AmericanMineralogist, Volume61, pages405408, 1976
Sanidinefrom the Mesa Falls Tuff, Ashton,Idaho
AsHurrBesu'nNr Cser.rEs J. Vtre.t-tnNo I ndianaUniue rs ity, Bloomingt on, I ndiana
Abstract
K-feldspar crystalsfrom different horizons and localitiesin the Mesa Falls member of the Yellowstone Tuff, in the vicinity of Ashton, Idaho, are unzoned and unaltered.Calculated formulae of representativecrystals, based on electron-microprobeanalysis, permit classifica- tion of a[ as sanidine,near Oru* Ab* An,.uin the ternary feldspardiagram. X-ray diffraction studiessuggest a uniformity of cell dimensionsand high albite-high sanidineseries structural state.Optical data are in agreementwith this classification.The data lead to the conclusion that all sanidinephenoclasts in the Mesa Falls Tuff appearto have crystallizedin the magma chamber during a single epoch prior to eruption. Eruptions from the magma chamber occurred in pulses to produce the different horizons in the tuff'
Introduction YellowstoneGroup, of Pleistoceneage (Christiansen and Blank, 1972, p. 86). The phenoclast-bearing Alkali feldsparcrystals constitute the major pheno- layer is as much as 15 m thick in the type locality, a clast component of an airfall crystal-richlayer in the road cut on U.S. Highway 20 and 9l (seeHamilton, Mesa Falls Tuff, the middle member of the Yellow- 1965,p. C5, fig. 3). There it restson a thin layer of stone Group (Christiansenand Blank, 1972,p. B3). loess. It crops out in severaladditional but smaller The unit crops out in the vicinity of Ashton, Idaho, road cuts along the highway, is presentin the cliffs at and mechanicalerosion continually affords an abun- Mesa Falls on the Henry Fork, and is exposedin a dant supply of crystalswhich accumulateat the break quarry about 6.8 km north of Ashton as well (Chris- in slope in several easily accessibleoutcrops (e.9. tiansenand Blank, 1972,p.87). Locationsof individ- along U.S. Highway #l9l). Microscopicexamination ual samplesare given in the appendix. shows that the feldsparcrystals are mainly sanidine. The sanidine crystals are white to colorless,sub- They are unaltered and unzoned and readily ame- hedral to euhedral and reportedly attain a length in nable to optical and chemical study. This and the excessof I cm (Christiansenand Blank, 1972,p. B6). lack, until recently2,of any detailed information in The crystals selectedfor study are clear and trans- the literature on the material from this occurrence parent, and despitea whitish dusty appearanceon the prompted us to study the sanidine.We haveprepared surface of some, microscopeexamination failed to this note on the basisof eight samplescollected and reveal any alteration products. Refractive indices kindly provided us by Mr. David Doherty of Wayne were determined for several crystals from each State University. sample. ln addition to the crystals selectedfor refrac- The airfall tuff tive index determination,eight moderate-sizesingle- feldspar phenoclasts were handpicked at random The zone which contains the sanidine crystals is from eachof the original samples.Each selectedgrain part of the Mesa Falls Tuff, the middle member of the was divided into two parts, one to be usedfor X-ray powder diffraction and the other for electronmicro- I Present address: Harvard and Smithsonian Astrophysical probe analysis. Observatory, 60 Garden Street, Cambridge Massachusetts02138 'Recentfy, (1974) Friedman el al , have studied the oxygen OPtical data isotopic composition of the sanidine crystals from the volcanic rocks in Yellowstone National Park. Among the results they The selected crystals were crushed and sized to presented are two determinations on sanidine crystals from the - 100 + 120 mesh and studiedby immersion method' The values for 6'"0 given for the Mesa Falls Tuff dated at 1.2 m.y. The refractive index of the matching liquid was im- two are *4.0 and *4.1 respectively.A K/Ar age of 1.2 m.y. has of the Abbe refractome- been determined by J. D. Obradovich (written communication to mediatelychecked by means Christiansenand Blank. 1972). ter (sodium light source) and adjusted,where neces- A BASU AND C J. VITALIANO
