American Mineralogist, Volume 65, pages 63-74, I9B0
Silica solid solution and zoningin natural plagioclase
Devn W. Beary eNo ARoBN L. ALBEE Division of Geologicaland Planetary Sciences' Califurnia Institute of Technology Pasadena,California 9I 125
Abstract
Microprobe analysesof plagioclasefrom somelunar basaltshave anomalousstoichiometry as comparedto terrestrialfeldspars. The anomaly indicatesthe presenceof the "excesssilica" substitution,[ ]SLO6.This coupled substitutionleaves a vacancyin the feldsparlattice and is effectivelySio, dissolvedin the feldspar.Detailed study indicatesthat core-rim [ ]SLo, zoning is a common phenomenonin plagioclasefrom lunar basalts.Anorthite-albite zoning is alsotypically present,so plots of [ ]SioO6as a function of anorthite contentprovide a con- venient measureof the [ ]SioO6variations present.Most lunar basalticplagioclase shows a monotonic increasein [ ]SioOrwith albite content, typically ranging from 0 molo percent [ ]SLOain the coresto 5-7 percentin the rims. This zoning is producedby two cooperating processes:a falling temperatureof formation, and growth from a progressivelysiliceous liq- uid. In addition, rapid growth and delayednucleation ofplagioclase and silica phaseappear to be important to the disequilibrium incorporation of [ ]StoO.. By contrast,plagioclase from a wide variety of terrestrialoccurrences lacks [ ]SioOr.The absenceof [ ]SLO8in many terrestrialigneous plagioclases is primarily related to the pres- enceof HrO, which decreasesthe liquidus temperaturesrelative to thosein lunar rocks.Ter- restrial basaltswhich have comparablecrystallization temperatures to their lunar counter- parts lack [ ]SLOgin part becauseof an absenceof strongin situ difrerettiation. In addition, plagioclasetypically crystallizesprior to pyroxene(the reverseof the mare basalts),so kinslio effectshave a reduced role. If a melt is in equilibrium with cristobalite (or tridymite or quartz), the silica activity is butrered,and the [ ]SLO, content of the plagioclaseis directly related to the temperature.[ ]SioO,incorporation, however, is typically not an equilibrium process,so there is little hope of using it directly as a geothermometer.
Introduction and Facchinelll(1974),Ito (1976),and Longhi and Hays (1979)all succeededin producing excess-silica- This paper is concernedwith the extent and signif- bearing feldspars in the laboratory, including both icanceofsilica solid solution in natural plagioclase,a anorthite and albite. The Longhi and Hays study subjectwhich hashad a long history in the geological presentedthe CaAlrSi,O,-SiO, phase diagram, lay- literature. Myrmekite was thought by Schwantke ing the foundation for the interpretationof excesssil- (1909) and Phillips (1964)to have formed by the sub- ica in natural feldspars.Consistent with the idea of solidus unmixing of feldspar and a dissolvedsilica- silica solid solution is the fact that feldspar has a rich component.The first discoveriesof excesssilica structure polytypic to that of coesite(Megaw, 1970; in natural feldsparswere those of Chayes and Zies Thompson and Hovis, 1979), a condition favoring (1962)and Carmen and Tuttle (1963, 1967).in sani- mutual solubility. dine from rhyolites.Luth and Tuttle (1966),however, The first demonstrationof excesssilica in natural found no evidencefor excesssilica in alkali feldspars plagioclasecame from studies of the lunar samples (mostly microcline and orthoclase)from a wide vari- (e.g., Weill et al., 1970).Many workers noted that geologic ety of environments.Wyart and Sabatier some lunar plagioclaseanalyses differed in stoichio- (1965), Kim and Burley (1971),Orville (1972),Bruno metry from their terrestrial counterparts. The "anomalous" analyseshad too much Si, too little Al, I Contribution No. 3260. and a deficiency of the singly- and doubly-charged 0003-004x/80/0 I 02-0063$02.0o BEATY AND ALBEE: SILICA SOLID SOLUTION IN PI'/IGIOCLASE cationswhich fill the larger site (Table l). Wenk and picture of the extent,causes, and signfficanceof silica Wilde (19'73)and Longhi et al. (1976a)demonstrated solid solution and zoning in plagioclase. that the feldsparsubstitution [ ]SLO' is necessaryto Analytical techniques balancethese anomalous analyses. This coupled sub- stitution. Small variationsin the amount of any of the major cationscan drasticallyeffect the amount of excesssil- (vacancy)+ Si: Na * A1,or [ ] ica which is subsequentlycalculated. Systematic dif- [ ]+2Si:Ca+'2Al ferences,such as those introduced by using different analytical methodsor eventhe samemethod but dif- is hereafter referred to as the excesssilica sub- ferent instruments,may affectthe excesssilica values. stitution. By routinely calculating the amount of For this reason,we have used only data collectedon in plagioclaseanalyses from the Apollo I I [ ]SLO, the Caltech microprobe. Analyses for all elements basalts,Beaty and Albee (1978) discoveredwell-de- were performed consecutively on a single spot with a fined core-rim excesssilica zoning in single crystals. MAC-5-SA3 electron microprobe interfaced to a Subsequentstudies of other groups of lunar basalts PDP-8/L computer for on-line control and data (Beaty and Albee, 1979; Beaty et al., 1979a,b, ci processing.Operating conditions were uniformly 15 Baldridge et al., 1979)showed that zoning [ ]SLO. kV acceleratingvoltage and either 0.05 or 0.005pA is a common phenomenonin lunar plagioclase.Our sample current (on brass) with beam current in- purposeis to synthesizethese data, supplementthem tegration and pulse height selection.Elements with new analysesfrom other rock types and other with peak to backgroundratios greaterthan five (about I geologic environments, and develop a meaningful weight percent of the oxide) are counted to a preci- l%oor better. Minor elements(
