Sr isotopic microsampling of magmatic rocks; a review (Invited)
Abstract
Sr isotopes have been used since the 1960s as powerful tracers of source for igneous rocks. In the past 10 years in-situ isotopic microsampling has afforded us tremendous progress in our capacity to understand magmatic processes. This progress is underpinned by analytical advances particularly in sample extraction through laser or micromill and in multicollector mass spectrometer improvements to sensitivity and precision. Perhaps the biggest surprise was the recognition in the 1990s that young magmatic rocks are commonly isotopically heterogeneous at the component (inter- or intra- crystal) scale. Given that melting and fractionation do not affect 87Sr/86Sr we would not a priori expect isotopic variations within or among crystals in a young igneous rock. This observation alone attests to open system behavior in magmas, and tells us that many of the crystals have been mechanically aggregated and not grown directly from the melt in which they are found solidified (a conclusion that can also commonly be drawn from cursory petrographic examination). This recognition in turn means that we can make use of the crystals as recorders of the isotopic environments in which they crystallise: If a crystal grows progressively from a melt which changes its isotopic composition through processes such as contamination and mixing, then the only record of the melt evolution is in the core-rim compositions of the crystals - analogous to the environmental record of tree rings. Plagioclase crystals in mafic enclaves from Lassen (CA) and Purico-Chascon (Chile), for instance, have isotopic records that reflect origination from the more silicic host. Core-rim records of evolution can also be integrated with textural measurements. At Stromboli we have shown how isotopic zoning correlates with crystal size distribution. The detailed records of single crystals can be complemented by multi crystal core analyses which can be used to distinguish specific populations. This approach was used on Santorini to identify magmatic processes associated with individual eruptive units. Since realising the power of in situ Sr isotope analysis practitioners have argued whether laser ablation or micromilling is the best approach. They are, in fact, complementary. The former allows for rapid accumulation of large datasets with statistical validity and is useful in pilot studies or where there are large ranges in 87Sr/86Sr . Micromilling, by virtue of the chemical separation and subsequent TIMS analysis, is time consuming but ensures better accuracy and precision.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V54A..01D
- Keywords:
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- 1036 GEOCHEMISTRY / Magma chamber processes;
- 1040 GEOCHEMISTRY / Radiogenic isotope geochemistry;
- 3620 MINERALOGY AND PETROLOGY / Mineral and crystal chemistry;
- 8439 VOLCANOLOGY / Physics and chemistry of magma bodies