Date of Award


Degree Name

Master of Science




Jasper G. Konter


The Pb isotope composition of volcanic rocks in the Rio Grande Rift (RGR) can be used to investigate the interaction of mantle melts with continental lithosphere, and has important implications for the composition of the lithosphere. We suggest some compositional signatures may pre-date current tectonic processes. Existing data from several volcanic fields in New Mexico show a converging pattern on a very limited Pb isotope range, corresponding to lower crustal xenolith compositions from Kilbourne Hole. The different volcanic fields show Pb isotope trends diverging from this lower crustal composition, likely depicting mixing with different upper crustal reservoirs. These very common, widespread lower crustal composition that overwhelmed the mantle isotopic signature, subsequently contaminated by variable upper crust. The geographically wide-spread lower crustal, low radiogenic Pb compositions are best explained with time-integrated low U/Pb ratios that imply a Proterozoic, large-scale event. In this study, we focus on the Pb isotope compositions of the Potrillo, Elephant Butte, Palomas, and the Hillsboro volcanic fields in the southern RGR in New Mexico to test our crustmantle mixing hypothesis. The studied volcanic fields represent a range in erupted volumes and are spread out over a significant area, therefore likely representing a range in extent of mixing, as well as variation in upper crustal compositions. The data obtained in this study is in agreement with all fields containing a similar lower crustal component that mixed with upper crustal compositions, the extent of which correlates with the erupted volumes. Observations suggest initial mixing of mantle melt with a lower crustal compostition. We also present results for the analysis of a set of USGS rock standards and show the measured compositions are in good agreement with high precision double-spike Pb analyses from the current literature.




Received from ProQuest

File Size

32 pages

File Format


Rights Holder

Lynnette Crocker

Included in

Geochemistry Commons