Documenting Mantle and Crustal Contributions to Flood Basalt Magmatism via Computational Modeling of the Steens Basalt, Southeast Oregon

Flood basalts are enormous volcanic events with volumes of volcanic cover and intrusive equivalents that are affected by and significantly affect the crust. Steens Basalt represents 31,800 km3 of flood basalt lavas that erupted in eastern Oregon ~16.8 Ma in less than 300,000 years. Analytical data of flows from a 1 km vertical exposure at Steens Mtn. documents time-transgressive changes in composition of two geochemically distinct units: (1) lower Steens, MgO-rich lavas with lower incompatible trace element concentrations and 87Sr/86Sr, and (2) upper Steens, MgO-poor, with higher incompatible trace element concentrations and 87Sr/86Sr. Results from the Energy-Constrained Recharge Assimilation Fractional Crystallization (EC-RAFC) computational modeling tool make predictions about the relative roles that magmatic processes (recharge, assimilation, and fraction crystallization) have in time (upsection), thus yielding estimates of the mantle vs. crustal mass contribution to the magmatic system. While all three processes are shown to be critical at Steens, results suggest sub-equal mantle-derived (magma recharge) and crustal (assimilation) input into lower and upper Steens basalts indicating that the fraction of inputs do not significantly change during the flood basalt event. In contrast, the modeled masses of crystals fractionated are significantly higher in the lower Steens compared to upper Steens, suggesting crustal thermal priming and substantial new mass added to the crust. Sub-equal mass balance of mantle and crust upsection doesn’t explain the more evolved character of upper Steens flows, perhaps the residual melt in the system has evolved with time. Also, the little variation in signature of the assimilant suggests the magma reservoirs shoaled to assimilate fresh crust. In support of the EC-RAFC results, four hypothetical magma behaviors are identified (from assessment of petrographic and select whole rock major and trace element data) that involve combinations of all magmatic processes and crystal entrainment. These behaviors are prevalent in the genesis of both lower and upper Steens lavas. EC-RAFC provides the first estimates of crustal vs. mantle mass input, as well as the first flow by flow modeling of geochemical changes. Additional work is needed to refine our understanding of the interaction between mantle and crust as the magmatic system matures

Dynamics of Pyroclastic Density Currents: Conditions That Promote Substrate Erosion and Self-Channelization - Mount St Helens, Washington (USA)

The May 18th, 1980 eruption of Mount St. Helens (MSH) produced multiple pyroclastic density currents (PDCs), burying the area north of the volcano under 10s of meters of deposits. Detailed measurements of recently exposed strata from these PDCs provide substantial insight into the dynamics of concentrated currents including inferences on particle-particle interactions, current mobility due to sedimentation fluidization and internal pore pressure, particle support mechanisms, the influence of surface roughness and the conditions that promote substrate erosion and self-channelization. Four primary flow units are identified along the extensive drainage system north of the volcano. Each flow unit has intricate vertical and lateral facies changes and complex cross-cutting relationships away from source. Each flow unit is an accumulation from an unsteady but locally sustained PDC or an amalgamation of several PDC pulses. The PDCs associated with Units I and II likely occurred during the pre-climactic, waxing phase of the eruption. These currents flowed around and filled in the hummocky topography, leaving the massive to diffusely-stratified deposits of Units I and II. The deposits of both Units I and II are generally more massive in low lying areas and more stratified in areas of high surface roughness, suggesting that surface roughness enhanced basal shear stress within the flow boundary. Units III and IV are associated with the climactic phase of the eruption, which produced the most voluminous and wide-spread PDCs. Both flow units are characteristically massive and enriched in vent-derived lithic blocks. These currents flowed over and around the debris avalanche deposits, as evidenced by the erosion of blocks from the hummocks. Unit III is massive, poorly sorted, and shows little to no evidence of elutriation or segregation of lithics and pumice, suggesting a highly concentrated current where size-density segregation was suppressed. Unit IV shows similar depositional features but typically has a basal lithic-rich region, is variably fines-depleted and contains pumice lobes, suggesting density segregation in a less concentrated current relative to Unit III. Deep, erosive channels cut by the Unit III current and thick lithic levee deposits within Unit IV occur in an area where debris avalanche relief is limited, suggesting self-channelization developed as a function of internal flow dynamics. An increase in the proportion and size of lithic blocks is found (1) downstream of debris avalanche hummocks, suggesting the PDCs were energetic enough to locally entrain accidental lithics from the hummocks and transport them tens of meters downstream, and (2) within large channels cut by later PDCs into earlier PDC deposits, suggesting self-channelization of the flows increased the carrying capacity of the subsequent channelized currents. Finally, the combination of thick, massive deposits with a high percentage of fine ash within Unit III and in the medial-distal depositional regions of Units II-IV suggests the PDCs developed and maintained a high internal pore pressure during transport and deposition. The most important include our ability to understand the role of internal pore pressure on current mobility, the influence of self-channelization on carrying capacity of the currents and the influence of surface roughness on substrate erosion. These observations have critical consequences for understanding the flow dynamics and hazard potential of PDCs

