Discussion of cathodoluminescence of Recent biogenic carbonates: an environmental and ontogenetic fingerprint

G. M. Friedman comments: Barbin et al. (1991) state 'most authors consider recent primary biogenic carbonates to be non-luminescent.' They quote several authors who have made such negative statements, but they do not reference studies that have demonstrated the usefulness of cathodoluminescence in the study of modern biogenic carbonates that make up reefs. Barbin et al. (1991) are not alone in questioning the uselessness of cathodoluminescence in modern biogenic carbonates as expressed in the work of their predecessors. Most recently Major (1991) likewise commented' cathodoluminescence in carbonates is primarily a result of diagenesis', especially 'postdepositional precipitation or recrystallization'. The purpose of this discussion is to explain the usefulness of cathodoluminescence in the study of modern biogenic carbonates in reefs, as developed by Hill in 1976 and Friedman (1985). The reason why these studies have not been quoted are (a) Hill's (1976) study has remained an unpublished Master's of Science thesis (C. M. Hill, unpub. Master's thesis, Renneselaer Polytechnic Institute) and (b) Friedman's (1985) paper was published under a title which does not include the word cathodoluminescence and was concerned with problems of carbonate classification rather than using luminescence as a tool. Blasting open a coral reef or drilling through such a reef rarely displays framework-building organisms in growth positions. Biting and boring and rasping or mechanical breakdown convert solid colonies of calcium carbonate skeletons secreted by the reef organisms into an everincreasing supply of skeletal particles that accumulate within the reef or in the vicinity of the reef. Boreholes through modern reefs reveal mostly debris; framework builders in a position of growth are sporadic or even absent. Both skeletal particles and interparticle cement in reefs consist of high-magnesium calcite and aragonite. High-magnesium calcite commonly luminesces, whereas aragonite does not luminesce and appears black under an electron beam. Three kinds of submarine cryptocrystalline cement occur in reefs. These are: (1) cements that luminesce brightly, (2) cements that luminesce dimly or faintly, and (3) cements that do not luminesce at all. Under plane-polarized light or crossed nicols these three kinds of cement appear identical. In places, a cement 'stratigraphy' can be demonstrated where two kinds of cement appear in sharp lateral stratigraphic contact, one luminescent and the other non-luminescent (Friedman, 1985, figs 1E, F). These differences in behaviour under an electron beam relate to differences in the concentration of activator (Mn) ions. Because the geochemical characteristics of manganese are similar to those of magnesium, manganese replaces magnesium in the highmagnesium calcite lattice in sufficient concentration to produce luminescence. The sharp boundaries between the three kinds of cement indicate variations in concentration of Mn ions and different times and/or rates of precipitation. The same principle of bright luminescence to non-luminescence applies to skeletal particles, especially those of coralline algae. In many reef samples the bulk of the skeletal particles are composed of aragonite which does not luminescence. By contrast the high-magnesium calcite cement shows strong luminescence. In such samples a particle-cement ratio can be measured under an electron beam. Under cathodoluminescence modern Red Sea reefs luminesce more than Pleistocene reefs. Pleistocene reef samples are weakly or non-luminescent. The reason for decreased luminescence is the diagenetic change from luminescent high-magnesium calcite to less luminescent lowmagnesium calcite. The presence of Fe ions in lowmagnesium calcite restricts luminescence. This observation is at variance with the experience of others who claim that diagenetic processes result in cathodoluminescence (Major, 1991). The colour plates of Barbin et al. (1991) showing cathodoluminescence and transmitted light photomicrographs are most instructive. They should be compared with those previously published (see Friedman, 1985).