dakitic in composition. Our data indicate that adakites in SW Japan are not related to melting of an eclogitic subducting slab at unusually high temperatures, but instead to melt evolution in the lower overriding crust in the presence of residual garnet, the stability of which increases as water contents of parental melts decrease. Lower crustal melt evolution at reduced water contents likely represent a unifying process for generating adakitic signatures in all tectonic settings that have previously been considered to enhance slab melting (subduction of young slabs, shallow subduction, or subduction of slab edges or windows). The results presented here by Zellmer and colleagues demonstrate that magmatic water plays a key role in the differentiation of arc magmas in modern and ancient subduction settings.
Stromatolite lamination frequency, Walker Lake, Nevada: Implications for stromatolites as biosignatures
Victoria A. Petryshyn et al., Dept. of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA. Posted online ahead of print 10 April 2012; doi: 10.1130/G32675.1.
Lamination in stromatolites (considered some of the oldest fossils on Earth) is commonly interpreted to record the periodic response of a microbial community to daily, seasonal, or perhaps yearly environmental forcing. This study by Victoria A. Petryshyn and colleagues uses high-resolution carbon-14 dating of Holocene stromatolites from Walker Lake (Nevada, USA) to construct a record of lamination rate over the course of accretion. The stromatolite lamination rates are found to form on a variable, multi-year time scale that closely tracks lake level and local climate factors (e.g., El Nio Southern Oscillation), suggesting that climate and lake geochemistry had a larger influence on lamination formation than did microbial communities. While it is clear that some modern and ancient stromatolites are built in response to a microbial-paPage: 1 2 3 4 5 6 7 8 9 Related biology news :1
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