Timing and Source of Alkali-Enrichment at Mt. Etna, Sicily using Clinopyroxene Geobarometry and \u3ci\u3ein situ\u3c/i\u3e Sr Isotope Data

Since 1971, Mt. Etna, Europe’s largest and most active volcano, has exhibited increased eruption frequency and explosivity. In association with this increased activity, researchers have documented higher abundances of alkali elements such as potassium and rubidium as well as elevated Sr isotopes (87Sr/86Sr) in Etnean lavas. The source of this alkali-enrichment has been hotly debated, with end-member hypotheses involving mantle vs. crust. While some researchers favor changes in the character of the mantle source region due to subduction, in situ plagioclase compositional data suggest the mineral crystallizes in the shallow crust (upper 12 km) and Sr isotopic data provide strong evidence for late-stage crustal assimilation as demonstrated by increasing 87Sr/86Sr in magma after plagioclase had begun to grow. To further evaluate the mantle vs. crustal debate, clinopyroxene, which forms at deep and shallow levels within the magma chamber, was targeted for in situ analysis. Compositional and isotopic data were collected for ten samples erupted between 1329 and 2004. The largest, most complexly-zoned clinopyroxene were analyzed for elemental concentration by electron microprobe, and these data were used to calculate pressures of formation for each crystal. Pressures range from deep (~27 km) to upper crust (~6.0-6.6 km). In situ 87Sr/86Sr of clinopyroxene data will be combined with this information to provide a window into the middle to lower crustal dynamics of the Etnean magma storage system, as well as a characterization of mantle and crustal contributions of the recent alkali-enrichment event

Topographic controls on pyroclastic density current dynamics: Insight from 18 May 1980 deposits at Mount St. Helens, Washington (USA)

Our ability to interpret the deposits of pyroclastic density currents (PDCs) is critical for understanding the transport and depositional processes that control PDC dynamics. This paper focuses on the influence of slope on flow dynamics and criticality as recorded in PDC deposits from the 18 May 1980 eruption of Mt. St. Helens (USA). PDC deposits are found along the steep flanks (10°–30°) and across the pumice plain (~ 5°) up to 8 km north of the volcano. Granulometry, componentry and descriptions of depositional characteristics (e.g., bedform morphology) are recorded with distance from source. The pumice plain deposits are primarily thick (3–12 m), massive and poorly-sorted, and represent deposition from a series of concentrated PDCs. By contrast, the steep flank deposits are stratified to cross-stratified, suggesting deposition from PDCs where turbulence strongly influenced transport and depositional processes. We propose that acceleration of the concentrated PDCs along the steep flanks resulted in thinning of the concentrated, basal region of the current(s). Enhanced entrainment of ambient air, and autofluidization from upward fluxes of air from substrate interstices and plunging breakers across rugged, irregular topography further inflated the currents to the point that the overriding turbulent region strongly influenced transport and depositional mechanisms. Acceleration in combination with partial confinement in slot canyons and high surface roughness would also increase basal shear stress, further promoting shear and traction transport in the basal region of the current. Conditions along the steep flank resulted in supercritical flow, as recorded by regressive bedforms, which gradually transitioned to subcritical flow downstream as the concentrated basal region thickness increased as a function of decreasing slope and flow energy. We also find that (1) PDCs were erosive into the underlying granular substrate along high slopes (\u3e 25°) where currents were partially confined in steep slot canyons, suggesting that basal shear stress is an important control on erosive capacity, and (2) bedform amplitude, wavelength and the presence of regressive bedforms increase with increasing slope and proximity to source along the steep flank, suggesting a link between bedform morphology, flow velocity, and flow criticality. While our results indicate that slope and irregular topography strongly influence PDC dynamics, criticality and erosive capacity, the influence of these conditions on ultimate flow runout distance is unclear. The work here also highlights the issue that relationships between the controls on bedform size and morphology in density stratified flows remain poorly constrained, limiting our ability to extract important information about the currents that produced them. These final two points warrant further exploration through the combination of field, experimental and numerical approaches