Depth of Magma Storage Under Iceland Controlled by Magma Fluxes

Abstract

Abstract The compositions of volcanic materials are sensitive to physical conditions in the underlying magmatic system. When basaltic melts are saturated in olivine‐plagioclase‐augite prior to eruption, their compositions can be used to estimate the pressure at which they last equilibrated. We developed PyOPAM, an open‐source tool that runs in Python, and use this refreshed liquid‐barometer to investigate the relationship between final depths of magma storage and magma flux. We first tested PyOPAM using 312 experimental glasses compiled from literature and found that the 1 σ uncertainty is 1.13 kbar (±3 km). PyOPAM was then applied to a data set of 13,400 analyses from Iceland, where suspected controls on magma flux are well constrained. Of these, 3807 analyses return robust pressure estimates, constraining final pre‐eruptive magma storage depths for 23 of the 30 Icelandic volcanic systems. Our results indicate that magma storage pressures on Iceland are linked to melt‐flux from the mantle. This finding is consistent with previous models linking storage depths and spreading rates on the global mid‐ocean ridge system. In addition, we provide clear evidence that the magma flux, rather than spreading rate alone, is the key control on the distribution of melt at spreading centers. Increased melt flux is associated with shoaling of pre‐eruptive storage depths, indicating that mantle melt fluxes dictate the long‐term stabilization of extensive magmatic storage regions at depths shallower than 10 km. Quantitative relationships between mantle melt flux and storage depths can be used to test computational models of transcrustal magmatic systems. Plain Language Summary An important part of monitoring volcanoes is understanding where magma is stored before an eruption. Some individual volcanic systems have been rigorously studied and their plumbing systems are well understood. However, the overall physical controls on the depths of magma storage under volcanic systems are not yet resolved. In a free python script (PyOPAM), we refreshed and streamlined a basaltic liquid barometer, that allows us to estimate the final depth of storage for a basaltic magma, ±1.13 kbar. We applied this tool to basaltic samples from Iceland. Of the original 13,400 analyses entered into PyOPAM, 3800 returned reliable estimates. We find that there is a relationship between the amount of magma being produced and moving through an area, and the final depth of storage for basaltic magma before eruption. With increasing magma production, magma storage depths decrease and shift into the shallow crust. When magma production is low, basaltic magma is stored in the lower crust or mantle. This pattern is modified by the temperature of the crust, as cold crust prevents shallow storage of basaltic magma. These findings can be applied to other basaltic volcanoes and magma systems throughout the world. Key Points We created pyOPAM, a python script for applying the OPAM barometer to basaltic liquids Using statistical filters we constrained the uncertainty to 1 σ of this barometer to 1.13 kbar We applied pyOPAM to Icelandic samples and find that final depths of basaltic magma storage is controlled by magma flux