Thick, Cold and Dry Roots: the Key to Longevity of Continental Arc Lithosphere?

Abstract:

In contrast to the continuity of mid-ocean ridge magmatism, arc volcanism is episodic, characterized by flareups lasting 10 – 50 My which, for reasons that remain unclear, end abruptly in <10 My. Key to understanding the origins of episodic arc behavior lie in constraining the roles of subducting vs. overriding lithosphere. Here, we show that upper mantle xenoliths from the Sierra Nevada arc, CA, USA represent mantle wedge residues that were thickened and rapidly cooled at ~3 GPa and 750 C, presumably at the slab-mantle interface. Pervasive melt infiltration from wedge-derived basalts transformed the depleted residues into refertilized lherzolite. Olivine crystal-preferred orientations (CPO) are weak and show predominantly axial-(010) and one lherzolite with B-type CPO. Measured water contents by SIMS in olivine and pyroxene are low, 5 – 9 ppm and 30 – 500 ppm, respectively. Assuming olivine lost water during eruption, recalculated olivine water in equilibrium with pyroxene does not exceed 35 ppm, resulting in reconstructed bulk rock water content similar to the MORB source. Extrapolation of experimental olivine water solubility to the xenoliths’ final PT conditions ranges from 30 to 270 ppm, indicating that the peridotites are water-undersaturated. Such low water contents are not sufficient to produce axial-(010) and B-type CPO. Instead, we propose that the observed CPO was inherited from the prior melt infiltration event, which deformed the peridotites via grain-size sensitive, diffusion creep (e.g., grain boundary sliding). Therefore, water played little role in deformation of arc mantle. Low water contents in thick, cold arc roots result in very high viscosities which preclude significant deformation at final PT. In the Sierran case, rapid cooling also helped to freeze in geochemical and microstructural evidence of earlier melt-assisted deformation, and allowed the preservation of arc mantle lithosphere for ~80 My after it was formed. Only when the Farallon slab rolled back in the Late Cenozoic was the Sierran arc root finally destabilized. Thus, thickening, dehydration, and cooling of lithospheric roots (part of the upper plate) can modulate the tempo of arc magmatism by episodically growing into the melt-producing mantle wedge, thereby shutting off volcanism.