Holli M. Frey

A thermodynamic model for the plagioclase-liquid hygrometer/thermometer

Abstract A new thermodynamic model for the plagioclase-liquid exchange reaction between the albite (NaAlSi3O8) and anorthite (CaAl2Si2O8) components is presented, which can be used as a plagioclaseliquid hygrometer or thermometer. The model incorporates calorimetric and volumetric data for the pure liquid and crystalline components, which permits the effect of temperature and pressure on the exchange reaction to be calculated independently from the effect of composition. This allows a more accurate assessment of the effect of melt composition (including dissolved water concentration) on the exchange reaction from plagioclase-liquid equilibrium experiments. Activity-composition relations for the plagioclase solid solution are taken from Holland and Powell (1992). The new hygrometer is calibrated on 71 plagioclase-liquid experiments, of which 45 are hydrous and 26 are anhydrous. Three filters were applied to the phase-equilibrium data: (1) crystallanities <30%; (2) pure H2O fluidsaturated; and (3) compositional totals (including H2O component) of 97-101% for hydrous quenched glasses. The final data set spans a wide range of liquid compositions (46-74 wt% SiO2), plagioclase compositions (An93-An37), temperatures (825-1230 °C), pressures (0-300 MPa), and dissolved melt water concentrations (0-7 wt% H2O). The standard error of estimate (SEE) for the model is ±0.32 wt% H2O, and all liquid compositions are fitted equally well. When the model is used as a thermometer, all measured temperatures are recovered equally well within ±14 °C on average. The model is only recommended for applications that fall within the compositional bounds of the calibration data set (i.e., metaluminous basalts through rhyolites in equilibrium with An95-An35). It is not yet calibrated for rhyolites crystallizing plagioclase more sodic than An30, owing to an absence of phase-equilibrium experiments on rhyolites that pass the required filters. The new plagioclase-liquid hygrometer/thermometer is available as a Visual Basic program that runs on Excel 2004.

Magma eruption rates constrained by 40Ar/39Ar chronology and GIS for the Ceboruco–San Pedro volcanic field, western Mexico

40Ar/39Ar geochronology is coupled with quantitative volume determinations (utilizing field mapping, digital elevation models, orthophotos, and geographic information system [GIS] software) to constrain magma eruption rates at the Ceboruco–San Pedro volcanic field (1600 km2) in the western Trans-Mexican arc. Ages are reported for 40 volcanic units, including Volcan Ceboruco (an active, andesitic stratovolcano), peripheral domes, shields, cinder cones, and fissure-fed flows. After a hiatus of ∼3 m.y., volcanic activity recommenced to produce 80.5 ± 3.5 km3 of magma at a rate of 63 m3/km2 per year over the past 0.8 m.y. However, 75% of this volume erupted in the past 100 k.y., including the 51 ± 2.5 km3 of Volcan Ceboruco, equivalent to an eruption rate of ∼377 m3/km2 per year. There have been at least two stages of cone-building activity at Volcan Ceboruco. The main edifice is composed of ∼38 km3 of precaldera andesites, the youngest dated at 45 ± 8 ka. Their eruption was followed by a hiatus, interrupted by a Plinian eruption at 1 ka. The Plinian eruption and subsequent lava flows are andesite to dacite in composition and constitute ∼13 km3 of the total volume of Volcan Ceboruco. Overall, the relative proportions of lava types erupted in the past 0.8 m.y. are 18%–19% basaltic andesite, 56%–60% andesite, 18%–22% dacite, and 3% rhyolite. The peripheral lavas are each of small volume, geochemically diverse, and show little evidence of prior storage in an upper-crustal chamber. The eruptive sequence, proportions of lava types, phenocryst assemblages, textures, and geochemistry imply that the lavas do not reflect the differentiation of a single parental liquid in a long-lived magma chamber. The distinct geochemical signatures were present prior to magma emplacement in the upper crust, whereupon subsequent degassing and crystallization led to variable phenocryst abundances and assemblages.

A Pliocene ignimbrite flare-up along the Tepic-Zacoalco rift: Evidence for the initial stages of rifting between the Jalisco block (Mexico) and North America

The Tepic-Zacoalco rift, a NW-trending corridor ~50 × ~250 km, is one arm of a triple-rift system in western Mexico. Together with the Colima rift and the Middle America Trench, it bounds the Jalisco block, a portion of western Mexico that may be moving independently of North America. The predominant basement rock types in the TepicZacoalco rift are rhyolitic ash-fl ow tuffs and lavas, which were previously assumed to be Oligocene-Miocene in age, related to the Sierra Madre Occidental volcanic province, or older. New 40 Ar/ 39 Ar dates on 41 volcanic samples reveal a previously unrecognized, voluminous fl are-up of rhyolitic ignimbrites between 5 and 3 Ma throughout the entire corridor of the Tepic-Zacoalco rift; they are often associated with Pliocene high-Ti basalts. The eruption rate during this Pliocene time period was an order of magnitude higher (hundreds of m/m.y.) than that documented in the Tepic-Zacoalco rift over the last 1 m.y. The Pliocene ash-fl ow tuffs have been faulted along NW-trending lineaments, producing vertical offsets up to at least 500 m. The voluminous ignimbrite fl are-up in the TepicZacoalco rift at 5–3 Ma may refl ect the initial stages of rifting of the Jalisco block away from North America, analogous to what occurred in the proto-gulf region at 12–6 Ma, prior to the transfer of Baja California from North America to the Pacifi c plate. Additionally, new 40 Ar/ 39 Ar dates show that the Sierra Madre Occidental volcanic province extends across the entire width of the Tepic-Zacoalco rift and terminates abruptly at the northern boundary of the Jalisco block near the Rio Ameca. In contrast, PaleoceneEocene basement from the Jalisco block extends northward into the Tepic-Zacoalco rift, where it is locally overlain by Sierra Madre Occidental rhyolites.

