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Crustal recycling beneath continental arcs: silica-rich glass inclusions in ultramafic xenoliths from the Sierra Nevada, California

Ducea, Mihai and Saleeby, Jason (1998) Crustal recycling beneath continental arcs: silica-rich glass inclusions in ultramafic xenoliths from the Sierra Nevada, California. Earth and Planetary Science Letters, 156 (1-2). pp. 101-116. ISSN 0012-821X. doi:10.1016/S0012-821X(98)00021-1.

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We describe silica-rich (up to ∼69.5% SiO_2) glass inclusions trapped as grain boundary films and within-grain pockets in ultramafic xenoliths hosted by Pliocene basalts from the Sierra Nevada, California. The ultramafic xenoliths are lherzolites which equilibrated in the Sierra Nevada upper mantle at 1150–1180°C and ∼1.4-1.8 GPa. The glass inclusions have trachytic compositions, similar to previously described silicic melts from mantle xenoliths [1–9]. We have determined the Sr and Nd isotope compositions of the grain boundary films using a leaching technique, and calculated the glass isotopic compositions. The glass^(87)Sr^(86)Sr (0.7077-0.7085) and^(143)Nd^(144)Nd (≈0.51244) ratios are higher than in the ultramafic xenoliths and distinct from the host basalt ratios. Glasses are characterized by Nb depletions (N/b_nNb_n^*∼0.15), enrichment of light rare earth elements (L/a_nYb_n≈ 50), and the presence of negative Eu anomalies (E/u_nEu_n^*≈ 0.7-0.86), indicating a crustal origin for the melt source. The Nd isotope ratios (ɛ_(Nd)≈ −4) are inconsistent with an oceanic crust as the source for these former melts. The source rocks must have been continental materials recycled in the mantle, either foundered lower crust or subducted sediment. LowRb/Sr (0.036-0.077) and high Sr/Nd (>35) ratios observed in the glasse are suggesting a lower crustal source. The Sierra Nevada lowermost crust (amphibole-bearing garnet pyroxenites and other dense Mesozoic cumulate mafic-ultramafic rocks), as defined by studies of older, Miocene xenolith-bearing volcanic rocks from the same area [M.N. Ducea, J. Saleeby, J. Geophys. Res. 101 (1996) 8229–8244], has isotopic compositions similar to the glass inclusions. Geologic [M.N. Ducea, J. Saleeby, J. Geophys. Res. 101 (1996) 8229–8244] and geophysical [G. Zandt, S. Ruppert, EOS Trans. AGU 77 (1996) 831] evidence indicate that the Sierra Nevada has lost its eclogitic arc root, probably by foundering in the mantle. We propose here that the silica-rich glasses were formed by low percent partial melting of the dense, cold Sierran batholithic lowermost crust during root delamination. Further tests need to be aimed at addressing the viability of the main alternative to our interpretation, i.e. derivation of glasses from melting subducted sediments.

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Ducea, Mihai0000-0002-5322-0782
Additional Information:© 1998 Elsevier Science B.V. Received 19 March 1997: revised version received 2 December 1997; accepted 31 December 1997. Available online 11 June 1998. This research was supported by National Science Foundation Grant EAR-95268549. M.D. acknowledges a research grant from the Geological Society of America (Grant 5810-96). This is California Institute of Technology Division of Geological and Planetary Sciences contribution 5915. We thank Tom Sisson for kindly providing a geologic description and a location map of the Scepter Creek xenolith-bearing lava outcrops, We are grateful for assistance from Mahmood Chaudry in glass separation. We acknowledge discussions with P.J. Wyllie, M. Baker, and B. Wernicke. An earlier version of this manuscript was reviewed by J. Spotila, J. Goreva, R. Brady, and S. Tulaczyk. Journal reviews by P. Schiano, B. Cousens and R. Rudnick have further improved the manuscript with their constructive criticism. [CL]
Group:UNSPECIFIED, Division of Geological and Planetary Sciences
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Geological Society of America5810-96
Subject Keywords:trachytes; glasses; inclusions; ultramafics; xenoliths; crust; delamination
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Caltech Division of Geological and Planetary Sciences5915
Issue or Number:1-2
Record Number:CaltechAUTHORS:20140827-132201270
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Official Citation:Mihai Ducea, Jason Saleeby, Crustal recycling beneath continental arcs: silica-rich glass inclusions in ultramafic xenoliths from the Sierra Nevada, California, Earth and Planetary Science Letters, Volume 156, Issues 1–2, 15 March 1998, Pages 101-116, ISSN 0012-821X, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:48995
Deposited By: Ruth Sustaita
Deposited On:27 Aug 2014 20:47
Last Modified:10 Nov 2021 18:38

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