End-menbet abundaDces percent (Table l), which would decreasethe calcu-
KFe" Sij08 0.0 0,0 0 0 0.0 0.0 0.0 0.0 0.0 lated excesssilica by 0.2-0.3 mole percent.The cal- ro1si308 0.9 1,3 1.0 1.0 7.4 0.5 0.5 0.5 culated uncertainties (lo, using Bernoulli counting NaA1Si3o8 28.4 34.8 32 3 34.9 r9.3 \6.7 L2.4 15,1 statistics)for this algorithm arc t0.2 mole percentfor 0,0 0.0 0.2 BaA12S12o8 0.0 0,1 0.0 0,0 00 +1.5 percentfor the excess cd12si208 68.7 60.8 63.6 61.3 71.0 75.6 73.2 68.0 the An content,and mole m1zSiz08 0.1 0.5 0.0 0.0 2.3 1.5 0.0 1.8 silica. Cahsi308 1.8 2.6 3.1 29 2.0 2.4 8.3 6.8 The tetrahedral Fe calculation [Ca(FeSi,)O'] is [ ]s14oS 0.0 0.0 0.0 0.0 4.o 3.3 basedon spectroscopicexperiments. Mtissbauer stud- (1971) and Appleman et al. (1)-(2) Skaersaard lntrusion (unpub dala fron R' Gregory); (3)-(a) New ies by Hafner et al. Mexico basalt (unpub. data fron w S' Baldridge); (5)-(6) 10047 (Beatv ahd (1971)suggest that, in Apollo ll and 12 plagioclase Albee, 1978); (7)-(8) 70215 (DyEek 4 a1., 197s). at room temperature,20-60Vo of the Fe'* is in tet- BEATY AND ALBEE: SILICA S0LID S1LUTI1N IN PLAGI)CI}ISE
rahedral coordination. Although these results were not confirmed in a near-infrared spectral study (using RM APOTLO1] LOW_K Luna 20 material) by Bell and Mao (1973),Sclar and BASALTS Kastelik (1979) were able to synthesize a complete >- -t\ isostructural series ranging in composition from .\ 10047
CaAlrSirO. to CaFeSirOr. Furthermore, Wenk and error
Wilde (1973),Longhi et al. (1976a,b),and our studies I all indicate that both [ ]SLO, and Ca(Mg,Fe)SirO, o I are necessaryto balance lunar plagioclaseanalyses. $ 30 Note, however,that the [ ]SirO, calculation is inde- pendent o of site-occupancyassumptions, depending I only on the total number of cations. If ferric iron ^o2 were presentin a plagioclasecrystal, the amount of ol excesssilica calculated using the above method would be about 0.1 mole percent less than that ac- tually presentfor every 0.5 weight percent FerOr. Our analytical techniquewas cross-checkedby an- alyzing in a single microprobe run a variety of plagioclasesamples that had been previously ana- lyzed,at different times. By relocating old analysis points we were able to determineour total reproduc- eo ibility, as well as to directly compare different feld- Morepercenr 38o,a=,routt
spar analyses.This experinent showedbetter repro- Fig. l. Variations in [ ]SiaO6 as a function of CaAl2Si2O, for ducibility in excesssilica determination than the plagioclase in Apollo ll low-K basalts 10047, 10092, and 10020. calculated 1.5 mole percent (o : 0.58 based on 26 In each case excess silica increases in a regular way from core to measurements).In addition, our analysesof well- rim of the plagioclase, and the three samples show the same zoning trend. This trend is produced by both falling characterizedterrestrial plagioclasestandards in- temperature and, in situ differentiation. The error bar is the calculated absolute dicated no excesssilica, even when analyzed alter- uncertainty, but the reproducibility (width of the data envelopes) nately with high-[ ]SLO, lunar feldspars. is somewhat less. One of our major conclusionsis that detectableex- cesssilica is not present in most terrestrial plagio- trend can be found in core-rim profilesof singlecrys- clase.This means that the crystallographicsites are tals, and eachgrain showsthe sameprofile. fully occupied.It is equally valid, therefore,that in- 10092 and 10020 are much finer-grained than steadof normalizing to a total positive chargeof 16, 10047, and in both rocks plagioclaseis the third terrestrial feldspar analysesbe normalized to 5 total phase to appear on the liquidus (after olivine and cations. The ferric/ferrous ratio, however, typically spinel).Nevertheless, both sampleshave excesssilica cannot be determinedfrom the total positive charge zoning profiles very similar to that of 10047.The becausethe uncertaintiesare too large (seeabove). principal diference lies in the extent of the zoning; rn si/z differentiation was not as efficient in 10092.The Silica solid solution in natural plagioclase remainder of the Apollo I I low-K basaltshave zon- ing patterns similar to those in 10047, 10092, Apollo II low-Kbasalts and 10020(Beaty and Albee, 1978;Beaty et al.,1979b). Figure I illustrates the compositionaldependence The simple excess-silicazoning profiles in plagio- of excesssilica in plagioclasefrom three Apollo I I clase from the low-K basalts can easily