New GSA Bulletin articles published ahead of print in May

Geological Society of America

Boulder, Colo., USA: The Geological Society of America regularly publishesarticles online ahead of print. For April, GSA Bulletin topicsinclude multiple articles about the dynamics of China and Tibet; newinsights into the Chicxulub impact structure; and the dynamic topography ofthe Cordilleran foreland basin. You can find these articles at

Tectonic and eustatic control of Mesaverde Group(Campanian-Maastrichtian) architecture, Wyoming-Utah-Colorado region,USA

Keith P. Minor; Ronald J. Steel; Cornel Olariu

Abstract:We describe and analyze the depositional history and stratigraphicarchitecture of the Campanian and Maastrichtian succession of the southerngreater Green River basin of Wyoming, USA, and surrounding areas to betterunderstand the interplay between tectonic and eustatic drivers that builtthe stratigraphy. By integrating new measured sections with publishedoutcrop, well-log, and paleogeographic data, two new stratigraphiccorrelation diagrams, 35 new paleogeographic reconstructions, and six newtectonic diagrams were created for this part of the Western InteriorSeaway. From this work, two time-scales of organization are evident: (1)100−300 k.y.-scale, mainly eustatically driven regressive-transgressiveshoreline oscillations that generated repeated sequences ofalluvial-coastal plain-shoreline deposits, passing basinward to subaqueousdeltas, then capped by transgressive estuarine/barrier lagoon deposits, and(2) 3.0−4.0 m.y.-scale, tectonically driven groups of 10 to 15 of theseeustatically driven units stacked in an offset arrangement to form largerclastic units, which are herein referred to as clastic wedges. Fourregional clastic wedges are recognized, based on the architectures of theseclastic packages. These are the: (1) Adaville, (2) Rock Springs, (3) Iles,and (4) Williams Fork clastic wedges. Pre-Mesaverde deposition in theWyoming-Utah-Colorado (USA) region during the Middle Cretaceous wascharacterized by thickening of the clastic wedge close to the thrust-front,driven primarily by retroarc foreland basin (flexural) tectonics. However,a basinward shift in deposition during the Santonian into the earlyCampanian (Adaville clastic wedge) signaled a change in the dominantstratigraphic drivers in the region. Shoreline advance accelerated in theearly to middle Campanian (Rock Springs clastic wedge), as the end ofactivity in the thrust belt, growing importance of flat-slab subduction,and steady eastward migration of the zone of dynamic subsidence led to lossof the foredeep and forebulge, with the attendant formation of alow-accommodation shelf environment. This “flat-shelf” environment promotedlarge shoreline advances and retreats during sea-level rise and fall.During the middle to late Campanian (Iles clastic wedge), deep erosion onthe crest of the Moxa Arch, thinning on the crests of the Rock Springs andRawlins uplifts, and subsequent Laramide-driven basin formation occurred asthe Laramide blocks began to partition the region. The next clastic package(Williams Fork clastic wedge) pushed the shoreline over 400 km away fromthe thrust belt during the late Campanian. This was followed by a verylarge and persistent marine transgression across the region, with theformation of a Laramide-driven deepwater turbidite basin with toe-of-slopefans into the early Maastrichtian. The Mesaverde Group in theWyoming-Utah-Colorado region is thus characterized by: (1) a succession offour tectonically driven classic wedges, each comprised of a dozen or soeustatically driven packages that preserve large basinward and landwardshoreline shifts, (2) broad regional sand and silt dispersal on alow-accommodation marine shelf setting, (3) a progressive, tectonicallydriven, basinward shift of deposition with offset, basinward stacking ofsuccessive clastic wedges, and (4) the gradual formation of various upliftsand sub-basins, the timing and sizes of which were controlled by themovement of deep-seated Laramide blocks. The Mesaverde Group in theWyoming-Utah-Colorado region provides an outstanding opportunity to studythe dynamic interaction among the tectonic control elements of a subductingplate (crustal loading-flexure, dynamic subsidence/uplift, and regionalflat-slab basin partitioning), as well as the dynamic interaction oftectonic and eustatic controls.

