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Z ircons -More than ages

Z ircons -More than ages. Add-ons. Other information to be coupled with age of zircon Need to be done in-situ, so analysis can be performed on the same spot or very near; Provide additional information on physical conditions or petrogenesis of the source rocks. Add-ons to zircon ages.

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Z ircons -More than ages

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  1. Zircons -More than ages

  2. Add-ons • Other information to be coupled with age of zircon • Need to be done in-situ, so analysis can be performed on the same spot or very near; • Provide additional information on physical conditions or petrogenesis of the source rocks.

  3. Add-ons to zircon ages • Radiogenic isotopes – Hf isotopes (model ages and time of extraction from mantle) • Stable isotopes – Oxygen isotopes – contributions from pre-existing crust; • Temperature from Ti-thermometers – physical conditions of zircon formation • Other (less common) – REE distributions other trace elements.

  4. Hf isotopes • Radiogenic isotopes (Lu-Hf decay) similar in behavior to Nd isotopes; less used conventionally; • The great advantage of Hf isotopes is that they can be measured in situ via laser ablation using MC-ICP-MS • Zircons have high Hf, low Lu concentrations; • Newer setups (with two instruments) can utilize split-stream technology to measure age and Hf isotopes simultaneously.

  5. Hf isotopes • Usually expressed as Epsilon Hf units, just like Nd isotopes; • Positive e(Hf) values reflect primitive mantle sources, whereas lower values, sometimes highly negative, are indicative of old crustal components; • Model ages represent approximate values of extraction from a mantle reservoir (such as DM or CHUR), and broadly represent lithospheric residence ages.

  6. Zhang et al., 2014, Lithos

  7. Delta 18O – a tracer of crust • Zircon d18O is a good indicator of pre-existing crust, just like quartz or other rock-forming minerals are in igneous rocks • Zircon is in some ways better because is not prone to secondary modifications • Mantle d18O=5.5 permil (relative to SMOW) • Crustal values are anywhere from 6 to 15 • Complementary tool to Hf isotopes, strictly identifies pre-existing crust.

  8. d18O– a tracer of crust • Zircon d18O need not be measured in-situ and is a common tool for igneous petrogenesis; • In provenance studies, the strategy is to measure oxygen isotopes in conjunction with age, Hf isotopes etc; • In-situ oxygen isotopes are being measured with high precision on small (few micron) spots using SIMS and equivalent instruments.

  9. Oxygen isotopes in the lithosphere

  10. Ti in zircon thermometry • Calibrated in the 1980s with major improvements over the past 5 years • Ti concentration (measurable in-situ by SIMS) is directly proportional to the temperature of zircon formation; • Spots dated by ICP can be assigned a physical property, namely temperature. • Published papers show zircon grew in the age range anywhere from 350 to >1000 C.

  11. Calibration and measurement • Ti concentration is directly proportional to the crystallization temperature (various calibrations, but mostly due to Harrison and Watson); • Measurement can be performed by SIMS at very fine scale, microns.

  12. El Teniente summary • Mantle-like, primitive Hf and d18O • Temperatures are 600-700 C, relatively low • Zircons from these rocks indicate that silica differentiation here took place by closed system fractionation of some mantle-derived basalts.

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