THE AMERICAN MINERALOGIST, VOL.51, MAY-JUNE. 1966 COMPOSITIONAL VARIATIONS OF PLAGIOCLASE FELDSPAR FROM A BASALTIC LAVA FLOW Jnnnv M. Honlnn, Departmentof Geology,Teras Western College, El Paso,Teras. Assrnecr A detailed study of the Rock Creek basalt flow reveals significant relationships between chemical composition of the plagioclase feldspar and texture. The An content of the ground- mass plagioclase varies inversely with size and directly with the amount of associated glass. The rate of cooling, alter extrusion, determined the composition of the groundmass plagio- clase. The plagioclase phenocrysts, averaging 20 percent more calcic than the corresponding groundmass plagioclase, are intratelluric. INrnooucrroN At the present time no detailed information is available concerning the variation in composition of any one mineral or the compositional varia- tion of a mineral present in two distinct sizes (i.e., phenocryst and groundmasscrystals) in a lava flow. In order to obtain such information a study has been completed concerningthe variation in composition of groundmass and phenocryst plagioclase from a thick porphyritic flow (250 to 470 feet), the Rock Creek flow, of the Columbia River Basalt in west-centralIdaho (Bond, 1963). Sampleswere collected from five measuredsections of the Rock Creek flow and the phenocryst and groundmassplagioclase in each sample were concentratedseparately with a Frantz Isodynamicseparator after crush- ing and screening.The composition of the plagioclase,expressed as mole per cent An content,was determinedby the glassfusion method (Foster, 1955).In addition, the textural relationshipsin eachsample were studied with thin sections,with specialemphasis on determinationof the sizeof the groundmass plagioclaseand the amount of associatedglass so as to obtain an index of the rate of coolingfor each sample.The factors that determine the texture of any igneousrock include the rate of cooling,bulk chemical composition, composition with regard to hyperfusibles,and pressure.The similarity in averagecomposition of the plagioclasepheno- crysts from all samples suggeststhat the first-formed crystals of plagio- claserepresent crystallization from the samemelt. There is no evidenceto indicate that the bulk chemical composition changed during crystalliza- tion. The content of hyperfusibles and the effect of pressureduring crys- tallization are hard to appraise, but it seemslikely that the influence of pressurewas small becauseof the crvstallizationat the surface.There- fore, the rate of cooling,after extrusion,was the primary factor in deter- mining the final texture. However, in small areas variations in volatile constituents and pressure may have influenced the rate of cooling and 807 808 JERRY M. HOFFER Tarr,r 1. CouposrrroN.tr, .a.NoToxrunnl Dera ol Pr-a.crocr-asr FRoM THE Rocr Cnaor Fr-ow Sample An Content An Content Average Size (width in mm) Per Cent Nunber Pheno.rJSt Groundmass of Groundmass Crvstals Glass w-1 600 o12 7 w-3 61.5 54.5 0.04 48 w-4 61 5 530 004 45 635 485 0.07 27 w6 59 .5 51 0 0.04 .57 w-7 670 540 007 7 w-8 675 395 0.09 t7 w9 645 500 0.07 23 w-10 620 s2.0 0 0.1 .3-5 W-11 600 ,13.5 009 28 w-12 625 56.0 0.06 53 SKC-1 620 ,t0 0 009 5 SKC-2 61 5 370 008 8 SKC-3 63 .5 360 0.09 8 SKC.4 620 400 0.07 17 SKC_6 61 5 56..5 0.02 62 630 570 002 67 SKC-8 62.0 420 0.07 17 SKC-9 630 505 003 SKC-IO 61 0 440 004 32 sKc-11 730 480 004 20 SK-1 635 4.3 5 006 8 SK-2 620 4r.5 008 10 SK,3 620 400 008 6 SK-4 630 490 0.04 35 SK-5 62.0 475 0.07 8 630 53.0 0.o2 38 SK-8 62.O 5l 0 0.03 SK-9 63.0 47,5 006 12 sK-10 635 54.0 003 SK-11 52.O 0.04 35 SK-12 620 530 007 SK-13 620 51 0 0.03 43 R-1 635 430 0.13 640 ,+5.0 0.13 R-3 640 41 5 0.13 R-4 640 445 0.12 R-5 650 52.0 0.11 R-6 635 0.10 R7 60 .5 45.0 009 R-8 620 .56.5 0.03 52 R-9 620 505 0.04 30 R-10 61 .5 51.5 003 R-11 61 5 505 004 28 s-1 620 ,15.0 008 7 s-2 640 435 009 10 5-.1 670 430 0.09 J s-4 650 460 010 5 660 .52 0 007 38 s-6 630 .5.5.0 006 s-8 635 4q5 009 7 s-9 625 48.0 006 12 s-10 660 550 0 0.5 s-12 62.5 003 20 s-13 62.5 .50.0 0.02 s-14 62.5 520 003 17 s-l t 62.5 .5S0 o02 )l PLAGIOCLASE COMPOSITIONAL VARIATION hencethe final texture. The averagewidth of the groundmassplagioclase rvasdetermined by micrometerocular measurements on two thin sections per sample.The percentageof associatedglass was estimatedfrom two thin sectionsper sampleby comparisonwith standardpercentage charts. To check the relative accuracyof theseabove determinations,each thin sectionwas ranked in order of increasingsize of the groundmassplagio- claseand the amount of associatedglass by projectionfrom a 35 mm slide projector. In all samples,excellent agreement was obtained by both methods.The resultsof