be under- low-K basalts.In eachrock, [ ]Si.O, showsa strong stood in terms of the phase diagram plagioclase- inverse correlation with anorthite content. plagio- SiOr. The An-SiO, join of this diagram has been ex- clase(Anr,) is on the liquidus in 10047,and contin- perimentally determined at low pressureby Longhi " ues to crystallize to the solidus (A&r). The first and Hays (1979) and is reproduced in simpliffed plagioclaseto form has no excesssilica, but as the form in Figure 2. Th;e excesssilica substitution is rock crystallizes,more and more excesssilica is in- equivalentto SiO, dissolvedin anorthite and is there- corporated,reaching a maximum of 4.6 mole percent fore governed by the univariant curves bounding the in the mesostasisareas (Fig. l). The entire zoning anorthite fietd. At temperaturesclose to its melting 66 BEATY AND ALBEE: SILICA SOLID SOLUTION IN PLAGIOCLASE
from G4 percent,and 12007zones from 0-2 percent (Baldridge et al.,1979). When theseexcess silica con- tents are comparedto the anorthite contents(Fig. 3)' the plagioclase analyses from all three samples fall Crstb along a common trend. For the reasonsdiscussed Anorihiie+ Crislobolile above,the inverserelation between[ ]SLO' and An content suggeststhat the three samplescrystallized /Anorthile + Tridymile /- -- ? ------'? ------from the same parental liquid along the same liquid 020 40 60 80 loo line of descent. The different positions along the CoAl2Si sio2 20s wt % si02 common trend suggestdi-fferent positions of plagio- Fig. 2. Experimentally-determined phas€ diagram for the clase saturation. binary system CaAl2Si2Os-SiO2 (after Longhi and Hays, in press). Petrologicevldence indicates that the seoling rate The width of the An field determines the maximum solubility of decreasesin the order l20l I > 1204.3> 12007(Bald- given temperature. Additional details involving [ ]SroOr at any the sameorder in which the silica polymorphs on the right side ofthe diagram have been idge et al.,1979), [ ]SLO' omitted for simplicity. decreases.This correlation is also found in the Apollo 12 feldspathic basalts (Beaty et al., 1979c). point, anorthite can contain very little dissolved sil- Plagioclasesaturation occurs after olivine, pigeonite, ica. As the temperaturefalls, the solubility of silica in and augitein theserocks. With increasedcooling rate anorthite steadily increases,reaching a maximum of all the saturationtemperatures are depressed,but the 9 mole percent [ ]Si.O' (8 weight percent) at temperature of plagioclaseappearance is depressed l370oc, the temperatureof the anorthite-cristobalite most (Walker et al., 1978; Grove and Raudsepp' eutectic.With further cooling, the anorthite will pro- 1978; Grove and Bence, 1979). Therefore, in the gressivelylose its [ ]SLO, to cristobalite,tridymite, more rapidly cooled samplesmore of the early ferro- and then quartz. magnesianphenocrysts form. This enrichesthe resid- In the Apollo 1l low-K basalts,the first plagioclase ual melt in silica, and depletes it in Ca and Al crystallizedat high temperaturefrom a melt that was through the incorporation of CaAlrSiO. in pyroxene. marginally quartz-normative. As predicted by the The excess-silicadata can thereforebe understoodin phase diagram, it contains negligible amounts of terms of a simple undercooling series.In l20ll [ ]SLO'. As the magma cooledit differentiated,and plagioclase began crystallizing at a lower temper- the residual liquid was enrichedin SiOr. The mono- tonic increasein [ ]SLO, from core to rim of the plagioclasecrystals is therefore causedby two fac- tors: increasing silica solubility in plagioclase at lower temperatures,and an increasinglysitceous pa- rental melt. Becauseboth temperatureand composi- 7 O tion determinethe amount of [ ]SLO' incorporated o6 in the feldspar,excess-silica zoning profiles must de- a -5 pend strongly on the liquid line of descent.The fact c4 that all of the Apollo 11 low-K basaltshave similar o zoning trends therefore indicates that they followed 83 tiquid lines of descent.This is consistentwith similar -)o the bulk chemical and petrologic similarities present within the suite (Beaty and Albee, 1978). I
Apollo 12pigeonite basalts 80 Like the low-K basalts,the Apollo 12pigeonite ba- Mole PercentCoAl25i2Og salts also show an inverse correlation between Fig. 3. Excess silica in plagioclase from the Apollo 12 pigeonite calcic and anorthite content (Fig. 3). In eachrock, basalts. In each sample, single crystals are zoned from [ ]SLO, cores to sodic rims. Superimposed plagioclaserims contain more excesssilica than the [ ]SlOs-poor [ ]Si+Oa-rich on this is a cooling rate effect: the slower the rock cooled, the less cores.In contrastto the low-K basalts,however, the overall excess silica its feldspar contains. This indicates that severalpigeonite basalts do not have similar profiles. kinetic factors are also responsible for the incorporation of excess l20ll rangesfrom 3-6 percent[ ]SLO', 12043has silica in plagioclase. BEATY AND ALBEE: SILICA SOLID SOLUT'ION IN PLaIGIOCITISE
