Geology topics

Olivier Bachmann

Publications and source records attributed to Olivier Bachmann.

7 recordsLinked to original sources

USGS2019

Geochemical and petrological diversity of mafic magmas from Mount St. Helens

Quaternary eruptive products in the Cascade arc include a variety of different basalt types. At Mount St. Helens (MSH), the most active volcano in the Cascades throughout the last 35 ka, three different mafic endmembers erupted at the end of the Castle Creek period (1900–1700 years B.P.): (1) high-field strength element (HFSE)-rich basalt enriched in K, Ti, P, and incompatible trace elements; (2) low-K olivine tholeiite (LKOT) with lower amounts of incompatible trace elements; and (3) calc-alkaline (arc-type) basaltic andesite with a typical subduction signature, i.e., enrichment in fluid-mobile large ion lithophile elements (LILE) relative to immobile high-field strength elements (HFSE). Each type has compositions projecting backwards to more primitive endmembers in the Cascades. Single units encompassing basaltic-to-basaltic andesitic compositions with intermediate trace-element abundances form two almost continuous trends towards basaltic andesite. These trends are interpreted to result from assimilation of pre-existing, more evolved, calc-alkaline material (and in one case mixing of different mafic magma types) during migration of the magmas through the crust. Most of the erupted basalts are porphyritic (10–30%) with an assemblage dominated by olivine and plagioclase and show disequilibrium textures preventing detailed reconstruction of mantle melting processes. Although typical hydrous arc basalt produced by flux melting in the mantle is absent in the eruptive products of MSH, arc-type basaltic andesite suggests its presence at depth. LKOT magmas are interpreted as decompression melts from the upper mantle, whereas HFSE-rich basalts are likely derived from the water-poor periphery of the main flux melting regime, potentially tapping a trace-element-enriched source. Primitive spinel compositions and whole-rock trace-element variations indicate at least two distinct, relatively fertile lherzolite sources for these two basalt types. Weak crustal zones associated with an old fracture system beneath MSH likely provide conduits for fast and isolated ascent of distinct batches of magma, bypassing the lower crustal mush zone. The eruption of the basalts through the upper crustal magma system and main edifice is consistent with an offset plumbing system suggested by geophysical data.

Washington↗

USGS2017

Thermal and petrologic constraints on lower crustal melt accumulation under the Salton Sea Geothermal Field

In the Salton Sea region of southern California (USA), concurrent magmatism, extension, subsidence, and sedimentation over the past 0.5 to 1.0 Ma have led to the creation of the Salton Sea Geothermal Field (SSGF)—the second largest and hottest geothermal system in the continental United States—and the small-volume rhyolite eruptions that created the Salton Buttes. In this study, we determine the flux of mantle-derived basaltic magma that would be required to produce the elevated average heat flow and sustain the magmatic roots of rhyolite volcanism observed at the surface of the Salton Sea region. We use a 2D thermal model to show that a lower-crustal, partially molten mush containing < 20 – 40 % "> <20–40% interstitial melt develops over a ∼ 10 5 "> ∼105 -yr timescale for basalt fluxes of 0.008 to 0.010 m 3 / m 2 / yr "> 0.010m3/m2/yr (∼0.0008 to ∼0.001 km / 3 yr "> /3yr injection rate) given extension rates at or below the current value of ∼0.01 m/yr ( Brothers et al., 2009 ). These regions of partial melt are a natural consequence of a thermal regime that scales with average surface heat flow in the Salton Trough, and are consistent with seismic observations. Our results indicate limited melting and assimilation of pre-existing rocks in the lower crust. Instead, we find that basalt fractionation in the lower crust produces derivative melts of andesitic to dacitic composition. Such melts are then expected to ascend and accumulate in the upper crust, where they further evolve to give rise to small-volume rhyolite eruptions (Salton Buttes) and fuel local spikes in surface heat flux as currently seen in the SSGF. Such upper crustal magma evolution, with limited assimilation of hydrothermally altered material, is required to explain the slight decrease in δ 18 O "> δ18O values of zircons (and melts) that have been measured in these rhyolites.

