Half a century ago, the first isotopic analyses were performed on grams of mineral separates. It is clear that the mineralogical characterization could only go as far as the assumption of average properties of the "mineral". Today's sensitive mass spectrometers have pushed down the detection limits of stable and radiogenic isotopes by several orders of magnitude, and it is routinely possible to produce intra-crystal maps of isotope ratios. However, the improved analytical capabilities only seldom have been matched by a cultural development, and many isotope practitioners still think of "minerals" as pure, homogeneous gems which obey simple laws formulated in pre-atomistic times for infinitely dilute solutions. By this incomplete understanding of the processes responsible for isotope transport, incorrect mathematical formulations and incorrect parameter fitting are obtained, leading to inaccurate geological inferences. In a parallel universe inhabited by mineralogists, the perception of "minerals" has been greatly modified by electron beam analyses. TEM and EMP have shown a whole new world in which minerals are no longer as simple as was believed 50 years ago: inclusions, inter/overgrowths, replacements and relics are now considered to be ubiquitous. How can geochronologists adapt to a changing paradigm, according to which most carrier phases of the isotopes used for dating are mixtures? The real progress in zircon dating was achieved when sensitive spot analyses were coupled to cathodoluminescence images (Gebauer et al., 1988). Petrographic and textural criteria were used to guide the interpretation of isotope analyses. Accretion of hydrothermal or metamorphic rims was shown to be one of the principal mechanisms causing discordance, thereby ending a 40-year-old dispute on the role of diffusive losses: it is now certain that diffusion is a very slow proces which always will be overrun by recrystallization whenever water is available. After the door was opened, other minerals also benefitted from detailed petrographic characterizations, and indeed showed a common pattern: monazite can record younger agers at the core of a crystal than at the rim - matching with recrystallization textures (Cocherie et al., 1998); muscovites give different 39Ar-40Ar ages by laser microprobe as a function of their shear-induced recrystallization (Hames & Cheney, 1997) or of their Si content, a monitor of their differential recrystallization under changing PTAX conditions (Di Vincenzo et al, 2001). A common conclusion of the new investigations, confirmed by a new generation of laboratory experiments, is that minerals are "more resistant to ¿ volume diffusion than previously thought" (Cherniak, 2000). What controls isotope systematics in all minerals for which detailed petrographic observations were made to date is not Fickian diffusion out of homogeneous gems, but instead the presence side by side of chemically distinct generations of the same mineral. Such diachronic generations are in mutual isotopic disequilibrium, showing that diffusive reequilibration was negligible, or at most incomplete. Accepting the petrographic evidence that many minerals consist of polygenic mixtures rather than of homogeneous gems can open up new paradigms for isotope geologists. If each generation can retain the isotopic and elemental record of its formation, it becomes necessary to discriminate the signature of each. When laser ICP or laser Ar-Ar analyses are not sufficiently precise, it is possible to exploit an alternative technique. Stepwise release experiments contain chemical information (e.g. the Th/U and Pb/U, or Ca/K and Cl/K element ratios given by Pb and Ar isotopes, respectively), which must be tied to electron microprobe analyses so as to firmly identify the isotope reservoirs and assign each of them an individual age. As an example, ore deposits were formed in the chemically open haloes of Hercynian granitoids in Sardinia; their timing is reconstructed by deconvolving the magmatic mica and feldspars from their hydrothermal overgrowths/replacements. The latter differ in their Pb isotopic composition and have higher Cl/Ar ratios (Boni et al., 2001). The ores were precipitated during a discrete Permian fluid circulation unrelated to the Carboniferous subvolcanic intrusions. The mineralizing fluid is associated to mass transfer and also to incomplete recrystallization of biotite to chlorite and of K-feldspar to sericite. K-feldspar cathodoluminescence confirms partial replacement textures.
Villa, I. (2002). Mete isotopes without petrography? / 'Tis but a jest, as sound chronology / Must needs base on complementarity.. Intervento presentato a: International Mineralogical Association Congress, Edinburgh.