ature and from a more siliceousmelt than the plagio- geonitebasalts, however, there is little assurancethat clasein 12007. these four samplescame from the same lava flow. When plagioclaseand one of the silica polymorphs The excesssilica data suggeststhat7O2l5 and 71055 are in equilibrium, their compositions are deter- followed the sameliquid line of descent(Fig. 4), but mined by the solvusportion of Figure 2.The position that 74255 crystallized along a different path. Note of the solvus is relatively insensitive to the liquid that each sample considered alone shows the antici- composition (for a more detailed discussion,see be- pated inverse correlation between [ ]SLO. and low). Therefore,since all the pigeonitebasalts are in anorthite content. equilibrium with either cristobalite or tridymite at their sohdi,it is theoreticallypossible to comparesol- Apollo 15 basalts idus temperaturesby examiningthe excesssilica con- We have analyzed plagioclase from three tents of the last plagioclaseto crystallize.This sug- Apollo 15 basalts.In both 15555and 15682,olivine-norma- geststhat l20ll, which has more [ ]Sr.O, had a tive and quartz-normativesamples respectively, higher solidus temperature than 12007. Figure 2, the excesssilica increasesfrom 0 percent at An86to 4 per- however, indicates that this temperature difference cent at An In cornmonwith the Apollo 12 pigeon- would have to be severalhundred degrees,which is u. ite basalts,plagioclase was the last major phase petrologically unreasonable.This evidence argues to crystallize.The trends suggestsimilar fiquid lines of that the high I lSioO, contentsof the more rapidly descentfor the two magmas. However, 15976, cooled samples,such as l20ll and 12043,are pro- an- quartz-normative duced by disequilibrium kinetic factors. other basalt, has a maximum of only about I mole percent excesssilica. Experimentalstudies (e.g. Grove and Bence,1979) show that with increased cooling rates the abun- Apollo I I high-K basalts dances of the minor constituents increase in each Most of the Apollo I I high-K basaltshave plagio- mineral. This is thought to be related to kinetic fac- clasecontaining large amountsof excess (Beaty tors: if the material in the magma at the mineral- silica and Albee, 1978).In somesamples is corre- melt interface does not have time to difuse away [ ]SLO, lated with anorthite content (Fig. from the growing crystal,it is incorporated(Albarede 5, 10ft24,l0ol7), but in othersit is not (e.g lOO72).100.24,the coarsest- and Bottinga, 1972).The high [ ]SLO, contents of grained rock ofthe suite,ranges from2-7 percent l20l I and 12043may in part be related to this proc- [ ] SLO, (Fig. 5). In order of decreasinggrain size ess.Also irnportant is the delayed nucleation of the 10072,10071,10017, 10069, and 10049contain l-4, silica phase(Longhi, personalcommunication, 1979), 0-7,0-2,0-1, and 0-7 percentexcess silica, respec- which could cause the metastableincorporation of tively. Unlike the Apollo 12 and 17suites, the overall [ ]SLO, in plagioclasein large amounts.The signifi- amount of excesssilica is not correlated udth the in- canceof kinetic effectsis confirmed by the observa- tion that most lunar basalts have solidi about 1050"C,and at that temperatureplagioclase in equi- librium (using Longhi and Hays' Fig. l) with tri- dymite containsabout 3 mole percent [ ]Si.Or. The high [ ]SioO, contents of mesostasisplagioclase, o therefore, appear to be kinetically controlled. rJ' (t
Apollo l7 basalts c o The positive correlation between o overall excesssil- (I ica content and cooling rate is also presentin a suite o of Apollo 17basalts (Fig. a).70215contains from 2- 7.5 percent [ ]Sr.O, 74255 has 0-6 percent, 71055 has l-4 percent,and 75055ranges from 0-2 percent. According petrographic to the observationsof Dy- Mole PercenlCoAl2Si2Og nek et al. (1975),the cooling rate decreasedin the Fig. sameorder (70215> 74255> 71055> 75055).In 4. Excess silica zoning in Apollo 17 basalts. In cach sample excess silica increases from core to rim ofthe plagioclase. contrastto the Apollo 12pigeonite basalt suite,these As in Apollo 12 basalts (Fig. 3), the bulk cooling rate also affects data do not define a corlmon trend. Unlike the pi- the amount of excess silica. BEATY AND ALBEE: SILICA SOLID SOLUT:ION IN PLAGIOCLASE ferred cooling rate. Neither does it depend on the APOLLO6 BRECCIA66075
U bulk composition.In short, the excesssilica contents E n o I E of these samplesvary in a way not easily understood J c F 70 P F F in terms of any of the argumentsadvanced above. F @ 0 ) ! = O ) d F As discussedin the sectionon Apollo I I low-K ba- - c I E z E = o z z 0 salts, a natural consequenc€of the anorthite-SiO, i- o ) z o U a o o t phasediagram (Fig. 2) is that the amount of excess 3 z c z F b40 ) j z ! ! plagioclase o i d i L silica in will steadily increase with crys- 3so E 6llization until cristobalite saturation occurs (at the l zzo eutectic),where crystallizationis completed.In mul- ticomponent systems cristobalite saturation will oc- to cur at a cotectic and plagioclase and cristobalite will o E LI t-- | | cocrystallizedown the cotectic.As long as the liquid o | 2 0 | 20 | ? 