Andean crystals separate erupted magma from material left underground
Crystals carried to the surface by volcanoes in the Central Andes preserve contrasting histories of erupted melt and older material held underground. A peer-reviewed geochemical study separated crystal populations before comparing their compositions. The distinction helps trace why lava can give an incomplete picture of deep magma reservoirs and how new magma entering those reservoirs can precede an eruption.
Science··Night
Crystals retained different magma histories
Minerals in volcanic rocks of the Central Andes helped researchers distinguish melt that erupted from older magma material retained within the crust. The peer-reviewed geochemical study focused on plagioclase, a feldspar mineral common in andesite. Small crystals formed from the final erupted melt differed chemically from much of the older crystal cargo. Lava therefore carried pieces of several underground histories to the surface.[1], [2]
The team led by Daniela Parra-Encalada at Australia’s University of Queensland examined 28 rock samples from seven volcanic systems in Chile and Argentina. They spanned approximately 230 kilometres. Two samples represented an older ignimbrite, a deposit from an especially large explosive eruption. Beneath these volcanoes, magma occupies branching systems containing melt, gas and crystals at different depths, providing several settings in which crystals can grow.[2]
Chemistry separated young grains from older interiors
Separating crystals by size and texture allowed the researchers to compare young grains with older interiors and outer rims. Electron microprobe and laser analyses measured elemental concentrations and strontium isotope ratios. The youngest crystals in erupted andesite averaged about 61 per cent on a molar basis anorthite, the calcium-rich component of plagioclase. A similar compositional range appeared across volcanoes differing in age and eruption style.[2]
Older interiors were generally poorer in calcium, consistent with prolonged crystallisation in silica-rich magma. Calcium-rich rims instead pointed towards arrivals of hotter, magnesium-rich magma. Thermodynamic models placed young-crystal formation at roughly 900 to 975 degrees Celsius and depths of about 11 to 19 kilometres. These are reconstructed formation conditions; the team did not directly measure them beneath individual volcanoes before an eruption.[2]
Large eruptions and global comparisons broadened the picture
Rocks from especially large explosive eruptions mostly lacked those characteristic rim changes. Their crystals indicated longer residence in cooler, silica-rich magma, suggesting that reservoir evacuation can follow other paths. The younger and older crystal populations thus preserve information that would become harder to distinguish if all grains were analysed together without attention to their different textures.[1], [2]
Comparisons with mineral analyses from 26 volcanic arcs worldwide showed related distributions but greater scatter. Crystal types had not always been distinguished in those datasets. Magma remaining underground can eventually solidify and contribute to crustal growth, while some reservoirs may be remobilized. The chemical patterns reconstruct past magma processes; applying them to other regions requires closer study, and they do not date a volcano’s next eruption.[2]
