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Introduction to Geochronology Part 2: Framing the problem (1) - what and why? Geochronology & Tracers Facility NERC Isotope Geosciences Laboratory British Geological Survey Framing the problem (1) • • • • Fit the geochronology to the research question Which rocks and minerals? Which methods? Sampling strategy Framing the problem (1) • Fit the geochronology to the research question Hypothesis Sampling & analytical strategy Interpretation of geochronological data Framing the problem (1) • Fit the geochronology to the research question • Which rocks and minerals? – – – – Provenance Magmatic evolution Metamorphism Deformation SCALE Framing the problem (1) • Fit the geochronology to the research question – Provenance Spencer et al., 2015, Geoscience Frontiers Framing the problem (1) • Fit the geochronology to the research question – Provenance Spencer et al., 2015, Geoscience Frontiers Framing the problem (1) • Fit the geochronology to the research question – Provenance Spencer et al., 2014, Precambrian Research Framing the problem (1) • Fit the geochronology to the research question – Provenance Spencer et al., 2014, Precambrian Research Framing the problem (1) • Fit the geochronology to the research question – Magmatic evolution Spencer et al., 2015, Precambrian Research Framing the problem (1) • Fit the geochronology to the research question – Magmatic evolution Spencer et al., 2015, Precambrian Research Framing the problem (1) • Fit the geochronology to the research question – Magmatic evolution SCALE Schoene et al., 2012, EPSL Framing the problem (1) • Fit the geochronology to the research question – Magmatic evolution SCALE Schoene et al., 2012, EPSL Framing the problem (1) • Fit the geochronology to the research question – Magmatic evolution SCALE Schoene et al., 2012, EPSL Framing the problem (1) • Fit the geochronology to the research question – Metamorphism SCALE Framing the problem (1) • Fit the geochronology to the research question – Deformation SCALE Framing the problem (1) • Fit the geochronology to the research question • Which rocks and minerals? – – – – – Provenance Magmatic evolution Metamorphism Deformation Mineral deposits Lithology/Mineralogy Framing the problem (1) • Fit the geochronology to the research question – – – – – Provenance Magmatic evolution Metamorphism Deformation Mineral deposits •Sedimentary units – across or along stratigraphy •Mature versus immature •Representative or looking for variation •Meta-sedimentary Lithology/Mineralogy Framing the problem (1) • Fit the geochronology to the research question – – – – – Provenance Magmatic evolution Metamorphism Deformation Mineral deposits Lithology/Mineralogy •Single versus multi-mineral •Zircon •Rutile •Titanite •Apatite •Monazite •Garnet •Model age Framing the problem (1) • Fit the geochronology to the research question – – – – – Provenance Magmatic evolution Metamorphism Deformation Mineral deposits •Evolution of single suite – mafic to felsic •Magmatic flux – batholith/terrane •Volcanic-plutonic connection •Cross-cutting relationships Lithology/Mineralogy •Magma mingling vs. mixing Framing the problem (1) • Fit the geochronology to the research question – – – – – Provenance Magmatic evolution Metamorphism Deformation Mineral deposits Lithology/Mineralogy •Minerals: •Zircon ubiquitous in felsic rocks •Baddelyite in some mafic rocks •Titanite & allanite •Monazite •Primary magmatic, alteration, metamorphic Framing the problem (1) • Fit the geochronology to the research question – – – – – Provenance Magmatic evolution Metamorphism Deformation Mineral deposits Dating ‘metamorphism’, a fallacy? Regis et al., 2014, Lithos Lithology/Mineralogy Dating a particular mineral reaction, or cooling path, or fluid event. Framing the problem (1) • Fit the geochronology to the research question – – – – – Provenance Magmatic evolution Metamorphism Deformation Mineral deposits Lithology/Mineralogy Dating a particular mineral reaction, or cooling path, or fluid event. Janots et al., 2008, J Met. Geology Framing the problem (1) • Fit the geochronology to the research question – – – – – Provenance Magmatic evolution Metamorphism Deformation Mineral deposits •‘Dating a shearzone’ •New mineral growth vs. disturbed isotopic systems •Low-temp – illite •Med-high temp – titanite •High-grade - monazite and zircon •Lead-loss •Resetting above closure temp Lithology/Mineralogy •Cross-cutting relationships Framing the problem (1) • Fit the geochronology to the research question – – – – – Provenance Magmatic evolution Metamorphism Deformation Mineral deposits •‘Dating a shearzone’ In general, deformation does not produce mineral growth or isotopic disturbance. Heat ± fluid Linked to shearing? Lithology/Mineralogy Framing the problem (1) • Fit the geochronology to the research question – – – – – Provenance Magmatic evolution Metamorphism Deformation Mineral deposits Lithology/Mineralogy Mahan et al., 2006, J. Met. Geology Framing the problem (1) • Fit the geochronology to the research question – – – – – Provenance Magmatic evolution Metamorphism Deformation Mineral deposits Lithology/Mineralogy •Dating an economic mineral? •Dating a hydrothermal event? •Dating host and/or cross-cutting rocks? Framing the problem (1) • Fit the geochronology to the research question Lithology/Mineralogy Framing the problem (1) • • • • Fit the geochronology to the research question Which rocks and minerals? Which methods? Sampling strategy Framing the problem (1) • Fit the geochronology to the research question • Which methods? What is the precision required to answer the hypothesis? Framing the problem (1) • Fit the geochronology to the research question • Which methods? What is the precision required to answer the hypothesis? 10% 2% 0.2% Framing the problem (1) • Fit the geochronology to the research question • Which methods? cost cost 10% cost What is the precision required to answer the hypothesis? 2% 0.2% Framing the problem (1) • Fit the geochronology to the research question • Which methods? Can two high precision dates answer the same question as five low precision dates? Framing the problem (1) • Fit the geochronology to the research question • Which methods? Restricted spatial resolution High precision e.g. ID-TIMS High spatial resolution Low precision e.g. LA-ICP-MS Framing the problem (1) • • • • Fit the geochronology to the research question Which rocks and minerals? Which methods? Sampling strategy Framing the problem (1) • Fit the geochronology to the research question • Sampling strategy Material can be dated in thin sections, thick sections, or after separation of minerals. Some techniques are limited (i.e. isotope dilution requires separated minerals). Is the context of the mineral crucial? Micro-drilling Framing the problem (1) • Fit the geochronology to the research question • Sampling strategy Laser microsampling FIB Selfrag selfrag.com Pollington & Baxter, 2011, Chemical Geology Wirth, 2004, Eur J Min. Take home messages • Fit the geochronology to the research question Hypothesis Sampling & analytical strategy Interpretation of geochronological data What scale? Which rocks? Which minerals? Which methods?
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