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Temperatures (Ti) and compositional characteristics of zircon: Early observations using high mass resolution on the USGS-Stanford SHRIMP-RG
Goldschmidt Conference Abstracts 2006
Temperatures (Ti) and compositional characteristics of zircon: Early observations using high mass resolution on the USGS-Stanford SHRIMP-RG J.L. WOODEN1, F.K. MAZDAB1, A.P. BARTH2, C.F. MILLER3, L.E. LOWERY3 1
U.S. Geological Survey, Menlo Park, CA 94025, USA (jwooden @usgs.gov; [email protected]) 2 Geology, Indiana-Purdue University, Indianapolis, IN 46202, USA ([email protected]) 3 Geology, Vanderbilt University, Nashville, TN 37235, USA ([email protected]; [email protected]) The high mass resolution of the SHRIMP-RG allows the measurement of a fairly complete set of trace elements for zircon including 48Ti, Sc, and all the REE. Using a spot size of 15– 20 lm allows analysis of numerous discrete CL zones from a single zircon with minimal contributions from unknown material below the exposed surface. Suites of zircons from individual granitoid samples suggest several general observations. Temperatures (T) from individual zircons and suites of zircons can vary by 150–200 °C and may be normal (high to low, core to rim), reverse (low to high) or complex with sharp changes (high and/or low). Many elements and element ratios show a co-variation with T. Increasing Hf concentrations and a decrease in Th/U and Er/ Yb with decreasing T is common. Decreasing REE, esp. LREE, and increasing positive Ce anomaly with decreasing T is a strong tendency. Hf concentrations can continue to increase after a minimum T is reached (eutectic?) indicating continuing zircon separation from the remaining melt. These trends and tendencies may result from co-fractionation of accessory minerals and/or be driven by the thermodynamics of crystal growth (like Ti). Process interpretations should be based on trends in individual samples as many samples have characteristics distinctive from general trends. Hydrothermal zircon as described by Hoskin is not unusual as a rim zone and may reflect a fine intergrowth of other minerals (i.e., apatite, titanite, and oxides). Ti temperatures from these zones are often unreliable, and all analyses for Ti should include some screen (i.e., Ca, Fe) for Ti bearing minerals in addition to zircon. Molar ratios of total 3+ ions over P range from <1 to >5 with the majority >1 suggesting the general need for charge compensation other than the xenotime substitution. While trace element concentrations of zircons may not be diagnostic of rock type in general, careful analyses as described above provide valuable information about processes in igneous as well as metamorphic petrology.
A707
Refining terrestrial palaeoclimate chronologies: New tools for old speleothems J. WOODHEAD, J. HELLSTROM, R. MAAS School of Earth Sciences, The University of Melbourne, Vic. 3010, Australia ([email protected]) Speleothems are widely used archives of palaeoclimate variation and have yielded some remarkable insights into climatic conditions during the late Quaternary. Compared to other proxy methods, they offer the possibility of furnishing palaeoprecipitation as well as palaeotemperature records and are eminently suited to dating by U-series analysis, a technique which is highly robust for carbonate materials up to 550 ka in age. Unfortunately, beyond this time their value has been limited by a lack of appropriate chronometers. Building upon the work of Richards et al. (1985), we have developed a method for precise dating of speleothems beyond the range of the U–Th technique using the U–Pb decay scheme. By coupling low-blank sample preparation procedures and MCICPMS analytical methodologies, we find that, under ideal circumstances, U–Pb dating of speleothems is not only possible but also produces excellent age resolution—often comparable to or better than U-series studies. Corrections for initial isotopic disequilibrium, however, remain necessary in most analytical situations and exert a strong control on the achievable age uncertainty. This technique will be of immediate benefit in extending speleothem-based climate proxy records beyond 550 ka and will also find other uses, such as the dating of associated sub-fossil remains, and providing constraints on rates of landscape evolution and neo-tectonic processes. Now that detailed chronologies can be established for older speleothems, our next goal is to document how well such materials preserve proxy records. Here, we demonstrate the potential of innovative elemental imaging techniques (by LA-ICPMS) to help distinguish true palaeoclimate signals from alteration and grainscale effects (see below).
doi:10.1016/j.gca.2006.06.1533
Figure shows a 5 mm section of Nullarbor speleothem contoured in U, Sr, and I content.
