Faculty Candidate Seminar - Emily Chin

01/21/2016 - 12:00pm


DATE:  January 21, Thursday, 12:00 p.m.  

LOCATION: Hubbs Hall 4500 
SPEAKER:  Emily Chin, Ph.D.
Brown University 

TITLE:  The role of deep lithosphere in episodic arc magmatism: constraints from Sierra Nevada, CA peridotite xenoliths

In contrast to the continuity of mid-ocean ridge magmatism, arc volcanism is episodic. Key to understanding the origins of episodic arc behavior lie in constraining the roles of subducting vs. overriding lithosphere.  Here, we show that peridotite xenoliths from the Sierra Nevada, 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 a hydrous arc melt 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.  SIMS measurements of water in olivine and pyroxene are low, 5 – 9 ppm and 30 – 500 ppm, respectively.  Such low water contents are unusual given the Sierran peridotites’ subduction zone provenance. Even after correcting for eruptive water loss, maximum olivine water does not exceed 30 ppm, indicating that water did not promote the transition from A- to axial-[010] and B-type CPO.  Reconstructed bulk water contents are ~100 ppm (similar to the MORB source) and constrain the last solidus associated with melt infiltration.  Thus, despite their mantle wedge provenance, the Sierran peridotites are not particularly hydrous, nor did water play an important role in deformation.  We propose that the observed CPO formed as a result of melt infiltration and precipitation of secondary pyroxene, which promoted grain-size sensitive creep.  Low water contents in thick, cold arc roots result in very high viscosities which prevent significant deformation once the root is stabilized.  Only when the slab rolled back ~80 Myr later did the Sierran root eventually founder.  Arc episodicity may therefore be modulated by cycles of melt depletion of the wedge, formation and thickening of residues, melt infiltration and deformation, and impingement on the slab resulting in rapid termination of magmatism.

Faculty Host:  James Day (jmdday@ucsd.edu)
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Hubbs Hall 4500