sary, to p 25"C. The index,determination of which Geology, Indiana University,was followed.Details presented some slight difficulty, was checkedin each of the procedureare given in Klein (1974). instanceby using severaldifferent orientations in ad- Average analysesfor each of the eight samplesare dition to the nearly centeredBxo figure.The optical given in Table I along with the chemicalformulae data are given in Table l. and the calculatedmolecular proportions Or, Ab, and An. The latter valueswhen plotted Electron probe microanalysis on a ternary diagram cluster closelyin the sanidinefield near the The portion grains of the set aside for chemical border betweensanidine and soda sanidine(see also analyseswere mounted with epoxy in small sleeves, Barth, 1969,p. a4). carbon coated, and analyzed with an srrc Auto_ probe. For each sample,several random single spots X-ray study wereanalyzed in order to obtain an averagecomposi_ X-ray data were obtained by meansof a phillips tion. An acceleration potential of 15 kV was used goniometer using Ni-filtered CuK-radiation. X-ray with a sample current of about 0.022uA. and the powder diffractograms were obtained from each effectivebeam diameter was about l-2 microns. A samplewith three oscillatoryscans at Vr" 20 per min- counting time of 10 secondswas used for each ele- ute from 60' to 20" 20 using a chart speedof % inch ment for the nine element analyses.Data were re- per minute. Each corrected peak positiona of the ducedon an pDp-ll on-line computerwith the cEo_ feldspars,if unaffectedby interfering linesand agree- wHIZ program (Finger and Hadidiacos, 1971,1972), using B factors calculated as per Bence and Albee s Pure semiconductorgrade (1968)and a factorsfrom Albee and Rav fi970) for crystallinesilicon metal (as supplied by Jarrel Ash, JM 424, spec-impurity (3 ppm) was used as an the standardsused. The general pro..dur" at the rnternal standard. Its unit cell parameter (D. R Waldbaum: electronmicroprobe laboratory of the Department of personalcommunication) is 0.543093+ 0.000008nm at 25"C
TAsLr l. Optical propertiesand electronmicroprobe analysesof Mesa Falls Tuffsanidine
IP-1 tP -2 1P-3 IP -ll 1l,f-4 TLP-6 TT.p-O -25 (ave. (avg. ILP of 3) of 2) (avg. of (avg, ( )+ 3) ol JJ avg. of ) (avg. of 3) (avg. of 3) (awg. of 3) si02 oo. J4 65.26 66.28 oo. I o 66.a7 66.28 A1203 18.jz 67.36 18.50 18' 5r r8.36 18. 5l+ 18.1+1 r8.76 Fe0* 0.13 0 .08 0,13 o. oB 0.01 o. oB 0.09 0.08 Mso 0.00 0.01 0.00 0.01 0.01 0.02 a.o2 0.0t Ca0 a .24 0. 33 0.31+ o .26 a .22 0.31 o .35 Na20 3. 8l+ !. 09 3.98 J.yo 3.92 3.97 Kzo 1o ' 3r 10.09 3.93 10.32 to.r6 10. Ll+ 9.9r .97 Mn0 0.05 0.00 9 0.00 0.00 0.01 0.02 0.03 0.01 Ti02 0.05 0.01 0 .02 0.01 0.01 0.01 0.03 0.01 Total 99.\8 99 .37 99.\' 99.L' 99.II 99 .66 99.50 100.47
a r,5t67 i . )a lo 1 .5168 r .5182 1 . tt85 r .rLBz 1.519r B r.5203 I .5203 \.52A)+ r.j225 L , v I
Nunbers of io basls of 8 si 3.01 3.01 3.0l 3.01 3.00 3.02 A1 o .99 o .99 3.O2 o .99 o.99 1 .00 I .00 0.98 0.99 Ca 0 .01 0,02 0.02 o.42 0 .0r 0.01 0.01 Na o .3)+ 0.02 0 .36 0 ,3, o .35 0.35 K 0 .50 o o'3)+ ,59 0 .50 o ,59 a.19 o .59 o .57
Mole !ercent Or 5I 6I 6I 61 6I Ab JO 37 10 3T 37 38 38 37 An I 2 2 2 I 1 t_ 2
