Evidence from accreted seamounts for a depleted component in the early Galapagos plume

David M Buchs,Folkmar Hauff,Peter O Baumgartner,Kaj Hoernle

doi:10.1130/g37618.1

Abstract

The existence of an intrinsic depleted component in mantle plumes has previously been proposed for several hotspots in the Pacific, Atlantic, and Indian Oceans. However, formation of these depleted basalts is often associated with unusual tectonomagmatic processes such as plume-ridge interaction or multistage melting at plume initiation, where depleted basalts could reflect entrainment and melting of depleted upper mantle. Late Cretaceous to middle Eocene seamounts that accreted in Costa Rica and are part of the early Galapagos hotspot track provide new insights into the occurrence and nature of intrinsic depleted components. The Paleocene (ca. 62 Ma) seamounts include unusually depleted basalts that erupted on the Farallon plate far from a mid-ocean ridge. These basalts closely resemble Gorgona komatiites in terms of trace element and radiogenic isotope composition, suggesting formation from a similar, refractory mantle source. We suggest that this source may be common to plumes, but is only rarely sampled due to excessive extents of melting required to extract melts from the most refractory parts of a heterogeneous mantle plume.

Highlights

It is generally assumed that ocean-island basalts (OIBs) are derived from mantle plumes and that these melts commonly have more enriched compositions than mid-oceanic ridge basalts (MORBs)
The origin of the outer Osa Igneous Complex (OIC) is determined based on a combination of immobile trace element discrimination diagrams (Fitton et al, 1997; Pearce, 2008), radiogenic isotope ratios that are relatively insensitive to alteration, geochemical comparison with a selection of possible volcanic analogues, field observations, and existing regional constraints
Support for a Galapagos Provenance A paleo–Galapagos hotspot origin for the Cretaceous to Paleocene seamounts accreted in the OIC is in good agreement with regional tectonic, age, and lithostratigraphic constraints

Summary

INTRODUCTION

It is generally assumed that ocean-island basalts (OIBs) are derived from mantle plumes and that these melts commonly have more enriched compositions than mid-oceanic ridge basalts (MORBs). The origin of the depleted component, remains controversial in most cases due to the similarity in composition between melts from entrained depleted upper mantle and from a depleted component intrinsic in the plume Both will melt due to shallow upwelling and highdegree melting of the plume beneath a ridge potentially coupled with previous extraction of enriched melts at depth. Due to ambiguities concerning the origins of depleted basalts formed in oceanic plateaus or at hotspots where a plume interacts with a mid-ocean ridge, it remains essential to provide additional constraints on the possible existence of an intrinsic depleted component in mantle plumes. Novel support for the existence of this component is provided here by new geochemical data from Late Cretaceous to middle Eocene accreted seamounts in the Osa Igneous Complex (Costa Rica), which formed at the early Galapagos hotspot far from a mid-ocean ridge. Full analytical methods, and detailed evaluation of whether initial or measured radiogenic isotopes provide the least biased source information are provided in the Data Repository, in and Tables DR1–DR4 therein

RESULTS

DISCUSSION

E Antilles Arc