Asimow, Paul D. and Langmuir, Charles H. (2005) Effect of water on magma and crustal density: Highly fractionated lavas in the Lau Basin and other wet spreading centers. Geochimica et Cosmochimica Acta, 69 (10). A149. ISSN 0016-7037 http://resolver.caltech.edu/CaltechAUTHORS:20120906-091801385
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Mid-ocean ridges worldwide typically erupt basalts with approximately 8% MgO. This can be understood if eruption is primarily controlled by the density of magmas relative to the crust: high-MgO primary MORB is denser than the porous layer 2 of oceanic crust, leading to ponding and fractionation. Stolper & Walker (1980) and Sparks et al. (1980) pointed out the density minimum along the tholeiitic liquid line of descent at plagioclase saturation, close to typical MORB composition. Certain regions, however, erupt anomalously fractionated lavas. Hotspot-affected and back-arc ridges tend towards values near 6% MgO and a few are dominated by andesites with <4% MgO. An outstanding example is the southern end of the Eastern Lau Spreading Center, particularly the Valu Fa Ridge, sampled by the Lau II Cruise (Leg 0417 of R/V Kilo Moana) in Fall 2004. Data show a systematic trend towards highly fractionated samples with proximity to the Tonga trench, including axial andesites and dacites. As pointed out for calc-alkaline lavas by Grove and Baker (1983), water lowers the density of magma and the effect increases with extent of fractionation because water is incompatible and because it suppresses plagioclase fractionation. MELTS calculations show that these effects together eliminate the density minimum for primary MORB lavas with >0.4% H_2O. Together with increasing porosity of the upper crust (confirmed by seismic data), this effect of water provides a neat explanation for the decrease in mean MgO with moderately elevated H_2O and the rather sudden transition to highly fractionated samples at higher H_2O concentrations. This is a regional effect and results from feedback between the density of roof-rocks derived from earlier dikes and eruptions with the material in the present magma chamber.
|Additional Information:||© 2005 Pergamon-Elsevier Science Ltd.|
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|Deposited By:||Tony Diaz|
|Deposited On:||06 Sep 2012 16:24|
|Last Modified:||06 Sep 2012 16:33|
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