Vanadium magnetite–melt oxybarometry of natural, silicic magmas: a comparison of various oxybarometers and thermometers

Abstract

To test a recently developed oxybarometer for silicic magmas based on partitioning of vanadium between magnetite and silicate melt, a comprehensive oxybarometry and thermometry study on 22 natural rhyolites to dacites was conducted. Investigated samples were either vitrophyres or holocrystalline rocks in which part of the mineral and melt assemblage was preserved only as inclusions within phenocrysts. Utilized methods include vanadium magnetite–melt oxybarometry, Fe–Ti oxide thermometry and -oxybarometry, zircon saturation thermometry, and two-feldspar thermometry, with all analyses conducted by laser-ablation ICP–MS. Based on the number of analyses, the reproducibility of the results and the certainty of contemporaneity of the analyzed minerals and silicate melts the samples were grouped into three classes of reliability. In the most reliable (n = 5) and medium reliable (n = 10) samples, all fO₂ values determined via vanadium magnetite–melt oxybarometry agree within 0.5 log units with the fO₂ values determined via Fe–Ti oxide oxybarometry, except for two samples of the medium reliable group. In the least reliable samples (n = 7), most of which show evidence for magma mixing, calculated fO₂ values agree within 0.75 log units. Comparison of three different thermometers reveals that temperatures obtained via zircon saturation thermometry agree within the limits of uncertainty with those obtained via two-feldspar thermometry in most cases, whereas temperatures obtained via Fe–Ti oxide thermometry commonly deviate by ≥50 °C due to large uncertainties associated with the Fe–Ti oxide model at T-fO₂ conditions typical of most silicic magmas. Another outcome of this study is that magma mixing is a common but easily overlooked phenomenon in silicic volcanic rocks, which means that great care has to be taken in the application and interpretation of thermometers and oxybarometers.

Appendix

Samples

Four samples were kindly provided by the Smithsonian Institution (NMNH 117450-58 Los Humeros volcano—Mexico; NMNH 117455-27 Mount Rano—Indonesia; NMNH 117462-1 Blackfoot Lava Field—USA; NMNH 116377-38a Nomlaki Tuff—USA), one by Sorin Silviu Udubasa (Oravita area—Romania) and one by John Hora (Parinacota volcano—Chile). Additional vitrophyres (Inyo-Mono craters, Banco Bonito volcano—both USA), silicic tuffs (Winter Park, Smelter Knolls, Guaje, Pine Grove, Crystal Peak, Lordsburg, Chalk Mountain, Amalia and Cottonwood—all USA) and fine-grained porphyritic rocks (Santa Rita rhyodacite dikes, Lordsburg rhyolite and granodiorite, Topaz Mountain, The Dyke—all USA) were collected by Andreas Audétat, whereas a sample from Kos (Greece) was kindly donated by Thomas Pettke.

Sample description

Oraviţa hyalodacite

Unfortunately, there is no detailed description (such as exact locality, rock unit, etc.) available for this specimen. Most probably it stems from one of the Permian rhyolitic flow units of the Oraviţa area in Romania. Publications about these Permian volcanics in the area are mainly in Romanian language and/or not accessible for the public, but a recent paper (Seghedi 2011) describes the Lower Permian volcanic occurrences of the neighboring Sirinia basin. The exclusively rhyolitic composition (74–80% SiO₂) and the glass-rich, dominantly hyaline texture of Sirinia basin rhyolites—similarly to our sample—let us to suggest that they are genetically related.

Mount Rano

The Mount Rano volcano is located on the southwestern part of the North Maluku Island, Indonesia. The volcano forms part of the Late Cretaceous-Eocene Oha Formation (Hakim et al. 1991), consisting of basalts and andesites. However, our vitrophyre sample, which stems from the southwestern flank of Mount Rano and was kindly provided by the Smithsonian institution (NMNH 117455-27), has a rhyolitic composition.

Parinacota

The Parinacota volcano is located in Chile, Central Andes, and its evolution can be subdivided into five main stages, spanning from 163 ka until 5 ka (K–Ar; Hora et al. 2007). A detailed description of the geochemistry, petrography and evolution history can be found in Wörner et al. (1988) and in Hora et al. (2007). Our sample stems from the Rhyodacite Dome Plateau sequence (47–40 ka), which represents the second stage of the volcano’s evolution history. In contrast to most other magmas produced at this stage, the rhyodacite sample originates from one of the non-mixing endmember reservoirs. This sample was thoroughly investigated in a case study involving a test of multiple thermometers (Hora et al. 2013). Despite some high amphibole–plagioclase temperatures obtained from glomerocrysts, the temperature estimates obtained by Hora et al. (2013) from phenocrysts/microphenocrysts provide convincing evidence that this unit is homogenous and free of magma mixing.

