Transition From Island Arc to Active Continental Margin: Evidence From The Neoproterozoic Fawakhir Ophiolite, Egypt
The Fawakhir ophiolite occurs in the central Eastern Desert of Egypt forming a suture zone separating the juvenile arc-related rocks from the Saharan Metacraton to the West. The ophiolite is composed of ultramafics, isotropic gabbros, dolerites and pillow lavas. The ophiolitic crustal rocks show a distinctive enrichment in large ion lithophile elements and depletion in high field strength elements (La/Smcn=0.40-1.22, Gd/Ybcn=0.85-1.23, Th/Nbpm=1.7-10.9, La/Nbpm=1.4-6.6). These geochemical characteristics indicate the derivation of magma from a depleted (N-MORB) mantle source that had been metasomatized by subduction-derived fluids. The geochemistry of the Fawakhir ophiolite is comparable to that of the Izu-Bonin-Mariana forearc oceanic crust. The mélange to the east of the Fawakhir ophiolite includes pillow lava blocks with a back arc-like geochemical signature. The spatial distribution of the Fawakhir ophiolitic rocks and the back arc mélange rocks suggests the formation of the oceanic arc system above an east- dipping subduction zone. The Fawakhir ophiolite is intruded by calc-alkaline dikes with geochemical characteristics similar to active continental margin rocks (La/Smcn=2.13-2.48, Gd/Ybcn=2.04-4.25, Th/Nbpm=3.2-5.8, La/Nbpm=2.5-4.9). The formation of the active continental margin followed the accretion/obduction of the arc-related rocks into the Sahara Metacraton, West Gondwana margin. The transformation of the West Gondwana margin from a passive continental margin to an active continental margin after the accretion process was associated with the subduction polarity reversal. The new west-dipping subduction zone might have caused the widespread of the calc-alkaline magmatism recorded in the Eastern Desert of Egypt.
Evaluation of Mantle Upwelling Beneath Iceland from U-series Disequilibria: Implication for Icelandic Mantle Plume Dynamics
The existence of mantle plumes has recently been questioned on the basis of geophysical, petrological and geochemical arguments (e.g., Meibom, et al., 2003), although there is a long-standing concession that hotspot volcanoes such as Hawaii or Iceland represent the surface expression of mantle plumes which is hot, buoyant upwelling regions beneath the Earth's lithosphere. In addition to that, the physical structures of plumes are still a subject of questions. For instance, there are models which advocate a relatively cool and broad plume beneath Iceland (Ribe, et al, 1995; Ito, et al., 1996), while others proposed hot and narrow plume beneath central Iceland (Mckenzie, 1984; White, et al., 1995; Wolf et al., 1997). Recently, it has been shown that U- series disequilibria in young hotspot lavas provide a relative measure of mantle upwelling velocity beneath ocean islands(e.g. Sims, et al., 1999; Kokfelt, et al., 2003; Bourdon, et al., 2005). U and Th fractionation produced during melting is a function of the melting rate. In turn, this parameter should scale with mantle upwelling velocities. Simply stated, a larger melting rate (larger mantle upwelling velocity) yields smaller Th excess relative to the parent nuclide. Using our new data together with previous works we modeled U-series disequilibria measured in recent lavas of Iceland volcanoes. For a reasonable range of mantle porosities (0.01-0.5 %) in a dynamic melting model the upwelling rates show sharp radial increase from 3 to 12 cm per year towards the presumed center of Iceland plume, but after about 135 km from the center of the plume the upwelling rates remain constant (2-3 cm per year). We suggest that the incremental upwelling rates towards the center can be associated with hot and buoyant upwelling that characterized by higher excess temperature than the surrounding mantle. Our observation is consistent with Wolf et al. (1997) seismic study that shows about 150 km radius of the low velocity anomaly beneath central Iceland, and it gives a strong supporting evidence for models of hot and narrow plume beneath central Iceland. Thus, geochemical data (U series disequilibrium) provide important complementary information that is not available from geophysical and petrological data alone in constraining the physical structure of mantle plumes.
