AN INTEGRATED PHYSICAL, GEOCHEMICAL AND GEOCHRONOLOGICAL INVESTIGATION OF THE JORDAN VALLEY VOLCANIC FIELD, SOUTHEASTERN OREGON: IMPLICATIONS FOR UNDERSTANDING SOURCE HETEROGENEITY AND MAGMATIC PROCESSES IN CONTINENTAL SETTINGS

Fields of small basaltic vents or monogenetic volcano-fields are very common on Earth as well as on Mars. On the former, they occur either in intraplate settings or in association with larger, polygenetic volcanoes. Recent studies have revealed that the eruptive products of several individual terrestrial monogenetic basaltic volcanoes are chemically and isotopically diverse. Since such volcanoes are typically the products of single, relatively short-lived (months to years) eruptions, the presence of geochemical diversity raises several questions regarding the nature of the sources and processes involved in monogenetic volcanism. Whether this implies small-scale heterogeneity in the mantle source and inefficient mixing of small magma batches or cryptic crustal contamination is something that has not yet been satisfactorily resolved by geochemical studies. However, physical and geochemical studies of such volcanoes have often been conducted in isolation from each other and few studies have considered the implications of compositional variations between monogenetic volcanoes. An integrated field, geochronologic and geochemical investigation of the Jordan Valley Volcanic Field (JVVF) in southeastern Oregon clearly indicates the value of such an approach.

The JVVF consists of ten scutulum-type shield volcanoes (very similar to those on the Snake River Plain) and two tephra cones. Some interesting aspects of JVVF volcanism include (a) Chemical and isotopic diversity within as well as between volcanoes (b) Lack of a clear relationship between composition, location and age. For example, among the 4 volcanoes that were constructed around 1.9 Ma, the compositions of two widely separated volcanoes are near-identical, whereas those of two closely spaced ones are rather different. Such information, in conjunction with limited modeling indicates that the diverse compositions primarily reflect heterogeneous sources. Furthermore, the spatial and temporal context of the geochemical data suggests that the scale of the heterogeneity is small (tens of meters to several kilometers?) and that the plumbing system is fairly complex. The study demonstrates that monogenetic volcanoes can provide important constraints on the generation and transport of magma that are difficult to discern in larger systems.