Analysis of 40Ar/39Ar eruption age and diffusion relaxation of Sr zoning in apatite to constrain the timing and processes of post-shield volcanism at Kohala volcano, HI

While post-shield hotspot volcanism typically represents <5% of the erupted volume of magma, the compositionally diverse alkalic lavas produced during this endstage offer important insights into how rapidly melt generation can transition from hotspot volcanism. The post-shield Hawi volcanics at Kohala volcano on the Island of Hawaii vary in composition from hawaiite to trachyte (45 to ~60 wt.% SiO₂). Despite being dominantly aphyric lavas, the groundmass has abundant anorthoclase microlites and whole-rock K₂O contents are higher (up to ~3.5 wt.%) in comparison to the shield stage tholeiitic basalts. Previous reconnaissance geochronology performed with the Hawi lavas mostly employed K-Ar methods and indicated that post-shield eruptions occurred over ~140 ka, from 260 to 120 kya. We have significantly expanded the existing dataset of eruption ages for the Hawi volcanics by applying new ⁴⁰Ar/³⁹Ar methods. Our inverse isochron age results fall within the previously determined bounds of the Hawi volcanics stage but are more tightly constrained to a ~75 ka window between 125 and 200 kya. To better understand the petrogenesis of these post-shield lavas at Kohala volcano within the new context of a narrower eruption window, trace element measurements of large (up to 500 um) euhedral apatite phenocrysts were performed using the SHRIMP-RG ion probe. Depth profiling via serial sectioning reveal that Sr concentrations in the apatite rims are markedly lower (500 to 1500 ppm decrease) relative to interior values (3500 ppm). We are currently refining the length scales of this abrupt decrease in Sr with NanoSIMS measurements. Our observations: (1) pronounced zoning in Sr in apatite; (2) the occurrence of rare resorbed xenocrystic feldspars; and (3) mingling of groundmass textures provides evidence that magma mixing is an important phenomenon that proceeds the incipient eruption of Hawi post-shield lavas at Kohala. Together, the refined geochronology and mineral chemistry provide a more complete understanding of the timing and processes of the oft-overlooked post-shield stage of volcanism at hotspot volcanoes.