Tephra glass chemistry provides storage and discharge details of five magma reservoirs which fed the 75 ka Youngest Toba Tuff eruption, northern Sumatra

The Youngest Toba Tuff contains five distinct glass populations, identified from Ba, Sr and Y compositions, termed PI (lowest Ba) – PV (highest Ba), representing five compositionally distinct pre‐eruptive magma batches that fed the eruption. The PI–PV compositions display systematic changes, with higher FeO, CaO, MgO, TiO₂ and lower incompatible element concentrations in the low‐SiO₂ PIV/PV, than the high‐SiO₂ PI–PIII compositions. Glass shard abundances indicate PIV and PV were the least voluminous magma batches, and PI and PIII the most voluminous. Pressure estimates using rhyolite‐MELTS indicate PV magma equilibrated at ~6 km, and PI magma at ~3.8 km. Glass population proportions in distal tephra and proximal (caldera‐wall) material describe an eruption which commenced by emptying the deepest PIV and PV reservoirs, this being preferentially deposited in a narrow band across southern India (possibly due to jet‐stream and/or plinian eruption transport), and as abundant pumice clasts in the lowermost proximal ignimbrites. Later, shallower magma reservoirs erupted, with PI being the most abundant as the eruption ended, sourcing the majority of distal ash from co‐ignimbrite clouds (PI‐ and PIII‐dominant), where associated ignimbrites isolated earlier (PIV‐ and PV‐rich) deposits. This study shows how analysis of tephra glass compositional data can yield pre‐eruption magma volume estimates, and enable aspects of magma storage conditions and eruption dynamics to be described

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

The Year Leading to a Supereruption

Supereruptions catastrophically eject 100s-1000s of km3 of magma to the surface in a matter of days to a few months. In this study, we use zoning in quartz crystals from the Bishop Tuff (California) to assess the timescales over which a giant magma body transitions from relatively quiescent, pre-eruptive crystallization to rapid decompression and eruption. Quartz crystals in the Bishop Tuff have distinctive rims (−8 and 10−10 m/s (depending on sample), revealing very fast crystal growth rates (100s of nm to 10s of μm per day). Our data show that quartz rims grew well within a year of eruption, with most of the growth happening in the weeks or days preceding eruption. Growth took place under conditions of high supersaturation, suggesting that rim growth marks the onset of decompression and the transition from pre-eruptive to syn-eruptive conditions

The Year Leading to a Supereruption.

Supereruptions catastrophically eject 100s-1000s of km3 of magma to the surface in a matter of days to a few months. In this study, we use zoning in quartz crystals from the Bishop Tuff (California) to assess the timescales over which a giant magma body transitions from relatively quiescent, pre-eruptive crystallization to rapid decompression and eruption. Quartz crystals in the Bishop Tuff have distinctive rims (<200 μm thick), which are Ti-rich and bright in cathodoluminescence (CL) images, and which can be used to calculate Ti diffusional relaxation times. We use synchrotron-based x-ray microfluorescence to obtain quantitative Ti maps and profiles along rim-interior contacts in quartz at resolutions of 1-5 μm in each linear dimension. We perform CL imaging on a scanning electron microscope (SEM) using a low-energy (5 kV) incident beam to characterize these contacts in high resolution (<1 μm in linear dimensions). Quartz growth times were determined using a 1D model for Ti diffusion, assuming initial step functions. Minimum quartz growth rates were calculated using these calculated growth times and measured rim thicknesses. Maximum rim growth times span from ~1 min to 35 years, with a median of ~4 days. More than 70% of rim growth times are less than 1 year, showing that quartz rims have mostly grown in the days to months prior to eruption. Minimum growth rates show distinct modes between 10-8 and 10-10 m/s (depending on sample), revealing very fast crystal growth rates (100s of nm to 10s of μm per day). Our data show that quartz rims grew well within a year of eruption, with most of the growth happening in the weeks or days preceding eruption. Growth took place under conditions of high supersaturation, suggesting that rim growth marks the onset of decompression and the transition from pre-eruptive to syn-eruptive conditions

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