Mete isotopes without petrography? / 'Tis but a jest, as sound chronology / Must needs base on complementarity.
VILLA, IGOR MARIA
2002
Abstract
Half a century ago, the first isotopic analyses were performed on grams of mineral separates. It is clear that the mineralogical characterization could only go as far as the assumption of average properties of the "mineral". Today's sensitive mass spectrometers have pushed down the detection limits of stable and radiogenic isotopes by several orders of magnitude, and it is routinely possible to produce intra-crystal maps of isotope ratios. However, the improved analytical capabilities only seldom have been matched by a cultural development, and many isotope practitioners still think of "minerals" as pure, homogeneous gems which obey simple laws formulated in pre-atomistic times for infinitely dilute solutions. By this incomplete understanding of the processes responsible for isotope transport, incorrect mathematical formulations and incorrect parameter fitting are obtained, leading to inaccurate geological inferences. In a parallel universe inhabited by mineralogists, the perception of "minerals" has been greatly modified by electron beam analyses. TEM and EMP have shown a whole new world in which minerals are no longer as simple as was believed 50 years ago: inclusions, inter/overgrowths, replacements and relics are now considered to be ubiquitous. How can geochronologists adapt to a changing paradigm, according to which most carrier phases of the isotopes used for dating are mixtures? The real progress in zircon dating was achieved when sensitive spot analyses were coupled to cathodoluminescence images (Gebauer et al., 1988). Petrographic and textural criteria were used to guide the interpretation of isotope analyses. Accretion of hydrothermal or metamorphic rims was shown to be one of the principal mechanisms causing discordance, thereby ending a 40-year-old dispute on the role of diffusive losses: it is now certain that diffusion is a very slow proces which always will be overrun by recrystallization whenever water is available. After the door was opened, other minerals also benefitted from detailed petrographic characterizations, and indeed showed a common pattern: monazite can record younger agers at the core of a crystal than at the rim - matching with recrystallization textures (Cocherie et al., 1998); muscovites give different 39Ar-40Ar ages by laser microprobe as a function of their shear-induced recrystallization (Hames & Cheney, 1997) or of their Si content, a monitor of their differential recrystallization under changing PTAX conditions (Di Vincenzo et al, 2001). A common conclusion of the new investigations, confirmed by a new generation of laboratory experiments, is that minerals are "more resistant to ¿ volume diffusion than previously thought" (Cherniak, 2000). What controls isotope systematics in all minerals for which detailed petrographic observations were made to date is not Fickian diffusion out of homogeneous gems, but instead the presence side by side of chemically distinct generations of the same mineral. Such diachronic generations are in mutual isotopic disequilibrium, showing that diffusive reequilibration was negligible, or at most incomplete. Accepting the petrographic evidence that many minerals consist of polygenic mixtures rather than of homogeneous gems can open up new paradigms for isotope geologists. If each generation can retain the isotopic and elemental record of its formation, it becomes necessary to discriminate the signature of each. When laser ICP or laser Ar-Ar analyses are not sufficiently precise, it is possible to exploit an alternative technique. Stepwise release experiments contain chemical information (e.g. the Th/U and Pb/U, or Ca/K and Cl/K element ratios given by Pb and Ar isotopes, respectively), which must be tied to electron microprobe analyses so as to firmly identify the isotope reservoirs and assign each of them an individual age. As an example, ore deposits were formed in the chemically open haloes of Hercynian granitoids in Sardinia; their timing is reconstructed by deconvolving the magmatic mica and feldspars from their hydrothermal overgrowths/replacements. The latter differ in their Pb isotopic composition and have higher Cl/Ar ratios (Boni et al., 2001). The ores were precipitated during a discrete Permian fluid circulation unrelated to the Carboniferous subvolcanic intrusions. The mineralizing fluid is associated to mass transfer and also to incomplete recrystallization of biotite to chlorite and of K-feldspar to sericite. K-feldspar cathodoluminescence confirms partial replacement textures.
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