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A new K-Ar illite dating application to constrain the timing ofsubduction in West Sarawak, Borneo

Qi Zhao; Yi Yan; Satoshi Tonai; Naotaka Tomioka; Peter D. Clift …

Abstract:The timing of subduction is a fundamental tectonic problem for tectonicmodels, yet there are few direct geological proxies for constraining it.However, the matrix of a tectonic mélange formed in a subduction-accretionsetting archives the physical/chemical attributes at the time ofdeformation during the subduction-accretion process. Thus, the deformationage of the matrix offers the possibility to directly constrain the periodof the subduction-accretion process. Here we date the Lubok Antu tectonicmélange and the overlying Lupar Formation in West Sarawak, Borneo by K-Aranalysis of illite. The ages of authigenic illite cluster around 60 Ma and36 Ma. The maximum temperatures calculated by vitrinite reflectance valuessuggest that our dating results were not affected by external heating.Thus, the ages of authigenic illite represent the deformation age of themélange matrix and the timing of the Rajang Unconformity, indicating thatthe subduction in Sarawak could have continued until ca. 60 Ma and thethermal and/or fluid flow events triggered by a major uplift of the RajangGroup occurred at ca. 36 Ma. Furthermore, this study highlights thepotential of using the tectonic mélange to extract the timeframe ofsubduction zone episodic evolution directly.

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Constraining the effects of dynamic topography on the development ofLate Cretaceous Cordilleran foreland basin, western United States

Zhiyang Li; Jennifer Aschoff

Abstract:Dynamic topography refers to the vertical deflection (i.e., uplift andsubsidence) of the Earth’s surface generated in response to mantle flow.Although dynamic subsidence has been increasingly invoked to explain thesubsidence and migration of depocenters in the Late Cretaceous NorthAmerican Cordilleran foreland basin (CFB), it remains a challenging task todiscriminate the effects of dynamic mantle processes from other subsidencemechanisms, and the spatial and temporal scales of dynamic topography isnot well known. To unravel the relationship between sedimentary systems,accommodation, and subsidence mechanisms of the CFB through time and space,a high-resolution chronostratigraphic framework was developed for the UpperCretaceous strata based on a dense data set integrating >600 well logsfrom multiple basins/regions in Wyoming, Utah, Colorado, and New Mexico,USA. The newly developed stratigraphic framework divides the UpperCretaceous strata into four chronostratigraphic packages separated bychronostratigraphic surfaces that can be correlated regionally andconstrained by ammonite biozones. Regional isopach patterns and shorelinetrends constructed for successive time intervals suggest that dynamicsubsidence influenced accommodation creation in the CFB starting from ca.85 Ma, and this wave of subsidence increasingly affected the CFB by ca. 80Ma as subsidence migrated from the southwest to northeast. During 100−75Ma, the depocenter migrated from central Utah (dominantly flexuralsubsidence) to north-central Colorado (dominantly dynamic subsidence).Subsidence within the CFB during 75−66 Ma was controlled by the combinedeffects of flexural subsidence induced by local Laramide uplifts anddynamic subsidence. Results from this study provide new constraints on thespatio-temporal footprint and migration of large-scale (>400 km × 400km) dynamic topography at an average rate ranging from ∼120 to 60 km/ the CFB through the Late Cretaceous. The wavelength and location ofdynamic topography (subsidence and uplift) generated in response to thesubduction of the conjugate Shatsky Rise highly varied through both spaceand time, probably depending on the evolution of the oceanic plateau (e.g.,changes in its location, subduction angle and depth, and buoyancy).Careful, high-resolution reconstruction of regional stratigraphicframeworks using three-dimensional data sets is critical to constrain theinfluence of dynamic topography. The highly transitory effects of dynamictopography need to be incorporated into future foreland basin models tobetter reconstruct and predict the formation of foreland basins that mayhave formed under the combined influence of upper crustal flexural loadingand dynamic subcrustal loading associated with large-scale mantle flows.

