American Mineralogist, Volume 77, pages 1242-1249, 1992 Sieve-texturedplagioclase in volcanic rocks producedby rapid decompression SrnpHnN T. NBr,soN, Anr MoNrANrA.* Department of Earth and SpaceSciences, UCLA, Los Angeles,California 90024-1567,U.S.A. Ansrnlcr Disequilibrium textures,particularly the coarsesieve texture ofplagioclase,are common in orogenic volcanic rocks. The textures are usually interpreted as resulting from magma mixing, but they may occur by rapid decompression,where heat loss is minor relative to the ascent rate. We conducted high-pressurepiston-cylinder experiments on an andesite to test this hypothesis. Experiments starting at 12 kbar, followed by isothermal pressure releasein increments of 2, 4, and 6 kbar, produce sieve textures in plagioclasevery much like those in many volcanic rocks. Therefore, the presenceof sieve-texturedplagioclase should not be taken as a priori evidencefor magma mixing. Many volcanic systemsprob- ably experienceconditions of decompressionsimilar to those simulated in this study, and decompressionis considered to be a simple mechanism to produce such textures, as it requires no addition of heat or mass. Rapid decompressionmay also operate in conjunc- tion with magmamixing. INrnonucrroN One interpretation of coarse sieve textures similar to Plagioclasecommonly exhibits a variety of disequilib- those in Figure I is rapid skeletal growth resulting from rium textures in volcanic rocks, especially in orogenic undercooling(Kuo and Kirkpatrick, 1982).However, melt andesites.These textures often include combinations of inclusions are commonly elongateparallel to, or abutted complex zoning patterns and resorption features in pla- against, polysynthetic twin planes (Fig. l), or they cut gioclasethat record changingphysical conditions in mag- boundaries betweenindividual crystals in glomerocrysts. matic systems.Thus, the potential exists to learn much Given that glomerocrystsand polysynthetic twin planes about magmatic processesinvolved in their formation if form after crystallization (Dungan and Rhodes, 1978), the textures can be interpreted, with the caveat that er- the cross-cuttingrelationships indicate that thesetextures roneous inferencesmay result if a particular texture can must form by resorption rather than rapid crystallization. arise through more than one process. This study deals Two other mechanismshave been proposedto explain with the origin ofcoarse sieve-texturedvolcanic plagro- the origin of sieve textures in plagioclasefrom volcanic clase (Fig. l) in the light of new experimental observa- rocks. They are magma mixing (e.g.,Dungan and Rhodes, tions and petrographiccharacteristics of resorbednatural 1978;Tsuchiyama, 1985) and magmaticdecompression plagioclase. (e.g.,Vance, 1965; Stormet 1972; Nelson, 1989). The We interpret the texture of the plagioclasein Figure I possible effectsof magma mixing have been experimen- as representingthe remnants of an interconnecting net- tally investigated, although only under a limited set of work ofchannels, developedas the crystal underwent dis- conditions (Tsuchiyama,1985). To date, these experi- solution in responseto physical and chemical changesin ments have not produced textures like those in Figure l, the magma reservoir or conduit. Initially, dissolution al- although future experiments may produce them if they lowed diffusive and convective communication of the liq- are carried out under an expanded range of pressures, uid in the channelswith the bulk of the melt. However, temperatures, and compositions. Many studies have in many volcanic rocks, sieve-textured plagioclasehas shown the widespreadoccurrence of magma mixing as a euhedral rims that may result from subsequentcrystal petrogeneticprocess, and it is not our intent to dispute growth in responseto loss of volatiles or cooling prior to its importance. However, our results indicate that coarse eruption. If the rims encapsulatethe crystal, this may sieve textures may result from the rapid ascent of mag- leave the resorbed plagioclasewith entrained liquid in- mas; therefore,invoking magma mixing solely on the ba- clusions that have no obvious means of communication sis of coarsesieve-textured plagioclase may not be war- with the bulk of the magma. Upon extrusion, quenching ranted. In the absence of other criteria indicative of of the magma leavesthe network of channelsfilled with magma mixing, such as trapped liquid enclaves,disequi- microcrystalline or glassymelt inclusions. librium phenocryst assemblages(e.g., Mg-rich olivine + quartz),or other chemical and isotopic criteria, magmatic decompressionis a geologicallysimpler model, as it does * Presentaddress: P.O. Box I139, Pecos,New Mexico 87552, not require an open system.However, the two processes U.S.A. need