Geochemical Fingerprinting of the Earth's Oldest Rocks

摘要

In this issue of Geology, Turner et al. (2014, p. 139) take a new look at the geochemistry of amphibolite-facies lavas from Nuvvuagittuq (O'Neil et al., 2011), possibly the Earth's oldest volcanic rocks (O'Neil et al., 2012). Geochemical fi ngerprinting had already been used to identify a possible subduction signature in some of these lavas (O'Neil et al. 2011), but Turner et al. additionally emphasize the similarity between the Nuvvuagittuq geochemical stratigraphy and that of the Izu-Bonin-Mariana (IBM) forearc (Ishizuka et al. 2011), the type area for intra-oceanic subduction initiation (Stern and Bloomer, 1992). The sinking of the slab following subduction initiation in the IBM forearc led to influx of mantle asthenosphere beneath an extending lithosphere, resulting in the eruption of forearc basalts (FAB) (Reagan et al., 2010), essentially mid-oceanicridge basalt (MORB) with a small but variable subduction component. In the IBM forearc, these are overlain by boninite-like rocks, marking the transition from a slab sinking vertically to a one subducting with a horizontal component, and then by island arc tholeiites or calc-alkaline rocks, marking the start of normal subduction (Ishizuka et al., 2011). Turner et al. interpret the lower (high-Ti) unit from Nuvvuagittuq as FAB, the middle (low-Ti depleted) unit as boninite and the upper (low-Ti enriched) unit as island arc, calc-alkaline lavas, They thus potentially strengthen the hypothesis for a subduction origin of the Nuvvuagittuq lava sequence by interpreting the sequence as a whole.