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Mid-Cretaceous thick carbonate accumulation in Northern Lhasa (Tibet):eustatic vs. tectonic control?

Yiwei Xu; Xiumian Hu; Eduardo Garzanti; Marcelle BouDagher-Fadel; GaoyuanSun …

Abstract:Widespread accumulation of thick carbonates is not typical of orogenicsettings. During the mid-Cretaceous, near the Bangong suture in thenorthern Lhasa terrane, the shallow-marine carbonates of the LangshanFormation, reaching a thickness up to ∼1 km, accumulated in anepicontinental seaway over a modern area of 132 × 103 km 2, about half of the Arabian/Persian Gulf. The origin ofbasin-wide carbonate deposits located close to a newly formed orogenic beltis not well understood, partly because of the scarcity of paleogeographicstudies on the evolution of the northern Lhasa. Based on a detailedsedimentological and stratigraphic investigation, three stages in themid-Cretaceous paleogeographic evolution of northern Lhasa were defined:(1) remnant clastic sea with deposition of Duoni/Duba formations (Early toearly Late Aptian, ca. 125−116 Ma); (2) expanding carbonate seaway ofLangshan Formation (latest Aptian−earliest Cenomanian, ca. 116−99 Ma); and(3) closure of the carbonate seaway represented by the Daxiong/Jingzhushanformations (Early Cenomanian to Turonian, ca. 99−92 Ma). Combined with dataon tectonic subsidence and eustatic curves, we emphasized the largelyeustatic control on the paleogeographic evolution of the northern Lhasaduring the latest Aptian−earliest Cenomanian when the Langshan carbonatesaccumulated, modulated by long-term slow tectonic subsidence and highcarbonate productivity.

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Early and middle Miocene ice sheet dynamics in the Ross Sea: Resultsfrom integrated core-log-seismic interpretation

Lara F. Pérez; Laura De Santis; Robert M. McKay; Robert D. Larter; JeanineAsh …

Abstract:Oscillations in ice sheet extent during early and middle Miocene areintermittently preserved in the sedimentary record from the Antarcticcontinental shelf, with widespread erosion occurring during major ice sheetadvances, and open marine deposition during times of ice sheet retreat.Data from seismic reflection surveys and drill sites from Deep Sea DrillingProject Leg 28 and International Ocean Discovery Program Expedition 374,located across the present-day middle continental shelf of the central RossSea (Antarctica), indicate the presence of expanded early to middle Miocenesedimentary sections. These include the Miocene climate optimum (MCO ca.17−14.6 Ma) and the middle Miocene climate transition (MMCT ca. 14.6−13.9Ma). Here, we correlate drill core records, wireline logs and reflectionseismic data to elucidate the depositional architecture of the continentalshelf and reconstruct the evolution and variability of dynamic ice sheetsin the Ross Sea during the Miocene. Drill-site data are used to constrainseismic isopach maps that document the evolution of different ice sheetsand ice caps which influenced sedimentary processes in the Ross Sea throughthe early to middle Miocene. In the early Miocene, periods of localizedadvance of the ice margin are revealed by the formation of thick sedimentwedges prograding into the basins. At this time, morainal bank complexesare distinguished along the basin margins suggesting sediment supplyderived from marine-terminating glaciers. During the MCO,biosiliceous-bearing sediments are regionally mapped within the depocentersof the major sedimentary basin across the Ross Sea, indicative ofwidespread open marine deposition with reduced glacimarine influence. Atthe MMCT, a distinct erosive surface is interpreted as representinglarge-scale marine-based ice sheet advance over most of the Ross Seapaleo-continental shelf. The regional mapping of the seismic stratigraphicarchitecture and its correlation to drilling data indicate a regionaltransition through the Miocene from growth of ice caps and inland icesheets with marine-terminating margins, to widespread marine-based icesheets extending across the outer continental shelf in the Ross Sea.

