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USGS2026

Magmatic source of the opening phase of Kīlauea’s 2018 Lower East Rift Zone eruption

The 2018 eruption of Kīlauea volcano in its Lower East Rift Zone began with the discharge of evolved high-Ti basalt as weak lava fountains and short, slow-moving lava flows. The lavas were quickly geochemically recognized as being derived from magmas stored within the rift zone and remobilized by a new intrusion, a sequence that is common at Kīlauea. This initial phase of the 2018 eruption, referred to as phase 1a, lasted for 6 days and was followed by extrusion of mixed magma after a 3-day pause. Even though remobilization of older rift zone magmas is common within Kīlauea’s rift zones, it is difficult to determine which past intrusion(s) may have initially emplaced those stored magmas. This difficulty stems from the tendency for Kīlauea magmas to follow very similar differentiation paths without significant variations in major, minor, or even trace element chemistry. We investigate possible magma sources for the lavas erupted during phase 1a of the 2018 eruption using whole-rock, mineral, and glass major and trace element compositions from historical East Rift Zone eruptions with adjacent fissures. We consider two primary hypotheses for the phase 1a source: magmas associated with the 1955 Lower East Rift Zone eruption or the nine eruptions in the Middle and Upper East Rift Zone during the 1960s. Our results suggest that magma associated with the earliest phases of Kīlauea’s 1955 eruption was the most likely source of the 2018 phase 1a remobilized magma. We determine volatile saturation pressures from melt inclusion chemistry and find similar storage depths for the 2018 phase 1a and early 1955 magmas. The phase 1a and early 1955 lavas are nearly indistinguishable in all of the compositional criteria considered, implying that the leftover 1955 magma body barely cooled and differentiated in the 63 years between eruptions (cooling rates of ~0.1 °C/year). This study sheds light on the potential for protracted storage of eruptible magmas in rift zones at Kīlauea, and highlights some of the challenges and solutions to identifying genetic relationships between magmas at Kīlauea.

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USGS1994

Water-quality, lithologic, and water-level data for wells in Tulare Basin, Kings, Kern, and Tulare Counties, California, August 1990 to February 1993

Much of the Tulare Basin is characterized by saline shallow ground water containing elevated concen- trations of arsenic, selenium, and other trace elements. A comprehensive investigation of ground-water geochemist, and, to a limited extent, hydrology was initiated in response to concerns related to (1) adverse effects on migratory waterfowl of high concentrations of selenium in agricultural drainwater disposed of in evaporation ponds and (2) adverse effects on human health due to potential downward migration of ground water containing high concentrations of arsenic to regional aquifers used as drinking-water sources. This report presents data collected during the initial phase of this investigation. A total of 20 observation wells were installed at 5 locations in the basin. Four observation wells in a cluster were installed at each location at depths of about 20, 50, 100, and 200 feet. Water samples collected from each site were analyzed for major ions, trace elements (including arsenic and selenium), nutrients, and selected isotopes. Lithologic logs and well-construction information for each site and water-level measurements for each well measured periodically from August 1990 to February 1993 also are reported.

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