Origin of Meter-Scale Submarine Cavities and Herringbone Calcite Cement in a Cambrian Microbial Reef, Ledger Formation (U.S.A.)

Abstract Meter-scale submarine cavities in Middle Cambrian shelf-margin microbial reef strata indicate large-scale dewatering processes, in conjunction with substrate instability related to interreef channeling and shelf-edge downslope creep and slip. Syndepositional cement precipitation within the cavities preserved delicate microbial fabrics and stabilized the reef system. Radiaxial fibrous calcite and herringbone calcite cements line the cavity interiors isopachously. The two phases cannot be discriminated on the basis of Fe, Mn, or Sr contents, but do have different isotopic signatures. Slightly more negative δ13C values in herringbone calcite suggest that abrupt transitions between radiaxial fibrous and herringbone calcite cement are the result of rapid and repeated changes in pore-fluid oxygen levels. Storm-driven pore-water circulation renewed oxygenated seawater flow into the cavities, resulting in precipitation of radiaxial fibrous calcite. A threshold level of oxygen reduction resulted in the change to herringbone calcite precipitation. The pore fluids associated with herringbone calcite did not have elevated Mn or Fe concentrations, as suggested in previous studies. Herringbone calcite appears to be more susceptible to diagenetic alteration than radiaxial fibrous cement however, as indicated by greater resetting of oxygen isotope values.

A new type of shelf margin deposit: rigid microbial sheets and unconsolidated grainstones riddled with meter-scale cavities

Middle Cambrian microbial limestone contains a network of unusual, predominantly horizontal cavities up to 2 m in length and 0. 5 m in height. The microbialite experienced rapid syndepositional lithification, but adjacent grainstone sediments remained unlithified during deposition. This juxtaposition contributed to sediment instability, resulting in fracturing and brecciation of the lithified microbialite while unconsolidated grainstones underwent slumping and injection into some cavities. Remaining space within the cavities was colonized by a series of encrustations: thin crusts (2‐8 mm) of laminated algal mats, followed by several generations of calcified Renalcis-like cyanobacteria up to 45 mm thick. Remaining void space was partially filled by internal sediment, and then sequentially occluded by banded radiaxial fibrous calcite, herringbone calcite, and finally saddle dolomite cements. The radiaxial and herringbone calcite cements precipitated from porewaters derived from seawater that became anoxic through the breakdown of organic matter in the microbialite. Noteworthy is the presence of herringbone calcite cement, not as a seafloor precipitate, but as an early cavity fill. We propose that the unusual bedding-parallel fractures were caused by gravity collapse along a shallow platform margin. Coeval foreslope sediments show syndepositional slumping, faulting, and mass flow deposits. These redeposited sediments contain boulders of microbialite and grainstone of platform margin provenance.

40Ar/39Ar geochronology of Volcán Tepetiltic, western Mexico: Implications for the origin of zoned rhyodacite-rhyolite liquid erupted explosively from an andesite stratovolcano after a prolonged hiatus

40 Ar/ 39 Ar geochronology is used to determine the eruptive history of Volcan Tepetiltic, a predominantly andesitic stratovolcano that underwent caldera collapse during explosive eruption of zoned rhyodacite–rhyolite. The main edifice was largely constructed between 560 and 450 ka, but it was not complete until ca. 416 ka, during which time ∼42 km 3 of phenocryst–rich (25–40 vol%) lavas ranging from 57 to 69 wt% SiO 2 were erupted. After a hiatus of ∼180 k.y., there was a climactic Plinian eruption of ∼4–8 km 3 of zoned magma (68–75 wt% SiO 2 ). Afterward, a small rhyodacite dome (69 wt% SiO 2 ; 190 ± 22 ka) and an andesite dome were emplaced on the caldera floor. There has been no subsequent volcanic activity. The crystal–poor (0–5 vol%) Plinian pumice could not be dated directly, owing to the hydration of glass and the absence of a K–rich mineral phase. Instead, the age of the climactic eruption was bracketed to be ca. 236 ± 52 ka from 40 Ar/ 39 Ar dates (±2δ) on peripherally erupted basaltic andesite flows, which are found underneath (241 ± 47 ka) and overlying (220 ± 36 ka) the associated fallout deposits. A volume of ∼9 km 3 of basaltic andesite was erupted from three peripheral vents surrounding Volcan Tepetiltic, most of it from a shield volcano that was active before, during, and after the Plinian event. Thermal models indicate that the magma chamber beneath Volcan Tepetiltic would have cooled below its solidus within 36 k.y. in the absence of new injections of magma. Given the long hiatus (∼180 k.y.) between the cone–building episode that built the andesite stratovolcano and the explosive eruption of rhyodacite–rhyolite, it is proposed that the influx of voluminous basaltic andesite into the upper crust drove partial melting of the subsolidus magma chamber beneath Volcan Tepetiltic, which explains the synchronicity of the basaltic andesite volcanism with the Plinian eruption of zoned, crystal–poor rhyolitic melt.