LITHIUM AND TRACE-ELEMENT INCORPORATION INTO METAPELITIC MINERALS: NEW DATA FROM LA-ICP-MS MEASUREMENTS
New LA-ICP-MS analyses of Li contents in coexisting minerals in 8 St-bearing samples, from NW Maine and the Truchas Mountains, NM, confirm the strong affinity of Li for St. In these samples, staurolite Li contents ranged from 347 - 4181 ppm, corresponding to 0.06 - 1.03 apfu. St with the highest Li content has over 25% of the 4-fold site occupied by Li, resulting in the formula: (Fe2.49Li1.03Mg.18Mn.1Zn.2Ni.01)(Al17.64Mg.24Ti.1V.01Cr.01)(Si7.8Al.16)O48H3.29. This sample has a low modal abundance of St. However, the second highest Li quantity, 3290 ppm, is in a large 1-cm St. Thus, it does not appear that Li concentrations necessarily reflect the modal amount of St (cf. Zn). No systematic core-to-rim Li zoning was apparent, nor did Li values differ consistently between St overgrowths and preexisting cores. Intragranular variability is < 20% relative. In addition to Li, these St contain: Ni = 474 - 754; Cr = 60 - 396; and Co = 23 - 89 ppm. REEs were below detection limits.
The impact of Li contents in St for petrogenetic calculations and subsequent interpretations stems from the ability of St to partition Li over other coexisting phases. Lithium contents of coexisting phases range from: biotite 137 - 1105 ppm; garnet 46 - 187; muscovite 20 - 369; chlorite ~ 160; tourmaline ~ 40 ppm; and chloritoid ~ 2 ppm. In all but one sample, garnet contained higher quantities of Li than muscovite. One sample contained cordierite with 1985 ppm Li, but St partitions Li more strongly, KDLi-Fe st-crd ~1.30. St partitions Li much more strongly than garnet, KDLi-Fest-grt ~ 260. In metapelites, Li contents may have significant consequences on the stability of phases and apparent phase relationships. This underscores the need for Li to be included in any petrogenetic analysis. LA-ICP-MS provides a powerful and rapid tool for analyzing Li in metapelitic minerals that will allow more accurate conditions of metamorphism to be deciphered.