California↗

USGS2016

The role of crystallization-driven exsolution on the sulfur mass balance in volcanic arc magmas

The release of large amounts of sulfur to the stratosphere during explosive eruptions affects the radiative balance in the atmosphere and consequentially impacts climate for up to several years after the event. Quantitative estimations of the processes that control the mass balance of sulfur between melt, crystals, and vapor bubbles is needed to better understand the potential sulfur yield of individual eruption events and the conditions that favor large sulfur outputs to the atmosphere. The processes that control sulfur partitioning in magmas are (1) exsolution of volatiles (dominantly H 2 O) during decompression (first boiling) and during isobaric crystallization (second boiling), (2) the crystallization and breakdown of sulfide or sulfate phases in the magma, and (3) the transport of sulfur-rich vapor (gas influx) from deeper unerupted regions of the magma reservoir. Vapor exsolution and the formation/breakdown of sulfur-rich phases can all be considered as closed-system processes where mass balance arguments are generally easier to constrain, whereas the contribution of sulfur by vapor transport (open system process) is more difficult to quantify. The ubiquitous “excess sulfur” problem, which refers to the much higher sulfur mass released during eruptions than what can be accounted for by amount of sulfur originally dissolved in erupted melt, as estimated from melt inclusion sulfur concentrations (the “petrologic estimate”), reflects the challenges in closing the sulfur mass balance between crystals, melt, and vapor before and during a volcanic eruption. In this work, we try to quantify the relative importance of closed- and open-system processes for silicic arc volcanoes using kinetic models of sulfur partitioning during exsolution. Our calculations show that crystallization-induced exsolution (second boiling) can generate a significant fraction of the excess sulfur observed in crystal-rich arc magmas. This result does not negate the important role of vapor migration in sulfur mass balance but rather points out that second boiling (in situ exsolution) can provide the necessary yield to drive the excess sulfur to the levels observed for crystal-rich systems. In contrast, in crystal-poor systems, magma recharge that releases sulfur-rich bubbles is necessary and most likely the primary contributor to sulfur mass balance. Finally, we apply our model to account for the effect of sulfur partitioning during second boiling and its impact on sulfur released during the Cerro Galan supereruption in Argentina (2.08 Ma) and show the potential importance of second boiling in releasing a large amount of sulfur to the atmosphere during the eruption of large crystal-rich ignimbrites.

Journal of Geophysical Research B: Solid Earth↗

USGS2016

In defense of Magnetite-Ilmenite Thermometry in the Bishop Tuff and its implication for gradients in silicic magma reservoirs

Despite claims to the contrary, the compositions of magnetite and ilmenite in the Bishop Tuff correctly record the changing conditions of T and fO 2 in the magma reservoir. In relatively reduced ( ∆NNO < 1) siliceous magmas (e.g., Bishop Tuff, Taupo units), Ti behaves compatibly (D Ti ≈ 2-3.5), leading to a decrease in TiO 2 activity in the melt with cooling and fractionation. In contrast, FeTi-oxides are poorer in TiO 2 in more oxidized magmas (∆NNO > 1, e.g., Fish Canyon Tuff, Pinatubo), and the d(a TiO 2 )/ dT slope can be negative. Biotite, FeTi-oxides, liquid, and possibly plagioclase largely maintained equilibrium in the Bishop Tuff magma (unlike the pyroxenes, and cores of quartz, sanidine, and zircon) prior ro and during a mixing event triggered by a deeper recharge, which, based on elemental diffusion profiles in minerals, took place at least several decades before eruption. Equilibrating phases and pumice compositions show evolving chemical variations that correlate well with mutually consistent temperatures based on the FeTi-oxides, sanidine-plagioclase, and ∆ 18 O quartz-magnetite pairs. Early Bishop Tuff (EBT) temperatures are lower (700 to ~780&lrm;°C) than temperatures (780 to >820°C) registered in Late Bishop Tuff (LBT), the latter defined here not strictly stratigraphically, but by the presence of orthopyroxene and reverse-zoned rims on quartz and sanidine. The claimed similarity in compositions, Zr-saturation temperatures and thermodynamically calculated temperatures (730-740°C) between EBT and less evolved LBT reflect the use of glass inclusions in quartz cores in LBT that were inherited from the low temperature rhyolitic part of the reservoir characteristic of the EBT. LBT temperatures as high as 820°C, the preservation of orthopyroxene, and the presence of reverse-zoned minerals (quartz, sanidine, zircons) are consistent with magma recharge at the base of the zoned reservoir, heating the cooler rhyolitic melt, partly remelting cumulate mush, and introducing enough CO 2 (0.4-1.4 wt%, mostly contained in the exsolved fluid phase) to significantly lower H 2 O-activity in the system.

