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Primary Igneous Analcime: the Colima Minettes

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Primary Igneous Analcime: the Colima Minettes American Mineralogist, Volume 74, pages216-223, 1989 Primary igneous analcime: The Colima minettes Jlnrns F. Lurrn Department of Earth and Planetary Sciences,Washington University, St. Louis, Missouri 63 130, U.S.A. T. Kunrrs Kvsnn Department of Geological Sciences,University of Saskatchewan,Saskatoon, Saskatchewan S7N 0W0, Canada Ansrnrcr Major-element compositions, cell constants, and oxygen- and hydrogen-isotopecom- positions are presentedfor six analcimes from differing geologicenvironments, including proposed primary @-type) analcime microphenocrysts from a late Quaternary minette lava near Colima, Mexico. The Colima analcimehas X.,.,",: 0.684, ao: 13.712A, and one of the lowest D'sOvalues yet recorded for analcime (+9.28m).The major difficulty in identifuing primary @-type) igrreousanalcime is distinguishing it from analcime formed by ion-exchangeconversion of leucite (L-type analcime) or other precursor minerals. Pet- rographic criteria are shown to be unreliable in discriminating P and L analcimes,although the higher KrO and Rb contents and D'8Ovalues of L-type crystals may be diagnostic. P- and L-type analcimescan be distinguishedfrom classichydrothermal varieties (H-type) by the lower Fe contents of the latter. H-type analcimes,however, can have 6t8Ovalues as low as 8.9Vmand cannot be distinguished from P-type analcimes by this criterion. Analcimes formed from volcanic glass or zeolite precursorsin saline, alkaline lakes (S- type) or metamorphic sequences(M-type) can be distinguished from other varieties by their higher silica contents, smaller cell constants,and higher d'8O values of > + 17.7Vm. For all types of analcime, d'8O of the channel water does not correlate with 6180of the framework oxygen. With the exception of the P-type Colima sample, analcime channel watershave D'8Oand 6D valuesthat fall on the meteoric water line, but differ from modern meteoric water at the individual sample sites. The channel waters may reflect fluids that enteredthe analcime shortly after the mineral formed and have not been replacedby more modern waters. The Colima analcime has D'8Oslightly higher than expectedfor magmatic analcime, based on exchangepartitioning with mafic minerals in the Colima minettes. This enrichment in framework D18Oand the distinct isotopic composition of the channel water in the Colima analcime indicates that exchangebetween channel water and frame- work oxygen has occurred. In previous discussionsof primary igneous analcime, most attention has focused on blairmorites and analcimitescontaining centimeter-sizedeuhedral phenocrysts of analcime in the absenceof other primary hydrous minerals. We consider theserocks to be unlikely hosts for P-type analcime, which is far more likely to occur as microphenocrysts and groundmassmicrolites in mica- or hornblende-bearinglamprophyres, magmascharacter- ized by high water contents and silica undersaturation. The Quaternary minettes from Colima are the youngestand freshestlamprophyres yet describedand appear to represent the strongestcase for primary igneousanalcime. INrnonucrron microscopic texture (Lonsdale, 1940). In the last few de- Historically, petrographershave interpreted euhedral cades,however, experimentalresults have turned the tide phenocrysts or microphenocrysts of analcime in mafic of opinion away from the concept of primary, magmatic alkalic igneousrocks as primary @-type) crystalsthat pre- analcime. Experiments have shown that leucite, nephe- cipitated directly from a silicate melt (Pirsson, 1896; line, albite, and other minerals can be rapidly converted Washington,l9l4; Tyrell, 1928;Larsen and Buie, 1938; to analcime by reaction with aqueous solutions at low Lonsdale,1940; Harker, 1954;Wilkinson, 1965, 19681 temperatures(Saha, l96l;Gupta and Fyfe, 1975).In ad- Pearce,1970; Williams et al., 1982).In this view, anal- dition, phase-equilibrium experiments in the synthetic cime is stable from late magmatic stagesthrough low- system NaAlSiOo-KAlSiOo-SiOr-HrO have demonstrat- temperature dueteric processes,and the different envi- ed that analcime and silicate melt cannot coexist above ronments of analcime formation can be distineuished bv 650 "C or below about 5-kbar pressureunder water-sat- 0003-004x/89 / 0 10242| 6s02.00 2t6 LUHR AND KYSER: PRIMARY IGNEOUS ANALCIME 2l'7 Fig. l. Photomicrographof proposedprimary @-type)analcime microphenocrysts(a) in Colima minette SAY-104. Also present are phlogopite (p), clinopyroxene (c), and titanomagnetite microlites in clear brown glass(g) with vesicles(v). Field ofview : 0.35 mm. urated conditions (Peters et al., 1966; Morse, 1968; cubic cell constants, and oxygen- and hydrogen-isotope Boettcherand Wyllie, 1969;Kim and Burley, l97l; Liou, compositions. 