HE-3 SURFACE EXPOSURE DATING AND ITS IMPLICATIONS FOR MAGMA EVOLUTION IN THE POTRILLO VOLCANIC FIELD, RIO-GRANDE RIFT, NEW-MEXICO, USA

Dating of very young igneous activity has been hampered by lack of suitable chronometers. We report here the results of He-3 surface exposure dating (KURZ, 1986a,b; CRAIG and POREDA, 1986) for lavas from the Potrillo volcanic field. This field is particularly suited to the technique because it has well-preserved, young (<700 ka) surfaces. The study applies He-3 dating in a systematic manner to a well-characterized volcanic field and shows the power of the technique in deciphering both relative and absolute chronologies. Dates are reproducible to within 14% for different samples from the same surface and are similar to within 8% for samples which, based on field relations, represent synchronous events. Cosmogenic He-3/Ne-21 confirms that He-3 is retained by the samples. Comparison of the relative chronology with compositional trends of the lavas suggests that eruptive centers within the volcanic field are fed by individual magma chambers. Ages range from 80 ka to 17 ka, consistent with estimates from geomorphology, soil profile development, and some K/Ar determinations.