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Late Quaternary aggradation and incision in the headwaters of theYangtze River, eastern Tibetan Plateau, China

Yang Yu; Xianyan Wang; Shuangwen Yi; Xiaodong Miao; Jef Vandenberghe …

Abstract:River aggradation or incision at different spatial-temporal scales aregoverned by tectonics, climate change, and surface processes which alladjust the ratio of sediment load to transport capacity of a channel. Buthow the river responds to differential tectonic and extreme climate eventsin a catchment is still poorly understood. Here, we address this issue byreconstructing the distribution, ages, and sedimentary process of fluvialterraces in a tectonically active area and monsoonal environment in theheadwaters of the Yangtze River in the eastern Tibetan Plateau, China.Field observations, topographic analyses, and optically stimulatedluminescence dating reveal a remarkable fluvial aggradation, followed byterrace formations at elevations of 55−62 m (T7), 42−46 m (T6), 38 m (T5),22−36 m (T4), 18 m (T3), 12−16 m (T2), and 2−6 m (T1) above the presentfloodplain. Gravelly fluvial accumulation more than 62 m thick has beendated prior to 24−19 ka. It is regarded as a response to cold climateduring the last glacial maximum. Subsequently, the strong monsoonprecipitation contributed to cycles of rapid incision and lateral erosion,expressed as cut-in-fill terraces. The correlation of terraces suggeststhat specific tectonic activity controls the spatial scale and geomorphiccharacteristics of the terraces, while climate fluctuations determine thevalley filling, river incision and terrace formation. Debris and colluvialsediments are frequently interbedded in fluvial sediment sequences,illustrating the episodic, short-timescale blocking of the channel ca. 20ka. This indicates the potential impact of extreme events on geomorphicevolution in rugged terrain.

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Late Neoproterozoic to early Paleozoic paleogeographic position of theYangtze block and the change of tectonic setting in its northwesternmargin: Evidence from detrital zircon U-Pb ages and Hf isotopes ofsedimentary rocks

Bingshuang Zhao; Xiaoping Long; Jin Luo; Yunpeng Dong; Caiyun Lan …

Abstract:The crustal evolution of the Yangtze block and its tectonic affinity toother continents of Rodinia and subsequent Gondwana have not been wellconstrained. Here, we present new U-Pb ages and Hf isotopes of detritalzircons from the late Neoproterozoic to early Paleozoic sedimentary rocksin the northwestern margin of the Yangtze block to provide criticalconstraints on their provenance and tectonic settings. The detrital zirconsof two late Neoproterozoic samples have a small range of ages (0.87−0.67Ga) with a dominant age peak at 0.73 Ga, which were likely derived from theHannan-Micangshan arc in the northwestern margin of the Yangtze block. Inaddition, the cumulative distribution curves from the difference betweenthe depositional age and the crystalline age (CA−DA) together with themostly positive εHf(t) values of these zircon crystals(−6.8 to +10.7, ∼90% zircon grains with εHf[t]> 0) suggest these samples were deposited in a convergent setting duringthe late Neoproterozoic. In contrast, the Cambrian−Silurian sediments sharea similar detrital zircon age spectrum that is dominated by Grenvillianages (1.11−0.72 Ga), with minor late Paleoproterozoic (ca. 2.31−1.71 Ga),Mesoarchean to Neoarchean (3.16−2.69 Ga), and latest Archean to earlyPaleoproterozoic (2.57−2.38 Ga) populations, suggesting a significantchange in the sedimentary provenance and tectonic setting from a convergentsetting after the breakup of Rodinia to an extensional setting during theassembly of Gondwana. However, the presence of abundant Grenvillian andNeoarchean ages, along with their moderately to highly rounded shapes,indicates a possible sedimentary provenance from exotic continentalterrane(s). Considering the potential source areas around the Yangtze blockwhen it was a part of Rodinia or Gondwana, we suggest that the source ofthese early Paleozoic sediments had typical Gondwana affinities, such asthe Himalaya, north India, and Tarim, which is also supported by theirstratigraphic similarity, newly published paleomagnetic data, andtectono-thermal events in the northern fragments of Gondwana. This impliesthat after prolonged subduction in the Neoproterozoic, the northwesternmargin of the Yangtze block began to be incorporated into the assembly ofGondwana and then accept sediments from the northern margin of Gondwanalandin a passive continental margin setting.

