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Strategies towards statistically robust interpretations of in situ U-Pb zircon geochronology

Geoscience Frontiers 2016年第4期7卷 581-589页

作者： christopher j.spencer christopher L.Kirkland Richard j.M.Taylor The Institute of Geoscience Research(TIGeR) Department of Applied GeologyWestern Australian School of MinesCurtin UniversityWA 6102Australia Centre for Exploration Targeting-Curtin Node Department of Applied GeologyWestern Australian School of MinesCurtin UniversityWA 6102Australia Australian Research Council Centre of Excellence for Core to Crust Fluid SystemsPerthWA 6102Australia

Zircon U-Pb geochronology has become a keystone tool across Earth science, arguably providing the gold standard in resolving deep geological time. The development of rapid in situ analysis of zircon （via laser ablation and secondary ionization mass spectrometry） has allowed for large amounts of data to be generated in a relatively short amount of time and such large volume datasets offer the ability to address a range of geological questions that would otherwise remain intractable （e.g. detrital zircons as a sedi- ment fingerprinting method）. The ease of acquisition, while bringing benefit to the Earth science com- munity, has also led to diverse interpretations of geochronological data. In this work we seek to refocus U -Pb zircon geochronology toward best practice by providing a robust statistically coherent workflow. We discuss a range of data filtering approaches and their inherent limitations （e.g. discordance and the reduced chi-squared; MSWD）. We evaluate appropriate mechanisms to calculate the most geologically appropriate age from both 238U/206pb and 207pb/206pb ratios and demonstrate the cross over position when chronometric power swaps between these ratios. As our in situ analytical techniques become progressively more precise, appropriate statistical handing of U-Pb datasets will become increasingly pertinent.

关键词： Zircon Geochronology U-Pb Geostatistics MSWD

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Timescales of geological processes: Preface

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Geoscience Frontiers 2019年第1期10卷 1-3页

作者： Andrew Putnis christopher j.spencer Tom Raimondo The Institute for Geoscience Research School of Earth and Planetary Science Curtin University School of Natural and Built Environments University of South Australia

*** One of the major challenges in Geoscience is to understand how the formation and evolution of the Earth System are governed by timescales-that is,how the various geological processes that continue to contribute to its present-day structure and composition operated in the deep *** traditional view of such processes refers to events that occur at immense spatial scales and over hundreds of millions of years,constrained in most cases by the ages of rocks determined using isotopic dating methods or the fossil ***,the modern view of geological processes has increasingly acknowledged that their durations can be significantly shorter than previously thought possible,or indeed detectable without recent analytical *** are a prime example of rapid,high energy and episodic events that have a profound effect on subsequent processes such as metamorphism,fluid transport,and ore formation e the evidence of which is written in microstructures,compositional zoning,and P-T *** studies have also revealed that the reaction rates between fluids and rocks can be extremely rapid relative to geological *** has led to the notion that geological processes are not necessarily continuous over millions of years but may,in fact,be sporadic,with long periods where essentially no reactions take place punctuated by periods of intense activity.

关键词： In Preface Timescales of geological processes

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Secular change in earth processes： Preface

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Geoscience Frontiers 2018年第4期9卷 965-966页

作者： Richard M.Palin christopher j.spencer Department of Geology and Geological Engineering Colorado School of Mines Golden CO 80401 USA The Institute of Geoscience Research (TIGER) School of Earth and Planetary Sciences Curtin University WA 6102 Australia

*** Uniformitarianism is a traditional approach used in many areas of geoscience that assumes the geologic processes observed today operated similarly throughout all of Earth history（Hutton,1788;Windley,1993）,with such an ideology commonly encapsulated by the maxim＂the present is the key to the past＂.This concept is closely associated with the philosophical principle of Occam’s razor.

