/rock ratio, which varied widely owing to different rock types and fracture control.

Section: Transformation Versus Simultaneous Formationmentioning

confidence: 98%

Genesis of Dioctahedral Phyllosilicates During Hydrothermal Alteration of Volcanic Rocks: I. The Golden Cross Epithermal Ore Deposit, New Zealand

Tillick

Peacor

Mauk

2001

“…Such a sequence is thus a necessary condition for time-dependent, sequential transformations, but it is not sufficient to prove they occurred. There is compelling geochemical evidence for episodic formation of clay minerals in Gulf Coast pelites (Morton, 1985;Ohr et al, 1991), which are commonly taken as a "type" example of burial metamorphism. We do not infer that such sedimentary systems are necessarily entirely episodic in nature.…”

Section: Conditions Of Formationmentioning

confidence: 99%

Genesis of Dioctahedral Phyllosilicates During Hydrothermal Alteration of Volcanic Rocks: II. The Broadlands-Ohaaki Hydrothermal System, New Zealand

Yan

Tillick

Peacor

et al. 2001

Abstract--The clay mineral textures, assemblages, formation mechanisms, and controlling geological parameters relating to alteration of silicic volcanic rocks by hydrothermal solutions, in core samples from the Broadlands-Ohaaki hydrothermal system, New Zealand, were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission and analytical electron microscopy (TEM/ AEM). Mineralogical and textural relations of this active hydrothermal system, for which temperatures and fluid relations are well known, are equivalent to those in the Golden Cross hydrothermal gold deposit as described in Part 1.XRD data show a sequence of clay minerals from smectite to a range of interstratified I-S to mica with increasing depth and temperature, on average. TEM observations are in general agreement with XRD data, especially with respect to relative proportions of illite (I)-and smectite (S)-like layers. TEM data also show that: (1) Smectite packets contain no discrete illite-like layers in samples identified as (Reichweite, R = 0) I-S by XRD. They coexist with separate packets of (R = 1) I~S. (2) A continuous range in I-S occurs from (R = 1) I-S with increasing proportion of illite-like layers, but at high illite-like layer contents there is a gap between I-S and illite. (3) 1M and 2M] polytypes of mica coexist in separate packets, but the rare 1M polytype has a larger VlMg content.The data imply that clay minerals formed by dissolution and neocrystallization directly from volcanic phases, although multiple reaction events can not be ruled out. Such "episodic" alteration produces a sequence of clay minerals identical to those of prograde diagenesis of pelitic sediments. This result implies that the presence of a continuous sequence is not definitive proof of continuous sequences of transformation as a function of time and continuous burial. Reaction progress of the clay-mineral sequence is in general accord with the known temperature gradient, but with significant and common exceptions. High porosity and permeability, both inherent in rock texture and local structure, are inferred to fbster local reaction progress, as consistent with metastability of phases and the Ostwald step rule.

“…This newly formed illite should have become older as it was buried deeper. Such observations have led to the idea of "punctuated diagenesis," in which illitization is hypothesized to have occurred across a thick sedimentary section in a single, short event (Morton, 1985;Ohr et al, 1991). This feature initially was postulated using Rb/Sr data, but it also appears in the deepest zone in K/Ar studies of the Gulf Coast basin (Aronson and Hower, 1976) and in the Bergen High area of the North Sea (Glasmann et el., 1989).…”

Section: Illite Recrystallization In the Deepest Zone (Zone 3)mentioning

confidence: 99%

Three Zones for Illite Formation During Burial Diagenesis and Metamorphism

Eberl

1993

Abstract--Reinterpretation of published data for shale cuttings from the Gulf of Mexico sedimentary basin identifies three reaction zones for illite formation with increasing depth for well CWRU6. In a shallow zone (1.85 to 3 km), non-expanding illite-like layers formed primarily by the coalescemce of smectite 2:1 layers around interlayer K + . In a middle zone (3 to 4 kin), illite crystals neoformed from solution as coarser K-bearing phases and smectite were dissolved by organic acids. In the deepest zone (>4 km), illite recrystallized as less stable illite crystals dissolved, and more stable illite crystals grew during mineral ripening. The progressive loss of radiogenic argon in the deepest zone yielded a constant apparent age for the clays with depth, an effect previously attributed to "punctuated diagenesis." The above hypothesis for illite formation emphasizes the need to establish the zone (i.e., the reaction mechanism) from which shales were derived before making detailed geologic interpretations based on illite mineralogy.