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Constraining the duration of the Tarim flood basalts (northwesternChina): CA-TIMS zircon U-Pb dating of tuffs

Yu-Ting Zhong; Zhen-Yu Luo; Roland Mundil; Xun Wei; Hai-Quan Liu …

Abstract:The Early Permian Tarim large igneous province (LIP) in northwestern Chinacomprises voluminous basaltic lava flows, as well as ultramafic and silicicintrusions. The age and duration of the Tarim LIP remains unclear, and thusthe rate of magma production and models of potential environmental effectsare uncertain. Here we present high-precision chemical abrasion−isotopedilution−thermal ionization mass spectrometry zircon U-Pb ages for threenewly discovered tuff layers interlayered with lava flows in theKupukuziman and Kaipaizileike formations in the Keping area (Xinjiang,northwest China). The volcanism of the Kupukuziman Formation is constrainedto a short duration from 289.77 ± 0.95 to 289.41 ± 0.52 Ma. An age for theoverlying Kaipaizileike Formation is 284.27 ± 0.39 Ma, bracketing theduration of the entire eruptive phase of the Tarim flood basalts at ∼5.5m.y. The low eruption rate and relatively long duration of magmatism isconsistent with a plume incubation model for the Tarim LIP.

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Late Pleistocene−Holocene flood history, flood-sediment provenance andhuman imprints from the upper Indus River catchment, Ladakh Himalaya

Choudhurimayum Pankaj Sharma; Poonam Chahal; Anil Kumar; Saurabh Singhal;YP Sundriyal …

Abstract:The Indus River, originating from Manasarovar Lake in Tibet, runs along theIndus Tsangpo suture zone in Ladakh which separates the Tethyan Himalaya inthe south from the Karakoram zone to the north. Due to the barriers createdby the Pir-Panjal ranges and the High Himalaya, Ladakh is located in a rainshadow zone of the Indian summer monsoon (ISM) making it a high-altitudedesert. Occasional catastrophic hydrological events are known to endangerlives and properties of people residing there. Evidence of such events inthe recent geologic past that are larger in magnitude than modernoccurrences is preserved along the channels. Detailed investigation ofthese archives is imperative to expand our knowledge of extreme floods thatrarely occur on the human timescale. Understanding the frequency,distribution, and forcing mechanisms of past extreme floods of this regionis crucial to examine whether the causal agents are regional, global, orboth on long timescales. We studied the Holocene extreme flood history ofthe Upper Indus catchment in Ladakh using slackwater deposits (SWDs)preserved along the Indus and Zanskar Rivers. SWDs here are composed ofstacks of sand-silt couplets deposited rapidly during large flooding eventsin areas where a sharp reduction of flow velocity is caused by localgeomorphic conditions. Each couplet represents a flood, the age of which isconstrained using optically stimulated luminescence for sand andaccelerator mass spectrometry and liquid scintillation counter 14C for charcoal specks from hearths. The study suggestsoccurrence of large floods during phases of strengthened ISM when themonsoon penetrated into arid Ladakh. Comparison with flood records ofrivers draining other regions of the Himalaya and those influenced by theEast Asian summer monsoon (EASM) indicates asynchronicity with the WesternHimalaya that confirms the existing anti-phase relationship of the ISM-EASMthat occurred in the Holocene. Detrital zircon provenance analysisindicates that sediment transportation along the Zanskar River is moreefficient than the main Indus channel during extreme floods. Post−LastGlacial Maximum human migration, during warm and wet climatic conditions,into the arid upper Indus catchment is revealed from hearths found withinthe SWDs.

