Some of the stuff we are doing in the study of volcanism:

 

Vulcan’s Throne, Grand Canyon, Arizona     

USGS image by Balsley, J.R., taken in 1950

Volcanoes are clearly a hazard when they erupt explosively, but they are also portals to the deep melting regions in the mantle.  We study both aspects of volcanism.  Our work on volatiles links to eruption, and diffusion profiles clock magma ascent prior to eruption.  We hope that our work on volatile inventories and magma ascent leads to a new understanding of why some volcanoes are more explosive and erupt more frequently than others.  Current projects focus on Kilauea and Central America.  Mafic magmatism is also a direct product of mantle melting, and we continue to try to read the melting record stored in volcanic rocks and their crystals.  Current work focuses on melting regions driven by continental extension (Basin and Range and Papua New Guinea) and water addition (subduction zones).

Mantle Melting During Continental Rifting

and the Mantle Control on Volcanic Eruptions

Magmatism & lithospheric destruction along the Colorado Plateau margin

Claire Bendersky, Terry Plank, Don Forsyth, Erik Hauri, Cin-Ty Lee, and Ben Holzman, Philipp and Plank, Terry

AGU Fall Meeting, 2012.


The process of cratonic lithosphere deformation remains mysterious. The Colorado Plateau (CP), including its underlying lithosphere, has persisted for over a 1Ga, while in parts of the adjacent Basin and Range (B&R) Cenozoic extension has thinned the lithosphere by half. Today, extensional processes are focused in the transition zone between these two regions, which is defined by a region of volcanism and active faulting. We combine seismic tomography models from EarthScope data with melt thermobarometry from mafic scoria erupted in three volcanic fields since 100ka to investigate lithospheric deformation in this actively rifting area. Our sample locations lie along the western and southwestern margin of the CP and record different evolutionary stages in the process of lithosphere destruction via melt infiltration. For each volcanic area studied we use seismic profiles of shear wave velocities (Vs) with pressures and temperatures of mantle-melt equilibrium calculated using the Si and Mg thermobarometer (Lee et al 2009). The thermobarometric results depend highly on the water and Fe3+ content of the melts, which were constrained separately for each volcanic field. Magmatic water contents have been determined by ion-microprobe measurements of olivine hosted melt inclusions. Fe/ΣFe+3 ratios were estimated for each volcanic field via LA-ICP-MS analysis of V in olivine and whole rock compositions (Canil 2002). In the northernmost volcanic field, Black Rock (BRVF), Utah, melts are hot (consistent with mantle potential temperature (Tp) >1400°C), dry (≤1 wt% H2O), and have equilibrated at shallow depths (<70 km), within the seismic lid. Shear velocities in this lid, however, are anomalously slow (4.1 km/s), and the mantle beneath (Vs ~ 3.9 km/s), is the slowest in the B&R, coincident with the highest and most focused extension rates (Wasatch Fault Zone). Together, these observations support high mantle temperatures, inefficient melt extraction, and a weak lithosphere due to melt corrosion, which has focused strain. Further south, in Snow Canyon Volcanic Field, Vs in the low-velocity zone and seismic lid are faster than at BRVF and magmas record equilibration depths that coincide with the seismically defined lithosphere-asthenosphere boundary (~70km). In this location both observations are consistent with a stronger lithosphere, which may allow rapid melt ascent by diking from the LAB to the surface, and limited melt-infiltration. Lavas from the most southern volcanic field, the Grand Canyon, record the deepest and largest range of melt equilibration depths (49-139km), the highest water contents (up to 3wt%) and have the most variable geochemistry (Nb/La ranging from asthenospheric to lithospheric values). Several vents contain abundant mantle xenoliths. Mantle Vs here is consistently higher (>4.1 km/s) than all the regions to the north. Here, lower mantle Tp, a stronger lithosphere, and wetter melts appears to enable more efficient melt extraction from all depths. Our results indicate that the destruction of the CP lithosphere occurs via a spectrum of processes along its margin, ranging from diking in the south to extensive melt infiltration and corrosion in the north. The spatial distribution of mantle heat and water content may exert a primary influence on the mode of lithosphere destruction, surface strain and volcanism.



