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 nonpolarizingmicroscope. 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 can be produced during deposition or vaderagranite consistsof plagioclase,K-feldspar, and quartz, all of during diageneticdissolution and redeposition.Figure 5 shows Parry which luminescered under CL. calcitethat has severaldistinctive layerswhose CL is probably due

FIGURES la & b-la is a zoned magnetite crystal (red and black) 2 mm. Exposuretime was 138sec using_Ektachrome ASA 400Day- next to smaller apatite (black) in carbonatite (sample LEM CC) light film.-Fig.1b is the sameview in unfilterednonpolarized trans- under CL. Matrix is carbonate (red). Field of view is approximatelv mitted light. Exposuretime was 0.5 sec.

FIGURES 2a & b-2a is a zoned apatite (blue) in carbonatite (sample ASA 400 Daylight film. Fig. 2b is the same view in unfiltered non- LEM CA) under CL Matrix is carbonate (orange). Field of view is polarizedtransmitted light. Exposuretime was 0.04 sec. approxirnately 2 mm. Exposure time was 43.7 sec using Ektachrome

May 7987 Nau Mexico Geology to trace-elementvariations. These layers are not prominent in the Wood, 1986a).Easily volatilized bonding agents must be avoided. sameview in nonpolarized transmitted light (Fig. 5b). Heating can also causeleakage or stretching of low-temperature fluid Figure 6a shows the ability of CL to highlight textural and com- inclusions;fluid inclusion work must be completedbefore CL anal- positional details.The siliclasticlayer in the Riley travertine is bounded ysesare done. Relativelyunstable minerals, such as some uranium by calcite,which exhibits different textures. The carbonatein the minerals, can be altered by heating. upper left corner of the photograph is more uniform than the com- Some additional problems encounteredwith CL include: 1) the quality of the image transmitted through the stage, particularly at high magnification, 2) the need for long-working-distance objective lensesand condensers,and 3) the low-intensity CL imageproduced in some minerals at the beam energies now available. Specific data on mineralidentification and chemistryare often not available.Stan- dards for reporting CL data, such as those proposed by Marshall (1978),should be incorporatedinto reports. Disadvantagesof CL microscopy Despite these problems, CL is an important petrologic tool, es- Somepro-blems inherent with CL havebeen mentioned previously. peciallywhen used in conjunction with other studies.Many of the Most samples,either thin sectionsor thin slabs,must have at least problems listed will be overcome with future research. one polished surface,which adds to preparationtime and expense. ACKNOWLEDGMENTS-Weare indebted to Charles Barker of the Perceptionof CL colors is subjectiveand difficult to comparewith Branchof PetroleumGeologt U.S. GeologicalSurvey (Denver, Colo- colors reported in other studies. CL work should be done in con- rado), for assistancein this study and for the use of CL equipment. junction with other methods if complexhypotheses are to be tested. PeterModreski of the U.S. GeologicalSurvey (Denver) assisted with Someminerals will not luminesceunder CL. the electron microprobe studies. We gratefully acknowledge reviews of this manuscript by CharlesBarker, PeterModreski, and _ Heatingof the samplewith the electronbeam can cause problems. Jacques Some organic-richsamples, such as coal, can volatilize and cause Renault.Technical assistance by Linda Frank and Robert Bolton is instability in, or even damage to, the CL instrument (Barker and appreciated. 13

FIGURES3a & b-3a is a vein cutting Polvaderagranite (LEM 531a) EktachromeASA 200 Daylight film. Fig. 3b is the same view in under CL. Nonluminescingmaterial (black)is hematite. Field of nonpolarizedtransmitted light Exposuretime was 0.3 sec view is approximately3.4 mm. Exposuretime was 19.5sec using

FICURES4a &b-4a is a fluorite vein (sampleLEM 813)associated film. Fig. 4b is the same view in nonpolarized transmitted light. with a carbonatiteunder CL. Field of view is approximately2 mm. Exposuretime was 0 12 sec. Exposuretime was 209 sec using Ektachromeesa 400 Daylight

