Fernandina Volcano's evolved, well‐mixed basalts: Mineralogical and petrological constraints on the nature of the Galapagos plume

Abstract

Fernandina Volcano, the most active of Galapagos volcanoes and the one most directly overlying the Galapagos hotspot, is one of a small number of active, plume‐related oceanic island volcanoes with a well‐documented recent eruptive record. Whole rock and glass analyses of lava and tephra from all 12 known eruptions from 1958 to the present show them to be evolved (Mg # (= Mg/Mg + Fe2+) of 0.47–0.56), plagioclase‐phyric, clinopyroxene‐ and olivine‐bearing tholeiites. Thermodynamic modeling indicates that this phase assemblage erupted at 1100–1130°C and is stable only at low pressures (<0.1–0.2 GPa), thereby representing crystallization within a shallow magma chamber underlying the well‐defined, 850‐m‐deep, 5 by 6.5 km caldera. These tholeiites are remarkably homogenous, as is shown by narrow ranges in Zr/Y (5.7–6.3), (La/Sm)N (1.4–1.7), and (La/Yb)N (3.8–4.3). Prehistoric Fernandina lavas, selected for petrographic and geographic variation over the entire subaerial volcano, are of remarkably similar composition (e.g., Zr/Y is 5.2–6.1, (La/Sm)N is 1.5–1.6, and (La/Yb)N is 3.4–4.1), with a single example of a more primitive lava having an Mg # of 0.63, with 8.5% FeO at 8% MgO. Cr‐spinel and glass melt inclusions in plagioclase, together with zoned olivine, give evidence for a range of parental melts in the recent shallow magmatic system, with Mg # as high as 0.62. Extensive, shallow magma mixing is required to explain these variations in mineral and inclusion chemistry, indicating more thermal variation within the shallow magmatic system than is implied by the relatively monotonous whole rock compositions. Apparently, Fernandina's shallow magma chamber has physically blocked passage of more primitive magmas to the surface and homogenized the erupted lavas by extensive mixing. Small variations in lava isotopic and compatible element composition, combined with the need to resupply melt to maintain thermal balance, indicate that replenishment of the magma chamber is semicontinuous, with resupply occurring of the order of a decade or less. Plagioclase‐dominated, evolved Fernandina lavas are in sharp contrast to more primitive, olivine‐dominated Hawaiian lavas; the total lack of primitive melts in the shallow Fernandina magmatic system implies that substantial fractionation and heat loss occurred during melt transportation from the mantle source to the edifice magma chamber, despite the fact that these melts were transported through relatively warm, young, and thin lithosphere adjacent to the Cocos‐Nazca ridge. The diffuse nature of Galapagos volcanism, with 20th century eruptions from eight distinct volcanoes over a 20,000 km2 area, further supports this interpretation and suggests that the Galapagos plume is relatively diffuse, weak, and significantly less thermally intense than the vigorous Hawaiian plume. It may in fact be waning.