关键词： Secular change in earth processes Preface

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Granular titanite from the Roter Kamm crater in Namibia:Product of regional metamorphism,not meteorite impact

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Geoscience Frontiers 2022年第3期13卷 156-163页

作者： Aaron j.Cavosie christopher j.spencer Noreen Evans Kai Rankenburg Robert j.Thomas Paul H.Macey Space Science and Technology Centre and The Institute for Geoscience Research School of Earth and Planetary ScienceCurtin UniversityPerthWA 6102Australia Dept.of Geological Sciences and Geological Engineering Queen’s UniversityOntarioCanada john de Laeter Centre Curtin UniversityPerthWA 6102Australia Council for Geoscience BellvilleWestern CapeSouth Africa

Accessory minerals with so-called granular texture have risen in importance as geochronological tools for U-Pb dating of meteorite impact events. Grain-scale recrystallization, typically triggered by a combination of high-strain deformation and post-impact heating, can create a polycrystalline microstructure consisting of neoblasts that expel radiogenic Pb, which are thus ideal for isotopic dating. While granular domains in zircon and monazite from shocked rocks have been demonstrated to preserve impact ages,few U-Pb dating studies have been conducted on granular microstructures in titanite(CaTiSiO;). Here we report the occurrence of granular-textured titanite from ~2020 Ma granite basement rock exposed in the rim of the 4–5 Ma Roter Kamm impact structure in Namibia. Orientation mapping reveals two microstructurally distinct titanite populations: one consisting of strained/deformed grains, and the other consisting of grains that comprise aggregates of strain-free neoblasts. In situ U-Pb geochronology on 37 grains shows that most grains from both titanite populations yield indistinguishable U-Pb dates of ca.1025 Ma, consistent with the observed microstructures forming during the Mesoproterozoic Namaqua Orogeny. Only four grains preserved older age domains, recording ca. 1875 Ma Paleoproterozoic metamorphism. Two significant observations emerge:(1) none of the analyzed titanite grains yield the 2020 Ma igneous crystallization age previously established from zircon in the same sample, and(2) no age-resetting was detected that could be attributed to the 4 to 5 Ma Roter Kamm impact *** the similarity of the neoblastic microstructure to minerals from other sites with an established impact provenance, the granular texture and near-complete Pb-loss in titanite from Roter Kamm granite instead records a Paleo-to Mesoproterozoic polymetamorphic history, rather than Miocene age shockrelated processes. These results highlight the critical importance of grain-scale cont

关键词： Namibia Roter Kamm Titanite EBSD Geochronology

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Deconvolving the pre-Himalayan Indian margin-Tales of crustal growth and destruction

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Geoscience Frontiers 2019年第3期10卷 863-872页

作者： christopher j.spencer Brendan Dyck Catherine M.Mottram Nick M.W.Roberts Wei-Hua Yao Erin L.Martin Earth Dynamics Research Group The Institute for Geoscience Research (TIGeR) Department of Applied Geology Curtin University NERC Isotope Geosciences Laboratory British Geological Survey Department of Earth Sciences Simon Fraser University University Drive Department of Environment Earth and Ecosystems The Open UniversityWalton Hall Milton Keynes MK7 6AA UK School of Earth and Environmental Sciences University of Portsmouth Schoolof Earth Sciences and Engineering SunYat-sen University

The metamorphic core of the Himalaya is composed of Indian cratonic rocks with two distinct crustal affinities that are defined by radiogenic isotopic geochemistry and detrital zircon age spectra. One is derived predominantly from the Paleoproterozoic and Archean rocks of the Indian cratonic interior and is either represented as metamorphosed sedimentary rocks of the Lesser Himalayan Sequence(LHS) or as slices of the distal cratonic margin. The other is the Greater Himalayan Sequence(GHS) whose provenance is less clear and has an enigmatic affinity. Here we present new detrital zircon Hf analyses from LHS and GHS samples spanning over 1000 km along the orogen that respectively show a striking similarity in age spectra and Hf isotope ratios. Within the GHS, the zircon age populations at 2800-2500 Ma,1800 Ma, 1000 Ma and 500 Ma can be ascribed to various Gondwanan source regions; however, a pervasive and dominant Tonianage population(～860-800 Ma) with a variably enriched radiogenic Hf isotope signature(eHf = 10 to-20) has not been identified from Gondwana or peripheral accreted terranes. We suggest this detrital zircon age population was derived from a crustal province that was subsequently removed by tectonic erosion. Substantial geologic evidence exists from previous studies across the Himalaya supporting the Cambro-Ordovician Kurgiakh Orogeny. We propose the tectonic removal of Tonian lithosphere occurred prior to or during this Cambro-Ordovician episode of orogenesis in a similar scenario as is seen in the modern Andean and Indonesian orogenies, wherein tectonic processes have removed significant portions of the continental lithosphere in a relatively short amount of time. This model described herein of the pre-Himalayan northern margin of Greater India highlights the paucity of the geologic record associated with the growth of continental crust. Although the continental crust is the archive of Earth history, it is vital to recognize the ways in which preservation

