Benjamin R. Edwards

Distribution, nature, and origin of Neogene–Quaternary magmatism in the northern Cordilleran volcanic province, Canada

The northern Cordilleran volcanic province encompasses a broad area of Neogene to Quaternary volcanism in northwestern British Columbia, the Yukon Territory, and adjacent eastern Alaska. Volcanic rocks of the northern Cordilleran volcanic province range in age from 20 Ma to ca. 200 yr B.P. and are dominantly alkali olivine basalt and hawaiite. A variety of more strongly alkaline rock types not commonly found in the North American Cordillera are locally abundant in the northern Cordilleran volcanic province. These include nephelinite, basanite, and peralkaline phonolite, trachyte, and comendite. The most MgO-rich nephelinites, basanites, and alkaline basalts from throughout the northern Cordilleran volcanic province show trace element abundances and isotopic compositions that are consistent with an asthenospheric source region similar to that for average oceanic island basalt and for post-5 Ma alkaline basalts from the Basin and Range. Our petrologic observations help constrain the origin of northern Cordilleran volcanic province magmatism as well as lithosphere changes between the four major basement terranes that underlie the province. Results from phase equilibria calculations and the spatial distributions of volcanic rock types and magmatic inclusions are more consistent with the existence of thicker lithosphere beneath Stikinia, which underlies the southern part of the northern Cordilleran volcanic province, than beneath the Cache Creek and Yukon-Tanana terranes, which underlie the northern part of the northern Cordilleran volcanic province. Our results support a model for initiation of northern Cordilleran volcanic province magmatism due to incipient rifting of the northern Cordillera, driven by changes in relative plate motion between the Pacific and North American plates ca. 15–10 Ma.

Time scales of magmatic processes: New insights from dynamic models for magmatic assimilation

We present a computational model that predicts the consequences of time-dependent, isenthalpic assimilation-fractional crystallization (AFC). Our model combines an existing paradigm for fractional crystallization based on equilibrium thermodynamics with a kinetic model for predicting rates of mineral dissolution. The isenthalpic constraint directly links the sensible heat of the magma and the latent heat of crystallization to the energy necessary for assimilation. Thus, we predict liquid lines of descent, rates of crystallization, rates of cooling, and ratios of assimilation to crystallization ( r ) as a function of time. Model simulations predict the time scale of AFC processes (weeks to years) and show the extent to which temperature and composition of the assimilant control these time scales.

A review and analysis of silicate mineral dissolution experiments in natural silicate melts

Abstract We present a database and a graphical analysis of published experimental results for dissolution rates of olivine, quartz plagioclase, clinopyroxene, orthopyroxene, spinel, and garnet in basaltic and andesitic melts covering a range of experimental temperatures (1100–1500°C) and pressures (10 5 Pa-3.0 GPa). The published datasets of Donaldson (1985, 1990) and Brearly and Scarfe (1986) are the most complete. Experimental dissolution rates from all datasets are recalculated and normalized to a constant oxygen basis to allow for direct comparison of dissolution rates between different minerals. Dissolution rates (ν) range from 5·10 −10 oxygen equivalent moles (o.e.m.) cm −2 s −1 for olivine in a basaltic melt to 1.3·10 −5 o.e.m. cm −2 s −1 for garnet in a basaltic melt. Values of ln ν are Arthenian for the experiments examined and activation energies range from 118 to 1800 kJ/o.e.m. for quartz and clinopyroxene, respectively. The relationship between calculated A/RT for the dissolution reactions, where A is the thermodynamic potential affinity, and values of ν is linear for olivine, plagioclase, and quartz. We interpret this as strong evidence in support of using calculated A as a predictor of ν for, at least, superliquidus melt conditions.

Northern Cordilleran volcanic province: A northern Basin and Range?