not be mutually exclusive.Both mechanismsdesta- 0003-004x/9 2 / | 1| 2-r 242SO2.OO 1242 NELSON AND MONTANA: SIEVE-TEXTURED PLAGIOCLASE t243 bilize relatively sodic plagioclase,and all magmasdecom- pressduring ascent. Magma mixing Some of the relative merits of magma mixing and mag- matic decompressionmodels can be evaluated by a re- view of pertinent phaseequilibria and experimental stud- ies. Tsuchiyama's experiments model a mafic magma mixing with a more silicic, sodic plagioclase-bearingmelt, which superheatsand destabilizes the sodic plagioclase (Tsuchiyama,1985). Any calcicplagioclase in the mafic a,'a i magma would be supercooled,leaving a compositionally bimodal population of plagioclasecrystals. Mixing experimentsproduced dusty calcic mantles sur- rounding sodic cores(see Fig. 2 of Tsuchiyama, 1985), an effect produced by diffusional exchangebetween pla- f,n gioclaseand melt, assistedby the development of micro- a channelsin the partially resorbingmantle. However, Tsu- chiyama noted that his resultsdid not produce the coarse sieve textures seenin Figure l, which are similar to tex- tures ascribed to magma mixing (see Fig. 5 of Dungan and Rhodes, 1978).Although it is apparentthat some disequilibrium texturesin lavas may result from the mix- ing of magmas, it seemsthat coarse sieve textures may also form by decompression. Magrnatic decompression Plagioclasephase equilibria provide a framework for predicting the possible effectsof rapid magmatic decom- pression and for designing experiments to model it. In Fig. 1. Photomicrographof the startingmaterial containing Figure 2a, decompressionof the bulk composition, rep- large resorbedplagioclase phenocrysts typical of many inter- resented by the filled squares,from A to B drives the mediateand mafic lavas. Arrow indicatesa memberof a second systemtoward a more calcic plagioclasecomposition and populationof euhedral,often more calcic,plagioclase mrcro- a greater proportion of liquid, as representedby the rel- phenocrysts. ative lengths of the tie lines connecting A and B to the solidi and liquidi. Our experiments were designed to mimic this effect and to test whether rapid decompres- of diopside(Wyllie, 1963).Thus, additional components sion can produce resorption in volcanic plagioclasephe- and crystallizing phasesin natural systemsmay enhance nocrysts. The vigor with which a decompressingplagio- resorption effectscaused by decompression.Our experi- clase-bearingsystem begins to reequilibratedepends upon mental data and thoseof Meen (1987)confirm that, un- the magnitude and rate of pressurerelease. Decompres- der nearly isothermal conditions, increasingpressure de- sion experiments modeled ascent rates >10 m/s (d.P/dt creasesthe An content of plagioclasein multicomponent > 0.2 kbar/min) over absolutepressure intervals of 2,4, natural systems(Fig. 3). This agreeswith the observations and 6 kbar. of other workers(Green and Ringwood, 1967, 1968;Co- The compositional changesproduced by decompres- hen et al., 1967) and supports our model for decompres- sion in the binary system(Fig. 2a) are not large.However, sion-induced dissolution of plagioclasephenocrysts. the effects of additional components in decompressing Although the curves in Figure 2 are for dry systems, systemscan be evaluated in Figure 2b. The curves rep- no magma is strictly anhydrous, making it necessaryto resent the plagioclaseloop with coprecipitating diopside evaluate briefly the effectsof volatiles. It is necessaryto in the system Di-Ab-An. For decompressionintervals of keep in mind that the processof magmatic decompres- equal magnitude, the simultaneous dissolution of diop- sion we describeis rapid, precluding a significantincrease side produces a greater change in the equilibrium com- in volatile pressuredue to crystallization of an anhydrous position of plagioclaseand a greaterproportion of liquid assemblage.Therefore, it is important to consider:(l) the at the final pressurethan in the system Ab-An (Fig. 2a). effects of decompression on vapor-undersaturated sys- The increased sensitivity of the system Di-Ab-An to tems, (2) the effectsof decompressionon vapor-saturated changesin pressureand temperatureresults from the low- systems,and (3) volatile loss. er slopesof the plagioclaseliquidus and solidus in both HrO lowers the temperatureof the liquidi and especial- P-X and T-X projections causedby the coprecipitation ly the solidi, but it does not significantly changethe over- 1244 NELSON AND MONTANA: SIEVE-TEXTURED PLAGIOCLASE IGL tr PL+CL tr PL+CPX+GL O PL+I,IT+GL (! Plagioclase + Liquid : l.i PL+CPX+L th th J H qJ 10 ti F< U) 1 atm a l-i 20 oC T = 1230 15 Plagioclase+ Diopside 1100 1200 1300 H Temperature('C) 10 Plagioclase+ Diopside+Liquitl Fig.3. P-Tprojectionfor andesitemelts modified from