OBTAINING GEOLOGICALLY MEANINGFUL 40AR-39AR AGES FROM ALTERED BIOTITE

Abstract

Biotite is the most used 40Ar-39Ar geochronometer yet two significant problems arise from Ar-Ar step-heating. Dating altered biotite can be problematic, producing disturbed age spectra that reflect 39Ar recoil. However, unaltered biotite can yield disturbed ages with apparently meaningful plateau ages as a result of mineral breakdown during stepped heating. Obtaining meaningful ages from such spectra is very difficult. Because alteration of biotite is common and widespread in nature, and because many biotites affected by alteration are nonetheless unique geologically and/or representative of key localities, the capability to obtain reliable ages from altered material is extremely important. In this study, the effects of alteration progress on biotite age spectra were tested using both IR laser step-heating and UV laser microprobe 40Ar-39Ar dating techniques. Our aims were to extract geologically meaningful ages from altered biotite and to identify cases where the ages had been influenced by alteration.Three variably altered biotites from the Precambrian metamorphic terrain of southwestern Montana were selected for argon isotopic analysis. Sample A is an unaltered rock containing pristine biotite, sample B is a highly altered rock with chlorite and prehnite interlayers within biotite, and sample C contains biotite with only incipient alteration. For each sample, the biotite ages obtained with IR and UV laser techniques were compared and the validity of the apparent ages was assessed.IR step-heating analysis of biotite from sample A yielded a well-defined plateau of 1776+/-6 Ma. UV laser microprobe analysis of the same biotite yielded a concordant weighted mean age of 1771+/-8 Ma, based upon 65 spot analyses of eight grains that ranged in age from 1819+/-54 to 1722+/-62 Ma. In contrast, IR step-heating of biotite from sample B resulted in disturbed spectra, with two separate fragments yielding total gas ages of 1505+/-12 and 1540+/-16 Ma, whereas UV laser microprobe analysis yielded 14 spot ages ranging from 1806+/-72 to 1565+/-78 Ma. UV laser analysis of sample C produced apparent ages ranging from 1772+/-52 to 1359+/-200 Ma.Detailed analysis of age variations occurring perpendicular to (001) cleavage planes of biotite in sample B was accomplished by depth profiling using the UV laser microprobe. Depth profiling of single grains revealed considerable age variation among the biotite layers, with profiles 1 and 2 yielding apparent ages ranging from 1730+/-226 to 1511+/-186 Ma, and from 1741+/-290 to 956+/-230 Ma, respectively. In both samples B and C, younger apparent ages correspond to higher 36Ar/39Ar ratios. The youngest ages, therefore, are believed to have derived from altered layers, whereas the oldest ages are related to layers with relatively little or no alteration.In summary, incremental step-heating of altered biotite single grains yields 40Ar-39Ar age spectra that are compromised by recoil and variable release patterns. However, extraction of small samples via the UV laser microprobe provides a simpler pattern of apparent ages, all younger than the ''true'' cooling age yet inversely correlated with atmospheric argon. This result suggests that true biotite ages can be recovered from areas of pristine material if they are sufficiently large to be ablated by the UV laser.