OBTAINING MONAZITE MICROPROBE AGES -- A NOVICE’S QUESTIONS AND NEED

Attempts to use the U-Th-Total-Pb chemical method of age dating by electron microprobe analysis on monazites from various localities (Australia, Norway, New Mexico, Colorado, Michigan, Southern Appalachians) reveal that under conditions of high electron beam current, small beam diameter, and long count time, a crater surrounded by a large contamination spot will form. When this occurs, calculated ages are generally higher (although perhaps geologically reasonable) and have a larger variance than when no beam damage occurs. Question 1: Under analytical conditions necessary for reasonable counting statistics is monazite sufficiently stable to accurately differentiate absolute ages of close spaced tectonic events?

Many monazites, regardless of Th concentration, contain sub-millimeter to sub-micrometer Th- and/or U-rich areas or grains (thorite, thorogummite, thorianite, uraninite, or hydrated U-oxide). Analyses on U-rich grains generally give calculated ages similar to the host monazite, but on Th-rich areas and grains the calculated ages are usually significantly lower than the host monazite. Question 2: Can Th migrate and U-Th-Pb systematics be disturbed in monazite? (at sub-metamorphic conditions?)

Perhaps the observations noted above stem from some system failure, faulty technique, and/or operator error. Many microprobe laboratories do not have ready access to samples dated by isotopic methods. Workers in such labs must assume that following published microprobe techniques will result in accurate analyses and real ages. Well characterized, not necessarily homogenous, inter-laboratory comparison samples are needed. In the mean time, caution is urged when interpreting monazite chemical ages.