John A. Wolff

Distinguishing strongly rheomorphic tuffs from extensive silicic lavas

High-temperature silicic volcanic rocks, including strongly rheomorphic tuffs and extensive silicic lavas, have recently been recognized to be abundant in the geologic record. However, their mechanisms of eruption and emplacement are still controversial, and traditional criteria used to distinguish conventional ash-flow tuffs from silicic lavas largely fail to distinguish the high-temperature versions. We suggest the following criteria, ordered in decreasing ease of identification, to distinguish strongly rheomorphic tuffs from extensive silicic lavas: (1) the character of basal deposits; (2) the nature of distal parts of flows; (3) the relationship of units to pre-existing topography; and (4) the type of source. As a result of quenching against the ground, basal deposits best preserve primary features, can be observed in single outcrops, and do not require knowing the full extent of a unit. Lavas commonly develop basal breccias composed of a variety of textural types of the flow in a finer clastic matrix; such deposits are unique to lavas. Because the chilled base of an ashflow tuff generally does not participate in secondary flow, primary pyroclastic features are best preserved there. Massive, flow-banded bases are more consistent with a lava than a pyroclastic origin. Lavas are thick to their margins and have steep, abrupt flow fronts. Ashflow tuffs thin to no more than a few meters at their distal ends, where they generally do not show any secondary flow features. Lavas are stopped by topographic barriers unless the flow is much thicker than the barrier. Ash-flow tuffs moving at even relatively slow velocities can climb over barriers much higher than the resulting deposit. Lavas dominantly erupt from fissures and maintain fairly uniform thicknesses throughout their extents. Tuffs commonly erupt from calderas where they can pond to thicknesses many times those of their outflow deposits. These criteria may also prove effective in distinguishing extensive silicic lavas from a postulated rock type termed lava-like ignimbrite. The latter have characteristics of lavas except for great areal extents, up to many tens of kilometers. These rocks have been interpreted as ash-flow tuffs that formed from low, boiling-over eruption columns, based almost entirely on their great extents and the belief that silicic lavas could not flow such distances. However, we interpret the best known examples of lava-like ignimbrites to be lavas. This interpretation should be tested through additional documentation of their characteristics and research on the boiling-over eruption mechanism and the kinds of deposits it can produce. Flow bands, flow folds, ramps, elongate vesicles, and probably upper breccias occur in both lavas and strongly rheomorphic tuffs and are therefore not diagnostic. Pumice and shards also occur in both tuffs and lavas, although they occur throughout ash-flow tuffs and generally only in marginal breccias of lavas. Dense welding, secondary flow, and intense alteration accompanying crystallization at high temperature commonly obliterate primary textures in both thick, rheomorphic tuffs and thick lavas. High-temperature silicic volcanic rocks are dominantly associated with tholeiitic flood basalts. Extensive silicic lavas could be appropriately termed flood rhyolites.

Anisotropy of magnetic susceptibility in welded tuffs: application to a welded-tuff dyke in the tertiary Trans-Pecos Texas volcanic province, USA

Consideration of published anisotropy of magnetic susceptibility (AMS) studies on welded ignimbrites suggests that AMS fabrics are controlled by groundmass microlites distributed within the existing tuff fabric, the sum result of directional fabrics imposed by primary flow lineation, welding, and (if relevant) rheomorphism. AMS is a more sensitive indicator of fabric elements within welded tuffs than conventional methods, and usually yields primary flow azimuth estimates. Detailed study of a single densely welded tuff sample demonstrates that the overall AMS fabric is insensitive to the relative abundances of fiamme, matrix and lithics within individual drilled cores. AMS determinations on a welded-tuff dyke occurring in a choked vent in the Trans-Pecos Texas volcanic field reveals a consistent fabric with a prolate element imbricated with respect to one wall of the dyke, while total magnetic susceptibility and density exhibit axially symmetric variations across the dyke width. The dyke is interpreted to have formed as a result of agglutination of the erupting mixture on a portion of the conduit wall as it failed and slid into the conduit, followed by residual squeezing between the failed block and in situ wallrock. Irrespective of the precise mechanism, widespread occurrence of both welded-tuff dykes and point-welded, aggregate pumices in pyroclastic deposits may imply that lining of conduit walls by agglutionation during explosive volcanic eruptions is a common process.