Magma recharge and ascent during episode 1 of the 1959 Kilauea Iki eruption


David Ferguson (Lamont), Terry Plank (Lamont), Meghan Crowley (Lamont), Erik Hauri (Carnegie), Helge Gonnermann (Rice U.), Bruce Houghton (U. Hawaii), Don Swanson (HVO)


Abstract for 2012 Chapman Conference on Hawaii


The 1959 eruption at Kilauea Iki crater produced the highest Hawaiian fountains yet recorded and resulted in the formation of the Kilauea Iki lava lake and an exceptionally well preserved tephra fall. Episode 1 lasted for 7 days and involved fountain heights of up to 380 m, which deposited material up to 5 km downwind of the vent [1]. This was also the only episode of the 1959 eruption that was not influenced by lava drain-back into the conduit. To understand the processes leading to the initiation of the eruption and the dynamics of magma ascent during fountaining episodes, we measured compositional profiles across olivine crystals erupted during the most vigorous part of episode 1 as well as the volatile contents of melt inclusions hosted within the crystals. To minimize the effects of post-entrapment re-equilibration or water-loss of the melt inclusions, we restricted our analysis to free crystals (<500 microns in diameter) found in fall deposits. Core to rim profiles of crystal composition measured by LA-ICP-MS reveal a population of crystals with reverse Fo zonation (Mg-rich crystal rims), implying mixing with a more mafic melt at a late stage in their growth. Modeling the timescales of Fe-Mg diffusion between these high Fo crystal rims and lower Fo cores allows us to estimate the timing between this mafic recharge and the eruption, which is on the order of months.  This is the same timescale as the onset of deep-seated volcanic tremor (~55 km depth), which has been linked to ascending melt from the mantle [2].  Thus both the diffusion chronometers and seismic records point to an influx of melt to shallow levels beneath Kilauea in the months leading up to the eruption.  Several volatile species (CO2, H2O, S, Cl, and F) derived from the melt inclusion data follow remarkably simple degassing trends, consistent with magma ascent from depths of ~3.5 km. We find no evidence for CO2 gas fluxing, and our results suggest that excess volatiles were not involved in driving the eruption. 


[1] W. K. Stovall et al. (2012), Bull. Volcanol.  73, 511-529

[2] J. P. Eaton & K. J. Murata. (1960), Science, 132, 3432

Lithosphere vs. Asthenosphere Mantle Sources

at Big Pine Volcanic Field


Esteban Gazel (Lamont & Virginia Tech), Terry Plank (Lamont), Don Forsyth (Brown Univ.), Claire Bendersky (Lamont), Cin-Ty Lee (Rice Univ.) and Erik Hauri (Carnegie)


Geochemistry, Geophysical, Geosystems (2012) 13 doi:10.1029/2012GC004060.

      

Here we report the first measurements of the H2O content of magmas and mantle xenoliths from the Big Pine Volcanic Field (BPVF) in order to constrain  the melting process in the mantle, and the role of asthenospheric and lithospheric sources in this westernmost region of the Basin and Range Province, western USA. Melt inclusions trapped in primitive olivines (Fo82-90) record surprisingly high H2O contents (1.5 to 3.0 wt.%). while lithospheric mantle xenoliths record low H2O concentrations (whole rock <75 ppm). Estimates of the oxidation state of BPVF magmas, based on V partitioning in olivine, are also high (FMQ +1.0 to +1.5).  Pressures and temperatures of equilibration of the BPVF melts indicate a shift over time, from higher melting temperatures (~1320 °C) and pressures (~2 GPa) for magmas that are >500 ka, to cooler (~1220 °C) and shallower melting (~1 GPa) conditions in younger magmas.  The estimated depth of melting correlates strongly with some trace element ratios in the magmas (e.g., Ce/Pb), with deeper melts having values closer to upper mantle asthenosphere values, and shallower melts having values more typical of subduction zone magmas. This  geochemical stratification is consistent with seismic observations of a shallow lithosphere-asthenosphere boundary (~55 km depth). Combined trace element and cryoscopic melting models yield self-consistent estimates for the degree of melting (~5%) and source H2O concentration (~1000 ppm). We suggest two possible geodynamic models to explain small-scale convection necessary for magma generation. The first is related to the Isabella seismic anomaly, either a remnant of the Farallon Plate or foundered lithosphere. The second scenario is related to slow extension of the lithosphere.

