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Three situations may be met: (1) monazites exhibiting significant Th/U ratio variation, (2) monazites exhibiting a fairly constant Th/U ratio, but significant U Th heterogeneity, and (3) monazites of constant U and Th concentrations.For the first case, a precise mean age can be calculated using a method of data reduction in the Th/Pb = f(U/Pb) diagram, whereby a precision of ±5−10 Ma (2σ) is commonly achieved.Uraninite (UO) have been used, the latter containing U and/or Th in minor proportions.Any stable U- or Th-bearing phase can be considered.also known as the Silverbell Road artifacts, were discovered between 19 in Arizona, and were the apparent relics of a lost Judeo-Christian Roman colony in North America, self-dating to around 800 "AD".This makes them unusual among alleged out-of-place artifacts in that the dating is incompatible not only with the timeline proposed by mainstream geology and history, but with that of Biblical creationism and flood geology as well.

Recent improvements in the statistical treatment of many in situ data now make it possible to decipher the related thermal events and so obtain reliable and precise ages.

of Geosciences, Univ of Massachusetts, Amherst, MA 01003, (4) Dept.

of Earth & Planetary Sciences, Univ of New Mexico, Northrop Hall, Albuquerque, NM 87131, (5) Department of Geology, Utah State Univ, 4505 Old Main Hill, Logan, UT 84322-4505, (6) New Mexico Bureau of Mines and Mineral Resources, New Mexico Tech, 801 Leroy Place, Socorro, NM 87801, (7) Massachusetts Institute Technology, 77 Massachusetts Ave Rm 54-816, Cambridge, MA 02139-4301, (8) Department of Geosciences, UMASS, 611, North Pleasant street, Amherst, MA 01003U/Th-Pb in situ dating of monazite offers rich potential for directly dating sedimentary sequences and for understanding their provenance.

Careful analysis is required with attention to interferences, background measurement, detection limits, and Pb-free sample preparation.

Extended counting times (600 s) at a probe current 200 n A are recommended.

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