Submarine Deep‐Water Lava Flows at the Base of the Western Galápagos Platform

Abstract

To investigate the initial phases of magmatism at the leading edge of the upwelling mantle plume, we mapped, photographed, and collected samples from two long, deep‐water lava flows located at the western base of the Galápagos Platform using the remotely operated vehicle Hercules. Lavas were recovered from four areas on the seafloor west of Fernandina volcano, including the western flow fronts of two deep‐water flows, heavily sedimented terrain between the two flows, and the eastern, shallower end of one flow. The sediment cover and morphologies are distinct between the western flow fronts and the eastern region based on seafloor imagery, suggesting that the long lava flows are not a single eruptive unit. Major and trace element concentrations reveal both tholeiitic and alkalic compositions and support the interpretation that multiple eruptive units comprise the deep‐water flows. Alkalic lavas have higher [La/Sm]N ratios (2.05–2.12) and total alkali contents (5.18–5.40) compared to tholeiitic lavas, which have [La/Sm]N ratios ranging from 1.64 to 1.68 and total alkali contents ranging from 3.07 to 4.08 wt%. Radiogenic isotope ratios are relatively homogeneous, suggesting a similar mantle source. We use petrologic models to assess three alternative mechanisms for the formation of the alkalic magmas: (1) high‐pressure crystallization of clinopyroxene, (2) mixing of high silica and mafic magmas, and (3) variable extents of melting of the same mantle source. Our modeling indicates that the alkalic samples form from lower extents of melting compared to the tholeiitic lavas and suggests that the deep‐water alkalic lavas are analogous to the initial, preshield building phase observed south of Hawaii and at the base of Loihi Seamount.

Monogenetic Near‐Island Seamounts in the Galápagos Archipelago

“…Combined, this suggests that the alkalic magmas bypass Fernandina's present-day shallow magma reservoir. Instead, the seamount alkalic samples are compositionally identical to alkalic deep water lava flows sampled in situ on the seafloor at the leading edge of the Galápagos plume (Figure 5c; Anderson et al, 2018) and similar, but more alkalic than, those sampled to the north at Roca Redonda (Standish et al, 1998).…”

“…Only samples reasonably cleaned of alteration were analyzed for major and trace elements (N = 81), and subsequent isotopic analysis (N = 31). The majority of our samples were recovered using the M/V Alucia, but we include in our discussion 13 samples collected on the R/V Nautilus (Anderson et al, 2018;Schwartz et al, 2018b) since three of these samples were analyzed for isotopes and are presented in this study. Trace elements from Schwartz et al (2018b) have been reprocessed using updated standard values and reported here, so they can be compared directly with our data.…”

“…The two seamounts with tholeiitic compositions are indistinguishable from those erupted on subaerially Fernandina, or samples with that affinity (Figure 3f). The alkalic samples are classified in the same affinity as those from deep water flows at the western extent of submarine sampling (Anderson et al., 2018, Figure 3c). The two seamounts are indistinguishable from these deep water flow samples, in that they are not next to each other at the lowest levels of the classification (Figure 3c).…”

“…The alkalic Fernandina samples were collected from seamounts on the submarine flanks of the volcano and have nearly identical geochemical characteristics to the deep water flows (Figure 5c), thus they were likely formed from a similar process. Therefore, for a detailed discussion into the origins of these alkalic lavas, the reader is directed to Anderson et al (2018). In summary, the trace element enrichment of the alkalic samples is accompanied by extreme enrichments in middle-to-heavy Rare Earth Element ratios (e.g., high [Sm/Yb] N ; Figures 6b and 6c), and very little isotopic variation, compared to tholeiitic samples from the same region (Figure 6a).…”

“…If true, this suggests that either (i) the earliest phase of volcanism in the Galápagos differs significantly from that of prototypical hotspot evolution, where initial island building begins with low‐volume, preshield building, alkalic magmatism (Clague & Dalrymple, 1987) or (ii) the evidence of this phase has been buried by subsequent volcanism. Recent analysis of long lava‐flows extending west of the platform are alkalic and are attributed to low degrees of partial melting (Anderson et al., 2018). It is unclear how exactly these sparse alkalic flows relate to island formation along the central hotspot track.…”