Contrib Mineral Petrol (1998) 130: 304±319 Ó Springer-Verlag 1998 Gordon Moore á I.S.E. Carmichael The hydrous phase equilibria (to 3 kbar) of an andesite and basaltic andesite from western Mexico: constraints on water content and conditions of phenocryst growth Received: 10 December 1996 / Accepted: 21 August 1997 Abstract We have conducted high pressure (to 3 kbar), ents containing greater than 6 wt% water. Indeed and- water saturated melting experiments on an andesite (62 esitic magmas could be related to a basaltic andesite wt% SiO2) and a basaltic andesite (55 wt% SiO2) from parent by hornblende-plagioclase fractionation under western Mexico. A close comparison between the ex- the same hydrous conditions. perimental phase assemblages and their compositions, and the phenocryst assemblages of the lavas, is found in water saturated liquids, suggesting that the CO2 content Introduction was minimal in the ¯uid phase. Thus the historic lavas from Volcan Colima (with phenocrysts of orthopyrox- Andesite lavas from the western Mexican volcanic belt ene, augite, plagioclase, and hornblende) were stored at contain a variety of phenocrysts ranging from only a temperature between 950±975 °C, at a pressure be- hornblende, to the most common, plagioclase-horn- tween 700±1500 bars, and with a water content of 3.0± blende-2 pyroxene-titanomagnetite assemblages. This 5.0 wt%. A hornblende andesite (spessartite) from diversity of phenocryst suites, despite similarity in bulk Mascota, of nearly identical composition but with only composition, indicates dierent conditions of magma amphibole phenocrysts, had a similar temperature but storage and phenocryst growth prior to eruption, and it equilibrated at a minimum of 2000 bars pressure with a is to quantify these conditions that this paper is con- dissolved water content of at least 5.5 wt% in the liquid. cerned. Similarly, basaltic-andesite lavas have an array Experiments on the basaltic andesite show that the most of phenocryst assemblages, typically without plagio- common natural phenocryst assemblages (olivine, clase, but their higher temperatures relative to andesites augite, plagioclase) could have precipitated at make hornblende phenocrysts rare among these lavas. temperatures from 1000±1150 °C, in liquids with a wide While we have known for some time the role that range of dissolved water content ( 2.0±6.0 wt%) and a water plays in the generation of continental subduction- corresponding pressure range. A lava of the same bulk related magmatism relative to that in island arc settings composition with phenocrysts of hornblende, olivine, (Osborn 1959; Ewart 1982; Merzbacher and Eggler plagioclase, and augite is restricted to temperatures be- 1984), the majority of hydrous phase equilibria studies low 1000 °C and pressures below 2500 bars, corre- on subduction-related lavas (e.g., andesites and basaltic sponding to <5.5 wt% water in the residual liquid. andesites) have focused on the high pressure assem- Although there is some evidence for mixing in the blages of these rocks (e.g., Baker and Eggler 1987; andesites (sporadic olivine phenocrysts), the broad Eggler and Burnham 1973; Eggler 1972; Holloway and theme of the history of both lava types is that the phe- Burnham 1972) in order to test hypotheses of possible nocryst assemblages for both the andesitic magmas and mantle sources (Osborn 1959; Green and Ringwood basaltic andesitic magmas are generated from degassing 1968). Fewer studies have been conducted concerning and reequilibration on ascent of initially hydrous par- the upper crustal conditions (1±3 kilobars) at which most subduction-zone magmas appear to reside prior to eruption (Luhr 1990; Rutherford and Devine 1988; G. Moore (&)1 á I.S.E. Carmichael Department of Geology and Geophysics, Rutherford et al. 1985; Merzbacher and Eggler 1984; University of California, Berkeley, CA 94720, USA Sisson and Grove 1993a, b). Small amounts of water Present address: dissolved in these magmas at crustal conditions can have 1Department of Geology, signi®cant eects on their phase assemblages, conse- Arizona State University, TEmpe, AZ 85287-14-4, USA quently in¯uencing evolution of melt composition. For Editorial responsibility: T.L. Grove example, water contents greater than 2 wt% in basaltic 305 melts lowers the crystallization temperature and pro- volcanic belt, and to quantify the water contents in the portion of plagioclase, and changes its equilibrium residual liquids required by their phenocryst assem- composition, while at the same time stabilizing olivine blages. (Sisson and Grove 1993a); as water changes the com- For this experimental study, we have chosen two position of the residual liquid, so will it change the representative calc-alkaline lavas from western Mexico: fractionation path taken by the magma. a basaltic