Strategies towards robust interpretations of in situ zircon oxygen isotopes

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.

Unravelling the protracted U-Pb zircon geochronological record of high to ultrahigh temperature metamorphic rocks:Implications for provenance investigations

The assessment of detrital zircon age records is a key method in basin analysis,but it is prone to several biases that may compromise accurate sedimentary provenance investigations.High to ultrahigh temperature(HT-UHT)metamorphism(especially if T>850℃)is herein presented as a natural cause of bias in provenance studies based on U-Pb detrital zircon ages,since zircon from rocks submitted to these extreme and often prolonged conditions frequently yield protracted,apparently concordant,geochronological records.Such age spreading can result from disturbance of the primary U-Pb zircon system,likewise from(re)crystallization processes during multiple and/or prolonged metamorphic events.In this contribution,available geochronological data on Archean,Neoproterozoic and Palaeozoic HT-UHT metamorphic rocks,acquired by different techniques(SIMS and LA-ICP-MS)and showing distinct compositions,are reassessed to demonstrate HT-UHT metamorphism may result in modes and age distributions of unclear geological meaning.As a consequence,it may induce misinterpretations on UPb detrital zircon provenance analyses,particularly in sedimentary rocks metamorphosed under such extreme temperature conditions.To evaluate the presence of HT-UHT metamorphism-related bias in the detrital zircon record,we suggest a workflow for data acquisition and interpretation,combining a multi-proxy approach with:(i)in situ U-Pb dating coupled with Hf analyses to retrieve the isotopic composition of the sources,and(ii)the integration of a petrochronological investigation to typify fingerprints of the HT-UHT metamorphic event.The proposed workflow is validated in the investigation of one theoretical and one natural example allowing a better characterization of the sedimentary sources,maximum depositional ages,and the tectonic setting of the basin.Our workflow allows to the appraisal of biases imposed by HT-UHT metamorphism and resulting disturbances in the U-Pb detrital zircon record,particularly for sedimentary rocks that underwent HT-UHT metamorphism and,finally,suggests ways to overcome these issues.

Corrigendum to “Strategies towards robust interpretations of in situ zircon oxygen isotopes”[Geosci.Front.14(2)(2023)101523]

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).

One size does not fit all: Refining zircon provenance interpretations via integrated grain shape, geochronology, and Hf isotope analysis

Sediment provenance studies commonly utilize isotopic signatures to resolve detrital mineral sources and routing.However,non-unique ages and geochemical characteristics across geographically distinct crystalline source regions can lead to significant ambiguities in mineral provenance interpretations.Such ambiguity is apparent in southern Australia’s Cenozoic Eucla Basin,which hosts world-class heavy mineral sand resources.Here,new Hf isotope data are provided from four heavy mineral prospects(N=8,n=844[N=samples,n=grains]).Zircon grain shape data are also presented for a suite of detrital Eucla Basin samples(N=22,n=35,604)and the basin’s underlying basement,the Coompana Province(N=13,n=824).The data are integrated with published detrital and non-detrital primary zircon data to investigate the efficacy of grain shape analysis to better resolve the basin’s mineral provenance.Zircon Hf isotope compositions indicate a primary Mesoproterozoic juvenile source for zircon melts(~1250-1000 Ma,-2.5<εHf>~+5)with additional contributions from a range of juvenile to evolved late Archean to Phanerozoic-aged zircon bearing magmas(-28.0<εHf>+11).U-Pb geochronology and Hf isotopes are incapable of differentiating Mesoproterozoic-aged source rocks bounding the region for the majority of heavy mineral deposits analyzed as potential sources express overlapping crystallization ages and similarities in Hf-isotope characteristics.However,distinct zircon grain shapes(i.e.,perimeter,major axis and circularity)facilitate improved differentiation across these Mesoproterozoic sources.Filtering of U-Pb age,Hf isotope and shape data implicate the underlying Madura and Coompana provinces as dominant sediment sources for Eucla Basin detritus aged~1400-1000 Ma.The lack of direct sediment pathways between the underlying basement provinces and placer sediments analyzed demonstrates the significance of zircon reworking from intermediate sedimentary basins in the formation of the economically significant Eucla Basin beach placers.Zircon grain shape represents a cheaply acquired and readily incorporated grain characteristic that can enhance provenance investigations.

A Laurentian affinity for the Embu Terrane,Ribeira Belt(SE Brazil),revealed by zircon provenance statistical analysis

New and compiled detrital zircon U-Pb ages from the southern Neoproterozoic-Cambrian Ribeira Belt,SE Brazil,demonstrate Laurentian affinity of the Embu Terrane which is statistically distinct from the adjoining Apiaíand São Roque terranes with cratonic affinity(e.g.,São Francisco Craton).Zircon provenance results indicate that the type-area of the Embu Terrane is dominated by detrital zircon age modes at ca.1200 Ma,1400 Ma,and 1800 Ma,with maximum depositional age of ca.1000 Ma.In contrast,the Apiaíand São Roque terranes are dominated by Paleoproterozoic detrital zircon ages(ca.2200-2000 Ma age dominant component),with maximum depositional ages of ca.1400 Ma and 1750 Ma,respectively.Multidimensional scaling(MDS)analysis of non-parametric similarity measurements on zircon age populations indicates for the first time that the Embu Terrane encompass two statistically distinct detrital zircon age spectra,which is also reflected in the metamorphic zircon age record.The statistical characterization of the Embu Terrane through populational metrics allow a quantitative comparison with surrounding tectonic domains and rock samples classified such as Embu-type.Our results clearly highlight the distinction between the statistically differentiated Embu Terrane from the Apiaíand São Roque terranes,supporting an allochthonous interpretation.In addition,we demonstrate that rocks samples previously classified as Embu-type are significantly dissimilar to the definition of Embu Terrane,failing to support alternative tectonic models(e.g.,intracontinental evolution).Detrital zircon age spectra reveal that the Apiaíand São Roque terranes have similar zircon provenance to domains sourced from the São Francisco Craton,whereas detrital zircon populations from the Embu Terrane have greater affinity with SW Laurentia basins(and their inferred sediment sources),consistent with previous findings.Therefore,we interpret the Embu Terrane as a Rodinia descendant developed along the active margin of the SW Laurentia that collided with the Ribeira Belt during early Neoproterozoic(810-760 Ma).