Featured Article
Featured Article: Salters et al., Domains of depleted mantle: New evidence from hafnium and neodymium isotopes
Geochem. Geophys. Geosyst., v. 12, Q08001, 18 PP., 2011
Featured Article: Yamagishi et al., 2011, Visualization of geochemical data for rocks and sediments in Google Earth: Development of a data converter application for geochemical and isotopic data sets in database systems
Geochem. Geophys. Geosyst. v. 12, Q03016. doi: dx.doi.org/10.1029/2010GC003490
Featured Article: Waters, C.L., et al., 2011, Perspective on the genesis of E-MORB from chemical and isotopic heterogeneity at 9-10ºN East Pacific Rise.
Journal of Petrology, v. 52, p. 565-602. doi: 10.1093/petrology/egq091
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“The study of chemically and isotopically enriched mid-ocean ridge basalts (E-MORB) has offered substantial insight into the origin of mantle heterogeneity, but the exact processes involved in producing this E-MORB enrichment are vigorously debated. Additionally, because the ages of E-MORB are not well constrained, the temporal petrogenetic and geological relationships between E-MORB and normal (N)-MORB are not known. To investigate these relationships, we measured major and trace elements, and Sr, Nd, Hf, Pb, and U-Th-Ra isotopes for a suite of E-MORB and N-MORB lavas that were collected off-axis from 9-10ºN along the East Pacific Rise (EPR).
These data show coherent mixing trends, implying that mixing of melts from different sources occurs at different depths. Forward modeling of the trace element data is consistent with mixing of deep, low-degree, enriched pyroxenite melts with shallow, high-degree, depleted peridotite melts. Previous studies of E-MORB at 9-10ºN have suggested that E-MORB compositions could be explained by off-axis magma transport and eruption. However, our U-series model age constraints, determined using U-series data available at PetDB for zero-age MORB from 9-10ºN EPR, indicate these E-MORB are old and were not erupted in situ. The asymmetric, off-axis distribution of E-MORB revealed by PetDB and ancillary data compiled for 9-10ºN EPR is more simply explained by a model in which E-MORB erupted within the axial summit trough (AST), flowed down the ridge flanks (~0-3 km), spread away from the AST, and were preserved on the seafloor through asymmetric construction of the extrusive layer. Taken together, the range of E-MORB ages and the geochemical mixing trends suggest that enriched melts are continuously supplied to the ridge axis, but because of their small proportions relative to the volumetrically and volcanically dominant N-MORB, E-MORB preservation and exposure is comparatively scarce.” –C.L. Waters
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Davies, G. F. (2009), Reconciling the geophysical and geochemical mantles: Plume flows, heterogeneities, and disequilibrium, Geochem. Geophys. Geosyst., 10, Q10008
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.
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
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.
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.
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
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.
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.
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
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.
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.
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.
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
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.