Regional trends and petrologic factors inhibit global interpretations of zircon trace element compositions

The trace element composition of zircon reveals information about the melt that they are derived from,as such,detrital zircon trace element compositions can be used to interrogate melt compositions,and thus the evolution of the continental crust in time and space.Here,we present a global database of detrital zircon compositions and use it to test whether average global trends for five common petrogenetic proxies truly represent secular changes in continental evolution.We demonstrate that the secular trend is broadly comparable across continental regions for Ti-in-zircon temperatures,but for other trace element ratios interrogated,secular trends are highly variable between continental regions.Because trace element ratios result from multiple petrologic variables,we argue that these petrogenetic proxies can be overinterpreted if projected to global geologic processes.In particular,we caution against the interpretation of crustal thickness from trace elements in zircon,and we argue that our results negate current hypotheses concerning secular changes in crustal thickness.

The replication crisis and its relevance to Earth Science studies:Case studies and recommendations

Numerous scientific fields are facing a replication crisis,where the results of a study often cannot be replicated when a new study uses independent data.This issue has been particularly emphasized in psychology,health,and medicine,as incorrect results in these fields could have serious consequences,where lives might be at stake.While other fields have also highlighted significant replication problems,the Earth Sciences seem to be an exception.The paucity of Earth Science research aimed at understanding the replication crisis prompted this study.Specifically,this work aims to fill that gap by seeking to replicate geological results involving various types of time-series.We identify and discuss 11 key variables for replicating U-Pb age distributions:independent data,global sampling,proxy data,data quality,disproportionate non-random sampling,stratigraphic bias,potential filtering bias,accuracy and precision,correlating time-series segments,testing assumptions and divergent analytical methods,and analytical transparency.Even while this work primarily focuses on U-Pb age distributions,most of these factors(or variations of them)also apply to other geoscience disciplines.Thus,some of the discussions involve time-series consisting ofεHf,δ18O-zircon,14C,10Be,marineδ13C,and marineδ18O.We then provide specific recommendations for minimizing adverse effects related to these factors,and in the process enhancing prospects for replicating geological results.

Strategies towards robust interpretations of in situ zircon Lu-Hf isotope analyses

The combination of U-Pb and Lu-Hf compositions measured in zircon crystals is a remarkably powerful isotopic couplet that provides measures on both the timing of mineral growth and the radiogenic enrichment of the source from which the zircon grew.The U-Pb age documents the timing of zircon crystallization/recrystallization and Hf isotopes inform on the degree to which the host melt was derived from a radiogenic reservoir(e.g.depleted mantle) versus an unradiogenic reservoir(e.g.ancient continental crust),or some mixture of these sources.The ease of generating large quantities of zircon U-Pb and Lu-Hf data has been in large part facilitated by instrument advances.However,the dramatic increase in time constrained zircon Lu-Hf analyses in the Earth science community has brought to the fore the importance of careful data collection and reduction workflows,onto which robust geological interpretations may be based.In this work,we discuss the fundamentals of Lu-Hf isotopes in zircon,which then allows us to provide a robust,accessible,methodology for the assessment of data quality.Additionally,we discuss some novel techniques for:data visualization—that facilitates better transparency of data interpretation;integration of geographic information—that may reveal spatial trends where temporal trends were only apparent before;and some novel statistical evaluation tools—that may provide more rigorous interand intra-sample comparisons.

Nomore blind dates with calcite: Fluid-flow vs. fault-slip along the Očkov thrust, Prague Basin

Dating of fracture-filling calcitewith U-Pb geochronology is becoming a rapidly adopted technique for determining the absolute timing of brittle deformation in the upper crust.Slickenfibre calcite is a desirable target,as it precipitates between individual fault slip displacement events,and provides additional kinematic information.Here we present a case study of slickenfibres formed on the Očkov thrust in the Lower Palaezoic Prague Basin,Bohemian Massif,utilising a combination of petrographic and in situ methods.We demonstrate that slickenfibre external textures can be preserved,whilst internally primary textures are removed by fluid infiltration and recrystallization,leading to variable U and Pb mobilisation.One slickenfibre yielded a date of ca.250 Ma,which we interpret as recording fault slip along the Očkov thrust.Another cross-cutting slickenfibre yielded more scattered U-Pb data,with an imprecise apparent age around ca.95 Ma.This slickenfibre is recrystallised,destroying the primary textures,and exhibits element mobility.The meaning of this younger apparent age is therefore questionable;whereas it likely reflects Cretaceous U and Pb mobility assisted by fluid-flow along the fault plane,it may not reflect a period of fault slip.Our results demonstrate that slickenfibre-based U-Pb dates do not unequivocally relate to fault motion,and that petrographic and elemental analyses are important requirements for interpreting calcite U-Pb data.

