Petrological Constraints on Magma Plumbing Systems along the Reykjanes and Juan de Fuca Ridges

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 depth of magma chambers that feed the dikes and flows is related to the heat flux - the higher the heat flux the shallower the depth of the magma chamber. As a preliminary test of this hypothesis, we have determined the depths of magma chambers beneath the slow spreading Reykjanes Ridge (RR) in the north Atlantic and the intermediate spreading Juan de Fuca Ridge (JdF) in the northeast Pacific. Pressures of partial crystallization were determined by quantitatively comparing the compositions of natural liquids (glasses) with those of experimental liquids in equilibrium with olivine, plagioclase, and augite at different pressures and temperatures using the method described by Yang and co-workers (1996). Published analyses of mid-ocean ridge basalt glasses sampled from along the RR and JdF 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 519 samples for the RR and 479 samples for the JdF were used to calculate the depths of partial crystallization and to identify the likely location of magma chambers. The RR results indicate that the pressure of partial crystallization decreases from 102 ± 33 MPa at the Charlie Gibbs Fracture Zone to 21 ± 12 MPa at 56°N, then increases to 367 ± 68 MPa as Iceland is approached. Four magma lenses were identified at depths of 2.5±0.8km, 5.2±0.8km, 5.9±1km, and 6.7±1. The magma lens at 2.46±.83 km agrees very well with seismically imaged sill at 2.5 km. The JDF results indicate that the pressure of partial crystallization decreases from 200 to100±50 MPa from the Blanco fracture zone to the north along the Cleft segment of the ridge. Calculated pressures remain approximately constant at 87±.53MPa along ridge segments to the north of Cleft. Two magma lenses were identified at depths of 4.47±.89km and 4.08±1.5km. Pressures calculated for samples from single lava flows along the Cleft segment allow identification of two magma chambers at depths of 4.91±.77km and 4.33±1.07km which agree well with the depth of 5 to 6 km inferred for a seismically imaged sill. The average depth of magma chambers along the slower spreading RR is only slightly higher than that along the intermediate spreading JdF , Moreover, he average depth of partial crystallization along the RR increases with increasing crustal thickness and this is thought to reflect increasing heat flux towards the Iceland hotspot. These results suggest that the relationship between magma chamber depth and heat flux maybe more complex that previously assumed. 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, and therefore the results support the many sill or crystal mush models for accretion of oceanic crust for both ridges. 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.