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New insights into the formation and emplacement of impact melt rockswithin the Chicxulub impact structure, following the 2016 IODP-ICDPExpedition 364

Sietze J. de Graaff; Pim Kaskes; Thomas Déhais; Steven Goderis; VincianeDebaille …

Abstract:This study presents petrographic and geochemical characterization of 46pre-impact rocks and 32 impactites containing and/or representing impactmelt rock from the peak ring of the Chicxulub impact structure (Yucatán,Mexico). The aims were both to investigate the components that potentiallycontributed to the impact melt (i.e., the pre-impact lithologies) and tobetter elucidate impact melt rock emplacement at Chicxulub. The impactitespresented here are subdivided into two sample groups: the lower impact meltrock−bearing unit, which intrudes the peak ring at different intervals, andthe upper impact melt rock unit, which overlies the peak ring. Thegeochemical characterization of five identified pre-impact lithologies(i.e., granitoid, dolerite, dacite, felsite, and limestone) was able toconstrain the bulk geochemical composition of both impactite units. Thesepre-impact lithologies thus likely represent the main constituentlithologies that were involved in the formation of impact melt rock. Ingeneral, the composition of both impactite units can be explained by mixingof the primarily felsic and mafic lithologies, but with varying degrees ofcarbonate dilution. It is assumed that the two units were initially part ofthe same impact-produced melt, but discrete processes separated them duringcrater formation. The lower impact melt rock−bearing unit is interpreted torepresent impact melt rock injected into the crystalline basement duringthe compression/excavation stage of cratering. These impact melt rocklayers acted as delamination surfaces within the crystalline basement,accommodating its displacement during peak ring formation. This movementstrongly comminuted the impact melt rock layers present in the peak ringstructure. The composition of the upper impact melt rock unit wascontingent on the entrainment of carbonate components and is interpreted tohave stayed at the surface during crater development. Its formation was notfinalized until the modification stage, when carbonate material would havereentered the crater.

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Isotopic spatial-temporal evolution of magmatic rocks in the Gangdesebelt: Implications for the origin of Miocene post-collisional giantporphyry deposits in southern Tibet

Chen-Hao Luo; Rui Wang; Roberto F. Weinberg; Zengqian Hou

Abstract:Crustal growth is commonly associated with porphyry deposit formationwhether in continental arcs or collisional orogens. The Miocene high-Kcalc-alkaline granitoids in the Gangdese belt in southern Tibet, associatedwith porphyry copper deposits, are derived from the juvenile lower crustwith input from lithospheric mantle trachytic magmas, and are characterizedby adakitic affinity with high-Sr/Y and La/Yb ratios as well as high Mg#and more evolved isotopic ratios. Researchers have argued, lower crust withmetal fertilization was mainly formed by previous subduction-relatedmodification. The issue is that the arc is composed of three stages ofmagmatism including Jurassic, Cretaceous, and Paleocene−Eocene, with peaksof activity at 200 Ma, 90 Ma, and ca. 50 Ma, respectively. All three stagesof arc growth are essentially similar in terms of their whole-rockgeochemistry and Sr-Nd-Hf isotopic compositions, making it difficult todistinguish Miocene magma sources. This study is based on ∼430 bulk-rockSr-Nd isotope data and ∼270 zircon Lu-Hf isotope data and >800whole-rock geochemistry analyses in a 900-km-long section of the Gangdesebelt. We found large scale variations along the length of the arc where theNd-Hf isotopic ratios of the Jurassic, Cretaceous, and Paleocene−Eocene arcrocks change differently from east to west. A significant feature is thatthe spatial distribution of Nd-Hf isotopic values of the Paleocene−Eocenearc magmas and the Miocene granitoids, including metallogenic ones, are”bell-shaped” from east to west, with a peak of εNd(t) and εHf (t) at ∼91°E. In contrast, the Jurassic and Cretaceous arcmagmas have different isotopic distribution patterns as a function oflongitude. The isotopic spatial similarity of the Paleocene−Eocene andMiocene suites suggests that the lower crust source of the metallogenicMiocene magmas is composed dominantly of the Paleocene−Eocene arc rocks.This is further supported by abundant inherited zircons dominated byPaleocene−Eocene ages in the Miocene rocks. Another important discoveryfrom the large data set is that the Miocene magmatic rocks have higher Mg # and more evolved Sr-Nd-Hf isotopic compositions than allpreceding magmatic arcs. These characteristics indicate that theinvolvement of another different source was required to form the Miocenemagmatic rocks. Hybridization of the isotopically unevolved primary magmaswith isotopically evolved, lithospheric mantle-derived trachytic magmas isconsistent with the geochemical, xenolith, and seismic evidence and isessential for the Miocene crustal growth and porphyry deposit formation. Werecognize that the crustal growth in the collisional orogen is a two-stepprocess, the first is the subduction stage dominated by typical magmaticarc processes leading to lower crust fertilization, the second is thecollisional stage dominated by partial melting of a subduction-modifiedlower crust and mixing with a lithospheric mantle-derived melt at thesource depth.

