Pierre-Simon Ross

Magmatic affinity of modern and ancient subalkaline volcanic rocks determined from trace-element discriminant diagrams

When dealing with ancient subalkaline volcanic rocks, the alkali – total iron – magnesium (AFM) diagram is of limited use in assigning a tholeiitic versus calc-alkaline affinity because these eleme...

Maar-diatreme geometry and deposits: Subsurface blast experiments with variable explosion depth

Basaltic maar-diatreme volcanoes, which have craters cut into preeruption landscapes (maars) underlain by downward-tapering bodies of fragmental material commonly cut by hypabyssal intrusions (diatremes), are produced by multiple subsurface phreatomagmatic explosions. Although many maar-diatremes have been studied, the link between explosion dynamics and the resulting deposit architecture is still poorly understood. Scaled experiments employed multiple buried explosions of known energies and depths within layered aggregates in order to assess the effects of explosion depth, and the morphology and compaction of the host on the distribution of host materials in resulting ejecta, the development of subcrater structures and deposits, and the relationships between them. Experimental craters were 1–2 m wide. Analysis of high-speed video shows that explosion jets had heights and shapes that were strongly influenced by scaled depth (physical depth scaled against explosion energy) and by the presence or absence of a crater. Jet properties in turn controlled the distribution of ejecta deposits outside the craters, and we infer that this is also reflected in the diverse range of deposit types at natural maars. Ejecta were dominated by material that originated above the explosion site, and the shallowest material was dispersed the farthest. Subcrater deposits illustrate progressive vertical mixing of host materials through successive explosions. We conclude that the progressive appearance of deeper-seated material stratigraphically upward in deposits of natural maars probably records the length and time scale for upward mixing through multiple explosions with ejection by shallow blasts, rather than progressive deepening of explosion sites in response to draw down of aquifers.

Experiments with vertically and laterally migrating subsurface explosions with applications to the geology of phreatomagmatic and hydrothermal explosion craters and diatremes

We present results of experiments that use small chemical explosive charges buried in layered aggregates to simulate the effects of subsurface hydrothermal and phreatomagmatic explosions at varying depths and lateral locations, extending earlier experimental results that changed explosion locations only along a vertical axis. The focus is on the resulting crater size and shape and subcrater structures. Final crater shapes tend to be roughly circular if subsurface explosion epicenters occur within each other’s footprints (defined as the plan view area of reference crater produced by a single explosion of a given energy, as predicted by an empirical relationship). Craters are elongate if an epicenter lies somewhat beyond the footprint of the previous explosion, such that their footprints overlap, but if epicenters are too far apart, the footprints do not overlap and separate craters result. Explosions beneath crater walls formed by previous blasts tend to produce inclined (laterally directed) ejecta jets, while those beneath crater centers are vertically focused. Lateral shifting of explosion sites results in mixing of subcrater materials by development of multiple subvertical domains of otherwise pure materials, which progressively break down with repeated blasts, and by ejection and fallback of deeper-seated material that had experienced net upward displacement to very shallow levels by previous explosions. A variably developed collar of material that experienced net downward displacement surrounds the subvertical domains. The results demonstrate key processes related to mixing and ejection of materials from different depths during an eruptive episode at a maar-diatreme volcano as well as at other phreatomagmatic and hydrothermal explosion sites.

Facies distribution of ejecta in analog tephra rings from experiments with single and multiple subsurface explosions

The volume, grain size, and depositional facies of material deposited outside an explosion crater, ejecta, are sensitive to the depth of the explosion, the explosion energy, and the presence or absence of a crater before the explosion. We detonate buried chemical explosives as an analog for discrete volcanic explosions in experiments to identify unique characteristics of proximal, medial, and distal ejecta facies and their distribution from a range of scaled depths in undisturbed and cratered ground. Ejecta are here discussed in terms of three facies: (1) proximal ejecta, which form a constructional landform around a crater; (2) medial ejecta, which form a continuous sheet deposit that thins much more gradually with distance; and (3) distal ejecta that are deposited as isolated clasts. The extent of proximal ejecta away from the crater, relative to crater size, is not sensitive to scaled depth, but the volume proportion of proximal ejecta to the total ejecta deposit is sensitive to the presence of a crater and scaled depth. Medial ejecta distribution and volume contributions are both sensitive to the presence of a crater and to scaled depth. Distal ejecta distance is dependent on scaled depth and the presence of a crater, while the volume proportion of distal ejecta is less sensitive to scaled depth or presence of a crater. Experimental facies and deposit structures inferred from observations of jet dynamics are used to suggest facies associations anticipated from eruptions dominated by explosions of different scaled depth configurations. We emphasize that significant differences in tephra ring deposits can result from the effects of scaled depth and preexisting craters on ejecta dynamics, and are not necessarily related to fundamental differences in explosion mechanisms or degree of magma fragmentation.

