Crustal recycling beneath continental arcs

Silica-rich glass inclusions in ultramafic xenoliths from the Sierra Nevada, California

Mihai N Ducea, Jason Saleeby

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69 Citations (Scopus)

Abstract

We describe silica-rich (up to ∼69.5% SiO2) 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 87Sr/86Sr (0.7077-0.7085) and 143Nd/144Nd (≈0.51244) ratios are higher than in the ultramafic xenoliths and distinct from the host basalt ratios. Glasses are characterized by Nb depletions (Nbn/Nb5n ∼0.15), enrichment of light rare earth elements (Lan/Ybn ≈ 50), and the presence of negative Eu anomalies (Eun/Eu*n ≈0.7-0.86), indicating a crustal origin for the melt source. The Nd isotope ratios (εNd ≈ -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. Low Rb/Sr (0.036-0.077) and high Sr/Nd (> 35) ratios observed in the glasses 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. Saleehy, 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 (1990) 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.

Original languageEnglish (US)
Pages (from-to)101-116
Number of pages16
JournalEarth and Planetary Science Letters
Volume156
Issue number1-2
StatePublished - Mar 15 1998
Externally publishedYes

Fingerprint

crustal recycling
continental arc
silica glass
recycling
Silicon Dioxide
Recycling
arcs
silica
glass
inclusions
Glass
crusts
Earth mantle
Bearings (structural)
rocks
melt
basalt
grain boundary
mantle
Chemical analysis

Keywords

  • Crust
  • Delamination
  • Glasses
  • Inclusions
  • Trachytes
  • Ultramafics
  • Xenoliths

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics

Cite this

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title = "Crustal recycling beneath continental arcs: Silica-rich glass inclusions in ultramafic xenoliths from the Sierra Nevada, California",
abstract = "We describe silica-rich (up to ∼69.5{\%} SiO2) 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 87Sr/86Sr (0.7077-0.7085) and 143Nd/144Nd (≈0.51244) ratios are higher than in the ultramafic xenoliths and distinct from the host basalt ratios. Glasses are characterized by Nb depletions (Nbn/Nb5n ∼0.15), enrichment of light rare earth elements (Lan/Ybn ≈ 50), and the presence of negative Eu anomalies (Eun/Eu*n ≈0.7-0.86), indicating a crustal origin for the melt source. The Nd isotope ratios (εNd ≈ -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. Low Rb/Sr (0.036-0.077) and high Sr/Nd (> 35) ratios observed in the glasses 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. Saleehy, 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 (1990) 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.",
keywords = "Crust, Delamination, Glasses, Inclusions, Trachytes, Ultramafics, Xenoliths",
author = "Ducea, {Mihai N} and Jason Saleeby",
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T2 - Silica-rich glass inclusions in ultramafic xenoliths from the Sierra Nevada, California

AU - Ducea, Mihai N

AU - Saleeby, Jason

PY - 1998/3/15

Y1 - 1998/3/15

N2 - We describe silica-rich (up to ∼69.5% SiO2) 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 87Sr/86Sr (0.7077-0.7085) and 143Nd/144Nd (≈0.51244) ratios are higher than in the ultramafic xenoliths and distinct from the host basalt ratios. Glasses are characterized by Nb depletions (Nbn/Nb5n ∼0.15), enrichment of light rare earth elements (Lan/Ybn ≈ 50), and the presence of negative Eu anomalies (Eun/Eu*n ≈0.7-0.86), indicating a crustal origin for the melt source. The Nd isotope ratios (εNd ≈ -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. Low Rb/Sr (0.036-0.077) and high Sr/Nd (> 35) ratios observed in the glasses 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. Saleehy, 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 (1990) 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.

AB - We describe silica-rich (up to ∼69.5% SiO2) 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 87Sr/86Sr (0.7077-0.7085) and 143Nd/144Nd (≈0.51244) ratios are higher than in the ultramafic xenoliths and distinct from the host basalt ratios. Glasses are characterized by Nb depletions (Nbn/Nb5n ∼0.15), enrichment of light rare earth elements (Lan/Ybn ≈ 50), and the presence of negative Eu anomalies (Eun/Eu*n ≈0.7-0.86), indicating a crustal origin for the melt source. The Nd isotope ratios (εNd ≈ -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. Low Rb/Sr (0.036-0.077) and high Sr/Nd (> 35) ratios observed in the glasses 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. Saleehy, 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 (1990) 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.

KW - Crust

KW - Delamination

KW - Glasses

KW - Inclusions

KW - Trachytes

KW - Ultramafics

KW - Xenoliths

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