American Mineralogist↗

USGS2015

Ignimbrites to batholiths: integrating perspectives from geological, geophysical, and geochronological data

Multistage histories of incremental accumulation, fractionation, and solidification during construction of large subvolcanic magma bodies that remained sufficiently liquid to erupt are recorded by Tertiary ignimbrites, source calderas, and granitoid intrusions associated with large gravity lows at the Southern Rocky Mountain volcanic field (SRMVF). Geophysical data combined with geological constraints and comparisons with tilted plutons and magmatic-arc sections elsewhere are consistent with the presence of vertically extensive (>20 km) intermediate to silicic batholiths (with intrusive:extrusive ratios of 10:1 or greater) beneath the major SRMVF volcanic loci (Sawatch, San Juan, Questa-Latir). Isotopic data require involvement of voluminous mantle-derived mafic magmas on a scale equal to or greater than that of the intermediate to silicic volcanic and plutonic rocks. Early waxing-stage intrusions (35–30 Ma) that fed intermediate-composition central volcanoes of the San Juan locus are more widespread than the geophysically defined batholith; these likely heated and processed the crust, preparatory for ignimbrite volcanism (32–27 Ma) and large-scale upper-crustal batholith growth. Age and compositional similarities indicate that SRMVF ignimbrites and granitic intrusions are closely related, but the extent to which the plutons record remnants of former magma reservoirs that lost melt to volcanic eruptions has been controversial. Published Ar/Ar-feldspar and U-Pb-zircon ages for plutons spatially associated with ignimbrite calderas document final crystallization of granitoid intrusions at times indistinguishable from the tuff to ages several million years younger. These ages also show that SRMVF caldera-related intrusions cooled and solidified soon after zircon crystallization, as magma supply waned. Some researchers interpret these results as recording pluton assembly in small increments that crystallized rapidly, leading to temporal disconnects between ignimbrite eruption and intrusion growth. Alternatively, crystallization ages of the granitic rocks are here inferred to record late solidification, after protracted open-system evolution involving voluminous mantle input, lengthy residence (10 5 –10 6 yr) as near-solidus crystal mush, and intermittent separation of liquid to supply volcanic eruptions. The compositions of the least-evolved ignimbrite magmas tend to merge with those of caldera-related plutons, suggesting that the plutons record nonerupted parts of long-lived cogenetic magmatic systems, variably modified prior to final solidification. Precambrian-source zircons are scarce in caldera plutons, in contrast to their abundance in some peripheral waning-stage intrusions of the SRMVF, implying dissolution of inherited crustal zircon during lengthy magma assembly for the ignimbrite eruptions and construction of a subvolcanic batholith. Broad age spans of zircons (to several million years) from individual samples of some ignimbrites and intrusions, commonly averaged and interpreted as “intrusion-emplacement age,” alternatively provide an incomplete record of intermittent crystallization during protracted incremental magma-body assembly, with final solidification only when the system began to wane. Analyses of whole zircons cannot resolve late stages of crystal growth, and early growth in a long-lived magmatic system may be poorly recorded due to periods of zircon dissolution. Overall, construction of a batholith can take longer than recorded by zircon-crystallization ages, while the time interval for separation and shallow assembly of eruptible magma may be much shorter. Magma-supply estimates (from ages and volcano-plutonic volumes) yield focused intrusion-assembly rates sufficient to generate ignimbrite-scale volumes of eruptible magma, based on published thermal models. Mid-Tertiary processes of batholith assembly associated with the SRMVF caused drastic chemical and physical reconstruction of the entire lithosphere, probably accompanied by asthenospheric input.