Improving lithological discrimination in exploration drill-cores using portable X-ray fluorescence measurements: (1) testing three Olympus Innov-X analysers on unprepared cores

Portable X-ray fluorescence (pXRF) analysers are increasingly popular tools for geoscientific applications, including mineral exploration. One promising application, illustrated in the companion paper, is to obtain high-spatial resolution down-hole geochemical profiles using pXRF on unprepared exploration drill-cores. However, the precision and accuracy of pXRF analysers on such samples is not well studied. We have tested three Olympus Innov-X analysers, both on a sediment standard (NIST 2702, ‘Inorganics in Marine Sediment’) and in-situ on unmineralized rock cores from volcanic and intrusive, mafic to felsic lithologies. We conclude that pXRF is quite precise for a number of elements, but not very accurate using factory calibrations. For example, the 1σ precision of one Delta Premium analyser tested on a basaltic core, in mining plus mode, with a 60 s integration time, is better than 5 % for Al, Ca, Fe, K, Mn, S, Si, Ti, Zn and Zr. The same analyser, tested on a range of volcanic and intrusive core samples, yielded the following average systematic errors: Al -23 %, Ca -4 %, Fe +1 %, K -9 %, Mg -17 %, Mn -15 %, P +218 %, Si +4 %, Ti -23 %, Cu +220 %, Zn +151 %, and Zr +17 %. These systematic errors can largely be removed by the application of correction factors, which are unique to each analyser and each project. Without such corrections, the three analysers tested, including two ‘identical’ Delta Premium models, yield different results on the same sample. Another important finding is that within 20 cm long core samples, the effect of mineralogical heterogeneity on in-situ pXRF data is much larger than that of the instrument precision. Finally, with the Delta analysers, both the ‘mining plus’ and the ‘soil’ modes are needed to determine as many elements as possible with the best data quality possible.

Debris avalanche deposits associated with large igneous province volcanism: An example from the Mawson Formation, central Allan Hills, Antarctica

An up-to-180-m-thick debris avalanche deposit related to Ferrar large igneous province magmatism is observed at central Allan Hills, Antarctica. This Jurassic debris avalanche deposit forms the lower part (member m 1 ) of the Mawson Formation and is overlain by thick volcaniclastic layers containing a mixture of basaltic and sedimentary debris (member m 2 ). The m 1 deposit consists of a chaotic assemblage of breccia panels and megablocks up to 80 m across. In contrast to m 2 , it is composed essentially of sedimentary material derived from the underlying Beacon Supergroup. The observed structures and textures suggest that the breccias in m 1 were mostly produced by progressive fragmentation of megablocks during transport but also to a lesser extent by disruption and ingestion of the substrate by the moving debris avalanche. The upper surface of the debris avalanche deposit lacks large hummocks, and sandstone breccias dominate volumetrically over megablocks within the deposits. This indicates pervasive and relatively uniform fragmentation of the moving mass and probably reflects the weak and relatively homogenous nature of the material involved. The avalanche flowed into a preexisting topographic depression carved into the Beacon sequence, and flow indicators reveal a northeastward movement. The source area is probably now hidden under the Antarctic ice sheet. Sparse basaltic bodies, which were hot and plastic during transport in m 1 , reveal the role of Ferrar magmatism in triggering the avalanche, possibly in relation to the emplacement of large subsurface intrusions. The documented deposits indicate that debris avalanches are among the various phenomena that can accompany the early stages of large igneous province magmatism, despite the common absence of large central volcanic edifices. Where large igneous provinces develop in association with faulting or slow preeruptive uplift accompanied by deep valley incision, there is a high probability that feeder dikes will approach the surface in areas of steep topography, allowing volcano-seismicity and fluid overpressures associated with intrusion to effectively trigger avalanches.

