Rapid cooling and cold storage in a silicic magma reservoir recorded in individual zircon crystals
Minerals such as zircon can record the storage conditions of magma before volcanic eruption. In this SCIENCE article the authors combined traditional 238U-230Th dating (using large radius SIMS) with lithium (and REEs) concentration profiles (using NanoSIMS 50L) in seven zircons from the Taupo supervolcanic complex in New Zealand to determine magma storage conditions. The zircons spent more than 90% of their lifetime in an uneruptible, mostly crystalline, and deep magmatic reservoir. The zircons were eventually transported to hotter, shallower, and eruptible magma bodies, where they spent only decades to hundreds of years before eruption. The result suggests a two-stage model for magmatic systems with large thermal variations.
The NanoSIMS was used here to acquire trace element line-scans using 16keV/1nA O- primary beam with spot size of 500 or 1000nm. The sub-micron lateral resolution coupled with excellent sensitivity (lithium down to ppb level) allowed then to compare and fit model diffusion profiles. No correlation was found with the recorded REEs. Two line-scans were recorded in multicollection mode for each zircon: first the light elements, namely 6Li+, 7Li+, 30Si+, 48Ti+, 48Ti16O+, 89Y+, 92Zr+, followed by heavy elements including 30Si+, 89Y+, 142Nd+, 152Sm+, 164Dy+, 174Yb+, and 180Hf+ .
Lithium and yttrium concentration profiles across zircon crystal G20 and modeling of diffusion of high-Li peaks.
(A) Cathodoluminescence image of a polished zircon interior with the location of the 238U-230Th age spot shown by the yellow circle. Brightness and contrast have been adjusted; The age of the unpolished surface is also indicated. Locations of trace element traverses are shown by red (light elements) and blue (heavy elements) arrows. kyr, thousand years.
(B) NanoSIMS concentrations measurements of Lithium [red squares, parts per billion (ppb)] and Yttrium [black circles, parts per million (ppm)] in the traverse shown in red in (A). Locations of modeled peaks are shown by labeled black boxes. Yellow bars at the top show the locations along the traverse of age analyses; surface age is shown as a 5-mm region at the rim of the crystal. Error bars, 1s.
(C and D) Li diffusion profiles for each modeled peak. Red squares represent measured Li concentrations with 1s Poisson uncertainties. The modeled initial step function is shown by the dashed blue line, and the modeled diffusion profiles at 700°C are shown as dashed lines for the best-fit diffusion time (black), 100 years (green), and 1000 years (red).
From: Rapid cooling and cold storage in a silicic magma reservoir recorded in individual crystals. Allison E. Rubin, Kari M. Cooper, Christy B. Till, Adam J. R. Kent, Fidel Costa, Maitrayee Bose, Darren Gravley, Chad Deering, Jim Cole. SCIENCE 356, 16 June 2017