3 0 r 2 0 | 2a 20 20 | 2a | 2 MolePercenl []Sioou in Plogioclose is in equilibrium with both cristobalite and plagio- silica in plagioclase from lunar highlands breccia clase,the amount of in the plagioclasewill Fig. 6. Excess [ ]SLO, 66075. Unlike those in the mare basalts, these feldspars are decreasewith falling temperature.Thus, [ ]SLO. re- virtually devoid of[ ]SiaO6. In all but the olivine basalt clasts ttre versalsought to be present in the rims of feldspars data lie within aaalytical uncertainty (1.5 mole percent) of zero. from cristobalite-bearing lavas. Such reversals were These plagioclases are also essentially unzoned (Ane7-e3). In the reported for six of the ten Apollo I I high-K samples otvine basalts, however, the plagioclase is zoned from Anea 63 and percent This illustrates the importance of rn (Beaty and Albee, 1978).Reexamination, however, from 0-3 [ ]SiaO6. sr?udifferentiation to the formation of [ ]SiaOs. indicates that the data are only suggestive,not con- clusive. lhere is probably a small (-0.5 percent) amount of Luna 24 andApollo 14 basalts [ ]SLO- actually present in the plagioclase.Grove In contrastto the abovedata. severallunar basalts and Bence (1979),howevet, report that in their dy- contain little or no excesssilica in their plagioclase. namic crystallizationexperfunents on a Luna 24 com- Ferrobasalt24170, for example,has excesssilica val- position, [ ]SLO' in plagioclaseincreases from rock uesclose to zero.The data rangefrom 0-2 percent[ ] to rock with both cooling rate and falling temper- SLO', and are scatteredwith no detectablezoning. ature. Like 24170,Apollo 14basalts 14310 and 14053 Since the data spread is somewhat shifted off zero, also contain very little excesssilica. It is difficult to understand why the plagioclase in these samplescontains no excesssitca. The pyroxene zoning trends indicate in situ differentiation (Gancarz APOTLO11 HIGH-K et al.,l97l; Wasserburget al.,1978),and the liquids enough near their solidi to pre- 7 BASALTS all becamesiliceous @ cristobalite. Furthermore, the parental liq- o.^ cipitate vo uids are thought to have crystallizedthrough temper- 2 ature intervals(Grove and Vaniman,1978;Walker e/ E o al.,1975) similar to the previouslydiscussed mare ba- o possibleexplanation lies in the observation L salts.One o1 (Longhi, personal communication 1979) that most o lunar basalts crystallized pyroxene before plagio- 2 clase,whereas in Luna 24 and Apollo 14 plagioclase precedesor is simultaneouswith pyroxene. Perhaps the delayednucleation of plagioclasefollowed by su- 65 70 75 80 85 persaturationand rapid growth is necessaryfor the Mole PercenlCoAl25irOa incorporation of large amountsof excesssilica. Fig. 5. [ ]SiaO6 in plagioclase from the Apollo ll high-K basalts 10024, 10o72, and, 10017. Unlike Figs. 3 and 4, there is no The lunar highlands(Apollo 16) correlation with bulk cooling rate; the variations within the suite silica in plagioclasefrom cannot be related to any known parameter. 10024 and 10017 show There is negligible excess the negative correlation between [ ]SroO. and CaAlrSirog found the anorthositic lunar highlands. Quick et al. (1978) in plagioclase from most lunar basalts (Figs. l, 3, and 4). divided the plagioclase in highlands soil breccia BEATY AND ALBEE: SILICA SOLID SOLUTION IN PLAGIOCI,IISE
66075 into nine populations: anorthosite-norite- Greater diferentiation can also explain the differ- troctolite (ANT) clasts, feldspathic basalts, olivine encesin [ ]SLO, contents--the more evolved resid- basalts,xenoliths in the olivine basalts,plagioclase ual liquids in the olivine basaltswere probably more vitrophyres,xenoliths in the plagioclasevitrophyres, siliceous than those in the plagioclasevitrophyres. feldsparclasts, maskelynite clasts, and matrix grains. Consequently, they were able to crystallize more All plagioclasesexcept those from the olivine basalts [ ]SLO.-rich plagioclase.Study of these melt-rocks, have compositionsin the rangeAne3-Ane7 and excess therefore, underlines the importance of in sl'rz differ- silica contents of between 0 and 1.3 mole percent entiation to increase[ ]Sr.Or; lt is a common means (Fig. 6). About 80 percent of theseanalyses indicate of producing the necessarilysiliceous parental melts. lessthan 0.2 percent[ ]SLO', and the data spreadre- flects analytic uncertainty about a true value of zero. Ten est rial p Ia gio c I as e In the olivine basalt clasts,however, the plagioclase In contrast to the lunar samplesdiscussed above, is zoned from An"o to An.. and the excesssilica in- none of the terrestrialrocks studiedcontain [ ]SLO'- creasessympathetically frorr 0-3 mole percent (Fig. bearing plagioclase.Figure 7 showsa set of plagio- 6). Becausethe olivine basaltsare extremelyrare, it is claseanalyses from a basaltflow in the JemezMoun- possibleto conclude from this that there is no mea- tains, New Mexico. Most data indicate a complete surable excesssilica in the large anorthosite-gabbro absenceof [ ]SioO*,and all analysesa1s wllhin lhe massesthat make up the lunar highlands. analytical uncertainly of zero. Similar results were Both the plagioclasevitrophyres and the olivine obtained from four other basalt flows in