INTRODUCTIONThe western margin of North America hosts adiverse assemblage of large Cenozoic igneousprovinces (Fig. 1 inset). The temporal,spatial,andpetrological characteristics of these provinces areattributed to a variety of magmatic processes andtectonic environments, including lithosphericextension (e.g., Basin and Range), migration ofmantle plumes (e.g., Snake River Plain), and sub-duction (e.g., Cascade magmatic arc). However,Neogene alkaline volcanic rocks in the Cordilleraof northwestern Canada, between 55° and 64° N(Fig. 1), currently lack a clear linkage to tectonicprocesses. These volcanic centers constitute thenorthern Cordilleran volcanic province and repre-sent a significant gap in our understanding of theCenozoic tectonomagmatic activity in westernNorth America (Fig. 1 inset).Our objective is to demonstrate that an exten-sional tectonic model adequately explains thepetrologic, radiometric, and tectonic data forNeogene-Quaternary volcanic rocks in the north-ern Cordillera. Furthermore, the model we pro-pose is consistent with models for Neogene-Quaternary interactions between the Pacific andNorth American plates.NORTHERN CORDILLERAN VOLCANICPROVINCEThe northern Cordilleran volcanic provinceencompasses a broad area of Neogene to Qua-ternary volcanism approximately 1200 km longand 400 km wide in the northern part of west-ern Canada (Fig. 1). The majority of the vol-canoes within this region formed east of theDenali fault system and west of the Tintinatrench (Fig. 1). Volcanic expressions of magma-tism in the province range from large basalticplateaus and stratovolcanoes to individual cin-der cones, lava flows, and eroded vents.PetrologyNeogene to Quaternary volcanic rocks withinthe northern Cordilleran volcanic province arealkaline and chemically bimodal. At least 70% ofthe volcanic rocks comprise alkali olivine basaltand hawaiite (Fig. 2A). Basaltic lavas are gener-ally olivine and plagioclase phyric with plagio-clase, olivine, titanaugite, and magnetite in thegroundmass.The province also hosts a variety of other alka-line and peralkaline rock types that are uncom-mon in the North American Cordillera, such asnephelinite, basanite, phonolite, trachyte, and

Dating young basalt eruptions by (U-Th)/He on xenolithic zircons

Accurate ages for young (e.g., Pleistocene) volcanic eruptions are important for geomorphic, tectonic, climatic, and hazard studies. Existing techniques can be time-consuming and expensive when many ages are needed, and in the case of K/Ar and 40Ar/39Ar dating, extraneous Ar often can limit precision, especially for continental basalts erupted through old lithosphere. We present a new technique for dating young basaltic eruptions by (U-Th)/He dating of zircons (ZHe) from crustal xenoliths. Single-crystal ZHe dates generally have lower precision than typical 40Ar/39Ar dates, but can be determined relatively easily on multiple replicate grain aliquots. We dated zircons from xenoliths from four volcanic centers in western North America: Little Bear Mountain, British Columbia (157 ± 3.5 [2.2%] ka weighted 95% confidence interval [CI], mean square of weighted deviates [MSWD] = 1.7) and Prindle Volcano, Alaska (176 ± 16 [8.9%] ka, MSWD = 13), in the northern Cordilleran volcanic province, and Fish Springs (273 ± 23 [8.6%], MSWD = 43) and Oak Creek (179 ± 8.1 [4.5%] ka, MSWD = 12), in the Big Pine Volcanic Field, California. All ZHe ages are either equivalent to or younger than previously determined K/Ar or 40Ar/39Ar ages, indicating the possibility of inherited 40Ar in some of the previous measurements. Zircons from upper crustal xenoliths in the Oak Creek and Fish Springs vents show poorer reproducibility and multiple apparent age distribution peaks, consistent with either intracrystalline U-Th zonation or <99.99% He degassing (assuming ca. 100 Ma pre-entrainment ZHe ages) of some zircons during magmatic entrainment. Removal of clear outliers in the older age-distribution peaks of the upper crustal xenoliths, most of which have extremely high U compared to other zircons of the same xenolith, improve the reproducibilities of Fish Springs to 4.7% (95% CI, MSWD = 4.8) and Oak Creek to 3.4% (95% CI, MSWD = 6.2). Coupled thermal and He diffusion modeling using appropriate xenolith sizes and magma temperatures and assuming published diffusion kinetics for zircon indicate that incomplete He degassing would require entrainment times <1 h. However, the observation of extremely high U in most zircons with older ages raises the possibility that zircons with high radiation dosages may have more retentive He diffusion characteristics.

