Across-arc geochemical variations in the Southern Volcanic Zone, Chile (34.5- 38.0°S): Constraints on Mantle Wedge and Input Compositions

Crustal assimilation (e.g. Hildreth and Moorbath, 1988) and/or subduction erosion (e.g. Stern, 1991; Kay et al., 2005) are believed to control the geochemical variations along the northern portion of the Chilean Southern Volcanic Zone. In order to evaluate these hypotheses, we present a comprehensive geochemical data set (major and trace elements and O-Sr-Nd-Hf-Pb isotopes) from Holocene primarily olivine-bearing volcanic rocks across the arc between 34.5-38.0°S, including volcanic front centers from Tinguiririca to Callaqui, the rear arc centers of Infernillo Volcanic Field, Laguna del Maule and Copahue, and extending 300 km into the backarc. We also present an equivalent data set for Chile Trench sediments outboard of this profile. The volcanic arc (including volcanic front and rear arc) samples primarily range from basalt to andesite/trachyandesite, whereas the backarc rocks are low-silica alkali basalts and trachybasalts. All samples show some characteristic subduction zone trace element enrichments and depletions, but the backarc samples show the least. Backarc basalts have higher Ce/Pb, Nb/U, Nb/Zr, and Ta/Hf, and lower Ba/Nb and Ba/La, consistent with less of a slab-derived component in the backarc and, consequently, lower degrees of mantle melting. The mantle-like δ18O in olivine and plagioclase phenocrysts (volcanic arc = 4.9-5.6 and backarc = 5.0-5.4 per mil) and lack of correlation between δ18O and indices of differentiation and other isotope ratios, argue against significant crustal assimilation. Volcanic arc and backarc samples almost completely overlap in Sr and Nd isotopic composition. High precision (double-spike) Pb isotope ratios are tightly correlated, precluding significant assimilation of older sialic crust but indicating mixing between a South Atlantic Mid Ocean-Ridge Basalt (MORB) source and a slab component derived from subducted sediments and altered oceanic crust. Hf-Nd isotope ratios define separate linear arrays for the volcanic arc and backarc, neither of which trend toward subducting sediment, possibly reflecting a primarily asthenospheric mantle array for the volcanic arc and involvement of enriched Proterozoic lithospheric mantle in the backarc. We propose a quantitative mixing model between a mixed-source, slab-derived melt and a heterogeneous mantle beneath the volcanic arc. The model is consistent with local geodynamic parameters, assuming water-saturated conditions within the slab

The influence of the IKEA effect on a value of good

In 4 experiments in which students assembled IKEA boxes, built LEGO-like construction sets and folded origami I demonstrate and examine the IKEA effect, which is defined as consumers' willingness to pay more for self-created products than for identical products made by others. I show that their increased valuation is led by the feelings of competence associated with self-created products and that affirming consumers sense of self decreases the value one derives from his creations. I found that students value more their labor only when labor results in successful completion of task regardless of whether they are "do-it-yourselfers" or not. I did not observe the IKEA effect in particular case of origami

Origin of the Tongbai-Dabie-Sulu Neoproterozoic low-δ18O igneous province, east-central China

Zircons from 71 diverse rocks from the Qinling-Tongbai-Dabie-Sulu orogenic belt in east-central China and, for comparison, eight from adjoining areas in the South China and North China blocks, have been analyzed for in situ 18O/16O ratio and/or U–Pb age to further constrain the spatial distribution and genesis of Neoproterozoic low-δ 18O magmas, that is, δ 18O(zircon) ≤4 ‰ VSMOW. In many metaigneous rock samples from Tongbai-Dabie-Sulu, including high-pressure and ultrahigh-pressure eclogites and associated granitic orthogneisses, average δ 18O values for Neoproterozoic “igneous” zircon cores (i.e., 800–600 Ma) vary from −0.9 to 6.9 ‰, and from −9.9 to 6.8 ‰ for Triassic metamorphic rims (i.e., 245–200 Ma). The former extend to values lower than zircons in primitive magmas from the Earth’s mantle (ca. 5–6 ‰). The average Δ 18O (metamorphic zircon − “igneous” zircon) values vary from −11.6 to 0.9 ‰.The large volume of Neoproterozoic low-δ 18O igneous protoliths at Tongbai-Dabie-Sulu is matched only by the felsic volcanic rocks of the Snake River Plain hotspot track, which terminates at the Yellowstone Plateau. Hence, the low-δ 18O values at Tongbai-Dabie-Sulu are proposed to result from shallow subcaldera processes by comparison with Yellowstone, where repeated caldera-forming magmatism and hydrothermal alteration created similar low-δ 18O magmas. However, the possibility of involvement of meltwaters from local continental glaciations, rather than global Neoproterozoic glaciations, cannot be precluded. Our data indicate that Neoproterozoic low-δ 18O magmas that are either subduction- or rift-related are present locally along the western margin of the South China Block (e.g., Baoxing Complex). It appears that Neoproterozoic 18O-depletion events in the South China Block as the result of hydrothermal alteration and magmatism affected a much larger area than was previously recognized