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Isotopic dating


Precise and accurate isotopic ages often represent the decisive argument to distinguish between different geological interpretations and they form the base to derive the rates of tectonic processes, i.e., of deformation, heating, cooling, and exhumation of metamorphic rocks in orogens, and the velocity of plates, to name a few. Over the last decades, developments in isotopic dating have gone in two major directions: (i) development of new analytical tools and – associated with it – a tremendous increase in data output and (ii) the use of an increasingly wider range of minerals and geochronometers in dating.

In the wake of these developments, a wide series of problems have emerged: (i) Different methods and geochronometers gave data of quite variable quality and dignity. (ii) There is an increasing number of conflicting ages, some of which have been brought into apparent agreement by involving a wide series of processes and concepts. Not all of them are necessary, if some of the published data are incorrect. The latter possibility, however, raises the question about which ages are problematic and why they are off the mark. There are three groups of geochronologic problems that have attracted my interest:


(i) U-Pb dating of niobates, wolframates & co.

(ii) Effects of initial isotopic heterogeneity

(iii) Role of nano-scale processes on dating at the micro-scale


(i) U-Pb dating oc niobates, wolframates & co.

There are many geologic problems that can not be solved by dating using minerals well-established in dating, either as such minerals are not present in the rock, are inherited from precursor rocks, or are genetically not linked with the process of interest. For such dating problems, it may be advisable to search for datable minerals previously (only little or) not used in geochronology. Over the years I have explored the geochronological potential of columbite-tantalite (and related niobates), hübnerite, and vesuvianite.


Associated publications:

Müller, A., Romer, R.L., Pedersen, R.-B. (2017) The Sveconorwegian Pegmatite Province – thousands of pegmatites without parental granite. Can. Mineral., 55: 1-33. DOI: 10.3749/canmin.1600075

Romer, R.L. and Lüders, V. (2006) Direct dating of hydrothermal W mineralization: U-Pb age for hübnerite (MnWO4), Sweet Home Mine, Colorado. Geochim. Cosmochim. Acta 70: 4725-4733. DOI: 10.1016/j.gca.2006.07.003

Lindroos, A., Romer, R.L., Ehlers, C. and Alviola, R. (1996): Late-orogenic Svecofennian deformation in SW Finland constrained through pegmatite emplacement ages. Terra Nova, 8: 567-574.DOI: 10.1111/j.1365-3121.1996.tb00786.x

Romer, R.L. and Smeds, S.-A. (1996): U-Pb columbite ages of pegmatites from Sveconorwegian terranes in southwestern Sweden. Precambrian Research, 76: 15-30.DOI: 10.1016/0301-9268(95)00023-2

Romer, R.L and Smeds, S.-A. (1994): Implications of U-Pb ages of columbite-tantalites from granitic pegmatites for the Palaeoproterozoic accrection of 1.90-1.85 Ga magmatic arcs to the Baltic Shield. Precambrian Res., 67: 141-158. DOI: 10.1016/0301-9268(94)90008-6

Romer, R.L. and Wright, J.E. (1992): U-Pb dating of columbites: a geochronologic tool to date magmatism, metamorphism, and ore deposits. Geochim. Cosmochim. Acta, 56: 2137-2142. DOI: 10.1016/0016-7037(92)90337-I


(ii) Effects of initial isotopic heterogeneity

With the increasingly more widespread use of low-µ (µ=238U/204Pb) minerals in isotopic dating, the isotopic composition of the initially incorporated Pb should be given a much higher attention, as the correction of initial Pb affects the age. Without knowledge of the isotopic composition of initial Pb, it is not possible to get accurate ages from low-µ minerals. In magmatic systems, the initial Pb isotopic composition can be estimated from cogenetic minerals with µ = 0. The initial Pb isotopic composition of metamorphic minerals, however, is more elusive as there is no initial homogenization among the metamorphic minerals, which already should be obvious from the sequential nature of minerals reactions preserved in the rock and widely used to constrain the pressure-temperature evolution of the metamorphic rock.

Actually besides Pb, it is quite likely that Nd and Hf do not homogenize either. The key question in not whether the initial isotopic composition of the daughter element of a geochronometer is homogeneous, but whether its heterogeneity is sufficiently large to affect the age. I have documented such initial heterogeneity and its effect on isotopic dating. Understanding how to distinguish systems where the initial disturbance does not affect the age from those where the age will be off the mark  contributes to avoid dating unsuitable samples.


Associated publications:

Romer, R.L. and Rötzler, J. (2011) The role of element distribution for the isotopic dating of metamorphic minerals. Eur. J. Mineral., 23: 17-23. DOI: 10.1127/0935-1221/2011/0023-2081

Romer, R.L. and Yilin Xiao (2005) Initial Pb-Sr(-Nd) isotopic heterogeneity in a single allanite-epidote crystal: implications of reaction history for the dating of minerals with low parent-to-daughter ratios. Contrib. Mineral. Petrol., 148: 662-674. DOI: 10.1007/s00410-004-0630-y

Romer, R.L. and Siegesmund (2003) Why allanite may swindle about its true age. Contrib. Mineral. Petrol., 146: 297-307. DOI: 10.1007/s00410-003-0494-6

Romer, R.L. and Rötzler, J. (2003) Effect of metamorphic reaction history on the U-Pb dating of titanite. In: Vance, D., Müller, A., and Villa I. (eds.) Geochronology: Linking the isotopic record with petrology and textures. Geol. Soc. London, Spec. Publ., 220: 147-158.DOI: 10.1144/GSL.SP.2003.220.01.08

Romer, R.L. (2001) Lead incorporation during crystal growth and the misinterpretation of geochronological data from low-238U/204Pb metamorphic mineral. Terra Nova, 13: 258-263.  DOI: 10.1046/j.1365-3121.2001.00348.x


(iii) Role of nano-scale processes on dating at the micro-scale

Since the enigma of contrasting Rb-Sr mineral and WR ages of metamporphic rocks had been solved more than four decades ago, it is known that geochronological systems are scale-dependent. For the Rb-Sr examples, metamorphism resulted in the redistribution of parent and daughter elements and concomitantly the resetting of the isotopic system at the mineral scale. As this redistribution occurred at scales smaller than the size of the WR samples used for dating, the WR samples did not record this redistribution and remained apparently closed systems. The same general interpretation also applies to other geochronologic systems, in particular systems involving α-decay (and, thus, also α-recoil).

Recoil of the daughter nucleus implies an open system, which eventually results in the decoupling of the parent-daughter system if the daughter (or intermediate daughters in the U and Th decay series) is mobile. As this process works at a scale of some tens of nanometer, the effect is irrelevant for dating, unless for microcrystalline material, ion sieves (e.g., zeolites), and metamict material. The key parameter is not the physical size of the investigated sample, but the distance to the next free surface that allows rapid transport of daughter isotopes out of the system.


Associated publications:

Romer, R.L. and Rocholl, A. (2004) Activity disequilibrium of 230Th, 234U, and 238U in old stilbite: effects of young U mobility and -recoil. Geochim. Cosmochim. Acta., 68: 4705-4719. DOI: 10.1016/j.gca.2004.04.026

Romer, R.L. (2003) Alpha-recoil in U-Pb geochronology: effective sample size matters. Contrib. Mineral. Petrol., 145: 481-491. DOI: 10.1007/s00410-003-0463-0

Romer, R.L. (2001) Isotopically heterogeneous initial Pb and continuous 222Rn loss in fossils: The U-Pb systematics of Brachiosaurus brancai. Geochim. Cosmochim. Acta, 65: 4201-4213. DOI: 10.1016/S0016-7037(01)00716-5


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