Gradients in physical parameters in zoned felsic magma bodies: Implications for evolution and eruptive withdrawal

Abstract Five diverse, well documented, chemically zoned magmas have been chosen from the literature to demonstrate the extent and patterns of density and viscosity gradients in zoned magma chambers. The patterns are used to assess implications for development of zonation, and withdrawal dynamics and preservation of systematic chemical variations in the final pyroclastic deposit. These examples are: Bishop Tuff, California (high-silica rhyolite); Los Humeros, Mexico (calc-alkaline rhyolite to andesite); Fogo A, Azores (trachyte); Laacher See, Eifel (phonolite) and Tenerife, Canary Islands (phonolite). It was necessary to make several simplifying assumptions in order to calculate viscosity and density profiles through each system; results are particularly sensitive to magmatic water and crystal contents. Nevertheless, the following conclusions can be drawn: 1. (1) Small, strongly zoned, alkaline magma systems which evolved through fractional crystallisation of a basaltic parent (Fogo A, Laacher See) have suffered a partial time-integrated volatile depletion prior to eruption. The most likely mechanism of volatile loss is degassing of the uppermost, highly differentiated, “cupola” magma layer. 2. (2) Eruption withdrawal dynamics are critically dependent on density gradients (and therefore on volatile content and phenocryst abundance), while viscosity variations play a subordinate role in the chosen examples. 3. (3) Formation of a chemically zoned tephra sequence by eruption of chemically zoned felsic magma requires a pre-eruptive volatile gradient in the magma chamber. 4. (4) Withdrawal-layer thicknesses during eruptions from naturally zoned magma chambers are of the order of 100 m. 5. (5) The quantitative treatment of gravitational liquid segregation processes by Nilson et al. (1985) successfully predicts times required for zonation of magma bodies: typically 103–104 years for small alkaline systems, and > 105 years for large silicic systems.

40Ar/39Ar dating of the Bandelier Tuff and San Diego Canyon ignimbrites, Jemez Mountains, New Mexico: Temporal constraints on magmatic evolution

Abstract The Jemez Mountains volcanic field (JMVF), located in north-central New Mexico, has been a site of basaltic to rhyolitic volcanism since the mid-Miocene with major caldera forming eruptions occurring in the Pleistocene. Eruption of the upper Bandelier Tuff (UBT) is associated with collapse of the Valles Caldera, whereas eruption of the lower Bandelier Tuff (LBT) resulted in formation of the Toledo Caldera. These events were previously dated by K-Ar at 1.12 ± 0.03 Ma and 1.45 ± 0.06 Ma, respectively. Pre-Bandelier explosive eruptions produced the San Diego Canyon (SDC) ignimbrites. SDC ignimbrite “B” has been dated at 2.84 ± 0.07 Ma, whereas SDC ignimbrite “A”, which underlies “B”, has been dated at 3.64 ± 1.64 Ma. Both of these dates are based on single K-Ar analyses. 40Ar/39Ar dating of single sanidine crystals from these units indicates revision of the previously reported dates. Isochron analysis of 26 crystals from the UBT gives a common trapped 40Ar/36Ar component of 304.5, indicating the presence of excess 40Ar in this unit, and defines an age of 1.14 ± 0.02 Ma. Isochron analysis of 26 crystals from the LBT indicates an atmospheric trapped component and an age of 1.51 ± 0.03 Ma. An age of 1.78 ± 0.04 Ma, based on the weighted mean of 5 individual analyses, is indicated for SDC ignimbrite “B”, whereas 3 analyses from SDC ignimbrite “A” give a weighted mean age of 1.78 ± 0.07 Ma. Evidence for xenocrystic contamination in the SDC ignimbrites comes from analyses of a correlative air-fall pumice unit in the Puye Formation alluvial fan giving ages of 1.75 ± 0.08 and 3.50 ± 0.09 Ma. The presence of xenocrysts in bulk separates used for the original K-Ar analyses could account for the significantly older ages reported. Geochemical data indicate that SDC ignimbrites are early eruptions from the magma chamber which evolved to produce the LBT, as compositions of SDC ignimbrite “B” are virtually identical to least evolved LBT samples. Differentiation during the 270-ka interval between eruption of SDC ignimbrite “B” and the LBT produced an array of high-silica rhyolite compositions which were erupted to form the LBT. Mixed pumices associated with eruption of the LBT indicated an influx of more mafic magma into the system which produced shifts in some incompatible trace-element ratios. Lavas and tephras of the Cerro Toledo Rhyolite record the geochemical evolution of the Bandelier magma system during the 370-ka interval between eruption of the LBT and the UBT. The combined geochronologic and geochemical data place the establishment and evolution of the Bandelier silicic magma system within a precise temporal framework, beginning with eruption of the SDC ignimbrites at 1.78 Ma, and define a periodicity of 270–370 ka to ash-flow eruptions in the JMVF. These intervals are comparable to those in other multicyclic caldera complexes and are a measure of the timescales over which substantial fractionation of large silicic magma bodies occur.

