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| 1 | Strategies towards statistically robust interpretations of in situ U-Pb zircon geochronology显示文摘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 sediment fingerprinting method).The ease of acquisition,while bringing benefit to the Earth science community,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 ^(238)U/^(206)Pb and ^(207)Pb/^(206)Pb 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. | Christopher J.Spencer Christopher L.Kirkland Richard J.M.Taylor | 2016 | Geoscience Frontiers2016，7，4： | 36 |
| 2 | Deconvolving the pre-Himalayan Indian margin-Tales of crustal growth and destruction显示文摘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 bias and destruction of continental crust informs geologic models. | Christopher J.Spencer Brendan Dyck Catherine M.Mottram Nick M.W.Roberts Wei-Hua Yao Erin L.Martin | 2019 | Geoscience Frontiers2019，10，3： | 3 |
| 3 | Generation and preservation of continental crust in the Grenville Orogeny显示文摘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 O analysis of detrital zircons from sedimentary successions deposited during the ~ 1.3-1.1Ca 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 εHf and higherδ^(18)O values from ~ 1800 to ~ 1200 Ma whereupon they have higher εHf and δ^(18) within the dominant1085-985 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 magmatism associated with the Crenville Orogeny,which is a typical collisional orogenic belt.The change from broad muted age peaks in the syn-orogenic strata to a single peak in the post-orogenic sedimentary successions and in the modern river sediments implies a significant shift in provenance following continental collision.This temporal change in provenance highlights that the source(s),from which detrital zircons within syn-orogenic strata were derived,was no longer available during the later stages of the accretionary and collisional stages of the orogenic cycle.This may reflect some combination of tectonic burial,erosion,or possibly recycling into the mantle by tectonic erosion of the source(s).During continental collision,the incorporated continental crust is isolated from crustal recycling processes operative at subduction margins.This tectonic isolation combined with sedimentaiy recycling likely controls the presence of the isotopic signature associated with the Grenville Orogeny in the modern Mississippi and Appalachian river sediments.These results imply that zircon age peaks,which developed in conjunction with supercontinents,are the product of selective crustal preservation resulting from collisional orogenesis. | Christopher J.Spencer Peter A.Cawood Chris J.Hawkesworth Anthony R.Prave Nick M.W.Roberts Matthew S.A.Horstwood Martin J.Whitehouse EIMF | 2015 | Geoscience Frontiers2015，6，3： | 1 |
| 4 | Strategies towards robust interpretations of in situ zircon oxygen isotopes显示文摘Oxygen isotopes are a versatile tool to address a wide range of questions in the Earth sciences.Applications include geothermometry,paleoclimatology,tracing of geochemical reservoirs,fluid-rock interaction,magmatic petrogenesis,and identification of extra-terrestrial materials.Zircon arguably provides one of the most robust records of primary magmatic O isotope ratio due to low diffusion rates in crystalline grains.The 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 evolution.Consequently,zircon O isotope geochemistry has found widespread usage to address fundamental questions across the earth and planetary sciences.The general apparent ease of O isotopic acquisition through the advancement of rapid in situ techniques(i.e.secondary 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 presentation.This paper presents a set of approaches for measurement,assessment,and reporting of zircon O isotope data.The 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 context.This work aims to provide an analytical framework necessary for geologically meaningful interpretation of O isotope data.In addition,we describe inherent geological(e.g.radiation-induced disturbance of the zircon O isotopic system)and analytical(e.g.fractionation 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. | Janne Liebmann Christopher L.Kirkland John B.Cliff Christopher J.Spencer Aaron J.Cavosie | 2023 | Geoscience Frontiers2023，14，2： | 1 |
| 5 | Secular change in earth processes: Preface显示文摘1.Introduction 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,as discussed by Gould(1987)in his critique of its usage in the natural sciences:'we should try to explain the past by causes now | Richard M.Palin Christopher J.Spencer | 2018 | Geoscience Frontiers2018，9，4： | 0 |
| 6 | Timescales of geological processes: Preface显示文摘1.Introduction 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 past.The 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 record.However,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 innovations.Earthquakes 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 records.Experimental studies have also revealed that the reaction rates between fluids and rocks can be extremely rapid relative to geological timescales.This 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. | Andrew Putnis Christopher J.Spencer Tom Raimondo | 2019 | Geoscience Frontiers2019，10，1： | 0 |
| 7 | Corrigendum to “Strategies towards robust interpretations of in situ zircon oxygen isotopes”[Geosci.Front.14(2)(2023)101523]显示文摘In the originally published version of our article(Liebmann et al.,2023),Table 2 incorrectly states that the publishedδ^(18)O value of reference zircon BR231 is 5.05‰±0.10‰.The correct value is 9.81‰±0.08‰(2 standard deviations)(Valley,2003). | Janne Liebmann Christopher L.Kirkland John B.Cliff Christopher J.Spencer Aaron J.Cavosie | 2023 | Geoscience Frontiers2023，14，6： | 0 |
| 8 | Corrigendum to'Strategies towards statistically robust interpretations of in situ U-Pb zircon geochronology'[Geosci.Front.7(2016)581-589]显示文摘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. | Christopher J.Spencer Christopher L.Kirkland Richard J.M.Taylor | 2019 | Geoscience Frontiers2019，10，6： | 0 |
| 9 | Granular titanite from the Roter Kamm crater in Namibia:Product of regional metamorphism,not meteorite impact显示文摘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 event.Despite 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 context for interpretation of U-Pb data in granular titanite, and the potential for misinterpreting inherited(pre-impact) microstructures as impact-related phenomenon in target rocks with a complex geological history. | Aaron J.Cavosie Christopher J.Spencer Noreen Evans Kai Rankenburg Robert J.Thomas Paul H.Macey | 2022 | Geoscience Frontiers2022，13，3： | 0 |
| 10 | Evolving mantle convection from bottom up to top down显示文摘When it comes to convection,what goes up must come down.Or is it,what goes down must come up?The truth is,it depends.Although 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 balanced.The balance,or imbalance,all depends on the top and bottom boundary layers. | Ross N.Mitchell Michael Brown Thomas M.Gernon Christopher J.Spencer | 2022 | The Innovation2022，3，6： | 0 |
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