Strontium Isotopes in Secondary Silicate Minerals Produced During Paleogroundwater Flow, Socorro, New Mexico

Physical microsampling of silicate alteration phases produced during past episodes of groundwater flow, combined with Sr isotopic data, can potentially be used to determine the age of the alteration phases and to define paleogroundwater flow paths. Recent studies of the K-metasomatized, mid-Tertiary Lemitar Tuff near Socorro, central New Mexico (Fritz and Farmer, Chem, Geol., 2006) suggested that secondary adularia produced during metasomatism may record the original Sr isotopic compositions of altering fluids, the later providing information regarding the source of the solutes in the fluids. Here we expand our microsampling of secondary minerals in the altered Lemitar Tuff to a wider geographic area with the intent of determining if spatial variations in fluid Sr isotopic compositions are recorded in these minerals and if these variations can be used prescribe the direction of paleogroundwater flow. Samples were obtained of K-metasomatized tuff and overlying Popotosa Conglomerate along along an ~40 km transect through the entire region of alteration from potential recharge area in Precambrian granitic rocks of the Magdalena Mountains to the west, and discharge region in similar rocks some 4 km to the east. Most of the new adularia microsamples plot on a steeper errorchron (~12 m.y.) with lower apparent initial ratios (~0.711) than determined for the adularia samples analyzed by Fritz and Farmer (2006) (~7 m.y., ~0.721), regardless of geographic position. Unlike the samples obtained in the early study, our new microsamples contain varying proportions of secondary clay minerals, which are difficult to avoid during sampling. Clay mineral-rich adularia microsamples have higher Sr contents (100 ppm) and lower measured 87Sr/86Sr (~0.711) than their clay poor counterpart, suggesting that Sr -errorchron" may be a mixing line between a high Rb/Sr, high 87Sr/86Sr adularia and low Rb/Sr, low 87Sr/86Sr clay minerals. If so, then the clay minerals must either have formed at later time than adularia and incorporated Sr from a fluid containing solely tuff-derived Sr, or the Sr incorporated into clay minerals must have come solely from altering, low 87Sr/86Sr, igneous plagioclase, unlike the adularia. In either case, the intimate association between clay minerals and adularia, and differences in either the timing or mode of their crystallization, renders it difficult to extract with our current methodology unambiguous information regarding the Sr isotopic compositions of the K-metasomatizing fluids.