France, Lyderic et al. (2010): Hydrous partial melting in the sheeted dike complex at fast spreading ridges; experimental and natural observations

ODP 206
ODP 111 504
ODP 137 504
ODP 140 504
ODP 148 504
ODP 206 1256


France, Lyderic
Universite Montpellier II, Geosciences Montpellier, Montpellier, France

Koepke, Juergen
Leibniz Universitaet Hannover, Germany

Ildefonse, Benoit
Universite Joseph-Fourier, France

Cichy, Sarah B.

Deschamps, Fabien

Hydrous partial melting in the sheeted dike complex at fast spreading ridges; experimental and natural observations
Contributions to Mineralogy and Petrology
Springer International, Heidelberg - New York, International
In ophiolites and in present-day oceanic crust formed at fast spreading ridges, oceanic plagiogranites are commonly observed at, or close to the base of the sheeted dike complex. They can be produced either by differentiation of mafic melts, or by hydrous partial melting of the hydrothermally altered sheeted dikes. In addition, the hydrothermally altered base of the sheeted dike complex, which is often infiltrated by plagiogranitic veins, is usually recrystallized into granoblastic dikes that are commonly interpreted as a result of prograde granulitic metamorphism. To test the anatectic origin of oceanic plagiogranites, we performed melting experiments on a natural hydrothermally altered dike, under conditions that match those prevailing at the base of the sheeted dike complex. All generated melts are water saturated, transitional between tholeiitic and calc-alkaline, and match the compositions of oceanic plagiogranites observed close to the base of the sheeted dike complex. Newly crystallized clinopyroxene and plagioclase have compositions that are characteristic of the same minerals in granoblastic dikes. Published silicic melt compositions obtained in classical MORB fractionation experiments also broadly match the compositions of oceanic plagiogranites; however, the compositions of the coexisting experimental minerals significantly deviate from those of the granoblastic dikes. Our results demonstrate that hydrous partial melting is a likely common process in the root zone of the sheeted dike complex, starting at temperatures exceeding 850 degrees C. The newly formed melt can either crystallize to form oceanic plagiogranites or may be recycled within the melt lens resulting in hybridized and contaminated MORB melts. It represents the main MORB crustal contamination process. The residue after the partial melting event is represented by the granoblastic dikes. Our results support a model with a dynamic melt lens that has the potential to trigger hydrous partial melting reactions in the previously hydrothermally altered sheeted dikes. A new thermometer using the Al content of clinopyroxene is also elaborated. Copyright 2010 Springer-Verlag
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Igneous and metamorphic petrology; Geochemistry of rocks, soils, and sediments; basalts; chain silicates; crystal fractionation; dikes; diorites; DSDP Site 504; East Pacific; Equatorial Pacific; experimental studies; geochemistry; granoblastic texture; granulites; Hess Deep; high-grade metamorphism; hydration; hydrothermal alteration; hydrothermal conditions; igneous rocks; intrusions; Leg 206; magma chambers; magmas; major elements; metamorphic rocks; metamorphism; metasomatism; mid-ocean ridge basalts; mid-ocean ridges; mineral composition; North Pacific; Northeast Pacific; Ocean Drilling Program; ocean floors; ODP Site 1256; ophiolite; ophiolite complexes; Pacific Ocean; partial melting; petrology; phase equilibria; plagiogranite; plate tectonics; plutonic rocks; prehnite; sea-floor spreading; sheeted dikes; silicates; spreading centers; textures; veins; volcanic rocks; whole rock;