Assimilation, contamination and felsic melt generation at the top of the axial melt lens at fast-spreading oceanic crust: Combined geochemical and experimental investigations at the dike/gabbro transition from IODP Site 1256 (East Pacific Rise)
Mineralogy, Petrology and Geochemistry
Final Report Abstract
IODP (International Ocean Discovery Program) drilled at Site 1256 in the eastern equatorial Pacific for the first time a section of intact fast-spreading oceanic crust from the sediments down to the uppermost gabbros, thereby penetrating the transition between the sheeted dikes and gabbros. In the lowermost sheeted dikes as well as in the gabbroic section are abundant intrusions of felsic rocks, which show different compositions and different modes of emplacement. In the performed project, we used these key rocks to evaluate the complex and intimate coupling between magmatic, metamorphic, and hydrothermal processes at the top of the AML in fast-spreading mid-ocean ridge systems. In a second theme we aimed to understand anatectic processes at the roof of axial magma chamber, recorded in the drilled rocks, by performing experimental simulations. The plutonic rocks including the felsic members consisting of quartz-bearing gabbros, diorites and tonalites are regarded to represent the upper part of a fossilized AML. Whole-rock major elements show a wide and continuous compositional range (e.g., Mg# 24~70) and apparent enrichments in Ti and Fe at intermediate MgO contents (4~6 wt%). Trace elemental characteristics are coherent for different lithology groups defined by petrography and mineral modes, i.e. gabbro, clinopyroxene-rich diorite, amphibole-rich or oxide-rich diorite, and tonalite. The gabbros and diorites are consistent with modeled products of MORB fractional crystallization, composed of mixed melt and cumulate with a varying ratio. Modeled trace elements (especially with respect to Eu) support a model in which the tonalites are originated from low-degree partial melting of the sheeted dikes overlying the AML rather than extreme fractional crystallization. Enrichments in rare earth elements (REEs) in clinopyroxenes of gabbroic and dioritic intrusives suggest strong assimilation of evolved MORB magmas by REE- rich tonalitic components. The hydrothermal alteration was pervasive during cooling of the plutonic system, which can be traced by petrography, mineral composition and bulk geochemistry. The upper part of AML, largely composed of low-density and high-viscosity felsic magmas, may serve as a barrier for the stopping and pooling of eruptible MORB melts in the lower part of AML. We also investigated the role of the volatiles CO2, S, F, Cl and Br for a variety of lithologies including evolved plutonic rocks in order to examine the magmatic imprint versus hydrothermal influence. The bulk CO2 contents (500-10000 ppm) show no lithological dependence and are similar to those of fresh lavas. Highly variable bulk S concentrations (3-900 ppm) show significant depletions compared to undegassed MORB melts, which might be caused by exsolution of magmatic fluids during crystallization. The plutonic samples have Cl concentrations of 500-1000 ppm, remarkably higher than the hornfelses (200-600 ppm Cl) and erupted lavas (average ~200 ppm Cl). Except for the cases that the albitites contain very low F (20-30 ppm) and the hydrothermal amphibole-rich vein contains extremely high F (~1000 ppm), the other lithologies have similar F concentrations within 100-350 ppm. The Br concentrations of different lithologies are largely overlapping within 0.5-3.5 ppm. Petrological and mineralogical evidence indicates that bulk F concentrations are dominated by the presence of amphibole and apatite, whereas Cl and Br are mainly hosted in fluid inclusions rather than in hydrous minerals. The variation of F/Cl and Br/Cl ratios may trace the mixing between MORB magmas and seawater-derived fluids, crystallization of apatite and amphibole, and/or extraction of magmatic fluids. Our data support the current S budget of oceanic crust, but indicate that the budgets of C and Cl may have been underestimated. Within a series of experimental investigations, we simulated those anatectic process at the top of axial magma chambers as recorded in the drilled Hole 1256D core. Hydrous partial melting experiments have been performed by different starting material of sheeted dikes and hornfelses of the drilled core. The protoliths span a lithological range from poorly to strongly altered basalts to partially or fully recrystallized granoblastic hornfels. The compositions of the anatectic melts correspond to the compositions of the felsic veins, while the residual minerals match the compositions of the two-pyroxene hornfels, evidencing that hydrous partial melting is an important magmatic process in the gabbro/dike transition of fast-spreading mid-oceanic ridges. Major and trace element composition of those tonalitic felsic veins intruding hornfelses and gabbros could be only reproduced in the experiments performed at low water activity. Additionally, bulk and mineral/melt trace element partition coefficients for different potential protoliths of anatectic processes are provided. Moreover, KD values are given for a large range of temperature and for different water activities. With these data, it is possible to decipher the different processes of felsic melt generation and to track MORB contamination by assimilation of highly evolved wall rocks. Calculations of mixing and assimilation fractional crystallization using the experimental partial melts as contaminant/assimilant showed that anatectic melts can only be a minor contributor to the contamination process.
Publications
- 2015. Anatexis at the roof of an oceanic magma chamber at IODP Site 1256 (equatorial Pacific): an experimental study. Contrib. Mineral. Petrol. 169:39
Erdmann, M., Fischer, L.A., France, L., Zhang, C., Godard, M., Koepke, J.
(See online at https://doi.org/10.1007/s00410-015-1136-5) - 2016. A Practical Method for Accurate Measurement of Trace Level Fluorine in Mg- and Fe-Bearing Minerals and Glasses Using Electron Probe Microanalysis. Geostandards and Geoanalytical Research, 40(3): 351-363
Zhang, C., Koepke, J., Wang, L.-X., Wolff, P.E., Wilke, S., Stechern, A., Almeev, R., Holtz, F.
(See online at https://doi.org/10.1111/j.1751-908X.2015.00390.x) - 2016. Trace element evidence for anatexis at oceanic magma chamber roofs and the role of partial melts for contamination of fresh MORB. Lithos 260
Fischer, L.A., Erdmann, M., France, L., Wolff, P.E., Deloule, E., Zhang, C., Godard, M., Koepke, J.
(See online at https://doi.org/10.1016/j.lithos) - 2017. Apatite in the dike-gabbro transition zone of mid-ocean ridge: Evidence for brine assimilation by axial melt lens. American Mineralogist
Zhang, C., Koepke, J., Albrecht, M., Horn, I., Holtz, F.
(See online at https://doi.org/10.2138/am-2017-5906) - 2017. Felsic Plutonics from IODP Hole 1256D, Eastern Pacific: Implications for the Nature of Axial Melt Lens at Fast-Spreading Mid-Ocean Ridges. Journal of Petrology
Zhang, C., Koepke, J., France, L., Godard, M.
(See online at https://doi.org/10.1093/petrology/egx064) - Trace elements in anatectic products at the roof of mid-ocean ridge magma chambers: an experimental study. Chemical Geology
Erdmann, M., France, L., Fischer, L.A., Deloule, E., Koepke, J.
(See online at https://doi.org/10.1016/j.chemgeo.2017.03.004)
