ORIGIN OF THE CE ANOMALY IN A GREEN RIDGE LAVA FLOW, CASCADE RANGE, OREGON

Some basaltic andesite lava flows of the Deschutes formation on Green Ridge, east of Mt. Jefferson in north-central Oregon, possess a chondrite-normalized negative Ce anomaly. Green Ridge is a 30 km long fault block along the eastern margin of the High Cascades graben. The Deschutes Formation at Green Ridge consists primarily of 7.3-5.0 Ma ash-flow tuffs and lavas representing the eastern flanks of volcanic edifices to the west. The Ce-anomalous lavas are marked by relatively few, but compositionally distinct, large plagioclase phenocrysts ranging in size from 0.3 mm to 5 cm across. Several hand samples from one of the basaltic andesites were ground and plagioclase phenocrysts were separated with a Franz magnetic separator. During final preparation, clear plagioclase grains, free of groundmass, were manually picked. The plagioclase separate and a powdered sample of phenocryst-free groundmass were analyzed with instrumental neutron activation analysis (INAA) at PSU. Further compositional and mineralogical data will be collected by petrographic microscopy, electron microprobe analysis, and inductively coupled plasma mass spectrometry. Preliminary INAA data reveal that the negative Ce anomaly is indeed present within the groundmass-free phenocryst separate. Proposed origins for such anomalies in volcanic arc lavas include weathering, the existence of a Ce anomaly in the subducted slab from which the magma formed, and contamination with Ce-anomalous crustal material during magma ascent. Weathering is an unlikely source as the anomaly is also present in the unweathered plagioclase phenocrysts. Among the two origins that are related to a deep source, we prefer the scenario in which the magma inherited the Ce anomaly through contamination with altered oceanic crust or oceanic sediments that contained a negative Ce anomaly. Seawater alteration of oceanic crust or deposition of sediments in equilibrium with sea water are known to introduce a negative Ce anomaly due to preferential loss or exclusion of Ce⁺⁴ during alteration and precipitation, respectively. A cerium anomaly is rare in High Cascades lavas, therefore lava flows with a Ce anomaly at Green Ridge provide an unusual chemical signature that will be explored to pinpoint their petrogenetic history.