the area as basalts are interpreted to be impact melt-rocks well as from tholeiites (Longhi and Hays, 1979). (Quick et al., 1978),and as suchhave undergonesim- The differencesbetween these samples and most of ilar cooling histories.Both melts were highly olivine- the mare basaltsare profound. Figure 7 also shows normative and were rapidly cooled from a high data for the Skaergaardintrusion, probably the best- temperature under similar conditions. The olivine understood igneous body in world. Like the New basalts,however, underwent fractional crystallization Mexican basalts, none of its plagioclase contains to a greater degree than did the plagioclasevitro- measurableamounts of [ ]SLO'. Although they are phyres, as indicated by the more sodic feldspars. not illustrated, excesssilica is also absentin plagio-
o.- ( OUARTZ zut I APATIT E MAGNE TITE 9el ILMENITE i.iz \ - PIGEONITE "t8= | CPX
t#jlit^i -- I* r r r. Rzo N E + l+r,rDD. r ro". -l -,-o*e. zo t. |";oop
SKAERGAARDINTRUSION o o o ol d c oo a "2 o
^ot NEW N4EXICO
o o
Mole PercentCoAl2SirOU in Plogioclose
Fig. 7. Excess silica mntent of plagioclase as a function of anorthite content for the Skaergaard intrusion and a basalt from the Jemez Mountains, New Mexico. Shown for reference are the coexisting minerals of the Skaergaard (solid : cumulus phase, dashsd : intercumulus or indeterminate) (Wager and Brown, 1967, p.26-27).The data represent analytic uncertainty about a value of zero, indicating an absence of [ ]SiaO6 in the plagioclase. This pattern is typical ofthat found in all terrestrial rocks examined in this study. BEATY AND ALBEE: SILICA SOLID SOLUTION IN PLAGIOCLASE clase analyses from amphibolites from Vermont, terrestrial basaltstypically do not show extensivein granulites from Greenland, lherzolite from Califor- situ ftactional crystallization.As discussedabove in nia, andesitefrom Paricutin, peridotite xenoliths in connectionwith Apollo 16, fractional crystallization New Mexico basalt flows, a granodiorite from Aus- is essentialto the production of [ ]SLO' in basaltic tralia, a tonatte from the Baja California batholith, plagioclase.In addition, terrestrial basalts,like Luna pelitic schists from Death Valley, and arkosic sand- 24 and Apollo 14, crystallize plagioclase before py- stonesfrom California. roxene,so there is lessopportunity for delayednucle- Note that if Na and K are lost by yslslilization ation effectsto generate[ ]SLO'. from the volume being analyzed,there will be defi- The liquid line of descent followed by the ciency in the larger site, and dependingon how the Skaergaardas it crystallized,however, was similar to end-membersare calculated,the resulting composi- that followed by the lunar mare basalts.The degree tion may have fictitious excesssilica. This alkali loss of silica saturation increasedsteadily with fractional can occur when the electron beam has too small a di- crystallization, culminating in quartz precipitation in ameter. Although such analysesneed to be elimi- the Upper Zone and SandwichHorizon (Fig. 7). Al- nated, it is difficult 1e dislinguish genuineexcess sil- though the liquid paths of the Skaergaardand the lu- ica from alkali loss or otherwise bad analyses. nar basaltswere similar chemically,they were differ- Zoning patterns like that in Figure 7, however,lead ent thermally. The lunar basaltswere dry, whereas us to conclude that for the terrestrial samples we the Skaergaardmagma, like all terrestrial magmas, havestudied,thepresenceoflargeamounts(>2per- contained a small amount of water, appreciably cent) of excesssilica is a good criterion for discarding depressingits liquidus temperature.The plagioclase the analysis.Furthermore, none of the high-l ]SLO, therefore crystallizedat lower temperature,a condi- analysescould be reproducedwhen analyzeda sec- tion sufficientto causethe plagioclaseto incorporate ond time. much less [ ]SLO,. This efect is schematicallyillustrated in Figure 8. Discussionof terrestrial data As other components,such as albite, orthoclase,py- There are severalpossible explanations for the ab- roxene, olivine, and particularly volatiles, are added senceof [ ]SLO, in terrestrialplagioclase. First of all, to the binary system anorthite-SiO, (Figure 2) to make a rock, the liquidus will be depressedto lower temperatures. The efect on the plagioclase-SiO, solvusis more difficult to predict. The presenceof a componentwhich is insoluble in either anorthite or the SiO, polymorphs at all temperatures will not af- betweenthem. The addition of Plogioclose fect the equilibrium confoining phaseslike pyroxene, ilmenite and water to the excess srlrc 0 simple binary system,therefore, will closelyapproxi- mate the behavior shown in Figure 8. The melting loop will be lowered, whereasthe solvuswill have a J' nearly constant position independent of the bulk composition. Varying the albite content of the plagioclase,how- ever, could either increase or decreasethe solvus temperature.According to Kim and Burley (1971), albite may contain up to 5 mole percent silica in solid Anorthite+ Quortz solution at 5.15 kbar and 670"C. From size and roo charge considerations,albite