Evidence for fractionation of Quaternary basalts on St. Paul Island, Alaska, with implications for the development of shallow magma chambers beneath Bering Sea volcanoes

The 40Ar/39Ar chronology and eruption rates of Cenozoic volcanism in the eastern Bering Sea Volcanic Province, Alaska

“…St. George Island has an area of 95 km 2 and neighboring St. Paul Island one of 115 km 2 . Average total thicknesses for the basaltic flows on St. George Island and St. Paul Island are 100 m and 65 m, respectively [ Barth , 1956; Feeley and Winer , 1999; Winer et al , 2004; authors' observations]. The total volume of the basaltic flows exposed on these two islands is therefore estimated to be about 17 km 3 (Table 2).…”

“…Most of St. Paul Island (Figure 1) consists of coalescing small volcanoes, each comprising a central cinder cone built on subdued shield‐like sequences of lava flows. Most flows are of the pahoehoe type, but at least three are aa, and alkali olivine basalts dominate [ Barth , 1956; Wood and Kienle , 1990; Feeley and Winer , 1999]. Basalts display elevated SiO 2 (44.0–46.2 wt %) and Al 2 O 3 (11.7–15.5 wt %), and low MgO (6.3–7.2 wt %) and total alkali (Na 2 O + K 2 O = 3.6–5.1 wt %) concentrations compared to the other lavas in the BSVP.…”

Quaternary faulting and volcanism in the Main Ethiopian Rift

“…This indicates that phase 1 basalts are more fractionated and, therefore, had greater crustal residence time of the magma than that of the late Pleistocene basalts. High-MG numbers (63-70) in some samples and an increase in compatible trace element contents, especially Cr and Ni with increasing MgO% and olivine phenocrysts (Feeley and Winer, 1999) suggest that, at least some of the youngest basalts represent relatively primitive magmas.…”