Improving lithological discrimination in exploration drill-cores using portable X-ray fluorescence measurements: (2) applications to the Zn-Cu Matagami mining camp, Canada

A new geoscientific application of portable XRF (pXRF) analysers is the acquisition of high-spatial resolution down-hole geochemical profiles obtained in-situ on exploration drill-cores. One advantage of such profiles over traditional laboratory geochemistry, apart from the non-destructive aspect of pXRF, is that they are obtained quickly, in the field. So they can help exploration companies take important decisions such as “has a target stratigraphic horizon been reached, or should we drill deeper?” For example, in the Matagami mining camp, pXRF data permits the rapid distinction of two visually similar and variably altered rhyolites in the Persévérance area, based on a plot of Ti/Zr vs Al/Zr. The corrected pXRF data plot within the same fields as the traditional geochemical analyses for these rhyolites. Another advantage of pXRF profiles for exploration companies, geological surveys or academic researchers is the ability to locate lithological contacts better, and in general improve down-hole lithological discrimination, especially for fine-grained and/or hydrothermally altered lithologies. For example, in the Caber volcanogenic massive sulphide deposit area, there are abundant intrusions which makes it difficult to follow the volcanic stratigraphy between drill-holes and sections. In the drill-hole studied, the pXRF data, plotted as down-hole profiles of elements/oxides and ratios, allow several previously unidentified altered dykes to be distinguished from altered rhyolites.

Unusually large clastic dykes formed by elutriation of a poorly sorted, coarser-grained source

Tuff dykes of unusual size invade coarser-grained volcaniclastic rocks of the Jurassic Mawson Formation at Coombs Hills and Allan Hills, Antarctica. We infer that the material for the largest dykes was elutriated from their hosts matrix. To our knowledge, this mode of genesis has not been invoked before. In this volcanic setting elutriation was driven by magmatic heat, but the production of relatively finer-grained clastic dykes by elutriation of material from coarser ill-sorted sources may be significant in many settings.

Portable X-ray fluorescence measurements on exploration drill-cores: comparing performance on unprepared cores and powders for ‘whole-rock’ analysis.

One geoscience application of portable XRF (pXRF) technology is acquiring ‘whole-rock’ analyses of unmineralized or weakly mineralized rock cores for major oxides and trace elements, to fill the gaps between traditional laboratory analyses and/or obtain geochemical data more quickly. But the question of whether the samples actually need to be crushed and pulverized before analysis to produce useful results has not been extensively studied. In this paper pXRF data quality is compared on unprepared rock cores and on powders in three ways: instrumental precision (relative standard deviation [RSD] of a series of measurements on the same spot), sample precision (for unprepared samples, RSD of a series of measurements on different spots on the core), and accuracy (average pXRF value versus laboratory geochemistry). Two Olympus Innov-X Delta Premium pXRF devices were tested on 27 core samples of dense, non-mineralized, fine- to medium-grained, Precambrian volcanic and intrusive rocks from Canada. In general, sample preparation does not improve instrumental precision or accuracy. The significant advantage of powders is to avoid mineralogical heterogeneity. However, sample precision for in situ data is improved by averaging multiple measurements of different points on the sample: a significant gain is obtained between three and seven measurements. The sample precisions at 25 points – which is about the most measurements one can make during the same amount of time used for powdering a rock core sample – are better than the instrumental precision on powders for most elements. For high spatial resolution down-hole element profiles on entire drill-holes, in situ pXRF measurements with smoothing (e.g. 3–5 point moving averages) provide fit-for-purpose data; the alternative of turning the entire drill-core into powder is not realistic.

The effects of the host-substrate properties on maar-diatreme volcanoes: experimental evidence

While the relationship between the host-substrate properties and the formation of maar-diatreme volcanoes have been investigated in the past, it remains poorly understood. In order to establish the effects of the qualitative host-substrate properties on crater depth, diameter, morphological features, and sub-surface structures, we present a comparison of four campaigns of experiments that used small chemical explosives buried in various geological media to simulate the formation of maar-diatremes. Previous results from these experiments have shown that primary variations in craters and sub-surface structures are related to the scaled depth (physical depth divided by cube root of blast energy). Our study reveals that single explosions at optimal scaled depths in stronger host materials create the largest and deepest craters with steep walls and the highest crater rims. For single explosions at deeper than optimal scaled depths, the influence of material strength is less obvious and non-linear for crater depth, and non-existent for crater diameter, within the range of the experiments. For secondary and tertiary blasts, there are no apparent relationships between the material properties and the crater parameters. Instead, the presence of pre-existing craters influences the crater evolution. A general weakening of the materials after successive explosions can be observed, suggesting a possible decrease in the host-substrate influence even at optimal scaled depth. The results suggest that the influence of the host-substrate properties is important only in the early stage of a maar-diatreme (neglecting post-eruptive slumping into the open crater) and decreases as explosion numbers increase. Since maar-diatremes reflect eruptive histories that involve tens to hundreds of individual explosions, the influence of initial substrate properties on initial crater processes could potentially be completely lost in a natural system.