New Mexico Geological Society Annual Spring Meeting — Abstracts

Evaporite minerals from southeastern New Nexico have not been prevlously extensively studied by geochronologic methods. The writer (Brookins, 1981) has summarized the earlier work of Schilling (1973), Tremba (1971), Register and Brookins (1980), and Brookins and others (1980). Based on this earlier work, several important conclusions were reached: (1) Primary evaporite minerals, including polyhalite, langbeinite. and sylvite yield Rb-Sr mineral and K-Ar (except for sylvite) mineral ages of 200-230 MYBP, which is interpreted as either a formational or diagenetic age. (2) Sylvites yield K-Ar ages from a few MYBP to 180 MYBP, and the amount of age lowering is random for this method. (3) Clay minerals from a Pleistocene Rubble Chimney are detrital as evidenced by scattered Rb-Sr ages in the range of 330-400 MYBP. (4) Evaporite mineral systematics in the vicinity of a 34 MYBP lamprophyre dike are reset within one meter of the contact with the dike. (5) No direct evidence for major tectonic or other events affecting Rb-Sr and K-Ar mineral systematics between about 200 and 34 MYBP has been noted.

We have now expanded our research to include systematic study of other evaporite minerals, as well as to continue to compile data on minerals studied earlier. New langbeinite analyses (n=4) yield K-Ar and Rb-Sr dates from 178 to 21.5 MYBP, while new sylvite K-Ar and Rb-Sr dates (n=3) range from 176 to 190 MYBP. Kieserite, due to its high water content, is difficult to analyze by the K-Ar method, as is leonite, and these minerals yield K-Ar apparent dates of 100 and 13 MYBP respectively. Yet Rb-Sr study of these minerals yields dates from 110 to 240 MYBP, showing that the Rb-Sr systematics are less affected than K-Ar systematics. An impure mixture of magnesite and talc, mixed with sylvite (180 MYBP), yields an anomalously high K-Ar date of 297 MYBP and a Rb-Sr date of 183 MYBP. It is interesting to note that the excess Ar in the carbonate-silicate mix can be explained by incorporation of that proposed to have been lost from the sylvite with which it is mixed. In support of earlier work, no geochronological evidence for thermal or other events disturbing the evaporites between near 200 MYBP to 34 MYBP is noted.