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Occurrence of Wide-Chain Ca-Pyriboles As Primary Crystals in the Salton Sea Geothermal Field, California, USA*

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Occurrence of Wide-Chain Ca-Pyriboles As Primary Crystals in the Salton Sea Geothermal Field, California, USA* Contributions to Contrib Mineral Petrol (1986) 94:127-134 Mineralogy and Petrology Springer-Verlag 1986 Occurrence of wide-chain Ca-pyriboles as primary crystals in the Salton Sea Geothermal Field, California, USA* Yu-Chyi Yau, Donald R. Peacor, and Eric J. Essene Department of Geological Sciences, The University of Michigan, Ann Arbor, MI 48109, USA Abstract. Amphiboles and pyroxenes occurring in the Sal- triple chains or both double and triple chains occurring ton Sea Geothermal Field were found to contain coherent in regular or irregular alternation. Isolated slabs with vari- intergrowths of chain silicates with other than double and able chain widths are also found within fibrous amphiboles single chain widths by using transmission and analytical (Chisholm 1973; Hutchison etal. 1975; Jefferson etal. electron microscopy. Both occur in the biotite zone at the 1976; Veblen etal. 1977; Jefferson et al. 1978). A wide- temperature (depth) interval of 310 ~ C (1,060 m) to 330 ~ C chain Ca-pyribole has been found in natural hydrother- (1,547 m) which approximately corresponds to tempera- mally altered clinopyroxene (Akai 1982) but has not yet tures of the greenschist facies. The amphiboles occur as been found in synthetic products. euhedral fibrous crystals occupying void space and are com- Although the existence and structure of wide-chain sili- posed primarily of irregularly alternating (010) slabs of dou- cates are well known, the conditions of their formation ble or triple chains, with rare quadruple and quintuple are only poorly understood. Veblen (1981) suggests that chains. Primary crystallization from solution results in eu- naturally occurring wide-chain pyriboles are typically pro- hedral crystals. Clinopyroxenes formed mainly as a pore- duced by replacement reactions from pyroxenes and amphi- filling cement and subordinately as prismatic crystals coex- boles. Nevertheless, an exsolution-like mechanism for fine isting with fibrous amphiboles. Fine lamellae of double and lamellae of amphibole within pyroxenes that originally had triple chains are irregularly intercalated with pyroxene. some solid solution toward amphibole composition has AEM analyses yield formulae (Cal.sMg2.9Fel.9Mno.1) been suggested (Desnoyers 1975; Smith 1977). To our Si8021.8(OH)1.8 (310 ~ C) and (Ca2.oFe2.sMgz.3) SisOzL8 knowledge, the primary growth of wide-chain silicates has (OH)2.o (330 ~ C) for amphiboles and (CaLaFeo.rMgo.~) never been reported in geological environments, although SizO6 for clinopyroxene. Thermodynamic calculations at the triple chain structures within fibrous amphiboles de- Pnuid = 100 bar of equilibrium reactions of (1) 3 chlorite scribed by Chisholm (1973) and Hutchison et al. (1975) may + 10 calcite + 21 quartz = 3 actinolite + 2 clinozoisite + 8 H20 have had such an origin. + 10 CO2 and (2) actinolite + 3 calcite + 2 quartz = 5 clino- On the other hand, formation of wide-chain pyriboles pyroxene+ H20 + 3 COz using Mg-end member phases in- synthesized under hydrothermal conditions implies that dicate that formation of amphibole and pyroxene require they should occur in analogous geological systems. On that very water-rich conditions (Xco2 < 0.06) at temperatures be- basis we anticipated their occurrence in sediments from the low 330 ~ C. Salton Sea Geothermal Field. We also surmised that the pyroxene formed in that environment would contain lamel- lae of amphibole or wide-chain pyriboles and accordingly searched for them. In this paper we report on observations Introduction of primary growth of disordered wide-chain pyribole in ac- Although their structures were modeled theoretically by tinolite and clinopyroxene. Actinolite (and/or tremolite) Thompson (1970, 1978), the wide-chain pyriboles with tri- and clinopyroxene have previously been reported from wells ple or wider chains such as chesterite and jimthompsonite in the Salton Sea sediments (Helgeson 1968; McDowell and (Veblen and Burnham 1978) were discovered only recently. McCurry 1977), but their structures and compositions were They have been identified in experimental syntheses of am- not well documented. They occur in association with an phiboles and micas (Drits et al. 1975; Tateyama et al. 1978; assemblage of quartz-epidote-chlorite-microcline-albite- Maresch and Czank 1983a, b) and in alteration products biotite-calcite-futile or titanite, similar to assemblages in of natural pyroxenes and amphiboles (Veblen and Buseck the greenschist facies (Zen and Thompson 1974). Because 1979, 1980, 1981; Nissen et al. 1980; Nakajima and Ribbe such materials occurred in an environment expected for 1981; Akai 1982). Most examples are composed of either pyriboles we have utilized the relatively high resolution ca- pabilities of scanning and transmission electron microscopy * Contribution No. 420 from the Mineralogical Laboratory, De- coupled with analytical