Hideaway Park tuff

The Hideaway Park tuff (also referred to as Winter Park tuff) is an extrusive volcanic unit of Oligocene age (27.77 ± 0.34 Ma, sanidine K–Ar; Mercer et al. 2015) that is genetically related to the Red Mountain intrusive complex in Colorado, USA, which hosts the giant Urad-Henderson porphyry Mo deposit. Its mineralogy and geochemistry is described in detail by Mercer et al. (2015), who provided also an extensive set of mineral– and melt inclusion analyses. In order to calculate the composition of the crystallized melt inclusions, the average Al₂O₃ content from all homogenized melt inclusions and pumice glasses analyzed by Mercer et al. (2015) was used as internal standard (13.30 wt% Al₂O₃).

Cottonwood Wash tuff

The Cottonwood Wash tuff belongs to the Indian Peak volcanic field that was active from ca. 30 to 29 Ma (plagioclase K–Ar; Best 2013) and comprises tens of calderas and related ash-flow sheets that cover about 50,000 km² across the Utah-Nevada state line, USA. The Cottonwood Wash tuff formed during a super-eruption 31.13 million years ago (plagioclase K–Ar; Best 2013), producing about 2000 km³ of crystal-rich, dacitic tuff, with a fairly homogenous composition over its entire extent. A detailed description about the geochemistry, petrography and mineralogy of the volcanic units of the Indian Peak volcanic field can be found in Best et al. (1989) and Best (2013). The rapidly quenched, fresh, glassy matrix and the presence of abundant Fe–Ti oxide microphenocrysts allow easy application of the mgt–ilm oxybarometer and the V partitioning oxybarometer. At the same time, the latter method could be applied also to inclusions, as both magnetite inclusions (in quartz) and melt inclusions (in quartz and feldspar) were available.

Lordsburg rhyolite

The Lordsburg mining district is located in the Pyramid Mountains, a north-trending range of Lower Cretaceous to middle Tertiary volcanic and plutonic rocks in the Basin and Range province of southwestern New Mexico, southwest of the city of Lordsburg. The Paleocene intrusive rocks of the area include a granodiorite stock (58.8 ± 2 Ma, biotite K–Ar; Thorman and Drewes 1978), rhyolitic vents, breccia pipes and dikes, the first two formations of which were investigated in this study. Although some early studies have been conducted on these rock units (Lasky 1938; Flege 1959; Thorman and Drewes 1978), nothing has been published about their major- and trace element composition.

Lordsburg granodiorite

This sample was collected from the most widespread intrusive unit of the Pyramid Mountains—the granodiorite—which intruded also the previously described Lordsburg rhyolite. The lack of quartz phenocrysts precluded any inclusion-based thermometry and oxybarometry. However, due to the relatively unaltered nature of this rock (unlike the Lordsburg rhyolite) the relevant information was obtained from the composition of fresh-looking microphenocrysts and the fine-grained rock matrix.

Smelter Knolls rhyolite

The Smelter Knolls rhyolite belongs to a bimodal association of silica-rich, often topaz-bearing rhyolites and contemporaneous basalts and basaltic andesites of Cenozoic age that are widespread in the Basin and Range province and along the Rio Grande Rift in Western USA and Mexico (Christiansen et al. 1986). Thorough geological, geochemical and age data about these topaz rhyolites can be found in the comprehensive study of Christiansen et al. (1986), whereas the detailed description of the Smelter Knolls complex was published by Turley and Nash (1980). The Smelter Knolls represent a single rhyolite flow-dome complex measuring 5 km in diameter and 2.2 km³ in volume, located north of the city of Delta in Utah, USA. The dome complex formed at 3.4 ± 0.1 Ma based on sanidine K–Ar dating (Turley and Nash 1980). A recent study on the Black Rock Desert volcanic field (Johnsen et al. 2010)—which includes the Smelter Knolls—contains a large amount of whole rock geochemical data, from which the average Al₂O₃ content of Smelter Knolls rhyolite was used as an internal standard for our melt inclusion analyses.

Amalia Tuff

The Questa caldera and the cogenetic volcanic and intrusive rocks of the Latir volcanic field in northern New Mexico, USA, were extensively studied by Lipman (1988), Johnson and Lipman (1988), Johnson et al. (1989), and Czamanske et al. (1990). Most of the Latir volcanic units and associated intrusive rocks were emplaced within a few million years (28–26 Ma), including the eruption of the only major ash-flow tuff—the Amalia Tuff—26.5 Ma ago (sanidine K–Ar; Lipman et al. 1986). This peralkaline, rhyolitic tuff can be subdivided into two subunits, and the upper sequence—according to similarities in composition and mineralogy—is thought to represent the erupted portion of resurgent peralkaline intrusions of the Questa caldera. Although plutons at exposed levels are texturally discrete and of contrasting composition, regional gravity data (Cordell et al. 1985) suggest that the entire Questa caldera is underlain by cogenetic batholithic rocks of 10 by 20 km size at shallow depth. The Amalia Tuff sample investigated in the present study was collected from an outflow sheet 40 km SW of the caldera rim near the town of Tres Piedras.