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Oxygen isotope (δ18O) trends measured from Ordovicianconodont apatite using secondary ion mass spectrometry (SIMS):Implications for paleo-thermometry studies

Cole T. Edwards; Clive M. Jones; Page C. Quinton; David A. Fike

Abstract:The oxygen isotopic compositions (δ18O) of minimally altered phosphateminerals and fossils, such as conodont elements, are used as a proxy forpast ocean temperature. Phosphate is thermally stable under low to moderateburial conditions and is ideal for reconstructing seawater temperaturesbecause the P-O bonds are highly resistant to isotopic exchange duringdiagenesis. Traditional bulk methods used to measure conodont δ18O includemultiple conodont elements, which can reflect different environments andpotentially yield an aggregate δ18O value derived from a mixture ofdifferent water masses. In situ spot analyses of individual elements usingmicro-analytical techniques, such as secondary ion mass spectrometry(SIMS), can address these issues. Here we present 108 new δ18O values usingSIMS from conodont apatite collected from four Lower to Upper Ordovicianstratigraphic successions from North America (Nevada, Oklahoma, and theCincinnati Arch region of Kentucky and Indiana, USA). The availableelements measured had a range of thermal alteration regimes that arecategorized based on their conodont alteration index (CAI) as either low(CAI = 1−2) or high (CAI = 3−4). Though individual spot analyses of thesame element yield δ18O values that vary by several per mil (‰), most forma normal distribution around a mean value. Isotopic variability ofindividual spots can be minimized by avoiding surficial heterogeneitieslike cracks, pits, or near the edge of the element and the precision can beimproved with multiple (≥4) spot analyses of the same element. Mean δ18Ovalues from multiple conodonts from the same bed range between 0.0 and 4.3‰(median 1.0‰), regardless of low or high CAI values. Oxygen isotopic valuesmeasured using SIMS in this study reproduce values similar to publishedtrends, namely, δ18O values increase during the Early−Middle Ordovician andplateau by the mid Darriwilian (late Middle Ordovician). Twenty-two of themeasured conodonts were from ten sampled beds that had been previouslymeasured using bulk analysis. SIMS-based δ18O values from these samples aremore positive by an average of 1.7‰ compared to bulk values, consistentwith observations by others who attribute the shift to carbonate- andhydroxyl-related SIMS matrix effects. This offset has implications forpaleo-temperature model estimates, which indicate that a 4 °C temperaturechange corresponds to a 1‰ shift in δ18O (‰). Although this uncertaintyprecludes precise paleo-temperature reconstructions by SIMS, it is valuablefor identifying spatial and stratigraphic trends in temperature that mightnot have been previously possible with bulk approaches.

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