Oscillatory Zoning in Plagioclase: Effect of P, T And Partial Molal Volumes of Na2O And CaO In The Melt
Melt inclusions in Fe oxide and phosphate tephra of El Laco volcano, Chile
The El Laco volcano of northern Chile is noted for its controversial iron ore deposits, which some researchers regard as examples of iron oxide lava flows but others regard as epithermal deposits replacing pre-existing silicate lava flows. Lava flows with textures and structures typical of ordinary silicate lavas are composed entirely of magnetite. Unconsolidated Fe oxide block and ash deposits show fine air-fall stratification and bomb sags. It has always been unclear how Fe-oxide melts could have formed by liquid immiscibility from intermediate silicate magmas. Some kind of fluxing component would seem to be required. We have examined blocks and ash from El Laco in polished grain mounts prepared without water to prevent loss of water-soluble minerals. The ash component of the tephra is composed of black magnetite and hematite crystals and polycrystalline aggregates ranging in size from several micrometres to several millimetres, commonly coated by soft dark olive green material with local orange and yellow crusts. Rare large euhedra of hematite up to one or two cm in size are also observed. Ash particle morphologies include euhedral Fe oxide crystals and crystal fragments as well as irregularly shaped polycrystalline Fe oxide aggregates. Most oxide crystals and aggregates display fenestral textures and locally highly convoluted margins. Scanning electron microscopy (SEM) suplemented by standardless semiquantitative energy dispersive X-ray spectrometry (EDS) of the grain mounts show that the re-entrant cavities are partially or completely filled by aggregates of Fe phosphate (probably beraunite or dufrenite), Fe-oxide, silica, and monazite. Some ash particles are wholly or partially composed of fine-grained aggregates of the same phosphate-rich material. Ovoid cavities within some of the oxide ash particles are wholly or partially occupied by the same polymineralic assemblage as the external cavities but also include domains interpreted to be finely crystallized melt as well as large voids interpreted to be trapped gas bubbles. The domains interpreted as quenched melt consist of submicron particles too small to identify individually. Analysis by EDS shows that these domains contain, in approximately descending order of abundance, Fe, P, Si, S, K, and Cl, with subsidiary and variable contents of Ti, Al, Ca, Mg, and light rare earths. Sulfur is assumed to be present exclusively as the sulfate ion, in light of the very high oxygen fugacity implied by the ubiquitous coexistence of magnetite and hematite. We interpret these results to indicate that the magma from which the magnetite and hematite tephra crystallized was an iron phosphate melt with important amounts of SiO2 and K2O and several weight % sulfate, as well as chloride and other dissolved volatiles represented by the voids in the melt inclusions. Fe phosphate melts have received much attention in recent years as matrices for nuclear waste storage due to their low solidus temperatures (as low as 800 C), the ease with which they can be quenched to glass, and their tremendous capacity to dissolve other oxide components (including Fe2O3 and H2O) while remaining liquid to low temperatures. We suggest here that the lavas and tephra were in equilibrium with a volatile-rich melt with a composition similar to that of the melt inclusions trapped in the tephra. Eruptive volatile loss left the assemblage magnetite-hematite-Fe phosphate-silica. There is very little phosphorus left in the massive lava flows, despite its abundance in the tephra deposits, requiring that some process during slow cooling of the lavas must have efficiently removed phosphorus.
Daly Lecture: How to Build a Habitable World (or, The Hospitable Hadean)
What are the ingredients for making an Earth-like planet? What are the interactions among these processes that lead to habitability? If the emergence of life requires water, energy and organic building blocks, when was Earth fit for habitation? These questions are addressed with reference to insights emerging from Hadean zircons - bits of terrestrial crust as old as ∼4.4 Ga. We know that these ancient zircons crystallized at remarkably low temperatures and that some are enriched in heavy oxygen, contain inclusion assemblages consistent with modern crustal processes, and show Hf isotope evidence of silicate differentiation by 4.51 Ga. These geochemical records are interpreted to reflect the existence of an early terrestrial hydrosphere, early felsic crust in which granitoids were produced and later weathered under high water activity conditions, and even the possible existence of Hadean plate boundary interactions - all this in profound contrast to the traditional view of an uninhabitable, hellish world. As there is near universal agreement that Earth-like life could not have emerged until liquid water was present at or near the Earth's surface, a significant implication is that our planet may have been habitable as much as 600 million years earlier than previously thought. Possible scenarios are explored with a view to reconciling our growing but fragmentary Hadean record with our knowledge of conditions then extant in the inner solar system.