Summary of estimates of the duration of crystallization for the Bishop Tuff.

<p>Zircon crystallization times are typically on the order of tens of thousands of years. Quartz interiors reveal crystallization timescales on the order of centuries to a few millennia. Quartz and orthopyroxene (Opx) rims have been previously inferred to have crystallized on decadal to centennial timescales. We show in this study that quartz rims mostly crystallized within a year of eruption, with most growth taking place in the days to weeks prior to eruption. Much of the discrepancy between the timescales results from different minerals, or different zones within the same mineral, recording different processes. Zircon records the evolution of the entire magmatic system, including long stages characterized by high crystal contents; quartz interiors and much of zircon crystallized within centuries to millennia of eruption, revealing the lifespan of crystal-poor, eruptible magma bodies; quartz rims reveal the onset of eruptive decompression. SIMS: secondary ionization mass spectrometry; TIMS: thermal ionization mass spectrometry.</p

Principle behind diffusion chronometry.

<p>We use a 1D model to calculate the time since the inception of a given compositional contact in a crystal. We assume an initially infinitely sharp “step function” boundary (indicated as “Initial condition” in the diagram), which becomes progressively less sharp with time (shown as a series of curves). This problem has a well-known solution, with the resulting damped curve being described by a complementary error function (erfc) as shown in the figure. We use a least-squares procedure to find the best-fit erfc function, from which we can extract the value of (); using experimentally determined values of the diffusion coefficient D, we can then calculate the residence time of a given contact and the growth time for the crystal region rimward from the contact. We can also measure the growth distance from our images, from which we calculate average crystal rim growth rates.</p

Effect of incident electron beam energy on excitation volume within a quartz crystal.

<p>Results shown are derived from Monte Simulations performed using the software MC X-Ray Lite (version 1.4.6.0, <a href="http://montecarlomodeling.mcgill.ca/software/mcxray/mcxray.html" target="_blank">http://montecarlomodeling.mcgill.ca/software/mcxray/mcxray.html</a>), assuming a quartz crystal with density of 2.65 g/cm³; calculations for 1000 electrons are shown as the depth-integrated paths on the plane perpendicular to incidence of the electron beam. Scale is the same for the three images shown. Horizontal bars are for reference; they show the approximate size of the beam at each energy. Notice the very strong effect of beam energy on the diameter of the excited volume, which translates into a strong dependency of maximum possible cathodoluminescence (CL) image resolution with beam energy.</p

Timescales of rim crystallization for the Bishop Tuff, CA.

<p>Top panel shows histogram of calculated times. Bottom panel shows cumulative distributions. Notice that bin sizes are on a logarithmic scale on both plots. Crystals from samples F8-15 (fall unit F8), Ib-A1 (flow unit Ig1Eb), and AB-6202 (flow unit Ig2NW) were imaged at 5 kV (red and orange colors), while those from F7-12 and F7-14 (both from fall unit F7) were imaged at 15 kV (blue colors); see text for details. Distribution has a mode at times of 10⁻² years (~3 days), and more than 50% of the calculated times are less than 0.1 years (~1 month), particularly so for the crystals imaged at 5 kV.</p

Correlation between CL intensity and Ti concentration in quartz.

<p>Central grayscale image is a detailed cathodoluminescence (CL) image of the rim region of a quartz crystal (fall unit F8 of the Bishop Tuff, following nomenclature of [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0159200#pone.0159200.ref002" target="_blank">2</a>]), including the boundary between the bright-CL rim and the duller-CL interior. Color inset is a Ti map obtained using the x-ray microprobe at insertion device beamline of GeoSoilEnviroCARS (Advanced Photon Source, Argonne National Laboratory); color scale corresponds to Ti concentration, as indicated on the top bar. The correlation between CL and Ti concentration is apparent, with largely homogeneous Ti concentrations in the rim and in the interior of the crystal, and a very sharp transition between the two. The bottom diagram shows a quantitative profile across the same boundary in a slightly different position, as indicated in the CL image. The transition between bright-CL, high-Ti rim and interior takes place within one step in the profile, which corresponds to 2 μm; this shows that the diffusional length scale of interest is on the order of 1 μm or less. We obtain a maximum growth time for this contact of 13 years (fitted complementary error function shown in black), which places an upper bound on the growth time for these rims.</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

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

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

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