Crystallisation of nepheline syenite in a subvolcanic magma system: Tenerife, canary islands

Abstract The late Quaternary phonolitic pumice deposits of Tenerife, Canary Islands, are the product of a periodically-tapped, periodically-replenished, zoned alkaline magma system. Nepheline syenite blocks occur as lithics in the deposits, and provide solidified representatives of the phonolite magmas. The blocks are considered to have crystallised in the roof zone of the active magma system at a depth of roughly 4 km beneath the Las Canadas caldera. Conversion of highly-differentiated phonolitic magma to solid nepheline syenite was achieved between 770° and 680°C. Quenched glass of extreme composition in one erupted block provides a sample of the last interstitial liquid; comparison of this sample with phonolitic pumice, and with fully crystallised syenites, allows reconstruction of the magmatic, interstitial and subsolidus crystallisation histories of nepheline syenite. Phases represented by the phenocryst assemblage of the most differentiated phonolitic pumices (sanidine, sodalite, nepheline, Na-poor pyroxene, biotite, sphene and magnetite) formed an initial crystal “mush”, containing abundant trapped liquid, on the walls of the magma chamber. Interstitial crystallisation, involving the conversion of pyroxene to Na-rich compositions, continued growth of felsic phases, and partial to complete consumption of biotite, sphene, magnetite and apatite, was accompanied by extreme sodium enrichment in the residual liquid. Stellate aegirine, lavenite, loparite and Mn-rich ilmenite are among the final products of magmatic crystallisation. Modification of feldspar primocrysts persisted into subsolidus conditions. It is suggested that complete solidification of an open-system alkaline magma chamber results in the formation of an alkaline alkaline ring complex.

Widespread, lavalike silicic volcanic rocks of Trans-Pecos Texas

Several silicic units of the Trans-Pecos volcanic field have outcrop and thin-section scale features of lava flows but areal extents and aspect ratios of ignimbrites. These voluminous rocks (up to hundreds of cubic kilometres per unit) are quartz trachytes to low-silica rhyolites (68% to 72%SiO 2 ). Lava flow features include flow banding and folding, elongated vesicles, and autobreccias and vitrophyres at the base and top of units. Pyroclastic flow features include sheetlike geometry, lateral extents up to 70 km, aspect ratios as low as 1:700, and areal extents up to 3000 km 2 . A few of these units are clearly rheomorphic ignimbrites, but others show no unambiguous evidence of a primary pyroclastic origin. Although no adequate explanation currently exists for the origin of the latter, we evaluate two end-member hypotheses: (1) they are ignimbrites in which extreme rheomorphism has obliterated primary internal features, and (2) they are highly viscous lavas with unusually high heat retention or effusion rates that allowed them to spread over great areas. Either origin requires a rock type and eruptive mechanism not commonly recognized.

Is the Valles caldera entering a new cycle of activity

The Valles caldera formed during two major rhyolitic ignimbrite eruptive episodes (the Bandelier Tuff) at 1.61 and 1.22 Ma, after some 12 m.y. of activity in the Jemez Mountains volcanic field, New Mexico. Several subsequent eruptions between 1.22 and 0.52 Ma produced dominantly high-silica rhyolite lava domes and tephras within the caldera. These were followed by a dormancy of 0.46 m.y. prior to the most recent intracaldera activity, the longest hiatus since the inception of the Bandelier magma system at ∼1.8 Ma. The youngest volcanic activity at ∼ 60 ka produced the SW moat rhyolites, a series of lavas and tuffs that display abundant petrologic evidence of being newly generated melts. Petrographic textures conform closely to published predictions for silicic magmas generated by intrusion of basaltic magma into continental crust. The Valles caldera may currently be the site of renewed silicic magma generation, induced by intrusion of mafic magma at depth. Recent seismic investigations revealed the presence of a large low-velocity anomaly in the lower crust beneath the caldera. The generally aseismic character of the caldera, despite abundant regional seismicity, may be attributed to a heated crustal column, the local effect of 13 m.y. of magmatism and emplacement of mid-crustal plutons. Seismic signals of magma movement in the deep to mid-crust may therefore be masked, and clear seismic indications of intrusion may only be generated within a few kilometres of the surface. We therefore encourage the establishment of a local dedicated volcanic monitoring system.

