Five million years of compositionally diverse, episodic volcanism: Construction of Davidson Seamount atop an abandoned spreading center

Abstract

[1] Davidson Seamount, a volcano located about 80 km off the central California coast, has a volume of 320 km 3 and consists of a series of parallel ridges serrated with steep cones. Davidson was sampled and its morphology observed during 27 ROV Tiburon dives. During those dives, 286 samples of lava, volcaniclastite, and erratics from the continental margin were collected, with additional samples from one ROV-collected push core and four gravity cores. We report glass compositions for 99 samples and 40 Ar-39 Ar incremental heating age data for 20 of the samples. The glass analyses are of hawaiite (62%), mugearite (13%), alkalic basalt (9%), and tephrite (8%), with minor transitional basalt (2%), benmoreite (2%), and trachyandesite (2%). The lithologies are irregularly distributed in space and time. The volcano erupted onto crust inferred to be 20 Ma from seafloor magnetic anomalies. Ages of the lavas range from 9.8 to 14.8 Ma. The oldest rocks are from the central ridge, and the youngest are from the flanks and southern end of the edifice. The compositions of the 18 reliably dated volcanic cones vary with age such that the oldest lavas are the most fractionated. The melts lost 65% to nearly 95% of their initial S because of bubble loss during vesiculation, and the shallowest samples have S contents similar to lava erupted subaerially in Hawaii. Despite this similarity in S contents, there is scant other evidence to suggest that Davidson was ever an island. The numerous small cones of disparate chemistry and the long eruptive period suggest episodic growth of the volcano over at least 5 Myr and perhaps as long as 10 Myr if it began to grow when the spreading ridge was abandoned.

Petrogenesis of Davidson Seamount lavas and its implications for fossil spreading center and intraplate magmatism in the eastern Pacific

“…The Davidson Seamount lavas were erupted from multiple vents over a span of ∼5 Myr (15–10 Ma), i.e., as much as 8 Myr after spreading ceased [ Clague et al , 2009]. However, although with some scatter, the lava composition generally behaves systematically with decreasing wt % MgO (Figure 4), consistent with differentiation trends.…”

“…The U‐shaped normalized REE pattern of the few differentiated samples is similar to that of some differentiated arc lavas from the southern Philippines, and this pattern has been modeled as the result of late stage (>70% crystallization) fractionation of a mineral assemblage that includes amphibole and accessory apatite or titanite phase from a basaltic parent [e.g., Castillo et al , 1999; see also Davidson et al , 2007]. In detail, fractional crystallization was most probably complex and likely occurred within the mantle at pressures that increased as the volcano grew [ Davis et al , 2002; Clague et al , 2009]. Such a complex fractional crystallization process is consistent with the observed mineralogy of the lavas, as the basalts contain microphenocrysts of olivine, clinopyroxene and plagioclase, plus amphibole in some mugearites.…”

“…For example, they have slightly higher SiO 2 and compatible trace elements but lower CaO and incompatible trace elements at a given MgO than the main rock series (Figure 4). There is also a general trend for the more differentiated and alkali‐rich lavas to occur near the seamount summit [ Clague et al , 2009].…”

“…This combined tectonic and mantle‐melting process produces voluminous mid‐ocean ridge basalt (MORB) with a fairly uniform major element composition that is distinct from magmatism at subduction zones and intraplate hot spots. The majority of fossil spreading centers in the eastern Pacific (Figure 1) are occupied by volcanic ridges that were built after the cessation of seafloor spreading activities [e.g., Batiza et al , 1982; Batiza and Vanko , 1985; Lonsdale and Castillo , 1998; Davis et al , 1995, 2002; Clague et al , 2009; Davis et al , 2010]. The composition of these ridges is distinct from that of normal MORB.…”

Phylogeography of Acesta clams from submarine seamounts and escarpments along the western margin of North America

“…The first type includes elongate volcanic seamounts in a wide range of sizes, ages, and depths, built on or near tectonically ‘abandoned’ mid‐ocean ridges where the California margin evolved from a subduction to a transform boundary (Davis et al. 2002, 2010; Clague et al. 2009; Paduan et al.…”