Reference Richards, D.A. et al., 1985. GCA 62, 3683–3688. doi:10.1016/j.gca.2006.06.1534
Temperatures (Ti) and compositional characteristics of zircon: Early observations using high mass resolution on the USGS-Stanford SHRIMP-RG J.L. WOODEN1, F.K. MAZDAB1, A.P. BARTH2, C.F. MILLER3, L.E. LOWERY3 1
U.S. Geological Survey, Menlo Park, CA 94025, USA (jwooden @usgs.gov; [email protected]) 2 Geology, Indiana-Purdue University, Indianapolis, IN 46202, USA ([email protected]) 3 Geology, Vanderbilt University, Nashville, TN 37235, USA ([email protected]; [email protected]) The high mass resolution of the SHRIMP-RG allows the measurement of a fairly complete set of trace elements for zircon including 48Ti, Sc, and all the REE. Using a spot size of 15– 20 lm allows analysis of numerous discrete CL zones from a single zircon with minimal contributions from unknown material below the exposed surface. Suites of zircons from individual granitoid samples suggest several general observations. Temperatures (T) from individual zircons and suites of zircons can vary by 150–200 °C and may be normal (high to low, core to rim), reverse (low to high) or complex with sharp changes (high and/or low). Many elements and element ratios show a co-variation with T. Increasing Hf concentrations and a decrease in Th/U and Er/ Yb with decreasing T is common. Decreasing REE, esp. LREE, and increasing positive Ce anomaly with decreasing T is a strong tendency. Hf concentrations can continue to increase after a minimum T is reached (eutectic?) indicating continuing zircon separation from the remaining melt. These trends and tendencies may result from co-fractionation of accessory minerals and/or be driven by the thermodynamics of crystal growth (like Ti). Process interpretations should be based on trends in individual samples as many samples have characteristics distinctive from general trends. Hydrothermal zircon as described by Hoskin is not unusual as a rim zone and may reflect a fine intergrowth of other minerals (i.e., apatite, titanite, and oxides). Ti temperatures from these zones are often unreliable, and all analyses for Ti should include some screen (i.e., Ca, Fe) for Ti bearing minerals in addition to zircon. Molar ratios of total 3+ ions over P range from <1 to >5 with the majority >1 suggesting the general need for charge compensation other than the xenotime substitution. While trace element concentrations of zircons may not be diagnostic of rock type in general, careful analyses as described above provide valuable information about processes in igneous as well as metamorphic petrology.
A707
Refining terrestrial palaeoclimate chronologies: New tools for old speleothems J. WOODHEAD, J. HELLSTROM, R. MAAS School of Earth Sciences, The University of Melbourne, Vic. 3010, Australia ([email protected]) Speleothems are widely used archives of palaeoclimate variation and have yielded some remarkable insights into climatic conditions during the late Quaternary. Compared to other proxy methods, they offer the possibility of furnishing palaeoprecipitation as well as palaeotemperature records and are eminently suited to dating by U-series analysis, a technique which is highly robust for carbonate materials up to 550 ka in age. Unfortunately, beyond this time their value has been limited by a lack of appropriate chronometers. Building upon the work of Richards et al. (1985), we have developed a method for precise dating of speleothems beyond the range of the U–Th technique using the U–Pb decay scheme. By coupling low-blank sample preparation procedures and MCICPMS analytical methodologies, we find that, under ideal circumstances, U–Pb dating of speleothems is not only possible but also produces excellent age resolution—often comparable to or better than U-series studies. Corrections for initial isotopic disequilibrium, however, remain necessary in most analytical situations and exert a strong control on the achievable age uncertainty. This technique will be of immediate benefit in extending speleothem-based climate proxy records beyond 550 ka and will also find other uses, such as the dating of associated sub-fossil remains, and providing constraints on rates of landscape evolution and neo-tectonic processes. Now that detailed chronologies can be established for older speleothems, our next goal is to document how well such materials preserve proxy records. Here, we demonstrate the potential of innovative elemental imaging techniques (by LA-ICPMS) to help distinguish true palaeoclimate signals from alteration and grainscale effects (see below).
doi:10.1016/j.gca.2006.06.1533
Figure shows a 5 mm section of Nullarbor speleothem contoured in U, Sr, and I content.
Reference Richards, D.A. et al., 1985. GCA 62, 3683–3688. doi:10.1016/j.gca.2006.06.1534