Colorado, New Mexico↗

USGS2002

The Fish Canyon magma body, San Juan volcanic field, Colorado: Rejuvenation and eruption of an upper-crustal batholith

More than 5000 km 3 of nearly compositionally homogeneous crystal-rich dacite (∼68 wt % SiO 2 : ∼45% Pl + Kfs + Qtz + Hbl + Bt + Spn + Mag + Ilm + Ap + Zrn + Po) erupted from the Fish Canyon magma body during three phases: (1) the pre-caldera Pagosa Peak Dacite (an unusual poorly fragmented pyroclastic deposit, ∼200 km 3 ); (2) the syn-collapse Fish Canyon Tuff (one of the largest known ignimbrites, ∼5000 km 3 ); (3) the post-collapse Nutras Creek Dacite (a volumetrically minor lava). The late evolution of the Fish Canyon magma is characterized by rejuvenation of a near-solidus upper-crustal intrusive body (mainly crystal mush) of batholithic dimensions. The necessary thermal input was supplied by a shallow intrusion of more mafic magma represented at the surface by sparse andesitic enclaves in late-erupted Fish Canyon Tuff and by the post-caldera Huerto Andesite. The solidified margins of this intrusion are represented by holocrystalline xenoliths with Fish Canyon mineralogy and mineral chemistry and widely dispersed partially remelted polymineralic aggregates, but dehydration melting was not an important mechanism in the rejuvenation of the Fish Canyon magma. Underlying mafic magma may have evolved H 2 O–F–S–Cl-rich fluids that fluxed melting in the overlying crystal mush. Manifestations of the late up-temperature magma evolution are: (1) resorbed quartz, as well as feldspars displaying a wide spectrum of textures indicative of both resorption and growth, including Rapakivi textures and reverse growth zoning (An 27–28 to An 32–33 ) at the margins of many plagioclase phenocrysts; (2) high Sr, Ba, and Eu contents in the high-SiO 2 rhyolite matrix glass, which are inconsistent with extreme fractional crystallization of feldspar; (3) oscillatory and reverse growth zoning toward the margins of many euhedral hornblende phenocrysts (rimward increases from ∼5·5–6 to 7·7–8·5 wt % Al 2 O 3 ). Homogeneity in magma composition at the chamber-wide scale, contrasting with extreme textural and chemical complexities at the centimeter–millimeter scale, is consistent with a dynamic environment, wherein crystals with a variety of growth and resorption histories were juxtaposed shortly before eruption by convective currents.

Colorado↗

USGS2000

Voluminous lava-like precursor to a major ash-flow tuff: Low-column pyroclastic eruption of the Pagosa Peak Dacite, San Juan volcanic field, Colorado

The Pagosa Peak Dacite is an unusual pyroclastic deposit that immediately predated eruption of the enormous Fish Canyon Tuff (~5000 km3) from the La Garita caldera at 28 Ma. The Pagosa Peak Dacite is thick (to 1 km), voluminous (>200 km3), and has a high aspect ratio (1:50) similar to those of silicic lava flows. It contains a high proportion (40-60%) of juvenile clasts (to 3-4 m) emplaced as viscous magma that was less vesiculated than typical pumice. Accidental lithic fragments are absent above the basal 5-10% of the unit. Thick densely welded proximal deposits flowed rheomorphically due to gravitational spreading, despite the very high viscosity of the crystal-rich magma, resulting in a macroscopic appearance similar to flow-layered silicic lava. Although it is a separate depositional unit, the Pagosa Peak Dacite is indistinguishable from the overlying Fish Canyon Tuff in bulk-rock chemistry, phenocryst compositions, and 40Ar/39Ar age. The unusual characteristics of this deposit are interpreted as consequences of eruption by low-column pyroclastic fountaining and lateral transport as dense, poorly inflated pyroclastic flows. The inferred eruptive style may be in part related to synchronous disruption of the southern margin of the Fish Canyon magma chamber by block faulting. The Pagosa Peak eruptive sources are apparently buried in the southern La Garita caldera, where northerly extensions of observed syneruptive faults served as fissure vents. Cumulative vent cross-sections were large, leading to relatively low emission velocities for a given discharge rate. Many successive pyroclastic flows accumulated sufficiently rapidly to weld densely as a cooling unit up to 1000 m thick and to retain heat adequately to permit rheomorphic flow. Explosive potential of the magma may have been reduced by degassing during ascent through fissure conduits, leading to fracture-dominated magma fragmentation at low vesicularity. Subsequent collapse of the 75 x 35 km2 La Garita caldera and eruption of the Fish Canyon Tuff were probably triggered by destabilization of the chamber roof as magma was withdrawn during the Pagosa Peak eruption. (C) 2000 Elsevier Science B.V. All rights reserved.

Journal of Volcanology and Geothermal Research↗