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Some Geological Applications of Cathodoluminescence

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Some Geological Applications of Cathodoluminescence Somegeological applications ofcathodoluminescence examplesfrom the Lemitar Mountains and Riley travertine, SocorroGounty, New Mexico byVirginiaT. McLemore and James M. Barker,New Mexico Bureau of Minesand Mineral Resources, Socorro, NM 87801 Introduction ratio of authigenic and detrital minerals (Sippel, 1968),stratigraphy, Cathodoluminescence(CL), the characteristicvisible radiation (color) siliclastic components, cementation history, and provenance (Owen produced in a mineral subjected to bombardment by electrons, was and Carrozi,1986). CL hasbeen used in paleontologyto study struc- first observed in the 1870's by Crookes (1.879).It has significant ture and chemistry of fossils (Nickel, 1'978;Batkerand Wood, 1986a). applications in petrology, although many petrologists do not utilize The CL of a sample reveals geochemical,rather than optical, var- it. Luminescencein CL is typically more intense than that produced iations in a sample, which provides important compositional,tex- by ultraviolet light (see pp. 25-30, this issue);CL is also observed tural, and structural information not readily obtained by other in mineralsthat do not luminesceunder ultraviolet light. methods. However, CL is most useful when used in conjunction Cathodoluminescence is produced by excitation of electrons in with other studies such as optical microscopy,x-ray diffraction, elec- specific chemical impurities (activators) within a crystal structure. tron microprobe, scanning electron microscopy, and geochemistry. When excited electronsreturn to their ground state, visible light may Unlike ultraviolet light, where an entire specimen can be exposed, be emitted.CL may be producedat sitesaltered by radiationdamage, CL requires a thin section or relatively thin slab, preferably with chemical heterogeneity, electron charge displacements, and struc- doubly polished surfaces.The doubly polished thin sectionsalso can tural defects (Mariano, 1976;Nickel, 1978), or by unidentified pro- be used in the electron microprobe or fluid inclusion stage. De- cesses(Barker and Wood, 1986a).CL may be inhibited in a mineral pending on the CL unit involved, samples can be several square by other impurities (quenchers), which counteract the activators to inchesin area,which is an advantagewith some rocks. preventor diminish luminescence.Other impurities (sensitizers)can Certain textural studies are only possible using CL. Textures such enhanceCL by aiding transfer of energy to activator electrons(Barker as intergrowths, overgrowths, growth rings, healed fractures (Sprunt and Wood, 1985a).The observedcolor of a mineral under CL de- and Nur, 7979), and radiation damage are readily observed by CL. pends upon complexinteractions among activators,quenchers, and Interpretation of these textures aids in relative dating of minerali- enhancers along with instrument conditions (manufacturer, accel- zation (Harwood, 1983)and in determiningmicrostratigraphy (Ebers erating voltage,beam intensity, vacuum, ambient gas, and others), and Kopp, 1979). specificmicroscopes and objective lens/condensercombinations, and CL can be used in mineral identification and mineral distribution an individual's perception of color. Heating of the sample by the becausemany minerals have diagnostic luminescenceproperties electronbeam can decreaseluminescence and/or changethe color. within a suite of rocks. Small mineral grains (such as apatite and Therefore,most descriptionsof CL color in the literature are sub- zircon) may be readily identified by CL when they cannot be iden- jective. Detailed CL studies that minimize this subjectivity require tified by optical microscopy due to their small size or when obscured an emission spectrograph to record the wavelength and relative in- by alteration effectsin transmitted light. However, minerals typically tensity of the luminescence(Mariano, 1976);this equipment was not cannot be identified only by their CL appearance;a detailed electron availableto us. microprobeand/or optical microscopystudy is needed.Mineral CL colors are not unique (Nickel, 1978) and can change due to slight Instrumental conditions compositionalchanges or to different origins (Mariano, 1976;Machel, Cathodoluminescencewas observed using a Technosyn Cath- 1986). odoluminescenceStage, Model 8200 MK II, attached to a Nikon Someaspects of geochemistrycan be determined on the basisof Optiphot nonpolarizing microscope.An electronmicroprobe can be CL color (Long and Agrell, 1965;Mariano, 1976;Mariano and Ring, used to observe CL; however, the electron beam ellipse is much 1975).Quartz, which has a high titanium content relative to iron larger (4 x 7 mm) with the Technosyn.The electronmiiroprobe has content, luminescesblue, whereas