Intraplate Basaltic Volcanism in the Basin and Range, USA: Relationship to Low-Velocity S- wave Anomalies and Asthenospheric Dynamics
Pliocene to Recent intraplate mafic volcanic rocks of the Basin and Range Province mostly formed by asthenospheric melting, as can be seen by melting temperatures. Here asthenosphere is defined by mantle rheology and temperature and not by geochemistry. The duration of melting in a volcanic field may be related to the size and shape of pockets of low velocity asthenosphere moving under the areas of volcanism. Seismic S- wave velocity profiles constrained by ambient noise and earthquake tomography of the mantle (Yang et al., 2008) show low velocity pockets, which may correspond to higher temperatures and/or higher water contents. By applying an asthenospheric flow velocity of 5 cm/yr east (Silver and Holt 2002, Conrad et al., 2007), the distance the mantle has moved since the time of volcanism can be calculated for basalts of known age. Past positions of low-velocity anomalies in the asthenosphere combined with depths and temperatures of melting calculated using the silica-liquid geobarometer (Lee et al., 2009) were used to determine if a low velocity anomaly existed under an area of volcanism at the depth of melting and time of eruption. The data constraints used for calculating depths and temperatures of melting are dry, MgO > 7.5 wt. %, SiO2 > 44 wt.%, and Fe as 90% Fe2+. Depths and temperatures of melting were calculated for several basalt fields of known age. Ages, temperatures, and depths are as follows: Death Valley 4 Ma, 1295-1350°C, and 42-63 km; Crater Flat 80 ka, 1 Ma, 3.8 Ma, 1388-1415°C, and 80-90 km; Lunar Crater 2.9-5.7 Ma, 1414-1480°C, and 80-121 km; Reveille 3.8-4.6 Ma, 1458-1516°C, and 110-140 km; Coso 0.23-5.3 Ma, 1244-1399°C, and 39.6-72 km; Big Pine 0.9-1.8 Ma, 1276-1356°C, and 42-72 km; Long Valley 0.4-3.2 Ma, 1289- 1323°C, and 44-50 km; Cima 0.3-8.3 Ma, 1330-1376°C, and 53-82 km; Snow Canyon <10,000 years, 1470-1485°C, and 75-85 km. Ages were converted to km of mantle motion and to degrees of longitude and plotted on seismic profiles for the appropriate latitude. Using the examples of Coso, Big Pine and Crater Flat, calculated depths correspond to low velocity pockets at the time of volcanism. A low velocity pocket entered the area under Coso at 5.3 Ma and will exit this area in about 0.4 m.y. explaining the span of volcanism in this field (5.3 to 0.23 m.y. and still active). At Big Pine the low velocity pocket entered at 1.8 Ma and exited about 0.9 Ma again similar to the span of volcanism (1.3 - 0.03 Ma, from Blondes et al., 2008). Crater Flat has undergone several episodes of volcanism, though the distance covered by the seismic profile only includes the most recent episodes. The leading edge of a low velocity pocket reached Crater Flat ∼ 6.75 Ma but the pocket is still beneath the area suggesting the possibility of future activity. It is possible that the concurrence of basaltic volcanism and low velocity seismic anomalies (hot and/or hydrous asthenosphere) at melting depths extends to the entire Basin and Range and that asthenospheric dynamics are an important driving factor in producing intraplate mafic volcanism.
Physical Volcanology and Hazard Analysis of a Young Volcanic Field: Black Rock Desert, Utah, USA
The Black Rock Desert volcanic field, located in west-central Utah, consists of ~30 small-volume monogenetic volcanoes with compositions ranging from small rhyolite domes to large basaltic lava flow fields. The field has exhibited bimodal volcanism for > 9 Ma with the most recent eruption of Ice Springs volcano ∼ 600 yrs ago. Together this eruptive history along with ongoing geothermal activity attests to the usefulness of a hazard assessment. The likelihood of a future eruption in this area has been calculated to be ∼ 8% over the next 1 Ka (95% confidence). However, many aspects of this field such as the explosivity and nature of many of these eruptions are not well known. The physical volcanology of the Tabernacle Hill volcano, suggests a complicated episodic eruption that may have lasted up to 50 yrs. The initial phreatomagmatic eruptions at Tabernacle Hill are reported to have begun ~14 Ka. This initial eruptive phase produced a tuff cone approximately 150 m high and 1.5 km in diameter with distinct bedding layers. Recent mapping and sampling of Tabernacle Hill's lava field, tuff cone and intra-crater deposits were aimed at better constraining the eruptive history, physical volcanology, and explosive energy associated with this eruption. Blocks ejected during the eruption were mapped and analyzed to yield minimum muzzle velocities of 60 - 70 meters per second. These velocities were used in conjunction with an estimated shallow depth of explosion to calculate an energy yield of ∼ 0.5 kT.
