Refined Petrological Constraints on Magma Plumbing Systems along the Reykjanes Ridge
Abstract
Plate spreading at mid-ocean ridges is accompanied by intrusion of dikes and eruption of lava along the ridge axis. It has been suggested that the primary control of the magma plumbing systems is the heat flux - the higher the heat flux the shallower the depth of the magma chamber. As a test of this hypothesis, we have determined the depths of magma chambers beneath the slow spreading Reykjanes Ridge (RR) in the north Atlantic. Pressures of partial crystallization were calculated from the compositions of natural liquids (glasses) lying along the pressure (and temperature) dependent olivine, plagioclase, and augite cotectic using the method described by Kelley and Barton (2008). Published analyses of mid-ocean ridge basalt glasses sampled from along the RR were used as liquid compositions. Samples with anomalous chemical compositions and samples that yielded pressures associated with unrealistically large uncertainties were filtered out of the database. The calculated pressures for the remaining 507 samples for the RR were used to calculate the depths of partial crystallization and to identify the likely location of melt lenses. The results indicate that the average pressure of partial crystallization decreases from 141 × 87 MPa near the Charlie Gibbs Fracture Zone to 64 × 73 MPa at 59°N, then increases to 354 × 75 MPa as Iceland is approached. Five magma lenses were identified from clustering of results for individual localities at depths of .15× 0.9km, 2.2×1.1km, 4.8×1km, 5.2×0.8km, and 6.7×1km. The depths of the lenses generally decrease, then increase to the north. The magma lens at 2.21×1.1 km agrees extremely well with seismically imaged sill at 2.5 km along the same part of the ridge (Sinha et al 1998). While calculated pressures of partial crystallization provide some evidence for crystallization in axial melt lenses, the results obtained for samples from virtually every locality also suggest partial crystallization in the crust beneath these lenses with remarkable agreement observed between the maximum depth of partial crystallization and the seismically imaged Moho (Foulger 2005) along the north RR, and therefore the results support the many sill or crystal mush models for accretion of oceanic crust for the RR. Use of the method described by Herzberg (2004) yields slightly lower pressures for most samples but differences between pressures calculated with both methods are within the uncertainties of the calculations. The results therefore indicate that the average depth of partial crystallization along the RR increases with increasing crustal thickness. Previous workers have suggested that this reflects increasing heat flux towards the Iceland hotspot as also suggested by the progressive decrease in values of Na8 for glasses from south to north along the ridge. However, values of K2O8, P2O58, TiO28, and K/Ti increase from south to north along the ridge suggesting that the relationship between magma chamber depth and heat flux maybe more complex that previously assumed. It appears that the magma flux reflects changes in both mantle composition and mantle temperature as the Iceland hot spot is approached, and that magma chamber depth is related to crustal thickness as well as heat flux.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.V53A2751S
- Keywords:
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- 8416 VOLCANOLOGY Mid-oceanic ridge processes;
- 8439 VOLCANOLOGY Physics and chemistry of magma bodies;
- 8410 VOLCANOLOGY Geochemical modeling