A new method fingerprinting magmatic processes using combined U/Pb ID-TIMS geochronology and accessory mineral geochemistry
Abstract
Zircon is commonly used as a recorder of magmatic processes because of its utility in geochronology and ability to retain primary growth information at prolonged magmatic conditions. Recent applications of U-Pb ID-TIMS geochronology have shown that precision on dates of single zircons are often far smaller than the time-scales of magmatic systems. The result is that dates on such grains record various processes occurring over an interval of time during the production, mobilisation, and emplacement of magmas. Combining age information with geochemical data from the same zircons has proved a useful tool, but thus far has been restricted to low-precision in situ dating techniques. Furthermore, these techniques have been unable to combine age data with trace element geochemistry on the same volume of zircon. We have developed a new technique that combines high-precision ID-TIMS dating with geochemical characterization on the exact same material. U and Pb are commonly separated from the other chemical constituents by ion exchange chemistry during sample preparation. We retain this fraction and analyse it using solution nebulization ICP-SFMS with matrix-matched external liquid calibration. We measured elements such as Zr, Hf, Y, Sc, and the REE, which are present in solution at between 10 and >105 ppt; Hf isotopes are subsequently measured on the same solution. Data obtained using this approach allow us to distinguish between models for melt generation, transport, and assembly of the ca. 40 Ma composite mafic to felsic Re di Castello pluton, Adamello batholith, northern Italy. Coupled with age uncertainties on single zircons as low as 10,000 years, zircon geochemistry preserves a rich record of fractional crystallization, crustal assimilation and magma mixing over timescales of <20,000 to >400,000 years. These data can also be used to distinguish between auto-, xeno-, and antecrystic zircon. Combined with field observation and U-Pb sphene thermochronology, we show that this pluton was assembled through the incremental intrusion of small magmatic pulses in the upper crust following the storage and modification of larger magma batches at depth. Application of our method to a wider range of U-Pb accessory minerals combined with analytical improvements will widen the breadth of its application and contribute to our understanding of magmatic processes as a function of time.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V42A..02S
- Keywords:
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- 1020 GEOCHEMISTRY / Composition of the continental crust;
- 1115 GEOCHRONOLOGY / Radioisotope geochronology