Refined Estimates of the Depths of Magma Chambers Beneath the Reykjanes and Kolbeinsey Ridges, and Implications for the Structure of Oceanic Crust
The mid-Atlantic ridge is the divergent plate boundary between North and South America to the west and Europe and Africa to the east. Plate spreading is accompanied by intrusion of dikes and eruption of lava along the ridge axis. The dikes are fed by magma chamber (s) located beneath the ridge. It has been suggested that the depth of magma chambers is related to the rate of spreading. In order to examine this hypothesis we determined the depths of magma chambers beneath the slow spreading Reykjanes Ridge that extends form the Charlie Gibbs fracture zone at 53° north to the southern tip of Iceland at 64° north and the Kolbeinsey Ridge that extends from north of Iceland at about 66° north to the west Jan Mayon ridge at about 71° degrees north in the North Atlantic . Pressures of partial crystallization were calculated from the compositions of natural liquids (glasses) with those of liquids in equilibrium with olivine, plagioclase, and clinopyroxene different pressures and temperature. Chemical analyses of mid-ocean ridge basalts (MORB) glasses collected along the Reykjanes and Kolbeinsey Ridge were used as liquid compositions. The glasses form by rapid cooling of magma when quenched by contact with seawater, and provide unambiguous samples of natural basalt liquids The calculated pressures were used to estimate the depths of partial crystallization of liquids in sub-crustal chambers or reservoirs. The results indicate that the depth of magma chambers of the Reykjanes Ridge decreases from 4 to 8 km (±0.8 km) near the Charlie Gibbs fracture zone to 1.2±0.5 km at 55.67° N. As the Ridge approaches Iceland the depth of chambers increases to 9.7±3 km. The limited data available for the Kolbeinsey Ridge provides only an approximate estimate of the depth of magma chambers (average, 8.2km) but the depths also seem to increase towards Iceland.. The shallow depths obtained for chambers beneath the southern part of the Reykjanes ridge and the average depth of chambers beneath the Kolbeinsey ridge is in contrast with results obtained for slow-spreading ridges elsewhere. This may reflect increased magma flux associated with the Iceland plume, and this is consistent with crustal thickening towards Iceland as suggested by the northerly increase in the maximum depths of chambers along the Reykjanus ridge. The influence of the Iceland plume is apparent from the chemical analyses of the glasses. The abundances of Ti, Na, K, P, and Fe increase whereas the abundances of Si, Mg, Al, and Ca decrease as Iceland is approached. These chemical data can also be interpreted in terms of increased magma flux reflecting the thermal effects of the Iceland plume.
Provisional Depths of Magma Chambers Below Kverkfjöll
Rifting along mid-ocean ridges (MOR) is facilitated by injection of dikes and formation of magma chambers within the crust in rifting events which occur at discrete times and locations along the axis of the rift zone. Volcano-tectonic events in Iceland's rift zones are often accompanied by outpouring of lava at the Earth's surface. We have used mineral-melt equilbria relationships to determine the pressure and temperature of magmas erupted from Mt. Upptyppingar and other localities along the Kverkfjöll volcanic system, which is located along the eastern margin of the Northern Volcanic Zone (NVZ). This volcanic system consists of a central volcano, Kverkfjöll, and lavas erupted from a series of fissures extending northward 50 km to Mt. Upptyppingar, a large mound of hyaloclastites and pillow lavas. Pressures and temperatures were calculated for melts lying along the olivine, plagioclase and augite cotectic. Input data included published glass analyses from the Kverkfjöll volcano as well as hyaloclastite ridges along the eruptive fissure system extending northward toward Mt. Upptyppingar. The results provide evidence for partial crystallization at an average pressure of 426 MPa. We conclude that these samples represent magmas that were erupted from magma chambers at a depth of ∼15 km. This depth is consistent with seismic data for recent swarms of microseismicity beneath Upptyppingar (February 2007-May 2008). The seismic activity is interpreted to reflect influx of magma into a dike at a depth of 15-18 km. Ongoing analysis of samples from Mt. Upptyppingar will lead to estimation of magma chamber depths beneath that locality. The seismic unrest at Upptyppingar might signal a rifting episode in the NVZ of Iceland. The latter accommodates the full ∼20 mm/yr spreading in this portion of the mid-Atlantic Ridge, but it has been over 20 years since the last major rifting episode in the NVZ at Krafla. Eruption of magmas from relatively deep chambers in Icelandic crust will promote rapid degassing and may lead to explosive activity that could inject volcanic aerosols sufficiently high in the atmosphere to affect the Earth's climate.
Oxygen isotope, TitaniQ, and cathodoluminescence analyses of the Alta Stock, UT: Preliminary insights into pluton assembly.
Volcanic deformation in the Andes
We present the results from an InSAR survey of volcanic activity in South America. We use data from the Japanese Space Agency's ALOS L-band radar satellite from 2006-2009. The L-band instrument provides better coherence in densely vegetated regions, compared to the shorter wave length C-band data. The survey reveals volcano related deformation in regions, north, central and southern, of the Andes volcanic arc. Since observations are limited to the austral summer, comprehensive coverage of all volcanoes is not possible. Yet, our combined observations reveal volcanic/hydrothermal deformation at Lonquimay, Llaima, Laguna del Maule, and Chaitén volcanoes, extend deformation measurements at Copahue, and illustrate temporal complexity to the previously described deformation at Cerro Hudson and Cordón Caulle. No precursory deformation is apparent before the large Chaitén eruption (VEI_5) of 2 May 2008, (at least before 16 April) suggesting rapid magma movement from depth at this long dormant volcano. Subsidence at Ticsani Volcano occurred coincident with an earthquake swarm in the same region.