might be expectedto ^1 wr% sio2 sio2 contain more [ ]SLO' than anorthite at any given Fig. 8. Schematic phase diagram anorthite-Sio2 illustrating temperature(J. B. Thompson,personal communica- the effect of adding H2O. The liquidus is significantly depressed, tion, 1979).Although both albite and anorthite are but the solvus is not This affected. in turn dramatically decreases therefore known to contain excesssilica, the detailed the amount of [ ]Sioosin the plagioclase(shown by the tie lines). This is one reason why (wet) terrestrial magmas such as the shapeof the "SiOr"-plagioclasesolvus as a function Skaergaard (Fig. 7) have no [ ]Si4O8, but in equally of plagioclasecomposition is uncertain. Note that differentiated(dry) lunar basalts(Figs. l, 3, 4, and 5) it abounds. this problem can be avoided by comparing feldspars BEATY AND ALBEE: SILICA SOLID SOLUTION IN PI}IGIOCLASE 7l of constant albite content. With theselimitations in aware of any reported in the literature. Observation mind, a great deal of insight can be gainedfrom Fig- is made more diftcult by the uncertainty in distin- ure 8. guishing magmatic from exsolvedquartz inclusions Figure 8 showsin a schematicway one reasonwhy (e.9.,Walker et al., 1973,p. 1018).Submicroscopic measurableamounts of [ ]SLO, are not present in exsolutionsmay be present,but if so they are in low the Skaergaardplagioclases. The solubility of silica abundance,otherwise they would show up in the mi- in plagioclasedecreases with temperature,and the croprobe analyses. temperatureinterval over which the Skaergaardcrys- Likewise there is little evidencethat [ ]Si.O, is lost tallized was too low for measurableamounts of by diffusion. The excesssilica gradientsin the Apollo [ ]Sioor. The effect is more irnportant for magmas ll low-K basalts(Fig. l) show no evidenceof being such as granitesand tonaliteswhich have high water flattened,even in the most slowly cooledsamples. Al- contentsand evenlower crystallizationtemperatures. though less [ ]SLO, is contained in the more slowly A low temperatureof formation can alsoexplain why cooled Apollo 12 and Apollo l7 basalts,this is prob- excesssilica was not found in any of the metamor- ably related to the physical conditions of growth phic plagioclasestudied. In addition to the temper- rather than diffusive loss (seeabove). The apparent ature effect,terrestrial plutonic feldsparis lesssubject low diffusion coefficientof [ ]SLO, is related to the to the disequilibrium kinetic factors which are im- fact that it is an integral part of the feldspar struc- portant in producing [ ]SLO, in lunar basalticplagio- ture. In order for [ ]SLO, to be lost, a crystal must clase. both lose Si and gain either Ca and Al or Na and Al, a complicatedcoupled diffusion process.If such Meteorites processeswere operating the anorthite-albite zoning We have analyzedplagioclase from the St. Severin of the crystalswould also be affected.Therefore, any chondrite (LL6) and found that it containsno excess plagioclasegrain which has preservedits anorthite- silica. The anorthite in the white inclusions in Al- albite zoning might also be expectedto preserveits lende also contain no excesssilica (Lee et al., 1977; excesssilica zoning. Bradley et al., 1978).Colomera, an unusual iron me- Thus far, excess silica has been confirmed in teorite with silicate globules, has both plagioclase plagioclasefrom relatively rapidly cooled lunar vol- and K-feldspar which are [ ]SioO*-free.Using data canic rocks.In the much more slowly cooledplutonic from other laboratories,Bence and Burnett (1969)re- rocks from both the moon and the earth, [ ]SLO, is ported that crypto-antiperthite from Kodaikanal absent.Plutonic plagioclaseis alsotypically unzoned, ranged from 0-4 percenthigh in SiOr. Partial (6-ele- hence it is not possibleto be certain whether or not ment) analysesof the plagioclasein enstatitechon- [ ]SLO, was originally present. drites (Keil, 1968)also suggestsmall amounts of ex- cesssilica. The enstatitechondrites are an extremely Summary reduced group containing Si dissolvedin the metal Most of the variations in the [ ]SLO, content of (Ringwood, 196l; Keil, 1968).The excesssilica in the natural plagioclasecan be understoodin terms of a plagioclasemay be relatedto the presenceof elemen- few simple factors.Most importantly, a high temper- tal silicon. Additional work is necessaryto more fully ature of formation and a siliceousparental melt are evaluatethe []Si.O, variations among meteorites. essential.Excess silica typically increasesmonotonic- ally from core to rim of the plagioclase in lunar ba- Excesssilica preservation salts. This trend is caused by two cooperative ef- Most of the preceding discussionhas treated the fects-falling temperature increasesthe solubility of variable incorporation of [ ]Si.O. in plagioclasecrys- [ ]SLO, in plagioclase,and fractional crystallization tals at high temperature.[ ]SioO, may or may not of the melt causesthe plagioclaseto grow from an in- have been preservedas the crystal cooled to room creasingly siliceous residual liquid. In addition to temperature,however. Two mechanismsare avail- these equilibrium processes,delayed nucleation of able by which the excesssilica could be lost: ex- plagioclaseand a silica phasefollowed by rapid feld- solution and diffusion. If the []SLO, were lost by ex- spar growth can causemetastable incorporation of solution, a distinctive myrmekite consisting of -5 [ ]SLO, in large amounts. percent qvartz and 95 percent plagioclase(as op- These effects are schematically illustrated in Fig- posedto a 50-50 mixture) would be formed. The au- ure 9. Lunar basalts,which crystallize 31 high tem- thors have neither seensuch an intergrowth nor are peratureswith large degreesof fractional crystalliza- 72 BEATY AND ALBEE: SILICA SOLID SOLUT:ION IN PL/IGIOCL/ISE
