Timescales of Quartz Crystallization and the Longevity of the Bishop Giant Magma Body

Supereruptions violently transfer huge amounts (100 s–1000 s km3) of magma to the surface in a matter of days and testify to the existence of giant pools of magma at depth. The longevity of these giant magma bodies is of significant scientific and societal interest. Radiometric data on whole rocks, glasses, feldspar and zircon crystals have been used to suggest that the Bishop Tuff giant magma body, which erupted ∼760,000 years ago and created the Long Valley caldera (California), was long-lived (>100,000 years) and evolved rather slowly. In this work, we present four lines of evidence to constrain the timescales of crystallization of the Bishop magma body: (1) quartz residence times based on diffusional relaxation of Ti profiles, (2) quartz residence times based on the kinetics of faceting of melt inclusions, (3) quartz and feldspar crystallization times derived using quartz+feldspar crystal size distributions, and (4) timescales of cooling and crystallization based on thermodynamic and heat flow modeling. All of our estimates suggest quartz crystallization on timescales of <10,000 years, more typically within 500–3,000 years before eruption. We conclude that large-volume, crystal-poor magma bodies are ephemeral features that, once established, evolve on millennial timescales. We also suggest that zircon crystals, rather than recording the timescales of crystallization of a large pool of crystal-poor magma, record the extended periods of time necessary for maturation of the crust and establishment of these giant magma bodies

Timescales of Quartz Crystallization and the Longevity of the Bishop Giant Magma Body

Supereruptions violently transfer huge amounts (100 s–1000 s km3) of magma to the surface in a matter of days and testify to the existence of giant pools of magma at depth. The longevity of these giant magma bodies is of significant scientific and societal interest. Radiometric data on whole rocks, glasses, feldspar and zircon crystals have been used to suggest that the Bishop Tuff giant magma body, which erupted ∼760,000 years ago and created the Long Valley caldera (California), was long-lived (>100,000 years) and evolved rather slowly. In this work, we present four lines of evidence to constrain the timescales of crystallization of the Bishop magma body: (1) quartz residence times based on diffusional relaxation of Ti profiles, (2) quartz residence times based on the kinetics of faceting of melt inclusions, (3) quartz and feldspar crystallization times derived using quartz+feldspar crystal size distributions, and (4) timescales of cooling and crystallization based on thermodynamic and heat flow modeling. All of our estimates suggest quartz crystallization on timescales of <10,000 years, more typically within 500–3,000 years before eruption. We conclude that large-volume, crystal-poor magma bodies are ephemeral features that, once established, evolve on millennial timescales. We also suggest that zircon crystals, rather than recording the timescales of crystallization of a large pool of crystal-poor magma, record the extended periods of time necessary for maturation of the crust and establishment of these giant magma bodies.</p

Phase-equilibrium geobarometers for silicic rocks based on rhyolite-MELTS. Part 2: application to Taupo Volcanic Zone rhyolites

© 2014, Springer-Verlag Berlin Heidelberg. Constraining the pressure of crystallisation of large silicic magma bodies gives important insight into the depth and vertical extent of magmatic plumbing systems; however, it is notably difficult to constrain pressure at the level of detail necessary to understand shallow magmatic systems. In this study, we use the recently developed rhyolite-MELTS geobarometer to constrain the crystallisation pressures of rhyolites from the Taupo Volcanic Zone (TVZ). As sanidine is absent from the studied deposits, we calculate the pressures at which quartz and feldspar are found to be in equilibrium with melt now preserved as glass (the quartz +1 feldspar constraint of Gualda and Ghiorso, Contrib Mineral Petrol 168:1033. doi:10.1007/s00410-014-1033-3. 2014). We use glass compositions (matrix glass and melt inclusions) from seven eruptive deposits dated between ~320 and 0.7 ka from four distinct calderas in the central TVZ, and we discuss advantages and limitations of the rhyolite-MELTS geobarometer in comparison with other geobarometers applied to the same eruptive deposits. Overall, there is good agreement with other pressure estimates from the literature (amphibole geobarometry and H2O–CO2 solubility models). One of the main advantages of this new geobarometer is that it can be applied to both matrix glass and melt inclusions—regardless of volatile saturation. The examples presented also emphasise the utility of this method to filter out spurious glass compositions. Pressure estimates obtained with the new rhyolite-MELTS geobarometer range between ~250 to ~50 MPa, with a large majority at ~100 MPa. These results confirm that the TVZ hosts some of the shallowest rhyolitic magma bodies on the planet, resulting from the extensional tectonic regime and thinning of the crust. Distinct populations with different equilibration pressures are also recognised, which is consistent with the idea that multiple batches of eruptible magma can be present in the crust at the same time and can be tapped simultaneously by large eruptive events

Evolution of crystallinity with time for the three solutions presented in Fig. 7 and discussed in the text.