Insights on lava–ice/snow interactions from large-scale basaltic melt experiments

Quantitative measurements of interactions between lava and ice/snow are critical for improving our knowledge of glaciovolcanic hazards and our ability to use glaciovolcanic deposits for paleoclimate reconstructions. However, such measurements are rare because the eruptions tend to be dangerous and not easily accessible. To address these difficulties, we conducted a series of pilot experiments designed to allow close observation, measurements, and textural documentation of interactions between basaltic melt and ice. Here we report the results of the first experiments, which comprised controlled pours of as much as 300 kg of basaltic melt on top of ice. Our experiments provide new insights on (1) estimates for rates of heat transfer through boundary layers and for ice melting; (2) controls on rates of lava advance over ice/snow; (3) formation of lava bubbles (i.e., Limu o Pele) by steam from vaporization of underlying ice or water; and (4) the role of within-ice discontinuities to facilitate lava migration beneath and within ice. The results of our experiments confirm field observations about the rates at which lava can melt snow/ice, the efficacy with which a boundary layer can slow melting rates, and morphologies and textures indicative of direct lava-ice interaction. They also demonstrate that ingestion of external water by lava can create surface bubbles (i.e., Limu) and large gas cavities. We propose that boundary layer steam can slow heat transfer from lava to ice, and present evidence for rapid isotopic exchange between water vapor and melt. We also suggest new criteria for identifying ice-contact features in terrestrial and martian lava flows.

Glacial influences on morphology and eruptive products of Hoodoo Mountain volcano, Canada

Abstract Hoodoo Mountain volcano (HMV), a Quaternary composite volcano in northwestern British Columbia, is a well-exposed example of peralkaline, phonolitic icecontact and subglacial volcanism. Its distinctive morphology and unique volcanic deposits are indicative of subglacial, within-ice, and/or ice-contact volcanic eruptions. Distinct ice-contact deposits result from three different types of lava-ice interaction: (1) vertical cliffs of lava, featuring finely jointed flow fronts up to 200 m in height, resulted from lava flows being dammed and ponded against thick masses of ice; (2) pervasively-jointed, dense lava flows, lobate intrusions, and domes associated with mantling deposits of poorly-vesiculated breccia are derived from volcanic eruptions contained beneath relatively thick ice; and (3) an association of pervasively-jointed, highly-vesicular lava flows or dykes encased by vesicular hyaloclastite of identical composition formed by eruption under and/or through relatively thin ice. The distribution of these three deposit types largely explains the distinctive morphology of Hoodoo Mountain and can be used to reconstruct variations in ice thickness surrounding the volcano since c.85 ka. Our analysis suggests that at c.85 ka Hoodoo Mountain erupted underneath ice cover of at least several hundred metres. At c.80 ka eruptions were no longer subglacial, but the edifice was surrounded by ice at least 800 m high that dammed lava flows around the perimeters of the volcano. After a period of eruptions showing no apparent evidence for ice interaction, from <80 to >40 ka, subglacial eruptions began again, signalling the build-up of regional ice levels. Local ice thickness during these eruptions may well have been over 2 km thick.

Pyroclastic passage zones in glaciovolcanic sequences

Volcanoes are increasingly recognized as agents and recorders of global climate variability, although deciphering the linkages between planetary climate and volcanism is still in its infancy. The growth and emergence of subaqueous volcanoes produce passage zones, which are stratigraphic surfaces marking major transitions in depositional environments. In glaciovolcanic settings, they record the elevations of syn-eruptive englacial lakes. Thus, they allow for forensic recovery of minimum ice thicknesses. Here we present the first description of a passage zone preserved entirely within pyroclastic deposits, marking the growth of a tephra cone above the englacial lake level. Our discovery requires extension of the passage-zone concept to accommodate explosive volcanism and guides future studies of hundreds of glaciovolcanic edifices on Earth and Mars. Our recognition of pyroclastic passage zones increases the potential for recovering transient paleolake levels, improving estimates of paleo-ice thicknesses and providing new constraints on paleoclimate models that consider the extents and timing of planetary glaciations.