[ ]SLO*. Secondly, terrestrial lavas contain water, QTZ NORI\IFIELD which sienificantly depressestheir liquidi compared to their lunar counterparts. Even if a terrestrial lava -o were to differentiate to a siliceous residual liquid, the z m temperaturewould be so low that [ ]Si.O, would be D below the detectionlimit (Fig. 9). The most siliceous - terrestrial volcanic rocks (most favorable composi- -l tionally for also have the lowest liquidus = [ ]SLO,) j temperatures(least favorable thermally for I [ ]SLO') --l z and vice versa. Plagioclases in terrestrial plutonic o o I rocks should contain even less [ ]SLO- than those in o n o C) the volcanic rocks, becausethey crystallize at even o i o lower temperatures(Fig. 9). Applying theseresults to l c z - actual rocks, the detailed diferences in [ ]SLO, zon- i ))O ing between samples can be used to compare the a temperatures,nucleation histories, and fiquid lines of t - descentof different magmas. r N Becauseof the kinetic complications, care is i o needed when applying equilibrium phase diagrams z to rapidly cooled samples.Once []SLO. has been in- corporated in a plagioclase crystal at high temper-
Mole 7" ature, however,it is preserved.There is no evidence that the excesssilica is lost, either through exsolution as the crystal cools to room temperature. o I or diffusion, SilicoActiv'ly in \,4ell For all but the most rapidly cooled samples,there- Fig. 9. Schematic diagram showing the relationship between fore, the amount of [ ]SLO. in the plagioclaseas we the field of excess silica and natural igneous rocks. By projectlng measureit today is directly relatedto the condition of different liquid lines ofdescent onto this diagram it is possible to its formation. responsible for the incorporation of illustrate the major factors If a maema is in equilibrium with cristobalite (or [ ]SroO* in plagioclase. If an olivine-normative basalt undergoes no fractional crystallization, it will follow the arrow so labeled and tridymite or quartz), the silica activity of the melt is never enter the [ ]SiaO6 stability field. With diferentiation at bufered, and [ ]SLO, can be directly used as a geo- high temperature (e.g. lunar igneous liquids), much [ ]SiaO6 will thermometer. This lsshnique appears to be most be present. At lower temperatures, the terrestrial volcanic promising as a means of comparing the solidus tem- is expected to have only rninor amounts of At spectrum [ ]SiaO3. peraturesof lunar basalts.Measurement of the effect even lower temperatures, plagioclase in terrestrial plutonic rocks should have even less [ ]Si4O8. In addition, the delayed of albite substitution (determinationof the An-Ab- nucleation and rapid growth of plagioclase can cause the Qtz ternary solvus) should enable this geothermome- disequilibrium incorporation of large amounts of [ ]SioOs. ter to be quantified. Alternatively, for magmas with the samesolidus temperatures,[ ]SLO, should allow measurementof the departure from equilibrium crys- tion, contain large amounts of [ ]Si.Or. The fact that tallization. the observed[ ]SLO. concentrations(up to 8Vo)ex- ceedthose predictedby Figure 2 (3Vo)can be attrib- uted to kinetic effects.In contrast to the lunar basalts, Acknowledgments terrestrialbasalts have no in their plagioclase. [ ]SLO, with John Longhi, Bob Dymek, and George Ross- This is relatedto two reasons.First, Discussions terrestrialbasalts man stirnulated the authors' interest in this study. The data were do not show the strong in situ differentiation of the collected by several Caltech graduate students between 1975 and lunar basalts-the residual liquids do not trayg high 1978as parts of their thesesand researchprojects. Bob Gregory silica activities. As shown on Figure 9, an olivine ba- and Scott Baldridge were particularly generouswith their unpub- salt which undergoesno fractional crystallizationwill lished data. The manuscript was typed by Lou Ann Cordell and Robinson. Early comments by Bob Gregory and George not enter the stability field. In Betty [ ]SLO, addition, Rossman and thorough reviews by Tim Grove and John Longhi plagioclase crystallizes before pyroxene in most ter- have significantly improved the manuscript. The support of restrial basalts, reducing the possibility of metastable NASA grant NGL-05-002-338 is gratefully acknowledged. BEATY AND ALBEE: SILICA SOLID SOLUTION IN PLAGIOCLASE t5
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