The dynamic Archean to Paleoproterozoic crustal evolution of Brazil:Preface

Brazil is the largest country in South America and contains the majority of the exposed Archean and Paleoproterozoic crust on the continent. As such, Brazil provides a natural laboratory for studies of the ancient Earth. In this Special Issue, contributions reflect the dynamic and protracted growth and reworking of former continents during a time frame of over a billion years, and depict how changes across the evolution of plate tectonics may have influenced the evolution of the interlinked crust-ocean-atmospheric Earth cycles over this period.

Metamorphism at convergent plate margins:Preface

1.Foreword The year 2020 will long be recognised as a period of sadness and frustration for academics worldwide,given the catastrophic impact that the spread of novel coronavirus“2019-nCoV”-or COVID-19-has had on research and teaching activities,as well as in their personal lives.This Special Issue was designed to represent an avenue for dissemination of many studies presented at the September2019 Geological Society of America session T23.

A continental back-arc setting for the Namaqua belt:Evidence from the Kakamas Domain

A study of the NW Kakamas Domain in South Africa/Namibia provides a new,unified lithostratigraphy and evolutionary history applicable to the whole Namaqua Sector.The Mesoproterozoic history ranges from~1350 Ma to 960 Ma,but isotopic evidence suggests it was built upon pre-existing Paleoproterozoic continental crust that extended west from the Archaean Craton.In eastern Namaqualand,early rift-related magmatism and sedimentation at~1350 Ma occurred in a confined ocean basin.Subsequent tectonic reversal and subduction at~1290–1240 Ma led to establishment of the Areachap,Konkiep and Kaaien Domains.In the Kakamas Domain,widespread deposition of pelitic sediments occurred at~1220 Ma(Narries Group).These contain detrital zircons derived from proximal crust with ages between~2020 Ma and 1800 Ma(western Palaeoproterozoic domains)and 1350–1240 Ma(eastern early Namaqua domains),suggesting pre-sedimentation juxtaposition.The pelites underwent granulite grade metamorphism at~1210 Ma(peak conditions:4.5–6 kbar and 770–850°C),associated with voluminous,predominantly S-type granitoid orthogneisses between~1210 Ma and 1190 Ma(Eendoorn and Ham River Suites)and low-angle ductile(D_(2))deformation which continued until~1110 Ma,interspersed with periods of sedimentation.This enduring P-T regime is inconsistent with the expected crustal over-thickening associated with the generally-accepted collision-accretion Namaqualand model.Rather,we propose the Namaqua Sector is a‘hot orogen’developed in a wide continental back-arc with subduction west of the present-day outcrop.The observed high geotherm resulted from thinned back-arc lithosphere accompanied by an influx of mantle-derived melts.Ductile D_(2)deformation resulted from"bottom-driven"tectonics and viscous drag within the crust by convective flow in the underlying asthenospheric mantle.This extended tectonothermal regime ceased at~1110 Ma when SW-directed thrusting stacked the Namaqua Domains into their current positions,constrained in the Kakamas Domain by late-to post-tectonic I-type granitoids intruded between~1125 Ma and 1100 Ma(Komsberg Suite).The thermal peak then shifted west to the Bushmanland and Aus Domains,where voluminous granites(1080–1025 Ma)were associated with high-T/low-P granulite facies thermal metamorphism and mega-scale open folding(D_(3)).Unroofing of the Namaqua Sector is marked by large-scale,NW-trending,sub-vertical transcurrent dextral shear zones and associated pegmatites and leucogranites at~990 Ma.