xFe recafcul ated as FeO SANIDINEFROM THE MESA FALLS TAFF, ASHTON,IDAHO ing within 0.03" 20 betweeneach scan,was averaged tions derivedby Luth (1974,revised,1975, personal for the three scansand tabulated.Miller indiceswere communication) also show that thesefeldspars have assignedaccording to Wright and Stewart(1968) and highest structural states with a mean Abc (Stewart Borg and Smith (1969).Usually more than 20 unique and Ribbe. t969) of 0.53 + .03. As might be ex- reflectionswere obtained for each sample. pected,maximum disorderis evidencedfrom the esti- A relatively precise method of approximating mated Al occupancyof the 7'(0) site in all the crys- Al/Si order-disorder involves the estimation of the tals (26.l%o L l.3Vo,Table 2). relative structural state of an alkali feldsparfrom its refined unit cell parameters(see Smith, 1974,for de- Discussion tails). Luth (1974) developeda set of equations in- volving the b and c cell edgesand the a* and 7* Optical properties, chemical compositions, and angles(reciprocal cell) to estimatethe proportion of structural states of the sanidine crystals in all the Al in the various T sites.We have used theseequa- eight samplessuggest that they are almost identical. tions to obtain an estimateof AllSi order-disorderof Individual oxides of the major cationsvary within * the sanidinephenoclasts from the Mesa Falls Tuff. 1 percent, refractive indices do not vary more than Unit cell parameteiswere refined by a least-squares 0.0024:and standard deviation of percentagesAl oc- method using a program written by Burnham (1962), cupancy at the ?"(0) site is only 1.3.All the crystals and modified by Waldbaum (1973,personal commu- studied are high-sanidineand the Or:Ab ratio of nication). The program was adapted and further up- approximately59:41 (estimatedfrom 201 peak posi- dated by Hazarika (1974, personal communication) tions, refined values of a and cell volumes) is in for use in Indiana University's cDc 6600 computer. agreementwith the results obtained through micro- Depending on the quality of the peaks they were probe analyses(K:Na : 62:38), weightedon a scalefrom one to three.As none of the The similarity of the physicaland chemicalproper- feldsparsshowed a l3l-l3l split only monoclinic ties of the sanidinephenoclasts collected from differ- solutionswere sought.Estimated Al occupancyln thq ent locations and different stratigraphiclevels of the various tetrahedralsites, Abc values,and variancesof Mesa Falls Tuff is remarkable. This similarity sug- each of the above quantities were calculated from the geststhat the physicochemicalconditions, including refined cell parametersusing a program written by pressure,temperature, chemical composition, and Mr. M. Hazarika(19?5, personal communication). probably the activitiesand chemicalpotentials of the Refinedcell dimensionsof all the crystals(Table 2) major cationsin the magma at the time of crystalliza- are nearly the same,and none of them appear to be tion of thesephenoclasts remained fairly constant.lt abnormal. The near constant valuesof cell volumes is likely that all the sanidinephenoclasts crystallized (X,ot = 705.44A3;ouor : 1.10A3)suggest a constancy more or lesstogether in the magma chamber before in composition. The b and c cell edgesindicate that being extruded in pulsesto produce the tuffs of the thesecrystals belong to the high albite-high sanidine different horizons and localities in the Mesa Falls seriesof the alkali feldspars.Calculations using equa- Tuff from which the sampleswere collected.
Tanlr 2. Cell dimensionsand order parametersof Mesa Falls Tuffsanidines