New Stuff !

Newly Published !

Feeding Andesitic Eruptions with a High-Speed Connection from the Mantle


Philipp Ruprecht and Terry Plank


In Press (2013)  Nature

      

Convergent margin volcanism is ultimately fed by magmas generated in the mantle, but the connection between the mantle and the eruption at the surface is typically obscured by cooling, crystallisation and magma mixing within the crust1,2,3. Geophysical techniques are also challenged in the lower and middle crust, where seismic events are rare and resolution is generally poor4,5. It has thus been unclear how fast mantle-derived magmas transit the crust and recharge crustal magma chambers. Here we use diffusion modeling of nickel zonation profiles in primitive olivines to show how mantle recharge may occur on timescales as short as eruptions themselves. In an example from Irazú volcano in Costa Rica, magmas apparently ascend from their source region in the mantle through ~ 35 km thick crust in months to years, recharging hybrid basaltic-andesites over the course of the eruption. Diverse olivine populations derive from distinct primary melts that mix on the same timescales. These results show that large stratovolcanoes with shallow magma chambers9,10 may still preserve the deep record of their mantle origin in olivine crystals. This is the first time that magma ascent timescales from the mantle have been documented beneath a convergent margin stratovolcano. This new approach can be applied to other eruptions that record magma mixing with recharge melts. Signs of volcanic unrest are typically monitored at the surface or upper crust; new efforts should set sights deeper, tracking magma movement from the base of the crust to the surface in the months to year run-up before eruptions.

Melting during late-stage rifting in Afar is hot and deep

Dave Ferguson, John MacLennan, Ian Bastow, David Pyle, S.M. Jones, Derek Kier, Jon Blundy, Terry Plank, G, Yirgu


Nature (2013) 499: 70-74  doi:10.1038/nature12292

      

Investigations of a variety of continental rifts and margins worldwide

have revealed that a considerable volume of melt can intrude

into the crust during continental breakup1–8, modifying its composition

and thermal structure. However, it is unclear whether the

cause of voluminous melt production at volcanic rifts is primarily

increased mantle temperature or plate thinning1,2,8–12. Also disputed

is the extent to which plate stretching or thinning is uniform or

varies with depth with the entire continental lithospheric mantle

potentially being removed before plate rupture13–16. Here we show

that the extensive magmatism during rifting along the southern

Red Sea rift in Afar, a unique region of sub-aerial transition from

continental to oceanic rifting, is driven by deep melting of hotterthan-

normal asthenosphere. Petrogenetic modelling shows that

melts are predominantly generated at depths greater than 80 kilometres,

implying the existence of a thick upper thermo-mechanical

boundary layer in a rift system approaching the point of plate

rupture. Numerical modelling of rift development shows that when

breakup occurs at the slow extension rates observed in Afar, the

survival of a thick plate is an inevitable consequence of conductive

cooling of the lithosphere, even when the underlying asthenosphere

is hot. Sustained magmatic activity during rifting in Afar

thus requires persistently high mantle temperatures, which would

allow melting at high pressure beneath the thick plate. If extensive

plate thinning does occur during breakup it must do so abruptly at

a late stage, immediately before the formation of the new ocean

basin16.