The open-system geochemical evolution of alkalic cap lavas from Haleakala Crater, Hawaii, USA

Abstract

Lavas ranging from alkalic basalt to mugearite are exposed in a continuous stratigraphic section in Haleakala Crater on the island of Maui. These lavas define a series of discrete magma batches separated by sharp compositional breaks characterized by upsection increases in incompatible element (e.g., K 2O, Nb, REE, Th, Ba, Rb) contents, Al 2O 3/CaO ratios, and La/Sm ratios and complementary decreases in compatible element (e.g., MgO, Ni, Cr, V, Sc) contents. Within magma batches, there are systematic upsection decreases in incompatible element contents, Al 2O 3/CaO ratios, and La/Sm ratios and systematic increases in compatible element contents. The observed compositional cyclicity can be explained as the result of alternating periods of low and high magma chamber recharge rates. The upsection interbatch compositional breaks represent periods of low or negligible magma chamber recharge, during which eruption is suppressed and the geochemical effects of crystal fractionation dominate those imposed by magma mixing. At the initiation of recharge, the influx of mafic magma into the magma chamber produces an eruption, resulting in partial evacuation of the magma chamber. Following eruption, recharge magma mixes with magma remaining in the magma chamber. Continued recharge and mixing during these eruptive periods cause the compositions of lavas to become more mafic with time. Thus, during a recharge period, the bulk composition of the magma chamber approaches that of the recharge magmas. The most mafic lavas (i.e., the youngest lavas within each magma batch) typically are relatively evolved (Mg# < 48, MgO < 7 wt%) and, therefore, cannot represent primary magmas. Evidence for polybaric fractionation suggests that these lavas represent recharge magmas that have undergone crystal fractionation in a deeper-level reservoir prior to injection into the shallow-level Haleakala magma chamber. The observed fine-scale geochemical cyclicity is superimposed over the larger-scale, systematic changes in isotope ratios and highly incompatible trace element ratios that are attributed to time-dependent changes in the proportions of distinct mantle components in the source of these lavas. These contrasting scales of temporal geochemical variability illustrate how shallow-level magma chamber processes have imposed short time period (≈ 2000 years) compositional controls on magmas whose compositions were also changing over relatively long time periods (≈ 800,000 years) due to source-related effects. The decrease in magma supply rates during the alkalic cap phase of Hawaiian volcanism probably results in increased magma chamber residence times, thus creating the greater magmatic diversity observed. The specifics of the petrologic and geochemical evolution of individual Hawaiian volcanoes during this period differ at least in part because of differences in magma supply rates.