Geochemical constraints on the formation of near-ridge Vance seamount chain at the Juan de Fuca Ridge (Invited)
Abstract
Observations and sampling of off-axis lava flows and near-ridge seamounts, coupled with the recent discovery of large melt bodies away from ridge axes, attest to the significance of off-axis magmatic phenomena for the formation of the oceanic lithosphere at mid-ocean ridges (MORs). One of the most crucial unsolved questions of oceanic volcanism surrounds the physical mantle processes that cause the initial formation of near-ridge seamounts and sustain volcanism over several million years to produce seamount chains. The Vance Seamounts are just one example of a series of near-ridge seamount chains on the Pacific Plate. The chain comprises six submarine mountains that sit more than 1km above the surrounding oceanic crust just west of the Vance segment of the Juan de Fuca Ridge (JdFR). Dive observations indicate that the seamount chain formed sequentially with the oldest seamount furthest from the ridge axis. The geochemical characteristics of the oldest seamount suggest that the initiation of seamount volcanism was associated with a localized chemical (× thermal) heterogeneity in the mantle. Trace element and isotopic signatures suggest that the chemical heterogeneity was progressively depleted as subsequent seamounts were formed. Central seamount lavas have N-MORB compositions with trace element and isotopic ratios that are significantly more depleted than N-MORB lavas erupted at the JdFR axis. Depletion in the most incompatible elements is so severe for the central seamounts that no physically realistic forward geochemical models involving average depleted MORB mantle can reproduce potential parental melt compositions. The lava compositions from seamounts closest to the ridge reverse the trend in trace element depletion becoming more similar to N-MORB erupted at the current axis. We suggest that excess melt is generated off-axis due to the impingement of a discrete chemical heterogeneity that is more fusible than the DMM matrix. A variety of 2D model ridge scenarios were used to investigate the geometry and nature of the mantle heterogeneity and predict the depth and degree of melting off-axis. Trace element compositions of potential off-axis parental melts were subsequently predicted using existing 1D geochemical models (INVMEL and alphaMELTS) and the results compared to the observed geochemical variations along the Vance seamount chain.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMOS42A..07W
- Keywords:
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- 1032 GEOCHEMISTRY Mid-oceanic ridge processes;
- 3610 MINERALOGY AND PETROLOGY Geochemical modeling;
- 1021 GEOCHEMISTRY Composition of the oceanic crust;
- 1065 GEOCHEMISTRY Major and trace element geochemistry