Twilight of a Volcanic Field: 11 Million Years of Basaltic Volcanism in the Southwestern Nevada Volcanic Field, USA
Abstract
Following the end of major caldera-forming silicic volcanism in the Southwestern Nevada Volcanic Field (SNVF), at least 10 episodes of alkalic basaltic volcanism have occurred over the last ~11 Ma. An understanding of the past behavior of the volcanic field provides insight for forecasting future eruptive behavior for use in hazard assessment for the high-level radioactive waste repository at Yucca Mountain. A program of geophysics, drilling, Ar-Ar dating and geochemistry conducted since 2004 by Los Alamos National Laboratory and the U.S. Geological Survey, combined with previous and ongoing petrogenetic and physical volcanology studies, sheds more light on the early and middle evolution of the volcanic field, much of which has been buried in alluvial basins. Volumes of erupted basalt have drastically declined over the history of the field, from as much as 50 km3 in the Miocene to about 0.5 km3 in the Pleistocene. The volume decrease is accompanied by a drastic decrease in extension rate, suggesting a close link between magmatism and tectonism. Neodymium and strontium isotopic analyses indicate that enriched lithospheric mantle has been the source of basalt throughout the history of the field. Decreasing eruption volumes are accompanied by an approximate doubling of Ce/Yb ratios, indicating that the volume decrease reflects a decrease in degree of partial melting of the lithospheric source. Eruption style has also changed with time, reflecting an increase in magma volatile content, consistent with decreased amounts of partial melting of a volatile-bearing source. These observations are consistent with a model in which the lithospheric mantle source was hottest during the period of major silicic volcanism and the presence of an active subduction system. After the breakdown of subduction, continued thermal input into the lithosphere ceased, and the lithosphere began to conductively cool. Melt accumulation in non-convecting, static lithosphere is probably related to the presence of mantle heterogeneities enriched in hydrous minerals that are partially melted. During regional extension, these zones are relatively weak and preferentially deform, forming melt bands of increased porosity that concentrate melt and lead to dike generation. Decreasing regional extension results in less melt accumulation and decreasing eruption volumes. Without a new source of heat and limited lithospheric extension, it is likely that the next million years of volcanic activity in the field will likely be characterized by eruptions of the type that have occurred during the past million years of activity: infrequent eruptions of small-volume (<0.1 km3), volatile-rich alkali basalt magmas within the most tectonically active southern and western margins of the volcanic field.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFM.V13C1492P
- Keywords:
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- 8410 Geochemical modeling (1009;
- 3610);
- 8488 Volcanic hazards and risks