Title:
 Oxygen isotope geochemistry of the Lassen volcanic center, California;resolving crustal and mantle contributions to continental arcmagmatism 
Reference Number:
 7582 
ISSN:
 0022-3530
Publication Year:
 2008
Publisher:
 Oxford University Press, Oxford, United Kingdom (GBR)
GeoREF Number:
 2008-130075 (View Original Data File)
Authors:
 Feeley, T C; Clynne, M A; Winer, G S; Grice, W C
Descriptors:
 andesitic composition; basaltic composition; California; continentalcrust; crust; geochemistry; igneous rocks; isotope ratios; isotopes;Lassen Volcanic Center; Lassen Volcanic National Park;lithogeochemistry; magmatism; mantle; O-18/O-16; oxygen; rhyoliticcomposition; stable isotopes; United States; volcanic rocks
Source:
 Journal of Petrology, vol. 49, no. 5, pp.971-997, May 2008
Abstract:
 This study reports oxygen isotope ratios determined by laserfluorination of mineral separates (mainly plagioclase) from basalticandesitic to rhyolitic composition volcanic rocks erupted from theLassen Volcanic Center (LVC), northern California. Plagioclaseseparates from nearly all rocks have delta (super 18) O values(6.1-8.4ppm) higher than expected for production of the magmas bypartial melting of little evolved basaltic lavas erupted in the arcfront and back-arc regions of the southernmost Cascades during thelate Cenozoic. Most LVC magmas must therefore contain high (super 18)O crustal material. In this regard, the delta (super 18) O values ofthe volcanic rocks show strong spatial patterns, particularly foryoung rhyodacitic rocks that best represent unmodified partial meltsof the continental crust. Rhyodacitic magmas erupted from ventslocated within 3.5 km of the inferred center of the LVC haveconsistently lower delta (super 18) O values (average 6.3ppm+ or-0.1ppm) at given SiO (sub 2) contents relative to rocks erupted fromdistal vents (>7.0 km; average 7.1ppm+ or -0.1ppm). Further, magmaserupted from vents situated at transitional distances haveintermediate values and span a larger range (average 6.8ppm+ or-0.2ppm). Basaltic andesitic to andesitic composition rocks showsimilar spatial variations, although as a group the delta (super 18) Ovalues of these rocks are more variable and extend to higher valuesthan the rhyodacitic rocks. These features are interpreted to reflectassimilation of heterogeneous lower continental crust by mafic magmas,followed by mixing or mingling with silicic magmas formed by partialmelting of initially high (super 18) O continental crust (approximately 9.0ppm) increasingly hybridized by lower delta (super18) O ( approximately 6.0ppm) mantle-derived basaltic magmas towardthe center of the system. Mixing calculations using estimatedendmember source delta (super 18) O values imply that LVC magmascontain on a molar oxygen basis approximately 42 to 4% isotopicallyheavy continental crust, with proportions declining in a broadlyregular fashion toward the center of the LVC. Conversely, the delta(super 18) O values of the rhyodacitic rocks suggest that thecontinental crust in the melt generation zones beneath the LVC hasbeen substantially modified by intrusion of mantle-derived basalticmagmas, with the degree of hybridization ranging on a molar oxygenbasis from approximately 60% at distances up to 12 km from the centerof the system to 97% directly beneath the focus region. These resultsdemonstrate on a relatively small scale the strong influence thatintrusion of mantle-derived mafic magmas can have on modifying thecomposition of pre-existing continental crust in regions of meltproduction. Given this result, similar, but larger-scale, regionaltrends in magma compositions may reflect an analogous but moreextensive process wherein the continental crust becomes progressivelyhybridized beneath frontal arc localities as a result of protractedintrusion of subduction-related basaltic magmas.
Copyright:
 GeoRef, Copyright 2008, American Geological Institute.

Sample Summary:
Sample ID: Major Data: Trace Data: Isotope Data:
LF01-002 Yes No No
LF01-003 Yes No No
LF01-004 Yes No Yes
LF01-005 Yes No No
LF01-006 Yes No No
LF01-007 Yes No No
LF01-008 Yes No No
LF01-009 Yes No No
LF01-010 Yes No No
LF01-048 Yes No No
LF01-049 Yes No No
LF01-050 Yes No No
LF01-051 Yes No No
LF01-052 Yes No No
LF01-024 Yes No No
LF01-017 Yes No No
LF01-018 Yes No No
LF01-019 Yes No No
LF01-020 Yes No No
LF01-021 Yes No No
LF01-013 Yes No No
LF01-012-1-A No No No
LF01-012-II-A Yes No No
LF01-012-II-B Yes No No
LF01-014 Yes No No
LM80-824 Yes No Yes
LM80-854 Yes No Yes
LC80-178 Yes No Yes
LC80-436B Yes No Yes
LC84-600 Yes No Yes
LF01-001 Yes No No
LF01-011 Yes No Yes
LF01-015 Yes No Yes
LF01-031 Yes No No
LF01-016 Yes No No
LF01-022 Yes No Yes
LF01-023 Yes No No
LF02-062 Yes No Yes
LF02-063 Yes No No
LF02-021 Yes No Yes
LF02-029 Yes No Yes
LF02-028 No No No
LF01-039 Yes No Yes
LF01-040iA No No No
LF01-040iA Yes No No
LF01-040iB Yes No No
LF01-041 Yes No Yes
LF01-042 Yes No No
LF01-037 Yes No Yes
LF01-038iA Yes No No
LF01-043 Yes No Yes
LF01-044iA No No No
LF01-044iA Yes No No
LF01-044iB Yes No No
LF01-045 Yes No Yes
LF02-043 Yes No No
LF02-043iA Yes No No
LF02-043iB Yes No No
LF02-049 Yes No No
LF02-050 Yes No No
LF02-050iA No No No
LF02-050iA Yes No No
LF02-051 Yes No No
LF02-052 Yes No No
LF02-053 Yes No No
LF02-053iA Yes No No
LF02-053iB No No No
LF02-053iB No No No
LF02-040 Yes No No
LF02-042 Yes No No
LF02-042iC Yes No No
LF02-064 Yes No No
LF02-060 Yes No No
LC84-454 Yes No Yes
LF02-061L Yes No No
LF02-061P Yes No No
LF02-055 Yes No No
LF02-054 Yes No No
LF02-047 Yes No Yes
LC81-706 Yes No Yes
LF02-056 Yes No No
LF02-066 Yes No No
LF02-067 Yes No No
LF02-065 Yes No No
LF02-045 Yes No Yes
LF02-046 Yes No Yes
LF02-026 Yes No No
LF02-024 Yes No No
LF02-057 Yes No No
LF02-058 Yes No No
LF01-026 Yes No Yes
LF01-027 Yes No No
LF01-028B No No No
LF01-028B m Yes No No
LF01-028C No No No
LF01-028C m Yes No No
LF01-028D No No No
LF01-028D m Yes No No
LF01-029 Yes No No
LF01-030A No No No
LF01-030A m Yes No No
LF01-030B Yes No No
LF01-030B m Yes No No
LF01-032 Yes No No
LF01-034 Yes No Yes
LF02-048A Yes No Yes
LF02-048B Yes No Yes
LF02-003 Yes No Yes
LF02-059 Yes No Yes
LF03-101Q No No No
CCPX-1 No No No
LF02-068 Yes No No