andesite and an andesite (Mas-22 and Mas-12 Due to the unique tectonic environment of the west- respectively; Table 1; described in Carmichael et al., ern Mexican volcanic belt (WMVB), the in¯uence of 1996). Lavas with similar compositions are found water on subduction-related magmatism is extensively throughout the entire Mexican volcanic belt (Fig. 1) and displayed. In addition to unusual hydrous, alkaline lava in continental volcanic arcs worldwide (Hasenaka and types (minettes, absarokites) not normally associated Carmichael 1987; Gill 1981). Lange and Carmichael with subduction (Carmichael et al. 1996; Wallace and (1990) noted the distinctive lack of plagioclase in basaltic Carmichael 1992; Wallace et al. 1992), calc-alkaline la- andesites (52±55 wt% SiO2) and the presence of Mg-rich vas of nearly identical composition have widely dierent (Fo70±90) olivine both as phenocrysts and in the phenocryst assemblages. We have used this phenocryst groundmass in many lavas from the San Sebastian area diversity to constrain experimentally the storage condi- (Fig. 2), and Carmichael et al. (1996) described a horn- tions prior to eruption of magmas from the Mexican blende lamprophyre of similar composition from the Table 1 Wet chemical analyses of whole rock compositions and Composition Spessartite Colima Hornblende Basaltic modal percent of phenocrysts in (Mas-12) andesite lamprophyre andesite lavas. Wet chemical analyses (Col-7) (Mas-405) (Mas-22) for Mas-12, 22, 405 from Car- michael et al. (1996); Col-7 SiO2 62.64 61.61 55.25 55.25 from Luhr and Carmichael TiO2 0.63 0.60 0.83 0.74 (1980). Modal data for Col-7 Al2O3 17.25 17.82 17.32 17.41 from Luhr and Carmichael Fe2O3 2.01 2.10 4.04 1.96 (1980), Mas-22, 12, 405 from FeO 2.01 2.96 2.06 4.22 Carmichael et al. (1996) MgO 2.65 2.54 5.59 6.68 CaO 5.64 5.70 7.09 7.28 Na2O 4.05 4.77 4.21 3.97 K2O 1.61 1.43 2.06 1.18 P2O5 0.24 0.20 0.23 0.27 H2O+ 0.80 0.10 0.49 0.61 Total 99.53 99.73 99.28 99.57 Plagioclase 2.4 22.2 0.1 6.5 Orthopyroxene ± 2.9 ± ± Augite ± 1.8 1.6 2.6 Olivine ± tr 1.1 6.5 Hornblende 3.8 1.5 5.6 ± Groundmass 76.0 61.3 84.5 72.0 Fig. 1 General map of Mexico showing the oceanic plates, spreading centers, and the Middle American trench. Box is area of western Mexico shown in Fig. 2. Lined area is the Sierra Madre Occidental ash¯ow province; shaded area labeled MVB is the Mexican Volcanic belt 306 Fig. 2 Geologic map of the Jalisco Block, western Me- xico, showing the major tec- tonic features. (WMVB the western Mexican volcanic belt, MGVF the Michoacan- Guanajuato volcanic ®eld). The numbers 1 and 2 res- pectively are the locations of Volcan Colima and the Mascota-San Sebastian vol- canic ®eld discussed in the text Mascota region with phenocrysts of hornblende, olivine, was found to be within 0.1 log units of the expected value of the augite and minor plagioclase. These assemblages con- oxygen fugacity (Huebner and Sato 1970). Temperature was mea- sured using a ``type S'', Pt-PtRh10 thermocouple that also served as trast with the postulated low-water assemblages (or- one of the leads for the zirconia oxygen sensor. The thermocouple thopyroxene plus plagioclase) of the basaltic andesites was calibrated against the melting point of gold, and was found to from the Michoacan-Guanajuato volcanic ®eld (MGVF be within 5° of the accepted value (1064 °C). in Fig. 2) in the central Mexican volcanic belt (Hasenaka The experimental sample consisted of a paste of polyvinyl al- and Carmichael 1987). Distinctive phenocryst assem- cohol (PVA) and rock powder placed on a Pt loop, which was subsequently hung from a Pt wire cage and suspended in the fur- blages in andesites are also thought to be related to nace using a thin Pt quench wire. Quenching was accomplished by preeruptive water contents. For example, the selected melting the quench wire and dropping the sample cage into a cup of andesite sample (62 wt% SiO2) was erupted as a distilled water placed at the bottom of the furnace. At no time did spessartite (a confusing name, but a variety of lamp- the samples leave the gas stream. All runs used Pt loops that were presaturated with iron to minimize iron loss. This was achieved by rophyre) with only phenocrysts of hornblende (Table 1), running the loop with sample for 24 hours at the oxygen fugacity near the town of Mascota (Fig. 2); in contrast, a nearly and temperature of interest, quenching it and dissolving the glass compositionally identical lava occurs as a lava ¯ow at o the loop by placing it in HF overnight. Fresh rock powder was Volcan Colima (Fig. 2), with phenocrysts of plagioclase, then placed on the ``iron saturated'' loop, and run at the same conditions. The quenched products were then analyzed on the augite, orthopyroxene, and hornblende (Table 1) (Luhr electron microprobe and the results are given in Table 2.