<p>Notice the dramatic differences in behavior between the solutions for invariant magmas (Continuous Source and Solidification Front) and for non-invariant magmas (Lovering). Curves for Lovering-type crystallization are for the center and the bottom of the 1 km column. In particular, notice that significant crystallization (e.g. 25 vol. %) is attained in <1 ka for invariant magmas, in accordance with geospeedometry estimates presented in the text. Much longer timescales are required to cause significant crystallization of the interior of non-invariant magma bodies.</p

Thermodynamic and heat flow modeling results.

<p>Temperature (°C) versus enthalpy change (J/g; top panel) and versus abundance (wt. %; bottom panel) plot shows results of MELTS <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0037492#pone.0037492-Ghiorso1" target="_blank">[22]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0037492#pone.0037492-Gualda4" target="_blank">[23]</a> simulations. Initial composition is average late-erupted pumice composition from Hildreth <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0037492#pone.0037492-Hildreth1" target="_blank">[6]</a>. Simulation assumes equilibrium crystallization at 175 MPa, under fluid-saturated conditions (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0037492#pone.0037492-Gualda4" target="_blank">[23]</a>), in agreement with melt inclusion data <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0037492#pone.0037492-Anderson1" target="_blank">[18]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0037492#pone.0037492-Wallace1" target="_blank">[24]</a>. Note short crystallization interval (<10°C). Nearly invariant condition is reached at 756.1°C when the system becomes saturated in quartz (in addition to fluid, sanidine, magnetite, and plagioclase), after which point crystallization is nearly isothermal. Quartz crystallization effectively locks the system at the nearly invariant temperature, given that, upon heating, temperature excursions above the nearly invariant temperature are only possible after complete resorption of quartz.</p

Shape evolution due to melt inclusion faceting.

<p>(a) Shape change of a melt inclusion inside a host crystal as a function of time due to faceting; initial inclusion is spherical, but with time gets transformed into a polyhedron with rounded edges; with sufficient time, inclusion may become a perfect anticrystal. (b) Evolution of shapes emphasizing the role of diffusion (green arrows) in transporting material to achieve faceting.</p

Melt inclusion faceting time.

<p>Time required for faceting versus inclusion radius plot for conditions relevant for Bishop magma crystallization. Vertical lines correspond to inclusion sizes estimated from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0037492#pone-0037492-g002" target="_blank">Fig. 2</a>. Using our best estimate of faceting time for the observed inclusions, we constrain the timescale for residence of the host crystals to be between ∼600–1,500 years. Even if faceting is significantly slower (t+2σ_t curve), residence times are within the range ∼2,200–5,300 years.</p

Examples of melt (glass) inclusions in quartz at different stages of faceting.

<p>(a) Quartz crystal in refractive index oil (cross-polarized light) showing several melt inclusions. (b–d) Detailed views of the three largest inclusions; scale bar is 50 µm and applies to all 3 images; area, radius (of a circle with same area), and faceting time are indicated for each inclusion. Note that (b) is non-faceted, (c) is partly faceted, and (d) is faceted. That only (d) is faceted suggests that crystal residence times are <1,500 years. Images (a–d) are from Anderson et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0037492#pone.0037492-Anderson1" target="_blank">[18]</a>, reproduced with permission.</p

Parameters used and estimated uncertainties for the computation of melt inclusion faceting time as a function of inclusion radius (r).

a<p>Approximate values.</p>b<p>Calculated using the CORBA Phase Properties applet (<a href="http://ctserver.ofm-research.org/phaseProp.html" target="_blank">http://ctserver.ofm-research.org/phaseProp.html</a>). Retrieved Nov 12, 2007. Calculations based on data from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0037492#pone.0037492-Berman1" target="_blank">[73]</a>.</p

Parameters used in heat-flow simulations.

a<p>Carslaw & Jaeger <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0037492#pone.0037492-Carslaw1" target="_blank">[51]</a>.</p>b<p>Whittington et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0037492#pone.0037492-Whittington1" target="_blank">[74]</a>.</p>c<p>Rhyolite-MELTS simulations.</p>d<p>Only for Lovering-type simulation.</p