low-titanium quartz (relativeto an electronbeam diameter as small as 1 micron. The samplesused iron content) luminescesred (Barker and Wood, 1986a).Apatites were uncoated,uncovered, polished thin sections.General operat- Iuminesce yellow, yellow-green, blue, or gray depending on the ing conditions were a beam energy (acceleratingvoltage) of 78-27 concentrationsof rare-earthelements and manganese(Mariano, 1976). kv, beam current of 460-600milliamps, and operating vacuum of Apatite without impurities does not luminesce.Calcite typically lu- 0.03-0.05torr. Air was the ambient gas used in the CL chamber. minescesbright yellow or orange compared to the red or reddish- Additional specificationsof the Technosynare discussedby Barker purple of dolomite (Kopp, 1981).Extreme caution must be used in and Wood (1986a,b). correlatingcolor with chemistry. Sprunt et al. (1978)have demon- Color slides were taken with a Nikon camerausing Ektachrome strated that quartz displays CL colors that vary according to differ- ASA 400Daylight, EktachromeP800/1600, and EktachromeASA 200 encesin its metamorphicgrade. Daylight. Slideswere alsotaken using EktachromeASA100 Daylight, but these generally were unsuitable. Exposure times varied from Examples lessthan a secondfor nonpolarizedlight to more than 300 seconds CL was observedon samplesfrom carbonatitesin the Precambrian depending on CL intensity. Normal commercialprocessing of the terrane of the Lemitar Mountains and from the Riley travertine of film sometimesyields slides that do not show the observedcolors the western Ladron Mountains as part of our ongoing researchin becauseof variationsin the developmentprocess. This probablycan these areas.It is beyond the scope of this report to describethe be mitigatedby supplying examplesof coloredslides with the correct geology of these regions, which is discussedby Mclemore (1982, color balanceto the processor.All slides (Figs. L-6) were processed 1,983,1,987) for the Lemitar Mountains and summarized by Barker at the samecommercial laboratory. (1983)for the western Ladron Mountains. Geologicalapplications Lemitar Mountains Cathodoluminescencecan be an important petrologic tool for many CL is especiallyuseful when studying carbonatitecomplexes (Mar- types of rocks, although most work has been done on sedimentary iano and Ring, 1975;Mariano, 1975).Numerous polished thin sec- rocks. CL has been used in sedimentary petrology to determine tions of carbonatitesfrom the Lemitar Mountains were examined texture,sediment source, degree of compaction,diagenetic history, with the CL microscope. New Mexica Geology N[ay 1987 Figure 1a is a typical exampleof 6 carbonatiteshowing CL. The When CL is used, fluorite from a vein associatedwith a carbonatite red luminescenceis due to the carbonatematrix and is brighter than exhibitszoning possibly due to compositionalchanges (Fig. 4a);bright most samplesunder CL. The large magnetite crystal exhibits CL orangeCL in calciteand yellowish-brownCL in quartz are observed. zoning, which is not observedin nonpolarizedtransmitted light (Fig. The black nonluminescingmaterial is probably hematite. 1.b).Examination of this crystalwith the electronmicroprobe reveals a complex intergrowth of magnetite, ilmenite, quaftz, rutile andior leucoxene,and calcite.Alteration of the xenolith is readily observed Riley travertine by a carbonatewith bright red CL rim. Apatite crystalsluminesce blue to blue-gray to green-gray.Due to variations in processingof the film, the apatites in Fig. La are darker than the blue-gray to green-grayobserved. Extremely small apatite crystals in carbona- tites, especiallyrodbergs, are readily identified by their CL when transmittedlight microscopyfails gatheredfrom numerous polished thin sections,The views in Figs. Zoning within an apatite crystal (Fig. 2a) may be due to trace- 5a, 5b, 6a, and 6b are of thin sectionscut from drill core taken from elementcompositional changes. This zoning is not observedin po- the north mesa portion of the Riley travertine (Barker,1983). The larized or nonpolarizedlight. Epoxy (brown in nonpolarizedlight) hole was drilled in the SW corner of sec. 15, T2N, R3W and the forms the edge of Figs. 2a and 2b and does not luminesceunder sectionswere cut from a core depth of 19 ft 10 inches.The textures CL. The patchesof nonluminescing(brown) material, not observed revealedby CL are consistentwith a spring origin for the Riley in nonpolarizedlight (Fig. 2b), have not been identified. travertine at North Mesa. Palered growth zonesof calcitecrystals are observed in a carbonate Two powerful uses of CL in carbonatesare to delineate trace- vein cutting Polvaderagranite (Fig. 3a). Thesezones are not easily element chemistry and to highlight obscureclastic content. Varia- distinguished by transmitted-light microscopy (Fig. 3b). The Pol- tions in carbonatechemistry
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