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

Abstract

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 2 20,000 km 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.

Anomalously thin transition zone beneath the Galápagos hotspot

“…While Fernandina has often been considered to overlie the Gala ¤pagos plume (e.g., [35]), our results suggest that the plume is located 40 km to the southwest of the center of this island and has a radial extent of about 100 km (Fig. 1).…”

1998 Eruption at Volc�n Cerro Azul, Gal�pagos Islands: I. Syn-Eruptive Petrogenesis

“…Garcia et al 1992;Carracedo et al 1992;Rowland and Munro 1993;Rhodes 1996), but heterogeneous lava flows in the Galµpagos Islands have not previously been reported, probably due to limited sampling of individual flows. Also, with the exception of Cerro Azul, and possibly Volcµn Ecuador , most of the western Galµpagos shields are characterized by exceedingly limited compositional variation over the entire volcano (Allan and Simkin, 2000;Reynolds and Geist, 1995). Nevertheless, it is important to recognize that single samples may not represent the compositional diversity of an entire lava flows or individual volcanic events.…”

Volumetric characteristics of lava flows from interferometric radar and multispectral satellite data: the 1995 Fernandina and 1998 Cerro Azul eruptions in the western Gal�pagos

“…Similar to the method above, two end members of the FLOWGO model, termed hot and cool, utilize parameter end members that respectively maximize and minimize heat loss down flow. Included in the parameter ranges are starting eruption temperatures of 1,130 and 1,100 C, typical of Galµpagos basalts (Allan and Simkin 2000), and these are used in the hot and cold models, respectively.…”

“…We applied this technique as well, using T erupt of 1,100-1,130 C (Allan and Simkin 2000) and T amb of 25 C. The total energy budget of the flow was calculated from the ATSR data combined with the conductive and convective models, and then used in Eq. (2) to calculate V, the volume of erupted lava visible to the sensor.…”