关键词： Himalaya Gondwana Zircon Subduction erosion

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Generation and preservation of continental crust in the Grenville Orogeny

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Geoscience Frontiers 2015年第3期6卷 357-372页

作者： christopher j.spencer Peter A.Cawood Chris j.Hawkesworth Anthony R.Prave Nick M.W.Roberts Matthew S.A.Horstwood Martin j.Whitehouse EIMF Department of Earth and Environmental Sciences University of St Andrews NERC Isotope Geosciences Laboratory British Geological Survey Swedish Museum of Natural History and Nordic Center for Earth Evolution(NordCEE) Edinburgh Ion Microprobe Facility University of Edinburgh

Detrital zircons from modern sediments display an episodic temporal distribution of U-Pb crystallization ages forming a series of ＇peaks＇ and ＇troughs＇. The peaks are interpreted to represent either periods of enhanced generation of granitic magma perhaps associated with mantle overturn and superplume events, or preferential preservation of continental crust during global collisional orogenesis. The close association of those peaks with the assembly of supercontinents implies a causal relationship between collisional orogenesis and the presence of zircon age peaks. Here these two end-member models （episodic periodicity of increased magmatism versus selective preservation during collisional orogenesis） are assessed using U-Pb, Hf, and 0 analysis of detrital zircons from sedimentary successions deposited during the - 1.3-1.1 Ga accretionary, -1.1-0.9 Ga collisional, and 〈 0.9 Ga extensional collapse phases of the Grenville orogenic cycle in Labrador and Scotland. The pre-collisional, accretionary stage provides a baseline of continental crust present prior to orogenesis and is dominated by Archean and Paleoproterozoic age peaks associated with pre-1300 Ma Laurentian geology. Strata deposited during the Grenville Orogeny display similar Archean and Paleoproterozoic detrital populations along with a series of broad muted peaks from - 1500 to 1100 Ma. However, post-collisional sedimentary successions display a dominant age peak between 1085 and 985 Ma, similar to that observed in modern North American river sediments. Zircons within the post-orogenic sedimentary successions have progressively lower EHf and higher -lSO values from - 1800 to - 1200 Ma whereupon they have higher EHf and -3180 within the dominant 1085-985 Ma age peak. Furthermore, the Lu-Hf isotopic profile of the Grenville-related age peak is consistent with significant assimilation and contamination by older crustal material, The timing of this dominant age peak coincides with the peak of metamorphism and

关键词： Zircon Continental crust Grenville Orogeny Crustal preservation

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Strategies towards robust interpretations of in situ zircon oxygen isotopes

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Geoscience Frontiers 2023年第2期14卷 241-255页

作者： janne Liebmann christopher L.Kirkland john B.Cliff christopher j.spencer Aaron j.Cavosie Timescales of Mineral Systems Group The Institute for Geoscience Research(TIGeR)School of Earth and Planetary SciencesCurtin UniversityPerthAustralia Pacific Northwest National Laboratory RichlandWashington 99354United States Department of Geological Sciences and Geological Engineering Queen’s UniversityKingstonONCanada Space Science and Technology Centre The Institute for Geoscience Research(TIGeR)School of Earth and Planetary ScienceCurtin UniversityPerthAustralia