Mechanisms of low-flux intraplate volcanic fields - Basin and Range and Northwest Pacific Ocean
Many fields of small-volume, scattered volcanoes that typically have alkaline affinities occur in intraplate settings. The underlying mechanisms of these intraplate volcanoes are enigmatic; they often do not correlate with anomalous heat sources or upwelling mantle (as in hot spots, mid-ocean ridges, and active continental rifts), or with fluids introduced by actively-subducting lithosphere. We compare the characteristics of two low volume-flux intraplate volcanic fields, one in a continental setting that is characterized by slow extension (western U.S.A.), and the other on the floor of the northwest Pacific Ocean in a region of plate flexure. The comparison supports an interpretation that the fundamental driving mechanism for low magma-flux volcanic fields, which episodically erupt scattered, small-volume volcanoes over millions of years, is regional-scale deformation of compositionally-heterogeneous upper mantle. Deformation serves to mechanically focus partial melts that might be present due to locally-depressed solidus temperatures caused by slightly higher volatile contents, creating sufficient melt buoyancy to trigger magma ascent via magma-driven fractures (dikes). The key role of deformation in collecting magmas and triggering dike ascent and eruption, without influx of new material or heat into the source region, supports the definition of such systems as tectonically-controlled, and is likely applicable at other low-flux, diffuse volcanic fields. Differences in the degree of fractionation and wall-rock contamination in the two fields is related to vertical variations in principal-stress orientation that may cause stalling of ascending dikes.
Mantle Source of Quaternary Basalt Volcanic Fields in Utah, USA
The magma source of Quaternary intraplate basaltic volcanic fields in the Basin and Range Province and Utah Transition Zone is currently unknown and could reside in either lithospheric mantle or asthenospheric mantle. These volcanic fields are approximately aligned with N-NE trending regional faults, which has led many to infer structural control on magma production and, by inference, a lithospheric mantle magma source. However, magma may also be produced in fertile asthenospheric mantle completely independent of tectonic regime by adiabatic melting. To test the hypotheses proposed by previous workers, a geochemical and petrologic study is being performed on two of the volcanic fields in Utah: Black Rock Desert in the Basin and Range and Markagunt Plateau in the Transition Zone. Geochemical signatures of basalts in both Black Rock Desert and Markagunt Plateau retain elements of arc basalts (e.g. Nb depletion relative to OIB), suggesting melting of a source enriched in subduction zone fluids. Epsilon-Nd values are highly negative (-6 to -11) with the exception of one flow in Markagunt Plateau which has an epsilon-Nd of -1. This suggests melting of an old source which, combined with the arc signature, indicates a magma source in lithospheric mantle. Markagunt Plateau basalt has several flows which show, with the exception of Nb depletion and slight Ba enrichment, trends similar to OIB rather than typical arcs. In comparison, most element concentrations in Black Rock Desert basalt are very close to average OIB and Sr isotopes are primitive (0.704); suggestive of an asthenospheric source. Depth of melting calculations using a Si-liquid geobarometer developed by Lee et al. (2009) on primitive basalts (> 45 wt. % SiO2, MgO >7.5 wt. %, 90% of Fe as Fe2+) in each field yield pressures of 2.2 -2.6 GPa (75-90 km depth) for melt generation in Markagunt Plateau and 2.7-3.0 GPa (95-105 km depth) in Black Rock Desert. These melting depths correspond to low velocity S-wave anomalies that can be interpreted as hot zones in the mantle (Yang et al., 2008). These depths place melting at or below the lithosphere-asthenosphere boundary, which is contrary to Nd isotopes that suggest an old, lithospheric mantle source. One explanation for this contradiction is that melting occurred in the asthenosphere and was later contaminated by lithospheric mantle. Melting may also have occurred at the base of the lithosphere, an area that may have recently been converted to more fertile asthenospheric mantle, but still retains isotopic characteristics of old lithosphere.
Inferring a Petrogenetic History of the Ethiopian Lithospheric Mantle Using the Geochemistry of Olivine Crystals in Mantle Xenoliths
Fragments of the fertile lithospheric mantle, represented by xenoliths, can be used to infer petrogenetic processes and interactions between the lithosphere and asthenosphere. Peridotitic xenoliths were examined from the Lake Tana Province and Wollega area, located on the western Ethiopian Plateau. The Ethiopian Plateau is part of the Ethiopian Flood Basalt Province and is associated with the 30 Ma impact of the Afar Plume head (Hoffmann et al., 1997). While the lithosphere is particularly thinned beneath the adjacent Ethiopian rift, lithospheric thickness of the plateau is less than that expected for a Proterozoic lithosphere (Dugda et al., 2007). We examined evidence for the interaction of the Afar plume with the subcontinental lithospheric mantle beneath the western Ethiopian Plateau. The geochemistry of lithospheric xenoliths can record events such as metasomatism associated with mantle plumes or subduction, regional enrichment events, and depletion due to partial melting. Although clinopyroxene, and to lesser extent orthopyroxene, have the highest concentration of incompatible trace elements, in order to better understand how trace elements are distributed amongst various peridotite minerals, this study will present new trace element data solely from olivine. Since some xenoliths may not contain any pyroxene, a study of trace elements in olivine can provide more information on mantle processes and can further explore the petrologic sequence of events. This study will constrain the geochemical evolution and fertility of the lithospheric mantle below the western Ethiopian Plateau by comparing the relative abundances of trace elements in olivine to known whole rock analyses (Roger et al., 1999; Roger et al., 1997). If trace elements are to be used accurately to constrain the geochemical behavior and processes in the upper mantle, a better understanding of how trace elements behave in olivine is much needed.