Uppermost Asthenosphere Research Articles

210. Low temperature techniques and aeronautics: W Baldus, Rep DGRR/WGLR, Oct 1966, 7 pages (in German) (Sci Tech Aerospace Reps, 5 (15), 2755, N67-28702)

The southernmost Izu–Bonin arc and northernmost Mariana arc are characterized by K-rich and shoshonitic lavas, referred to as the alkalic volcano province (AVP). These compositions are unusual for intra-oceanic arcs and the interpretation of the AVP is controversial. Rifting to form the Mariana Trough back-arc basin occurs just south of the AVP although back-arc seafloor spreading has not begun. Here we report the results of dredge sampling of the West Mariana Ridge (WMR) in the region of rift propagation; this recovered exclusively medium K to shoshonitic basalts that show clear arc-like geochemical signatures.Ar–Ar ages of WMR shoshonitics systematically young northward. Age of c. 6 Ma was obtained at 21.5°N, c. 3 Ma at 23–23.5°N, and zero-age shoshonites occur on Io-to Island (formerly Iwo Jima) at 24.8°N. Shoshonitic magmatism migrated northward at 4.3 cm/year, in advance of northward-propagating Mariana Trough rifting. This implies that AVP shoshonitic magmatism manifests processes and sources that are uniquely associated with earliest back-arc basin rifting.High-precision Pb isotopic analyses reveal that WMR lavas form a single trend between 2 components, one with lower 206Pb/204Pb and high Δ7/4 (arc-like), and another with high 206Pb/204Pb as well as low Δ 7/4 and 8/4 (HIMU-like). These components could correspond respectively to subducted pelagic sediment and subducted seamounts and volcaniclastics with HIMU isotopic signature. These slab-derived components alone, however, cannot fully explain chemical characteristics of WMR shoshonitic lavas. These lavas require a component with high Δ7/4 and high Ce/Pb, which is not likely to be either pelagic sediment or seamount volcanics. This component is only expressed when rifting begins, suggesting that it resides in enriched lithosphere or uppermost asthenosphere, which is easily melted due to decompression caused by rifting, when the lithosphere is first ruptured. This component might be linked to slow Vs anomalies in the mantle wedge beneath the AVP.