IP-1 IP -2 IP-3 ILP-4 tLr-o 8.)+35()+) 8.)+35(5) (8) B.\09 (4 )" 8.l+19(3 ) 8.)+r5(T) 8.)+30(l+) 8.)+31+(2) 8.\26(2) 12.995(r) " (Ri 13.008(2) r3.ooT(2) :-2,99\(r) 12.996(rJ 13.006(r) 13.008(1) 13.0r-5(2) u (r) c (X) 7.166(2) 7.166(r) z.:-58(t+) 7.r7r(2) r .L69 7.170(r) t.tTo(2) ?.170(3) pO rr6.o5(e) rr6.o3(r) rr5.ro(t+) L\6.2r(2) 116.oT(1) 1r6.05(1) r15.08(2) 116.o8(3) irEEl 7o\.2(5) 7oi. a(3) 703.9(r2) 701+.911; 7o6.)+(2) To5. o ( 3) ToT.r(l+) 705.9(8) Abc 0.515(20) 0.519(r4) o.j6\(i) o .iT7(12) 0.538(9) o.5l+o(T) 0,525(11+) o.5't5(r2) ttfrin 28.7(3) r, (o) z>.8(>) 26.o(3) 28.2(r) 28.9 (3) 26.9(2) 2T.012) 26.2(\) n** 22 2)+ 18 2? 22 )5 23 22
* Estinated standard d.eviation in parentheses rt Nunber of reflectlons used. in ceII refinement 408 A BASU AND C. J. VITALIANO
Appendix:sample documentation Benrn, Tou F. W. (1969) Feldspars. Wiley-lnterscience, New York.26l p. IP-l From Quarry (71/zmin. Ashton Sheet). Brxcr, A. E eNo A. L. ALBEE(1968) Empirical correctionfactors From bottomof quarry. for the electron microanalysis of silicates j6, This materialcon_ and oxides."/. Geol. tainspumice fragments up to 4,,in size.It 382-403. is poorly sortedand crudelycrossbedded. Bonc, I. Y. lNo D. K. Slrrrn (1969)Calculated powder patterns. lP-2 l' below Part II. Six potassium feldspars and barium feldspar. Am. Min- baseof pumicebreccia layer. eral.54.163-181. IP-3 Road cut on Hwy 90-127(See: Hamilton, BuRrHrm, C. W. (1962) Lattice constant refinement. CarnegieInst. 1965,Fig. 3, p. C-5)2' belowbase of top ll'ash. Year Book,6l, 132-135. pumicebreccia. Good cross-bedding. CnnrsrreNsrN,R. L. lNo H. Rrcueno BLANK,JR. (1972)Volcanic plateau IP-4 (8-22-3b)Road cut on Hwy 90-127(same stratigraphy of the Quaternary Rhyolite in yellowstone National Park. localityas IP-3). U.S. Geol. Suru. Prof. Pap.729-8. Flncrn, L. W. eNo C. G. Heororecos ( l97l Aspectsof computer ILP-25 Sand and ) Gravel euarry, Ashton euad- automation of an electron microprobe. Carnegie Inst. Wash rangle,one-half mile north of the Snake Year Book,70, 269-275. River,just eastof Highway 19l. From the - AND - (1972) Electron microprobe automation, Car- top of the exposure. negie Inst. lAash. Year Book,71.598-600 Fnrrol,nN, I., P. W. LlelreN, J. D. OsnnoovrcH, ILP-9 Samelocation; from middleof exposure. J. D . Gt-slsoN ero R. L. CHnrsrrlNsrN (1974) Meteoric water ln magma. ILP-4 Samelocation; from middleof exposure. Science, 184, 1069-1072. ILP-6 Samelocation; base of exposure. Hlurr-toN, W. (1965) Geology and petrogenesisof the lsland park cafdera of rhyolite and basalt, eastern Idaho. U.S. Geol. Suru, Acknowledgments Prof. Pap.504-C. Klrru, Connelrs, Jn. (1974) Greenalite, stilpnomelane, min- nesotaite, crocidolite, and carbonates in a very low-grade meta- morphic Precambrian iron-formation. Can. Mineral. 12, 475498. Luru, W. C. (1974)Analysis of experimentaldata on alkali feld- spars:unit cell parametersand solvi. In, W. S MacKenzie and J. Zussman Eds., The Feldspars. Manchester University press, 249-296. tory. The electron microprobe and X-ray diffraction equipment SIunH, J. Y . (1974) FeldsparMinerals. Springer-Verlag, New york. wereobtained by joint funds from N.S.F. grantsGA_37109 (Klein) Yol. l,627 p., Vol. 2, 690 p. and GY-10055 (Klein) respectivelyand the Indiana University Strwlnr, D. B. lNo P. H. Rrsse (1969) Structural explanation for Foundation. The electron microprobe analyses were covered by variations in cell parameters of alkali feldspar with AllSi order- N.S.F. Grant C'{-36186 (Klein). rng.Am. J. Sci.267-A,444-462. WRIGHT,T. L. rNo D. B. Srrwrnr (1968)X-ray and optical study of alkali feldspar. I. Determination of composition and struc- References tural state from refined unit-cell parameters and 2V. Am. Min- eral. 53, 38-87. Alnrr, A. L. lNo Lu-y Rlv (1970) Correction factors for electron probe microanalysis of silicates,oxides, carbonates,phosphates, Manuscript receiued, October 14, 1975; accepted and sulfates.Anal. Chem.42(tZ), t40g-1414. for publication, February 4, 1976.