Compositional variations in mafic phyllosilicates from regional low‐grade metabasites and application of the chlorite geothermometer

317

210

Abstract--Chlorite minerals, found in a great variety of rocks and geological environments, display a wide range of chemical compositions and a variety of polytypes, which reflect the physicochemical conditions under which they formed. Of particular importance for studies dealing with ore deposit genesis, metamorphism, hydrotherrnal alteration or diagenesis is the paleotemperature of chlorite crystallization. However, in order to understand the relationship between chlorite composition and formation temperature and hence use chlorite as a geothermometer, one must determine how other parameters influence chlorite composition. These parameters may include fO2 and pH of the solution and Fe/(Fe + Mg) and bulk mineral composition of the host rock.Four approaches to chlorite geothermometry, one structural and three compositional, have been proposed in the past: 1) a polytype method based on the (largely qualitative) observation that structural changes in chlorite may be partly temperature-dependent (Hayes, 1970); 2) an empirical calibration between the tetrahedral aluminum occupancy in chlorites and measured temperature in geothermal systems (Cathelineau, 1988), which has subsequently been modified by several workers; 3) a six-component chlorite solid solution model based upon equilibrium between chlorite and an aqueous solution, which uses thermodynamic properties calibrated with data from geothermal and hydrothermal systems (Walshe, 1986); and 4) a theoretical method based on the intersection of chlorite-carbonate reactions and the CO2-H20 miscibility surface in temperature-XCO2 space, which requires that the composition of a coexisting carbonate phase (dolomite, ankerite, Fe-calcite or siderite) be known or estimated (Hutcheon, 1990). These four approaches are reviewed and the different calculation methods for the compositional geotherrnometers are applied to a selection of chlorite analyses from the literature. Results of this comparative exercise indicate that no single chlorite geothermometer performs satisfactorily over the whole range of natural conditions (different temperatures, coexisting assemblages, Fe/(Fe + Mg), fO2, etc.).. Therefore, chlorite geothermometry should b e used with caution and only in combination with alternative methods of estimating paleotemperatures.

Section: Discussion: Comparison Of Chlorite Geothermometersmentioning

confidence: 99%

Section: Description Of Chlorite Non-stoichiometry and Calculation Ofmentioning

confidence: 99%

Section: The Empirical Calibrationmentioning

confidence: 99%

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Chlorite Geothermometry: A Review

Caritat

Hutcheon

Walshe

1993

317

210

“…The gradual or continuous transformation of smectite to chlorite, as accommodated through randomly interlayered S/C (with various percentages of expandable layers) and regularly interlayered C/S (corrensite), has been reported from metabasalts (Bettison and Schiffman 1988;Bevins et al 1991) and sediments (Helmhold and van der Kamp 1984;Chang et al 1986). However, a discontinuous decrease in smectite proportion, whereby smectite (with less than 20% chlorite), corrensite and chlorite (with less than 10% smectite) occur with increasing temperature, has been reported from diagenetic and hydrothermal environments (Inoue et al 1984;Inoue 1987;Inoue and Utada 1991;Schiffman and Staudigel 1995).…”

Section: Introductionmentioning

confidence: 99%

The Role of Randomly Mixed-Layered Chlorite/Smectite in the Transformation of Smectite to Chlorite

Bettison-Varga

Mackinnon

1997

Abstract--Vesicular and groundmass phyllosilicates in a hydrothermally altered basalt from the Point Sal ophiolite, California, have been studied using transmission electron microscopy (TEM). Pore-filling phyllosilicates are texturally characterized as having coherent, relatively thick and defect-free crystals of chlorite (14 _A) with occasional 24-.~ periodicities. Groundmass phyllosilicates are texturally characterized as 1) randomly oriented crystals up to 200 A in width and 2) larger, more coherent crystals up to 1000 in width. Small crystallites contain predominantly 14-.~ layers with some 24-.~ units. Large crystals show randomly interlayered chlorite/smectite (C/S), with approximately 50% chlorite on average. Adjacent smectite-like layers are not uncommon in the groundmass phyllosilicates. Electron microprobe analyses show that Fe/Mg ratios of both groundmass and vesicular phyllosilicates are fairly constant.Termination of brucite-like interlayers has been identified in some of the TEM images. The transformation mechanisms represented by these layer terminations are 1) growth of a brucite-like interlayer within smectite interlayer regions and 2) the dissolution and reprecipitation of elements to form chlorite layers. Both mechanisms require an increase in volume as smectite transforms to chlorite.The data, combined with that from previously published reports, suggest that randomly interlayered C/S is a metastable phase formed in microenvironments with low water/rock ratios. Chlorite forms in microenvironments in the same sample dominated by higher water/rock ratios. The relatively constant number of Mg's in the structure (Mg#) of both structures indicates that in both microenvironments the bulk rock composition has influence over the composition of phyllosilicates.

“…Among several potential chlorite geothermometers, that using the tetrahedral Al content of trioctahedral chlorite has been increasingly used in low-grade regimes in recent years (Cathelineau 1988;Bevins et al 1991). However, chemical analysis of phyllosilicates in low-grade rocks is fraught with difficulty because of the small grain sizes, mixed layering, and complex intergrowths that exist at scales beyond the resolution limits of the electron microprobe and conventional analytical techniques (Peacor 1992;Warren and Ransom 1992).…”

Section: Introductionmentioning

confidence: 99%

Chlorite Geothermometry?—Contamination and Apparent Octahedral Vacancies

Jiang

Peacor

Buseck

1994

Abstract--The large contents of octahedral vacancies in published formulae of chlorite from hydrothermal systems and clastic sequences are shown to be largely caused by inclusion of other minerals. Verification is provided by analytical electron microscope (AEM) analyses of chlorite in pelitic rocks from the Gasp6 Peninsula, Quebec and the Gulf Coast, Texas. The Gasp6 chlorite occurs as discrete crystals locally coexisting with corrensite, and the Gulf Coast chlorite is free of mixed layers other than local serpentinelike 7-~ layers. Unlike most electron microprobe analyses (EMPA), but like other AEM analyses, the reported chlorite formulae do not have significant octahedral vacancies, are not Si-rich and (Fe + Mg)-poor relative to classic metamorphic chlorite, and have nearly equal amounts oftetrahedral and octahedral AI. The studied chlorites and those in metabasites and clastic rocks that could be positively identified as containing no or minimal mixed layering or submicroscopic intergrowths have little or no Ca or alkalis. In contrast, EMPA of chlorite reported for other clastic sequences show variable amounts ofNa + K + 2Ca that exhibit a poorly defined positive correlation with the proportion of octahedral vacancies. The EMPA of chlorite from the Salton Sea and Los Azufres geothermal fields that were suggested to contain temperature-dependent amounts of tetrahedral A1 (and thus used as "'chlorite geothermometers") show compositional characteristics similar to those reported for several saponite-chlorite transition series in metabasites.Continuous increases in octahedral occupancy and tetrahedral Al with increasing metamorphic grade are attributed to decreases in abundance of mixed layers or fine-grained intergrown minerals that commonly occur as a result of increasing crystal size and homogeneity in prograde sequences. Use of"chlorite geothermometry'" based on the proportion of apparent octahedral vacancies or tetrahedral A1 is therefore unwarranted and leads to inaccurate temperature estimates.