Melt and Chemical Transport in the Mantle: Insights from Deglaciation-Induced Melting Perturbations in Iceland
Abstract
Eruptive products represent a time-averaged view of the melting region and melt migration processes, making numerous fundamental parameters of the melt system difficult to constrain. Temporal and spatial variations in melting provide potential windows into this obscure region of the Earth by preferentially sampling melts from different regions of the mantle or mixing melts over different length-scales. We present a newly extended geochemical time series from the Western Volcanic Zone (WVZ) of Iceland, which experienced a short-lived melting perturbation due to glacial unloading during the last major deglaciation (~15-10 ka). Glacial unloading during this period led to increased degrees of melting particularly in the shallow mantle, which is manifest as an observed increase in volcanic production up to 30 times the steady-state value, decreased levels of highly to moderately incompatible element ratios (e.g., a 35-50% decrease in Nb/Y, with the greatest change occurring in the northernmost WVZ), and elevated SiO2 and CaO concentrations (~0.8 wt. % and ~1.9 wt. % increase in average oxide concentrations respectively) during and immediately following deglaciation. Although eruptive productivity returns to steady-state values within ~3000 yr following deglaciation, the incompatible element concentrations in erupted lavas gradually increase throughout the post-glacial period. We exploit this short-lived melting perturbation to examine and constrain knowledge of fundamental characteristics of melt generation and transport, including mantle permeability, melt ascent rates, depth-dependent melting functions (dF/dP), and the nature of chemical transport and melt mixing in the system. Using conservation equations describing the generation and porous flow of melt in a viscous matrix, we model melt migration in the mantle during and after ice sheet removal, as well as trace element transport for both equilibrium and disequilibrium transport end members. The predicted geochemical time series at the surface is particularly sensitive to the mode of chemical transport, with trace element compositions predicted for disequilibrium transport exhibiting a greater dependency on melt ascent rate than for equilibrium transport, as well as a greater overall magnitude of compositional change. We propose and examine other potential geologic settings in which short-lived perturbations in the melting region might be exploited to examine time- and depth-dependent melt processes. The results of this case study emphasize the potential importance of the nature and rate of melt migration and chemical transport in controlling compositional variability at spreading centers and ocean islands.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.V32B..01E
- Keywords:
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- 1009 GEOCHEMISTRY / Geochemical modeling;
- 3614 MINERALOGY AND PETROLOGY / Mid-oceanic ridge processes;
- 3621 MINERALOGY AND PETROLOGY / Mantle processes;
- 3653 MINERALOGY AND PETROLOGY / Fluid flow