Banco Bonito vitrophyre

The Banco Bonito Flow forms part of the East Fork Member of the Valles Rhyolite in New Mexico, USA. It represents the youngest volcanic unit of the Valles caldera complex that was active between 45 ka and 35 ka (ESR ages, ²¹Ne exposure ages, regional constraints; (Goff and Gardner 2004; Ogoh et al. 1993; Phillips et al. 1997). Detailed descriptions of the mineralogy, petrography and geochemistry of the East Fork Member can be found in Eichler (2012). Based on the trace element geochemistry of the phenocrysts and the volcanic glass, the East Fork Member rhyolites had a complex evolution history, starting with fractional crystallization of a basalt, followed by magma ascent to lower crustal levels and crustal assimilation, and finally fractional crystallization in a granitic magma chamber in the upper crust.

Santa Rita rhyodacite dike

The porphyry-copper deposit at Santa Rita (Chino Mine) in southwestern New Mexico formed during the Laramide orogeny (45–75 Ma) as a result of subduction along an Andean-type continental margin. There were several stages of the igneous activity in the region, starting with dioritic to quartz–dioritic sills intruded into the Precambrian basement rocks, followed by the eruption of basaltic–andesitic to andesitic magma and the formation of mafic dikes, and subsequently the intrusion of granodioritic to quartz–monzodioritic magma. The last stage of magmatic activity is represented by dikes of rhyodacitic to rhyolitic composition, which cut across all other lithologies. Details on the geology and petrography of the complex can be found in Rose and Baltosser (1966) and Jones et al. (1967), whereas a more recent study (Audétat and Pettke 2006) focuses on the chemical analysis of mineral and melt inclusions. We investigated two samples (SR15, SR9) from two rhyodacite dikes.

The Dyke

The West Elk laccolite cluster occupies the northern part of the West Elk Mountains, northwestern Gunnison County, Colorado. The laccoliths are located along a dike swarm related to a NNE-SSW trending, at least 40 km long fracture zone (Godwin and Gaskill 1964). The mafic, early stage of each stock is crosscut by SiO₂-rich dikes or stock-internal granodiorites (Mutschler et al. 1981). “The Dyke” represents this second stage and forms a prominent outcrop of ca. 2.5 km length south of Ruby Peak. There are no age data available from The Dyke, but considering that it intrudes the Early Eocene Wasatch formation and the genetically related Crested Butte laccolith and Paradise stock, which yielded biotite K–Ar ages of 29.0 ± 1.1 and 29.1 ± 1.0 Ma, respectively (Obradovich et al. 1969), it is probable that The Dyke granite formed in the Oligocene.

Nomlaki Tuff

This sample was kindly provided by the Smithsonian Institution (NMNH 116377-38a). The Nomlaki Tuff (4.2–3.6 Ma) formed by a Plinian eruption in the Pliocene and covers areas in California, Nevada, Utah, Arizona and New Mexico. The deposits consist of widespread ash-fall and proximal ash-flow units and are commonly used as marker horizons in the region (Knott and Sarna-Wojcicki 2001). Information about the distribution and regional correlation of the Late Cenozoic tuffs of the Central Coast Ranges of California can be found in Sarna-Wojcicki et al. (1984), in Knott and Sarna-Wojcicki (2001) and in Poletski (2010). Whole rock geochemical data can be found in Knott and Sarna-Wojcicki (2001), whereas an extensive dataset regarding the geochemistry of glass fragments and pumices, as well as regarding mgt–ilm thermometry results—both from the occurrences in the Sacramento Valley—are published in Poletski (2010). Poletski (2010) distinguished two chemo-types within the tuff based on the major element composition of glass shards and magnetite–ilmenite microphenocryst thermometry results, and explained the phenomenon by a zoned magma chamber, containing a more evolved and cooler magma in its upper part, and a less evolved, hotter magma at its bottom.

Kos granite enclave

The geochemical evolution of the Kos Plateau Tuff and related magma chamber processes have been exceptionally well studied. Petrographic observations (Keller 1969), sanidine Ar–Ar dating (Smith et al. 1996), zircon U–Pb dating (Bachmann et al. 2007), melt inclusion analysis (Bachmann et al. 2009), and a recent comprehensive study on the Kos Plateau Tuff (Bachmann 2010) all contributed to a comprehensive understanding of the region’s magmatism.