Nau Mexico Geology May 7987 References Mariano, A. N., 1976, The application of cathodoluminescencefor carbonatite explo- ration and characterization;in Proceedingsof the first intemational symposium on Bachman, G. O., and Machette, M. N Calcic soils and calcretesin the south- ,7977, carbonatites:Pocos de Caldas,Minas Gerais,Ministerio das Minas & Eneregia,Brazil, western United States:U S. GeologicalSurvey, Open-file Report 77-794,1,63pp pp.39-57. Barker, C E , and Wood, T , 1,986a,A review of the Technosyn and Nuclide cathodo- Miriano, A. N and Ring, P 1975, Europium-activated cathodoluminescencein luminescencestages and their application to sedimentary geology; in Processmin- , J., minerals: Geochimicaet CosmochimicaActa, v 39, pp 6a9-660. eralogy: The Metallurgical Society of the American Institute oI Mining Engineers, v. Marshall, D. 1978, Suggestedstandards for the reporting of cathodoluminescence Y[,22 pp. J., results: of Sedimentary Pehology, v 48, p. 651'-653 Barker, C. E., and Wood, T., 1986b,Notes in cathodoluminescencemicroscopy using Journal Massingill, G. L., 7977,Geology of the Riley-Puertecito area, southeasternmargin of the TechnosynStage and a bibliography of applied cathodoluminescence:U.S Geo- the Colorado Plateau, Socorro County, New Mexico: New Mexico Bureau of Mines logical Survey, Open-file Report 86-85, 35 pp and Mineral Resources,Open-file Report 107, pp. 1'27-1'28 Barker, M, 1983, Preliminary investigation of the origin of the Riley travertine, J. Mclemore, V. T., 7982,Geology and geochemistry of Ordovician carbonatite dikes in northwestem Socorro County, New Mexico: New Mexico GeologicalSociety, Guide- the Lemitar Mountains, Socono Counry New Mexico: New Mexico Bureau of Mines book to 34th Field Conference,pp 269276 and Mineral Resources,Open-file Report 158, 112 pp Crookes, W,1879, Contributions to molecular physics in high vacua: Philosophical Mclemore, V T, 1983,Carbonatites in the Lemitar and Chupadera Mountains, Socorro Transactionsof the Royal Society of London, pp 641-662. County, New Mexico: New Mexico GeologicalSociety, Guidebook to 34th Field Con- Denny, C. 5.,1941,, geology of the San Acacia area, New Mexico: Quaternary Joumal ference,pp. 235-240 of Geology, v. 49, pp 225J60 Mclemore, V. T., 1987,Geology and regional implications of carbonatitesin the Lemitar Ebers,M.L.,andKopp,O C,l9T9,Cathodoluminescentmicrostratigraphyingangue Mountains, central New Mexico: of Geology, v 95, pp 249252 dolomite, the Mascot-Jeffersoncity district, Tennessee:Economic Geology, v.74, pp fournal Nickel, E 7978,The present status of cathodoluminescenceas a tool in sedimentology: 908-918 , Minerals Scienceand Engineering, v. 10, pp. 73-100. Hamood, G , 1983,The application of cathodoluminescencein relative dating of barite Owen, M. R., and Carozzi, A. V, 1985,Southern of upper Jackfork Sand- mineralization in the lower MagnesianLimestone (Upper Permian), United Kingdom: Provenance stone, southern Ouachita Mountains: Cathodoluminescencepetrology: Geological EconomicGeology, v 78, pp 1.022-1027. Societyof America Bulletin, v. 97, pp. 110-115. Kopp, O. C., 1981, Cathodoluminescencepetrography-a valuable tool for teaching Sippel, R. F., 1968, Sandstone petrology, evidence from luminescence petrography: and research: of GeologicalEducation, v 29, pp.108-113. Journal j6urnal SedimentaryPetrology, v. 38, no. 2, pp. 530-554. Kottlowski, F- E ,7962, Reconnaissanceof commercialhigh-calcium limestonesin New Sprunt, E. S., Dengler, L. A., and Sloan, D., 1978,Etrects of metamorphism on quartz Mexico: New Me{co Bureau of Mines and Mineral Resources,Circular 50, pp 9, 19- cathodoluminescence:Geology, u. 5, 305-308 27,28. PP. Sprunt, E. S., and Nur, A.,1979, Microcracking and healing in granites: New evidence Long, V P, and Agrell, S O 1965,The cathodoluminescenceof minerals in thin J , from cathodolurninescence:Science, v. 205, pp. 495-497 section:Mineralogical Magazine, v 34, pp. 318-326 ! Machel, H G , 1986, Cathodoluminescencein calcite and dolomile and its chemical interpretation: GeoscienceCanada, v 12, pp 139-1,47.

FIGURES5a & b-5a is a radial sparry calcite showing one prom- was about 90 sec using Ektachrome Daylight 400 film. Fig. 5b is inent and severallesser luminescent bands (yellow to reddish range) the same view in nonpolarized transmitted light. Exposure time under CL. Field of view is approximately3.4 mm. Exposuretime was lessthan 1 sec.

FIGURES6a & b--6a is a siliclastic layer (blue luminescent clasts) ASA 400 Daylight film. Fig. 6b is the same view in nonpolarized bounded by luminescent calcite (reddish orange) under CL. Fieli transmitted light. Exposure time was less than L sec. of view is 0.85 mm. Exposure time was 331 sec using Ektachrome

4() May 1987 New Mexico Geology