Chapter 20 – Glaciovolcanism

Glaciovolcanism (a.k.a., subglacial volcanism, volcano–ice interaction) involves all interactions between magmatic-volcanic systems and ice in any form, including meltwater derived directly from ice melting. The earliest studies to recognize connections between characteristic volcanic deposits and an ice-rich environment were conducted in Iceland (1920s) and British Columbia, Canada (1940s). Almost a century later, glaciovolcanic deposits have been identified on all terrestrial continents, except Australia, and on Mars. The “classic” tuya model provides a generalized account of the dominant processes extant during a volcanic eruption beneath ice, including (1) transitions in eruptive style from effusive to explosive (or vice versa), (2) growth and emergence of volcanoes from subaqueous to subaerial environments (i.e., passage zones), and (3) waning of the eruption commonly expressed by effusion of lavas to cap the tuya. While the model has general application to the entire magmatic compositional spectrum, intermediate to felsic glaciovolcanic eruptions appear to preserve a less diagnostic record of interactions with ice owing to their higher viscosities and to their capacity to form glassy deposits even in subaerial environments. Recognition of glaciovolcanic origins for volcanic edifices and deposits is important well outside of classical volcanology. Glaciovolcanic deposits serve as valuable proxies for paleoclimate reconstructions by delineating the spatial and temporal limits of Quaternary ice sheets on Earth and also are used to map ice sheet distributions on Mars. Additionally, as demonstrated by recent eruptions at Gjalp (1996), Eyjafjallajokull (2010), and Redoubt (2009), volcano–ice interactions produce a unique set of local and regional hazards in both remote and urban areas that are critical to understand in order to protect strategic infrastructure and growing human populations.

ASTER and DEM Change Assessment of Glaciers Near Hoodoo Mountain, British Columbia, Canada

Hoodoo Mountain ice cap, Hoodoo Glacier, and Twin Glacier are located about 250 km southeast of Juneau, Alaska, in the Coast Mountains (near 56.8°N, 131.3°W, northwestern British Columbia). Several outlet valley glaciers flow towards the south from an ice cap centered approximately 16 km northeast of Hoodoo Mountain; some glaciers are relatively clean ice, while others are heavily debris covered. Hoodoo and Twin glaciers have a Pleistocene and Early Holocene record of interaction with a trachyte volcano, Hoodoo Mountain (which is still ice capped), though they have retreated far enough that future eruptions are unlikely to produce direct lava–ice interactions from anything other than long-lived lava flows. Our analysis shows retreat and accelerating thinning for valley glaciers within this study; this behavior appears to be climatically driven. However, the small ice cap on Hoodoo Mountain seems to be insensitive to climate change; rather, the ice cap’s extent is controlled mainly by the shape and elevation of the landform. The overall average mass balance of the combined set of glaciers in the study region was about −840 ± 180 kg m−2 yr−1 for the period from 1965 to 2005, though different glaciers have specific mass balances ranging from near zero (i.e., in local balance) to −2,400 kg m−2 yr−1. Furthermore, the documented increase in the rate of thinning indicates an increasing magnitude of negative balances over the four decades of the study period. Aside from the Hoodoo Mountain ice cap (which is close to a balance state, except at the very edges on the cliffs, where retreat and thinning have taken place), the prevalent glacier thinning and retreat of the Hoodoo Mountain area is similar to most other maritime parts of the Canadian Cordillera (see Chapter 14 of this book by Wheate et al.). Hoodoo Mountain is a classic flat-topped glaciovolcanic edifice (tuya), and was shaped when ice was much thicker within the massive Cordilleran Ice Sheet. Continuing glacial retreat from the flanks of Hoodoo Mountain offers new possibilities for the study of fresh exposures of materials formed by ice interactions with a rare lava type.