electron microscopy to characterize partment of Geological Sciences, University of Michigan, Ann them, and have confirmed the primary growth of wide- Arbor, Michigan chain pyriboles crystallizing under hydrothermal conditions Offprint requests to." D.R. Peacor corresponding to temperatures of the greenschist facies. Fig. 1. Low magnification image showing pyribole fibers within casting resin. Arrows indicate apparent strain contrast in a curved fiber Fig. 2. Low magnification image of ion-thinned edge, showing euhedral pyribole crystals superimposed on a mica crystal Fig. 3. An electron diffraction pattern of pyribole from crystal A in Fig. 2 showing streaking parallel to b* due to irregular intercalation of different chain widths Fig. 4. An a-axis lattice fringe image of part of the pyribole crystal A in Fig~ 2. Irregular intercalation of double (2)-, triple (3)-, and quadruple (4)-chain widths in the b direction are shown Fig. 5. Lattice fringe image of part of a pyribole fiber containing a quintuple (5)-chain unit. Unlabeled portions are double (2)-chain units Fig. 6. Lattice fringe image showing straight external crystal edges and coherent boundaries between different chain widths 129 Sample characterization and experimental procedures Table 1. Chemical compositions and cation populations on the ba- sis of a total of 8 Si cations The samples studied are siltstone and sandstone cuttings recovered from the No. 2 Imperial Irrigation District (IID No. 2) well in Phase Amphibole Pyroxene the Salton Sea Geothermal Field, Southern California. The geology of this area has been studied in detail by Muffler and Doe (1968) T (~ 310 320 330 320 and the chemistry of the aqueous system by Helgeson (1968). The IID No. 2 well penetrated Pliocene and Pleistocene sand, silt, and Depth (m) 1,060 1,384 1,547 1,515 clay deposits of the Colorado River to a depth of 1,547 m. Under the elevated temperature and pressure of the geothermal system, ~- analyses 9 19 30 18 sediments are actively undergoing transformation with conditions ranging from diagenesis to Iow-grade metamorphism correspond- Si 8.0 8.0 8.0 8.0 ing to temperatures of the greenschist facies (Muffler and White Mg 2.9 2.4 2.3 1.2 1969). McDowell and Elders (1980) further divided the overall Fe +z 1.9 2.2 2.5 2.4 progression of mineral assemblages in order of increasing depth Mn 0.t tr. tr. tr. (temperature) into four mineral zones: (1) dolomite/ankerite zone Ti - 0.t tr. -- (< 190 ~ C), (2) chlorite/calcite zone (190-325 ~ C), (3) biotite zone A1 tr. tr. tr. tr. (325-360 ~ C) (4) andradite zone (>360 ~ C). The andradite zone Ca 1.8 2.0 2.0 4.4 of McDowell and Elders (1980) was not encountered in the IID O 21.8 ~ 21.83 21.8 ~ 24.0 u No. 2 well. In addition, Yau et al. (1983, 1984) have separately OH b 1.8 2.0 2.0 - described the diagenesis of illite and chlorite in shale samples in the same well. This paper is principally concerned with samples M r 4.9 4.6 4.8 5.6 in the biotite zone. Cuttings were impregnated with casting resin (Castolite-AC) a calculatedonthebasisofassumedcompositionof67% actinolite to preserve the original textures, and ion-thinned by argon beam and 33% triple-chain Ca-pyribole bombardment. The ion-thinned samples were then examined by b calculated for charge balance scanning electron microscopy using backscattered imaging (SEM) c M=Mg+Fe+Mn operating at 15 kV to identify the amphibole/pyroxene and other mineral phases associated with them within a 3 mm diameter area. One dimensional (010) and (100) lattice fringe images and selected sharp. The streaking is due principally to the irregular inter- area electron diffraction patterns were obtained using a JEOL calation of double-, triple- and wider-chain units. All chain JEM-100CX transmission electron microscope (TEM) with a side- entry stage operating at 100 kV. Chemical compositions were deter- units share a common subunit of 4.5 A (a single-chain unit) mined using an energy dispersive X-ray analysis system attached and therefore give rise to the sharp reflections having k= to the TEM. The analytical data were obtained by rastering the 4n, the differences between chain units being reflected in beam over a square area (400 x 400 A.) for 100 seconds. Well ana- diffuseness for reflections with k = 4n + 2. lyzed ion-thinned clinochlore adularia, margarite and fayalite were Figure 4 shows an (010) lattice fringe image of the crys- used as standards for Mg/Si, AI/Si, Ca/Si and Fe/Si, respectively. tal which gave rise to the selected area diffraction pattern Procedures for preparation and analysis of samples are described of Fig. 3. The image shows three kinds of spacing between in detail by Blake et al. (1980). pairs of dark fringes. They are designated as 2, 3 or 4, corresponding to the multiplicity in chain width. Similar features have been observed in samples from all three Amphibole/wide-chain pyribole intergrowth depths (Fig. 5, 6) in which wide-chain pyriboles were de- Amphiboles were observed by TEM in siltstone samples tected. The widths of the images in these figures correspond from depths (temperatures) of 1,060 m (310 ~ C), 1,384 m to the entire width of the crystals.
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