Redistribution of Pb and other volatile trace metals during eruption, devitrification, and vapor-phase crystallization of the Bandelier Tuff, New Mexico

A diverse suite of micron-scale minerals was deposited from vapor during eruption and post-emplacement crystallization of the Bandelier Tuff, New Mexico. The mineral suite is rich in sulfides, oxides, and chlorides of both common and rare metals (e.g., Fe, Pb, Bi, Cu, Ag, Re), and oxides and silicates of incompatible elements (e.g., P, Zr, Y, Nb, Ba and LREE). Minerals preserved in glassy samples grew from magmatic vapor trapped during emplacement, or from vapor migrating along contacts with more impermeable rocks; minerals observed in devitrified samples also grew from crystallization of glass and vapor liberated during this process. In devitrified samples, mafic silicate phenocrysts were partially replaced by an assemblage dominated by smectite and hematite. n nThe syn- to post-eruptive mineral assemblage observed in upper Bandelier Tuff (UBT) samples bears striking similarity to those deposited by cooling gases near active volcanic vents. However, several differences exist: (1) the mineral suite in the UBT is disseminated throughout the unit, and formed over a broad temperature range (> 700 to < 150 °C) at higher rock:gas ratios; (2) the highly evolved composition of the UBT yielded a greater abundance of minerals rich in incompatible elements compared to sublimates from less evolved volcanoes; and (3) the UBT has suffered over 1 million years of post-emplacement exposure, which resulted in solution (or local re-precipitation in fractures) of soluble compounds such as halite, sylvite, and gypsum. n nPb was enriched toward the roof of the UBT magma body due to its affinity for the melt and vapor phases relative to crystals (Bulk Dpb < 0.2). Micron-scale Pb minerals appear to have grown from vapor exsolved during eruption, as well as vapor liberated during later devitrification. Additional Pb was scavenged by smectite and hematite that probably formed during the later stages of the devitrification and cooling process. Up to ten-fold increases in Pb concentrations are seen in zones of fumarolic concentration in the UBT, however, most bulk tuff samples have Pb values that appear to preserve magmatic values, indicating only very local trace-metal redistribution. The concentration of Pb and other heavy metals in micron-scale mineral coatings in porous tuff indicates that these metals could be readily mobilized and transported by acidic groundwaters or hydrothermal fluids, and thus locally concentrated into ore-grade deposits in long-lived systems.

Trapped liquid from a nepheline syenite: a re-evaluation of Na-, Zr-, F-rich interstitial glass in a xenolith from Tenerife, Canary Islands

Abstract Interstitial glass and pyroxene in a nepheline syenite described by Wolff (1987) have been more precisely and accurately re-analysed using a cryogenic microprobe technique. The glass compositions, with Na2O = 13.4−17.0 wt.% and (Na+K) Al = 1.81−2.26 at SiO2 = 53−56.5 wt.% (on a 100% normalised basis) are more extreme than any known erupted magma. ZrO2 contents of the glasses are 1.4%–1.8% by weight, while F ranges between 1% and 1.4%. The high Zr content is due to the failure of a Zr-silicate phase to precipitate, despite predicted zircon saturation of the liquids above their probable quench temperatures. Partitioning of Zr between pyroxene and liquid is highly variable and apparently controlled by sector zoning; Zr is preferentially incorporated into aegirine {001} growth sectors. The glasses demonstrate the extent to which peralkaline phonolitic liquids can be fractionated in closed magma systems.

The slot technique for rock magnetic sampling

Abstract We have developed a new technique for paleomagnetic sampling that allows one to sample extremely friable, poorly to moderately lithified sedimentary and volcanic materials. The method uses a portable electric drill and a dual blade masonry wheel (or two sharp steel cutting blades) with a 2 cm separation, which cuts two parallel slots in the material being sampled. A second set of cuts at right angles to the first produces a fixed, cubic pedestal around which a plastic sample box can be inserted, oriented and extracted. The method has allowed us to take samples at localities where we were not able to sample using other methods. To test the method we have sampled the Bandelier Tuff in northeast New Mexico and have measured the anisotropy of magnetic susceptibility (AMS) on boxes taken using the new method and on cylinders drilled at the same sites using standard techniques. Results show better AMS within-site precision for box samples taken using the new method. We conclude that the method is convenient, reliable and may be preferred in certain types of paleomagnetic studies.