thesedeterminations are summarizedin Table 1. Rrsur,rs ANDDrscussroN The compositionof the groundmassplagiociase ranges from An36to An5eand shows no s1'stematicspatial variation vertically or laterallv (l) .. u, bo y=O.2445-O.OO38x tro o.t5 - E ctl r= 0.68 ?o '-(L o.to fo 33 o.o5 3eE q)c o.oo N= cn9 (9 MolePer CentAn Content Fro. 1. Variation of mole per cent An content of the groundmass plagioclase samples with size (width) of the groundmass plagioclase (y: equation of the line and r : Iinear corrclation coefficient). Each point represents a sample. through the flow. Marked correlationsexist, however, between the An content and cr1'stalsize of the groundmassplagioclase (Fig. 1) and be- tween the An content of the groundmassplagioclase and the amount of associatedglass (Fig.2). As the An content of the groundmassplagioclase increases,the sizeof the crystalsdecreases (r: -0.68), and the amount of associatedglass increases (r: +0.75). In addition, a marked degreeof negative correlation (r: -0.i9) exists between the proportion of asso- ciated glassand the sizeoI the groundmassplagioclase crystals. The plagioclasephenocrysts range in compositionfrom An5e5 to An73, with 98 per cent of the samplesin the interval An5e5 to Anozr. Textural relationships and composition (the phenocrysts average20 per cent more 810 IERRV M. HOFFER calcic than the corresponding groundmass plagioclase)suggest that the plagioclasephenocrysts are intratelluric and crystallized at depth under conditions of slow cooling, before the crystallization of the groundmass plagioclase.The relationship of the size of the groundmass plagioclase and proportion of associatedglass indicates that there were variationsin the rate of cooling after extrusion of the magma. In samplesrepresenting conditions of relatively slow cooling the proportion of glassis small and the average size of the groundmass plagioclaseis relatively large. Con- versely, small groundmassplagioclase crystals, formed under condition of rapid cooling, are found in samples containing a large amount of glass. Rock Creek samplesrepresenting rapid cooiing, inferred from the small 60 y=42.OOO1.O.2332x. E55 :so o4so E 35 o l5 30 45 60 75 PerCent Gloss Frc. 2. Variation of mole per cent An content of the groundmass plagioclase samples rnith per cent glass (y:equation of the line and r:linear correlation coefficient). Each point represents a sample. crystal size and abundant glass, possessgroundmass plagioclaseof high An content (An66to An6e)whereas those that cooled more slowly contain groundmassplagioclase of low An content (Anaeto Anro). With regard to the plagioclasephenocrysts, no significant relationship was found to exist between the composition and the rate of cooling, as indicated by the proportion of glass. The relationship between compositional variations of the groundmass and phenocryst plagioclaseand cooling history, as indicated by textural features, during crystallization of the Rock Creek flow can best be ex- plained by referenceto the plagioclasephase diagram in Fig. 3 (Bowen, 1913).In this simplifiedscheme aII componentsof the Rock Creekmagma are neglectedexcept plagioclase.The temperaturesindicated on the phase diagram do not imply the temperatures of crystallization of the magma of the Rock Creek flow. PLAGIOCLASE COMPOSI TION A L VARI AT ION 811 t540 () o t420 (l) 5 E, q) CL E t300 o 20 40 60 80 too An Content Frc. 3. The phase relationships of the plagioclase feldspars (A:composition of the sim- plified hypothetical Rock Creek magma. B:composition of the most calcic plagioclase phenocryst (Ana), C:composition of the most sodic plagioclase phenocryst (Anrr.u), D:composition of the most calcic groundmass plagioclase (An5e), E:composition of the most sodic groundmass plagioclase (Aq6), and F:composition of the hypothetical Rock Creek magma after extrusion to the surface). When the temperature of the hypothetical Rock Creek magma of com- position A intersected the liquidus, the first plagioclaseto crystallize had a composition of An73, the composition of the most calcic plagioclase phenocryst (B in Fig.3). As cooling continued more sodic crystals of plagioclasecrystallized and some of the earlier more calcic crystals were converted to more sodic composition by reaction with the melt. The relatively small range of composition of the majority of the Rock Creek plagioclasephenocrysts suggeststhat, for the most part, close approach to equilibrium between the crystals and the melt was attained during 812 JDRRY M. HOFFI'R crystailization.Weak zoning in manv of the phenocrystsindicates, how- ever, that equilibrium betweenthe melt and crystalswas not maintained throughout the crvstallizationof the plagioclasephenocrysts.