Science

Featured Article

C. Johan Lissenberg and Henry J. B. Dick (2008). "Melt-rock reaction in the lower oceanic crust and its implications for the genesis of mid-ocean ridge basalt." EARTH AND PLANET SCI LETT  In Press doi:10.1016/j.epsl.2008.04.023

"Transport of melt through the lower oceanic crust is a poorly understood phenomenon. However, it may have a large effect on the compositions of both lower crustal cumulates and the most abundant magma on Earth, mid-ocean ridge basalts (MORB). We studied gabbroic rocks from the Kane Megamullion, on the Mid-Atlantic ridge, and found evidence that melt was transported in diffuse cm-wide channels. Mineral compositions and textures indicate that melt flow in these channels is reactive. We modeled this reaction to constrain its effect on melt composition, and compared the results with MORB data. Using PetDB allowed a rigorous comparison of our model melt composition with the full spectrum of melts erupted along the Mid-Atlantic Ridge. Results suggests that MORB may owe part of their compositional variation, previously attributed to fractional crystallization at elevated pressures in the mantle, to reactive transport in the lower crust." - C. J. Lissenberg

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Previously Featured Articles

J. Blichert-Toft and F. Albarede (2007). "Hafnium isotopes in Jack Hills zircons and the formation of the Hadean crust." EARTH AND PLANET SCI LETT 265(3-4): 686-702. doi:10.1016/j.epsl.2007.10.054
photo credit: NASA Earth Observatory

"One of the fundamental issues of Earth evolution is to understand when growth of continental crust began. Constraining this question will help us date the timing of onset of plate tectonics and thus ultimately the origin of life. We have addressed this problem by dating single Hadean zircons from Jack Hills in Western Australia and comparing their inferred geochemical properties with a variety of modern rock types (MORB, OIB, plateau basalts, subduction zone lavas etc.) in order to deduce the nature of the source rock of the Jack Hills granites. This approach, which was based on using real data of worldwide distribution from PetDB and which could not have been undertaken in such an efficient manner without access to this data base source, helped us exclude all modern candidates as the protolith of the Jack Hills granites and come up with a unique alternative explanation involving a ~4.35 Ga TTG-like (tonalite-trondhjemite-granodiorite) crust which itself may have been produced by melting of the last remains of the magma ocean." - J. Blichert-Toft

Rubin, K. H. and J. Sinton (2007). "Inferences on mid-ocean ridge thermal and magmatic structure from MORB compositions." EARTH AND PLANET SCI LETT 260(1-2): 257-276. doi:10.1016/j.epsl.2007.05.035

"Interplays among magma accumulation, differentiation, heat loss, and eruption as functions of space and time at mid ocean ridges are key for understanding the creation and structure of oceanic crust. Using new observations of systematic, regional compositional variations in a global MORB dataset (>11000 samples compiled from PetDB), we propose a new ocean ridge magma chamber model in which the number, size and depth of shallowest melt segregations vary smoothly with spreading rate and magma supply. Magma reservoir depth in this model is dynamically controlled by the long-term melt flux to the crust. Highest melt supplies promote shallower and thermally more variable magmatic differentiation conditions, accompanied by a dramatic reduction in chemical variance for parameters related to parental melt compositions (i.e., those obtained in Earth's mantle). In contrast, low melt supply promotes the eruption of less differentiated magma from deeper magma chambers of more uniform differentiation degree but more variable mantle parentage. This new perspective on crustal magma bodies provides a framework for understanding structural, geophysical, hydrothermal and volcanological attributes of ridges. The major element, trace element, and radiogenic isotope data within PetDB made this research possible." - K. Rubin

Herzberg, C., P. D. Asimow, N. Arndt Y. Niu, C. M. Lesher, J. G. Fitton, M. J. Cheadle, A. D. Saunders (2007). "Temperatures in ambient mantle and plumes: Constraints from basalts, picrites, and komatiites." Geochemistry, Geophysics, Geosystems 8: Q02006. doi:10.1029/2006GC001390

The temperature of ambient mantle and the existence of any possible thermal anomalies are fundamental properties of the Earth. Although they cannot be directly measured, they can be inferred from their effects on both seismic wave velocities and the compositions of magmas that are produced at oceanic ridges, islands, and plateaus; hot mantle slows down seismic waves and increases the MgO contents of primary magmas. MORB production from ambient mantle is critical in this discussion because it provides the background reference frame for understanding all thermal anomalies. However, the extraction of reliable temperature information requires that a petrological analysis be conducted on a large number of glass samples. This would not have been possible without PetDB.

"Results of primary magma compositions we obtain for MORB and various oceanic islands and plateaus generally favor ... ambient mantle potential temperatures in the range 1280-1400ºC and thermal anomalies that can be 200-300ºC above this background. Our results are consistent with the plume model."