Mantle plume capture, anchoring, and outflow during Galápagos plume‐ridge interaction

Abstract

Compositions of basalts erupted between the main zone of Gal apagos plume upwelling and adjacent Gal apagos Spreading Center (GSC) provide important constraints on dynamic processes involved in transfer of deep-mantle-sourced material to mid-ocean ridges. We examine recent basalts from central and northeast Gal apagos including some that have less radiogenic Sr, Nd, and Pb isotopic compositions than plume-influenced basalts (E-MORB) from the nearby ridge. We show that the location of E-MORB, greatest crustal thickness, and elevated topography on the GSC correlates with a confined zone of lowvelocity, high-temperature mantle connecting the plume stem and ridge at depths of $100 km. At this site on the ridge, plume-driven upwelling involving deep melting of partially dehydrated, recycled ancient oceanic crust, plus plate-limited shallow melting of anhydrous peridotite, generate E-MORB and larger amounts of melt than elsewhere on the GSC. The first-order control on plume stem to ridge flow is rheological rather than gravitational, and strongly influenced by flow regimes initiated when the plume was on axis (>5 Ma). During subsequent northeast ridge migration material upwelling in the plume stem appears to have remained ''anchored'' to a contact point on the GSC. This deep, confined NE plume stem-to-ridge flow occurs via a network of melt channels, embedded within the normal spreading and advection of plume material beneath the Nazca plate, and coincides with locations of historic volcanism. Our observations require a more dynamically complex model than proposed by most studies, which rely on radial solid-state outflow of heterogeneous plume material to the ridge.

A 1.5 Ma record of plume-ridge interaction at the Western Galápagos Spreading Center (91°40′–92°00′W)

“…1a-c), ridge jumps are likely in the vicinity of hotspots as they are able to capture the ridge (e.g., Mittelstaedt et al, 2012;Gibson et al, 2015). There is however no obvious evidence for ridge jumps or overlapping spreading centers in the sampled area from the bathymetry or from synthetic magnetic anomaly profiles (Mittelstaedt et al, 2012).…”

“…1; e.g., Handschumacher, 1976;Hey and Vogt, 1977;Wilson and Hey, 1995;Canales et al, 1997;Werner et al, 1999Werner et al, , 2003. Over the last 5 Ma, the GSC, located north of the Galápagos hotspot, has been progressively drifting northwards but has been repeatedly captured by the hotspot, as reflected by numerous ridge-jumps (Wilson and Hey, 1995;Mittelstaedt et al, 2012;Gibson et al, 2015).…”

“…These models are broadly divided into 1) transport to the GSC along channels within the base of the lithosphere (e.g., Morgan, 1978;Schilling et al, 1982;Verma and Schilling, 1982;Braun and Sohn, 2003); 2) deflection of the Galápagos plume head primarily eastwards due to eastward migration of the Nazca Plate but with some of the material in the north reaching the ridge axis (Richards and Griffiths, 1989;Geist, 1992;White et al, 1993;Harpp and White, 2001); 3) gravity driven plume dispersal along the base of the lithosphere (Bercovici and Lin, 1996;Hoernle et al, 2000); 4) radial outflow of plume material away from its stem along the base of the lithosphere to the spreading center (Schilling et al, 2003;Shorttle et al, 2010); 5) subsolidus transport of plume material beneath a viscous residual plug to the ridge (Kokfelt et al, 2005;Ito and Bianco, 2014;Villagomez et al, 2014;Byrnes et al, 2015); and 6) melt transport via veins and channels below the anhydrous peridotite solidus (Gibson et al, 2015).…”

“…Both areas are characterized by axial high morphology, increased crustal thickness (up to 8-9 km; Ito and Lin, 1995;Canales et al, 2002;Mittelstaedt et al, 2014) and excursions in various geochemical plume signals (Schilling et al, 2003;Christie et al 2005;Kokfelt et al, 2005;Ingle et al, 2010;Gibson et al, 2015).…”

“…Although low resolution of the available seismic array does not explicitly identify plume material reaching the GSC, a general inference can be made that plume material is transported to the western and eastern GSC consistent with maximum plume signals west and east of the 91°W transform. Bending of the plume to the north rather than the east (direction of Nazca Plate motion) is likely to reflect capture of the GSC as it migrated over the plume (Gibson et al, 2015).…”