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Davies, G. F. (2009), Reconciling the geophysical and geochemical mantles: Plume flows, heterogeneities, and disequilibrium, Geochem. Geophys. Geosyst., 10, Q10008, doi:10.1029/2009GC002634.

"Geophysical evidence and numerical models of mantle stirring imply the source of mid-ocean ridge basalts (MORBs) comprises most of the mantle, excepting only the D″ region and the “superpile” anomalies deep under Africa and the Pacific. Geophysical evidence is also strong that the mantle is heated substantially from within. Geochemical inferences of a strongly depleted MORB source are inconsistent with this picture because they would require the MORB source to be heated mainly from below and because they cannot accommodate all of the Earth's incompatible elements. Lacking any other large mantle reservoir, the MORB source is required to balance the global uranium budget, which implies a U concentration of about 10 ng/g, more than double recent estimates. The MORB source would then have been depleted only by a factor of two in highly incompatible elements, rather than four or more, relative to is primitive composition. Both geophysical and geochemical evidence support a heterogeneous, multicomponent MORB source. Surprisingly, former plume material may comprise 25% of the MORB source, and this alone could add 50–100% to previous inventories of incompatible elements. Previous geochemical estimates may also be less secure because of a continuing focus on the more common, more depleted MORBs, because of long chains of geochemical inference, and because of a reliance on peridotites that may not have equilibrated with the mean composition of the heterogeneous source. Mean compositions are of most geophysical relevance, rather than putative end-member compositions, but mean compositions will be difficult to estimate accurately because more enriched components are less common and more variable. Nevertheless, a reconciliation of geochemical and geophysical inferences seems possible." - G. Davies

Previously Featured Articles

S. Escrig, A. Bezos, S. L. Goldstein, C. H. Langmuir, P. J. Michael (2009). "Mantle source variations beneath the Eastern Lau Spreading Center and the nature of subduction components in the Lau basin-Tonga arc system" G-CUBED 10, Q04014, doi:10.1029/2008GC002281

"New high-density sampling of the Eastern Lau Spreading Center provides constraints on the processes that affect the mantle wedge beneath a back-arc environment. PetDB MORB data provide a compositional range for typical mid-oceanic ridges and allow to distinguish the effect of the subduction input on back-arc basalt petrogenesis and the change in subduction input with distance from the Tonga arc. The trace element and Pb-Sr-Nd isotopic compositions of 64 samples distributed between 20.2°S and 22.3°S do not vary simply with distance from the arc and reflect variations in the mantle wedge composition and the presence of multiple components in the subduction input. The comparison with the East Pacific Rise and Central Indian Ridge basalts compiled in PetDB confirms that the mantle wedge composition varies form north to south, owing to the southward migration of Indian-like mantle, progressively replacing the initially Pacific-like mantle wedge. The mantle wedge compositions also require an enriched mid-ocean ridge basalt-like trace element enrichment that has little effect on isotope ratios, suggesting recent low-degree melt enrichment events. The composition of the subduction input added to the mantle wedge is geographically variable and mirrors the changes observed in the Tonga arc island lavas. The combination of the back-arc and arc data allows identification of several components contributing to the subduction input. These are a fluid derived from the altered oceanic crust with a possible sedimentary contribution, a pelagic sediment partial melt, and, in the southern Lau basin, a volcaniclastic sediment partial melt. While on a regional scale, there is a rough decrease in subduction influence with the distance from the arc, on smaller scales, the distribution of the subduction input reflects different mechanisms of the addition of the subduction input to a variable mantle wedge." - S. Escrig

J.P. Brandenburg, Erik H. Hauri, Peter E. van Keken, Chris J. Ballentine (2008). ""A multiple-system study of the geochemical evolution of the mantle with force-balanced plates and thermochemical effects." EARTH AND PLANETARY SCIENCE LETTERS 276: 1-13. doi:10.1016/j.epsl.2008.08.027

"Isotope ratios measured in oceanic basalts may provide important information on the fate of oceanic crust after subduction.  We couple a dynamic model of thermochemical convection in the mantle to a geochemical model for isotopic evolution.  Mechanically strong plates are included, extraction of oceanic crust is explicitly linked to divergent plate boundaries and secondary removal to a continental crust reservoir is linked to convergent plate boundaries.  The isotopic evolution of the U-Pb, Th-Pb, Nd-Sm, Rb-Sr, and Re-Os systems are simulated in this framework.  As in similar studies, changes in the extraction systematic of continental crust with time are necessary to reproduce the distribution of present day Pb isotopes.  However, the simultaneous and consistent use of these five isotope systems reveals that the part of the mantle array not involving subducted sediments is perhaps best explained as a mixture between DMM, HIMU and EM-1 like components.  In order to make a comparison with actual data rather than an interpretation, PetDB was an excellent source of information for the more depleted end of the spectrum." - J. P. Brandenburg

M. Carpentier, C. Chauvel, and N. Mattielli (2008). "Pb-Nd isotopic constraints on sedimentary input into the Lesser Antilles Arc system." EARTH AND PLANETARY SCIENCE LETTERS 272:199-211. doi:10.1016/j.epsl.2008.04.036

"Lesser Antilles lavas exhibit a large range of Pb-Nd isotopic compositions, which includes the most radiogenic Pb isotopic compositions known for intra-oceanic arcs. They are also characterized by a strong chemical zoning from north to south along the arc, with the highest Pb isotope ratios being found in the southern islands. We studied the geochemical characteristics of subducting sediments at different latitudes, in order to constrain a potential north-south chemical gradient in the sediments. We show that mixing of local depleted mantle with sediments subducting under the northern part of the arc could explain the northern island lava Pb-Nd isotopic compositions. In the south, subducting sediments display higher Pb isotopes, and mixing of local depleted mantle with these sediments satisfactory explains the very radiogenic Pb isotopic composition of lavas from the southern part of the Lesser Antilles arc. Using PetDB, we compiled data for Atlantic MORB to determine the Pb and Nd isotopic compositions of the local depleted mantle. When combined with our measurements on the subducting sediments, these data helped us to evaluate the sedimentary contribution in the Lesser Antilles magma genesis." - M. Carpentier

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 271(1-4): 311-325 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

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."