CATHODOLUMINESCENT AND TRANSMITTED LIGHT PETROGRAPHY

CL microscopy is a complementary technique to standard petrographic methods that has revolutionized the way in which carbonates are interpreted. Three areas of application are common: (1) making fabrics visible that are not visible by standard petrographic microscopy, such as recrystallization nuclei; (2) "cement stratigraphy," that is, the correlation in carbonate rocks by interpreting equally luminescent zones as diagenetically coeval; and (3) geochemical interpretation of redox-sensitive trace elements incorporated into the crystal structure at the time of precipiation (Machel, 1985). Mn2+ appears to be the most important element causing luminescence in natural carbonates, because it is relatively abundant and generates intense emission (Machel et al., 1991). Fe2+ is the single most important element in quenching luminescence (Machel and Burton, 1991; Machel et al., 1991). For a detailed overview of CL and its petrographic applications, the reader is referred to Barker and Kopp (1991) and references therein. Three types of dolomite were identified within samples based on their mode of occurrence in thin section and CL properties. These are (1) dolomicrite; (2) blocky, void-filling dolomite; and (3) fine silt-sized, zoned, rhombohedral dolomite. These phases record progressive degrees of induration associated with horizon development and burial.

Dolomicrite

Dolomicrite is the dominant horizon-forming phase and consists of sucrosic mosaics of interlocking subhedral, equigranular dolomite rhombs that form olive-green authigenic horizons with a "fitted fabric." Rhombs are typically 10 Ám in diameter, have grown within pore spaces as an intergranular cement, and have planar, irregular, compromise crystal boundaries. Hypidiotopic crystalline aggregates commonly envelop diatom and foraminifer tests, fish bones, subhedral pyrite grains, and luminescent, pyrite-rich organic blebs (Fig. F2E). Pyrite grains are a common accessory phase and are commonly enveloped by a 1- to 2-Ám-thick rim of luminescent dolomite (Fig. F2C). Foraminifer tests are typically well preserved. Recrystallized skeletons are present only within lithified horizons where the outer margins of shell walls are altered, forming a thin, luminescent intraparticle lining (Fig. F2D). Fish bones enveloped by dolomite crystals show evidence of dissolution along their margins.

Dolomicrites can be divided, based on their CL properties, into luminescent, weakly luminescent, and nonluminescent varieties. CL spectra and microprobe analysis indicate that cathodoluminescence is Mn-activated and Fe-quenched (Fig. F3). Luminescent layers are characterized by an orange-brown color and a blotchy, mottled texture when viewed with CL (Fig. F2H). Weakly luminescent dolomicrites are a dull brown-orange color and exhibit a shift in their emission spectra towards longer wavelengths caused by crystallographic distortions in the coordination of Mn2+ induced by high quencher concentrations of Fe2+ (Machel, 1985; Machel et al., 1991).

Blocky Dolomite

Blocky dolomite occurs only within lithified dolomite horizons and becomes increasingly more abundant in samples from deeper stratigraphic levels. It consists of blocky nonluminescent dolomite crystals with planar boundaries that infill foraminifer and diatom tests (Fig. F2F, F2G, F2H). Crystals are larger than those forming dolomicrite, ranging in size from 15 to 20 Ám. In many instances, the intraparticle pores within tests have been entirely occluded by blocky dolomite. Blocky dolomite was not observed in semilithified horizons recovered from Site 1081.

Zoned Dolomite

Zoned dolomite is found only in the Angola Basin at Site 1078 within the interval of laminated sediments at 131 mbsf. It occurs as disseminated, euhedral rhombohedra within the bases of authigenically cemented Bouma DE turbidites. Rhombs consist of an abraded, detrital core and an outer authigenic rim. Crystal size is more variable than in the dolomicrites and void-filling dolomites, typically ranging from 5 to 12 Ám.

CL study of zoned dolomites confirms what transmitted light petrography suggests, that euhedral dolomite crystals within turbidites consist of a central core of detrital dolomite and an outer rim (Fig. F2C). Rims are distinguished from cores by differences in luminescent character and zoning discontinuities across core margins. Cores typically consist of concentrically or oscillatory zoned silt-sized dolomite grains. Rims consist of a single luminescent zone of dolomite 1 to 2 Ám thick. These characteristics are easily viewed by the naked eye under CL but are difficult to discern in photomicrographs of zoned dolomites because the small crystal size and long exposure times required prevents adequate resolution of these features. The common presence of luminescent dolomite rims around subhedral pyrite grains and detrital dolomite cores suggests that dolomite authigenesis commenced in the zone of sulfate reduction soon after the formation of pyrite. This interpretation is supported by studies that indicate that iron concentrations within this zone are maintained at low levels because of its incorporation into diagenetic pyrite (Berner, 1984, 1985; Burns and Baker, 1987; Hesse, 1990; Lyons and Berner, 1992; Murata et al., 1972). Pyrite forms through a series of metastable reactions when Fe2+, derived from redox-controlled cation exchange reactions of clay minerals and dissolution of Fe oxides, combines with sulfide produced from the microbial reduction of pore-water sulfate. These early diagenetic reactions preclude the incorporation of Fe2+ in dolomite precipitating within the zone of sulfate reduction, producing the luminescent dolomite rims observed in thin section.

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