DETERMINATION OF 25 ELEMENTS IN THE COMPLEX OXIDE MINERAL ZIRCONOLITEBY ANALYTICAL ELECTRON-MICROSCOPY

In this paper we describe a technique for the determination of 25 elem ents in natural zirconolite using energy-dispersive analytical electro n microscopy (AEM). The method presented here allows one to quantitati vely investigate the chemistry of submicron-scale zones in complex oxi de minerals. The effects of electron channeling, thickness variability and variations in detector resolution were minimized by using a contr olled set of operating procedures and instrument parameters. To provid e a high level of accuracy, k(ATi)-factors were determined from standa rds for most of the 25 elements of interest, including all of the majo r elements. Each analytical spectrum is reduced to a set of raw peak c ounts (and errors) using a digital top-hat filter to suppress backgrou nd followed by multiple least squares fitting of reference spectra. Co unting times of 12-15 min per analysis were required to provide suitab le counting statistics. Results are presented for zirconolite samples from the contact metamorphic aureole of the Bergell granodiorite intru sion, Switzerland-Italy. A comparison of 43 AEM analyses with 15 analy ses obtained by wavelength-dispersive electron probe microanalysis (EP MA) shows that there is excellent agreement between the two data sets in the amounts of individual elements present, chemical trends and ove rall stoichiometry. An assessment of the combined data set shows that the major substitution mechanisms in the Bergell samples are coupled s ubstitutions involving the M5,6- and M8-sites of the zirconolite struc ture: Ca-M8(2+) + Ti-M5,6(4+) --> (M8)REE(3+) + (M5,6)(Al,Fe)(3+) and Ca-M8(2+) + Ti-M5,6(4+) --> (M8)(Th,U)(4+) + (M5,6)(Mg,Fe)(2+).