The Kos Plateau Tuff formed 160,000 years ago (Smith et al. 1996), and represents one of the largest Quaternary explosive eruptions. The non-welded tuff consists mainly of juvenile ash, different types of pumice, and lithic fragments, and locally contains equigranular granitic enclaves. The rhyolitic magma most probably evolved from a more mafic parent dominantly by fractional crystallization, as shown by the lack of inherited zircons (Bachmann et al. 2007). The magma was probably in a crystal mush state before the eruption, with some completely crystallized units at the edge of the magma body having been entrained in the form of granitic enclaves. The system was partly reheated by injection of a more mafic magma batch, which triggered the eruption and resulted in the formation of andesitic bands within pumice, plagioclase overgrowths on K-feldspar, and inverse zonations in plagioclases (Bachmann 2010).

Los Humeros

This sample was obtained from the Smithsonian Institution (NMNH 117450-58). The Los Humeros volcanic center of Pleistocene age (K–Ar) is located 180 km east of Mexico City and represents the easternmost expression of the late Tertiary to Quaternary Mexican Neovolcanic Belt (Ferriz and Mahood 1984). The volcanic province can be characterized by a series of lava flows, large eruptions (resulting in large scale ignimbrite deposition) and subsequent caldera collapse and dome-forming events. The most common compositions are rhyolitic and rhyodacitic, however, andesites and basalts (the latter mainly at the last stage of volcanism) occur as well. A detailed description of the volcanics in the region can be found in Ferriz and Mahood (1984). Our sample was obtained from the Smithsonian Institution (NMNH 117450-58).

Samples from the Mono-Inyo volcanic field

The Long Valley Volcanic Field is located at the intersection of the Sierra Nevada and the Basin and Range tectonic province in east-central California. Volcanism in the region started about 4 million years ago (Gilbert et al. 1968) and continued in multiple phases until recently. It can be separated into a pre-caldera and a post-caldera episode, the first of which peaked at 0.76 ka with the eruption of the Bishop Tuff, followed by a caldera collapse and the formation of multiple craters along the N–S trending Mono-Inyo fissure system (Bailey 2004). The composition of magmas erupted from the Mono-Inyo crater system ranges from basaltic to rhyolitic, with a compositional gap between trachyandesite and dacite, and includes several high-silica varieties containing andesitic inclusions. These andesitic inclusions lie on the mixing line between finely porphyritic rhyolites and typical post-caldera mafic magmas. Their xenocryst assemblage is identical to the phenocryst assemblage of the host rhyolites, which provides strong evidence for the involvement of mafic magma and magma mixing during silicic eruptions (Varga et al. 1990). Extensive geochemical datasets, detailed geological observations and interpretation of the complex geological evolution of the region can be found in Bailey (2004) and in Bray (2014).

Tunnel Spring Tuff

The Tunnel Spring Tuff was erupted 35.4 million years ago (K–Ar age) from a vent that probably was close to Crystal Peak, Utah, where the tuff forms a canyon fill of more than 400 m thickness (Steven 1989). The P–T–x history of the parental magma was investigated by Audétat (2013) based on zircon saturation thermometry of melt inclusions and Ti-in-quartz thermobarometry.

Blackfoot lava field vitrophyre

The Blackfoot lava field is located in southeastern Idaho, USA. It is characterized by bimodal volcanism, consisting of five rhyolitic domes located in a predominantly basaltic volcanic field. The rhyolites, which belong to the group of Cenozoic Topaz Rhyolites (Christiansen et al. 1986), contain inclusions of older basalts and andesites, but they are older than the basalts exposed on the surface. Our vitrophyre sample was kindly provided by the Smithsonian Institution (NMNH 117462-1) and was collected on the northern side of a rhyolite dome named China Hat (also referred to as China Cap or Middle Cone), the age of which was determined at 61 ± 6 ka (sanidine K–Ar; Pierce et al. 1982). The geology of the lava field, including whole rock data, is described in Christiansen et al. (1986).

Pine Grove Tuff

The Pine Grove intrusions and associated tuffs are located in the southern Wah Wah Mountains, southern Utah, USA. Whole rock and K-feldspar K–Ar data suggests that the Pine Grove system, which consists of rhyolitic (and partly dacitic) tuffs, and intrusions ranging from rhyolitic to mafic compositions, formed 23 to 22 m.y. ago (Keith et al. 1986). According to Keith et al. (1986) the tuffs were erupted from a magma chamber that was intruded by a trachyandesitic magma multiple times, and was compositionally zoned from dacite to rhyolite. Detailed petrography and K–Ar age data of the Pine Grove system can be found in Keith et al. (1986), an extensive dataset of melt inclusion analyses was published by Lowenstern et al. (1994), whereas thermometry and oxybarometry data can be found in Keith and Shanks (1988) and Audétat et al. (2011). Our sample was collected from the rhyolitic air-fall unit.