Eclogites and basement terrane tectonics in the northern arm of the Grenville orogen,NW Scotland

The presence of eclogites within continental crust is a key indicator of collisional orogenesis.Eclogites within the Eastern Glenelg basement inlier of the Northern Highland Terrane(NHT)have been redated in order to provide more accurate constraints on the timing of collision within the northern arm of the Grenville Orogen.The eclogites yield dates of ca.1200 Ma which are interpreted to record the onset of continent–continent interaction,and the NHT as a whole is thought to represent the lower plate in successive 1200–1000 Ma collision events.The Eastern Glenelg basement inlier is viewed as a fragment of the leading edge of the NHT continental basement that was partially subducted along a suture and then exhumed back up the subduction channel.Differences in ages of igneous protoliths and intrusive histories,and metamorphic events(this paper)between the NHT basement and the Laurentian foreland,suggests that they were separate crustal blocks until after ca.1600 Ma.We therefore suggest that:(1)the NHT represents a fragment of Archean–Paleoproterozoic crust that was reworked within the ca.1.7–1.6 Ga Labradorian-Gothian belt,although whether it was derived from Laurentia or Baltica is uncertain,and(2)amalgamation of the NHT with the Laurentian foreland did not occur until the terminal stages of the Grenville collision at ca.1000 Ma.

In situ calcite U-Pb geochronology by high-sensitivity single-collector LA-SF-ICP-MS

U-Pb geochronology of calcite using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is an emerging method,with potential applications to a vast array of geological problems.Due to the low levels of U and Pb in calcite,measurement using higher-sensitivity instruments,such as sector field (SF) ICP-MS,have advantages over more commonly used quadrupole (Q) ICP-MS instruments.Using a Thermo Element XR ICP-MS,we demonstrate that the Jet+X cone combination with the N_(2) enhancement technique provides the best sensitivity for measuring U and Pb isotopes.This higher sensitivity improves the precision of calcite U-Pb isotope measurements,and permits dating at high spatial resolutions (<110μm) and for samples containing low contents of^(238)U (<1μg g^(–1)) and/or^(207)Pb (i.e.,young samples<10 Ma).Using a spot size of 85μm with a low fluence (~2.0 J cm^(–2)),the laser-induced elemental fractionation of^(206)Pb/^(238)U in the NIST SRM 614,ARM-3 and WC-1 reference materials are insignificant (<2.2%).Adopting the optimized instrument parameters,we analysed four commonly-used calcite U-Pb reference materials (WC-1,Duff Brown Tank,JT,and ASH-15).The results match well with published isotope dilution data,demonstrating the reliability of our technique.ARM-3,an andesitic glass,is shown to be an appropriate reference material for both^(207)Pb/^(206)Pb calibration and instrument optimization because of its moderate contents of U(~3.75μg g^(–1)) and Pb (~12.7μg g^(–1)).We further demonstrate that the image-guided approach using LA-ICP-MS elemental mapping is an efficient tool in obtaining robust ages.

高灵敏度-单接收杯LA-SF-ICP-MS原位方解石U-Pb定年

激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)方解石U-Pb定年是一种新兴的地质年代学方法,在解决一系列关键地质问题中具有广阔的应用前景.由于方解石中U和Pb含量低,采用高灵敏度扇形磁场电感耦合等离子体质谱仪(SF-ICP-MS)比四极杆电感耦合等离子体质谱仪(Q-ICP-MS)更有优势.研究发现对于Thermo Element XR型号的SF-ICP-MS仪器,当采用Jet+X锥组和N_(2)增敏技术时,U和Pb的灵敏度最佳.提高灵敏度不仅能明显改善方解石定年分析精度,而且可在高空间分辨率条件下(<110μm)实现低^(238)U(<1μg g^(−1))或^(207)Pb(如<10Ma)含量样品的定年测试.对于NIST SRM 614、ARM-3和WC-1这三个标准物质来说,在激光束斑85μm和能量密度~2.0J cm^(−2)的条件下,由激光剥蚀产生的^(206)Pb/^(238)U动态分馏不显著(<2.2%).在最佳仪器条件下,系统分析了四个常用的方解石U-Pb标准物质(WC-1、Duff Brown Tank、JT和ASH-15).结果与已发表的同位素稀释热电离质谱(ID-TIMS)数据相吻合,验证了方法的可靠性.研究进一步证明,基于LA-ICP-MS二维元素成像技术辅助选取样品定年区域可提高方解石U-Pb定年的成功率.