Seiko Yamasaki | Search USGS

USGS2026

Prolonged cooling of andesitic-dacitic lava flows produces optimal groundmass material for 40Ar/39Ar dating

Organizing the eruptive histories and petrogenetic models that underpin hazard assessments at active volcanoes typically requires high-precision ages for late Pleistocene to Holocene lava flows. In this study, features of an existing 40Ar/39Ar dataset for andesitic-dacitic rocks from Ruapehu volcano, Aotearoa New Zealand, were interrogated with new microanalytical data to identify the optimal groundmass material for age determination. To understand the behavior of K and Ar within the cooling melt of lavas during their emplacement, we examined and compared the petrological features of samples from the ice-chilled margin and interior of a lava flow. The groundmass of the rapidly cooled sample comprises non-vesicular rhyolitic glass and microlites of plagioclase, orthopyroxene, and magnetite. The sample from the flow interior has minimal amounts of groundmass glass, and interstices are instead occupied by sanidine and tridymite. Modelling supports the interpretation that sanidine crystallized as a stable groundmass phase during slow cooling of degassed melt in the interior zones of lavas that were insulated behind their glassy margins. Our evaluation of the age data shows that sanidine contributed high yields of radiogenic argon during step-heating experiments for samples collected from lava interiors, which enabled high-precision 40Ar/39Ar ages to be determined. Using Ruapehu as an example of an arc volcano affected by lava-ice interaction, we provide a basic field guide for identifying lava flows that contain groundmass sanidine at the expense of glass. Target outcrops also offer opportunities for reconstructing geomagnetic dynamics and volcano–climate relationships during the Pleistocene and Holocene.

Ruapehu volcano↗

USGS2010

Low‐productivity Hawaiian volcanism between Kaua‘i and O‘ahu

The longest distance between subaerial shield volcanoes in the Hawaiian Islands is between the islands of Kaua‘i and O‘ahu, where a field of submarine volcanic cones formed astride the axis of the Hawaiian chain during a period of low magma productivity. The submarine volcanoes lie ∼25–30 km west of Ka‘ena Ridge that extends ∼80 km from western O‘ahu. These volcanoes were sampled by three Jason2 dives. The cones are flat topped, <400 m high and 0.4–2 km in diameter at water depths between ∼2700 and 4300 m, and consist predominantly of pillowed flows. Ar‐Ar and K‐Ar ages of 11 tholeiitic lavas are between 4.9 and 3.6 Ma. These ages overlap with shield volcanism on Kaua‘i (5.1–4.0 Ma) and Wai‘anae shield basalts (3.9–3.1 Ma) on O‘ahu. Young alkalic lavas (circa 0.37 Ma) sampled southwest of Ka‘ena Ridge are a form of offshore secondary volcanism. Half of the volcanic cones contain high‐SiO 2 basalts (51.0–53.5 wt % SiO 2 ). The trends of isotopic compositions of West Ka‘ena tholeiitic lavas diverge from the main Ko‘olau‐Kea shield binary mixing trend in isotope diagrams and extend to lower 208 Pb/ 204 Pb and 206 Pb/ 204 Pb than any Hawaiian tholeiitic lava. West Ka‘ena tholeiitic lavas have geochemical and isotopic characteristics similar to volcanoes of the Loa trend. Hence, our results show that the Loa‐type volcanism has persisted for at least 4.9 Myr, beginning prior to the development of the dual, subparallel chain of volcanoes. Several West Ka‘ena samples are similar to higher SiO 2 , Loa trend lavas of Ko‘olau Makapu‘u stage, Lāna‘i, and Kaho‘olawe; these lavas may have been derived from a pyroxenite source in the mantle. The high Ni contents of olivines in West Ka‘ena lavas also indicate contribution from pyroxenite‐derived melting. Average compositions of Hawaiian shield volcanoes show a clear relation between 206 Pb/ 204 Pb and SiO 2 within Loa trend volcanoes, which supports a prominent but variable influence of pyroxenite in the Hawaiian plume source. In addition, both Pb isotopes and volcano volume show a steady increase with time starting from a minimum west of Ka‘ena Ridge. The entrained mafic component in the Hawaiian plume is probably not controlling the increasing magma productivity in the Hawaiian Islands.

Hawaii↗