How Important Is Small-Scale Geochemical Variability To Constraining Sources And Processes?
The low viscosity of mafic magmas generally promotes effusive to mildly explosive eruptions; however, highly explosive mafic volcanic activity does occur. Recent models suggest that rapid ascent rates leading to significant disequilibrium drastically impact the rheology leading to fragmentation of the mafic magma. However, the magmatic conditions preceding magma ascent initiation are primarily unknown. There is a rich rock and mineral record for understanding melt generation in the mantle wedge and open system processes stored in the juvenile products of these highly explosive mafic eruptions. This study uses the pyroclasts from the Curacautin ignimbrite, a large volume (3.5–4.5 km3 DRE), mafic (50.4-57.3 wt% SiO2) ignimbrite at Llaima volcano in Chile.
Of the 68 ignimbrite samples collected and previously analyzed, we targeted four for small volume (5 mg vs. standard 50 mg) characterization via ICPMS: (1) the top and (2) base of the ignimbrite, and the most (3) depleted and (4) enriched samples. We prepped and analyzed ten randomly selected pyroclasts from the highlighted samples (a total of 40 analyses). This study aimed to determine how sampling (e.g., composite vs. individual and the number of samples) impacts the signal of geological processes in trace element compositions. We used the χ2 distribution to test if the observed standard deviation (corrected for noise by subtracting analytical error) of all 40 samples, as well as each sample set’s ten analyzed pyroclasts, is larger than the analytical error. We identified true variability for at least 13 of the 30 trace elements measured for the base of the ignimbrite and enriched end member sample sets and the combined 40 analyses. For the top of the ignimbrite sample and the depleted end member sample, there is true variability in almost all 30 trace elements. Small volume whole rock geochemical characterization of individual pyroclasts detects previously unseen small-scale geochemical variability in compatible and REE elements and constrains new enrichment end-member compositions for geochemical models. Using principal component analysis, we investigate melt source heterogeneity and fractional crystallization as controls on the observed variability.