197l; Roux and Hamilton, 1976).In hydrothermal ex- periments on a natural analcime-bearing from crinanite Slrr.rpr.Bs sruDIED AND TECHNIQUES the Dippin sill, analcimewas absentabove 400'C (Hen- mineral from derson and Gibb, 1977).Because the crystallization tem- Analcime characteristicallyforms asa secondary alteration of zeolites,volcanic glass,feldspathoids, or feldspars peratures of most analcime-bearing igneous rocks are in a broad rangeofgeologic and hydrologic environments.These much higher, thought to be euhedral analcime crystalsin include saline,alkaline lakes (S-type),hydrothermal veinlets (H- igneous rocks are now generally explained as low-tem- type), low-gradeburial metamorphic sequences(M-type), pelagic perature alteration products ofleucite, nepheline,or oth- sedimentaryrocks (Hay, 1966, 1978, 1986),and from precursors er precursor minerals (Rock, 1977; Wilkinson, 1977; of primary leucite and perhaps other feldspathoids in igneous Comin-Chiaramonti et al., 1979). rocks (L-type). In addition to the proposedP-type analcime from Although we concur with this interpretation for most Colima, five other analcimeswere investigated,representing L, analcime-phyric samples described in the literature, we S, and H varieties: argue that the most likely rock types to contain primary l. Colima minette SAY- 104 was chosento representthe pro- location is shown igneous analcime are lamprophyres. These are mafic al- posed primary @-type) analcime. The sample on the map of Luhr and Carmichael(1981). Whole-rock and kalic rocks containing phenocrysts of Mg-mica or am- glass analysesfor lava sample SAY-104 are given in Table 1. phibole groundmassfeldspars (Williams and et al., 1982), SAY-104 difers from other Colima minettes in its lack of feld- features consistent with high magmatic water contents. spars. In addition to olivine (5.1 volo/o)with spinel inclusions, Unfortunately, most lamprophyresoccur as dikes that are clinopyroxene(l6.6oh), phlogopite (0.40lo),and apatite, SAY-104 very old and substantially altered, thus rendering them contains more than 8.30/oeuhedral analcime microphenocrysts ill-suited to test for primary analcime. Luhr and Carmi- (up to 0.16 mm diameter) encasedin clear brown glass(Fig. 1). chael (1981) described Quaternary basanitesgradational These crystals are completely homogeneousand unaltered and to analcime-bearing minettes near Volcan Colima in may contain small clinopyroxene and titanomagnetite inclu- western Mexico. Minettes are the most common variety sions, identical to microlites in the surrounding glass.The lack in may of mica lamprophyre, with K./Na > 1 and characteristi- of feldspars and the large size of analcimes SAY-104 have resultedfrom the failure offeldspars to nucleateupon erup- cally high Ba and Sr contents. The Colima samples are tive quenching,a common development in experimental cool- the youngest freshestminettes yet reported, and and we ing-rate studies(Lofgen, 1980).A histogram of K,/Na ratios for consider them the best candidatesfor primary analcime. the various Colima alkaline samplesis shown in Figure 2. Sur- In this paper, the proposed primary P{ype) analcime prisingly, there is no systematicdifference in K,/Na betweenthe from Colima is comparedwith analcimesfrom other geo- basanites and minettes, although the three highest ratios are logic environments, using major-element compositions, clearly related to high abundancesof phenocrystic phlogopite. 2r8 LUHR AND KYSER:PRIMARY IGNEOUS ANALCIME SAY 1O4 Trele 1. Colimaminette SAY-104: whole-rock and olass anal- WBG++ yses (wt%) -tr_'- _.La a 2% o.7% 11.8% (rililtilcretmtmmn nffi n SAY-104 Whole rock Glass o.5 1.O 1.5 sio, 50.20 53.91(0.6s) Tio, 1.80 1 83(0.10) K/N^ Alr03 13.05 18.03(0.26) FeO. 7.79 2.74(0.13}, Fig. 2. Whole-rock molar K,/Na ratios for 27 basanite-mi- MnO 0.14 0.11(0.01) nette samplesfrom Colima (Luhr and Carmichael, 1981; Luhr, Mgo 8.s8 1.2s(0.16) 1980). Open circlesare basanites,open squaresare minettes,and CaO 2.58(0.27]. Naro 379 5.s3(0.14) closed triangles show three leucite-bearingbasanites. The three KrO 3.72 6.76(0.24) minettes with the highest K,/Na values have the highest modal So. n.o. 0.19(0.04) phlogopitecontents as labeled(2 volo/o,0.7o/o, 11.8o/o). The whole- Total 10000 95.32 rock (WR) and glass(G : star) analysesof SAY 104 from Table LOI 2.94 n.o. K/Na(molar) 0.65 0.80 I are indicated. [The NarO and KrO values for Colima samples SAY-5A and SAY-6E were reversed in the above references: Nofe; Whole-rock analysis by xRF(normalized anhydrous) on pressed SAY-5A-2.55 wto/oNa,O, 3.850/oK,O; SAY-68-2.770loNa,O, powder with wet-chemicallyanalyzed minette standards used to define 4.160/oKrO.l workingcurves. Loss on ignition(LOl) determined at 950'C. Meananalysis (8 points)and 1 std dev (inparentheses) of brownglass in SAY-104(Fig. 1) determined by
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