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Direct evidence for the origin of low-18O silicic magmas: quenched samples of a magma chamber's partially-fused granitoid walls, Crater Lake, Oregon

Earth and Planetary Science Letters

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Abstract

Partially fused granitoid blocks were ejected in the climactic eruption of Mount Mazama, which was accompanied by collapse of Crater Lake caldera. Quartz, plagioclase, and glass in the granitoids have much lower ??18O values (-3.4 to +4.9???) than any fresh lavas of Mount Mazama and the surrounding region (+5.8 to +7.0???). Oxygen isotope fractionation between phases in granitoids is consistent with equilibrium at T ??? 900??C following subsolidus exchange with hydrothermal fluids of meteoric origin. Assimilation of ??? 10-20% of material similar to these granitoids can account for the O and Sr isotopic compositions of lavas and juvenile pyroclasts derived from the climactic magma chamber, many of which have ??18O values ??? 0.5??? or more lower than comparable lavas of Mount Mazama. The O isotope data provide the only clear evidence for such assimilation because the mineralogy and chemical and radiogenic isotopic compositions of the granitoids (dominantly granodiorite) are similar to those of erupted juvenile magmas. The granitoid blocks from Crater Lake serve as direct evidence for the origin of 18O depletion in large, shallow silicic magma bodies. ?? 1989.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Direct evidence for the origin of low-18O silicic magmas: quenched samples of a magma chamber's partially-fused granitoid walls, Crater Lake, Oregon
Series title:
Earth and Planetary Science Letters
Volume
96
Issue:
1-2
Year Published:
1989
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
First page:
199
Last page:
208
Number of Pages:
10