Oxygen isotopes are a versatile tool to address a wide range of questions in the Earth *** include geothermometry,paleoclimatology,tracing of geochemical reservoirs,fluid-rock interaction,magmatic petrogenesis,and identification of extra-terrestrial *** arguably provides one of the most robust records of primary magmatic O isotope ratio due to low diffusion rates in crystalline *** ability to correlate zircon O isotopes with temporal and petrogenetic information(e.g.U-Pb geochronology,Lu-Hf isotopes,and trace elements)makes this mineral a key archive for understanding Earth’s crustal ***,zircon O isotope geochemistry has found widespread usage to address fundamental questions across the earth and planetary *** general apparent ease of O isotopic acquisition through the advancement of rapid in situ techniques(*** ion mass spectrometry;SIMS)and associated dedicated national laboratories has led to the generation of large O isotopic data sets of variable quality,highlighting the importance of a coherent workflow for data collection,reduction,and *** paper presents a set of approaches for measurement,assessment,and reporting of zircon O isotope *** focus in this contribution is on in situ analysis via secondary ion mass spectrometry using large geometry instruments,but other commonly used techniques are briefly reviewed for *** work aims to provide an analytical framework necessary for geologically meaningful interpretation of O isotope *** addition,we describe inherent geological(***-induced disturbance of the zircon O isotopic system)and analytical(*** due to sample topography effects)challenges and outline means to identify and avoid such issues as a prerequisite to the generation of robust primary O isotopic signatures for geological interpretation.

关键词： In situ Oxygen isotopes Zircon SIMS Secondary ion mass spectrometry Off-mount standard

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Evolving mantle convection from bottom up to top down

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The Innovation 2022年第6期3卷 13-14页

作者： Ross N.Mitchell Michael Brown Thomas M.Gernon christopher j.spencer State Key Laboratory of Lithospheric Evolution Institute of Geology and GeophysicsChinese Academy of SciencesBeijing 100029China Laboratory for Crustal Petrology Department of GeologyUniversity of MarylandCollege ParkMD 20742USA School of Ocean and Earth Science University of SouthamptonSouthampton SO224JRUK Department of Geological Sciences and Geological Engineering Queen’s UniversityKingstonON K7L 2N8Canada

When it comes to convection,what goes up must come *** is it,what goes down must come up?The truth is,it *** convection must be mass-balanced,there is no reason that it must be force-balanced:the positive and negative buoyancy forces driving convection up and down,respectively,do not necessarily need to be *** balance,or imbalance,all depends on the top and bottom boundary layers.

关键词： necessarily boundary hough

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Low-δ18O A-type granites in SW China: Evidence for the interaction between the subducted Paleotethyan slab and the Emeishan mantle plume

Low-δ18O A-type granites in SW China: Evidence for the inte...

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2020年中国地球科学联合学术年会

作者： jian Xu Xiao-Ping Xia Qiang Wang christopher j.spencer Bin He Chun-Kit Lai State Key Laboratory of Isotope Geochemistry Guangzhou Institute of Geochemistry Chinese Academy of Sciences Department of Geological Sciences and Geological Engineering Queen's University Faculty of Science Universiti Brunei Darussalam

Mantle plumes are geochemically and isotopically heterogeneous. Although incorporation of subducted slabs may account for such heterogeneity, the mechanism and process by which subducted slabs are involved into magmas has long been controversial. Here, we first report Late Permian-Early Triassic A-type granites on the margin of the Emeishan Large Igneous Province(ELIP) in SW China.

关键词： A-type granites in SW China Evidence for the interaction between the subducted Paleotethyan slab and the Emeishan mantle plume Low

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Corrigendum to"Strategies towards statistically robust interpretations of in situ U-Pb zircon geochronology"[***.7(2016)581-589]

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Geoscience Frontiers 2019年第6期10卷 2329-2329页

作者： christopher j.spencer christopher L.Kirkland Richard j.M.Taylor The Institute of Geoscience Research(TIGeR) School of Earth and Planetary SciencesCurtin UniversityWA6102Australia Centre for Exploration Targeting Curtin Node Department of Applied GeologyWestern Australian School of MinesCurtin UniversityWA6102Australia Australian Research Council Centre of Excellence for Core to Crust Fluid SystemsPerthWA6102Australia

Unfortunately,we failed to notice a typesetting error that resulted in mistakes in the equations published in spencer et al.(2016);see also spencer et al.,(2017).The correct equation for the weighted arithmetic mean is:-x=∑^n/i-1(xiσi^-2)/∑^n/i-1σi-2The correct equation for weighted uncertainty is:σ-x=√1/∑^n/i-1σi-2The equations used in the KDX program are not affected by this(spencer et al.,2017).The authors express gratitude to Paul Green for bringing this error to our attention.

关键词： error equation bringing

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