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Preliminary results on the oxygen isotopic composition of the lower crust, Kilbourne Hole Maar, New Mexico
Author(s) -
James David E.,
Padovani Elaine R.,
Hart Stanley R.
Publication year - 1980
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/gl007i005p00321
Subject(s) - xenolith , geochemistry , protolith , granulite , geology , partial melting , mafic , crust , isotopes of oxygen , mantle (geology) , continental crust , metamorphic rock , facies , paleontology , structural basin
18 O/ 16 O ratios measured in deep crustal xenoliths from Kilbourne Hole maar, New Mexico show that the oxygen isotopic composition of the lower crust of that region is grossly heterogeneous. Xenoliths derived from similar crustal depths vary in δ 18 O by nearly 6‰, although the xenoliths are individually in internal oxygen isotopic equilibrium. Mafic granulites (including charnockite), probably of mantle origin, have δ 18 O of about 6.2 to 6.3‰ and correspondingly low 87 Sr/ 86 Sr ratios (0.703 to 0.707). Garnet granulites, apparently the residue of partial fusion of pelitic sedimentary rock, have δ 18 O ranging from about 9‰ to 12‰ and correspondingly high 87 Sr/ 86 Sr ratios (0.71 to 0.78). No evidence is found for extensive oxygen exchange over large distances in the anhydrous lower crust beneath Kilbourne Hole. Absence of regional isotopic equilibrium indicates that even at melting or near melting temperatures the oxygen isotopic identity of the protolith can be preserved over long periods of time. We conclude that there is little reason to postulate that massive oxygen exchange with a mantle‐like reservoir is a general phenomenon of the lower crust.