A recently discovered Bison-bearing fossil locality at Térapa, Sonora, Mexico, had previously been dated to 440 ± 130 ka using whole rock 40Ar/39Ar on a basalt flow that impounds the deposit. This age is considerably older than the accepted age of about 240-160 ka for the migration of Bison into greater North America. The Térapa deposit also contains a mixture of fossils from extralimital or extinct tropical animals and temperate animals. Constraining the age of the deposit is critical to interpret the paleontologic and paleoclimatologic implications of this unique Sonoran fossil locality. Three additional geochronological methods have been applied to this deposit (infrared stimulated luminescence (IRSL), amino acid racemization (AAR), and radiocarbon) and the data from the original 40Ar/39Ar age were revisited. The IRSL data suggest that the impounding basalt flow and the sediments that abut it were emplaced 43 ka ago and that the oldest sediments were deposited shortly after. Two radiocarbon ages suggest the fossiliferous sediments were emplaced by 42 ka. Effective diagenetic temperatures inferred from the AAR results, combined with AAR data from a similar-age deposit in southern Arizona, are in accordance with the 40-43 ka age estimates. For the AAR results to corroborate the 40Ar/39Ar age, the effective diagenetic temperature for the area would need to be approximately 3 °C, which is unrealistically low for northern Mexico. The new geochronological results suggest the Térapa deposit and fossils are 40-43 ka old. The anomalously old 40Ar/39Ar age for the impounding basalt is probably the result of low 40Ar* concentrations and inherited 40Ar.

The Scandinavian Caledonides formed as a result of collision between the continents Baltica and Laurentia, in Silurian and Early Devonian time. The evolution of the orogen has been a topic of research since before the turn of the last century. However, there are still uncertainties regarding the character and timing of the orogenic processes involved in the formation of the Caledonian orogen. Identification and study of high-pressure terranes are a key to understanding the processes involved, and such terrains are found in Jämtland, central Sweden. The most well-known location is Mt. Åreskutan. This study focuses on the Helags Mountain, a locality potentially equivalent to Mt. Åreskutan. It has combined structural and mineralogical studies, pressure and temperature esti-mates, and monazite geochronology, in an attempt to obtain an overview of the metamorphic his-tory.The Helags Mt. geology, as on Åreskutan, is dominated by a klippe of high grade gneisses, overlying lower grade schists and amphibolites, both typical of the middle and lower part of the Seve Nappe Complex in the Swedish Caledonides. The gneisses are dominantly felsic and contain garnet. Two episodes of garnet growth, likely separated in time, are observed in the gneisses. The first episode probably took place in the presence of melt, as is evident from the presence of inclusion of so called nanogranites. This is further supported, but not fully confirmed, by observed homo-genization of the garnet core chemistry. Such processes take place at high temperature, above 700°C. Pressure estimates are less well defined and indicate about 1 GPa during this first garnet growth event. This event may be related to the observed migmatisation. The second garnet growth episode took place at lower pressure and temperature conditions, and similarities with garnet observed in studies elsewhere indicate a connection with shearing and emplacement of the Middle Seve unit. However, no garnets were observed in the studied shear zone, and it is with the available data not possible to confirm a relation to a specific event. Monazite geochronology has contributed Caledonian ages (400-480 Ma) but has not yielded any precise results with regard to the timing of the migmatisation and thrusting. / Den svenska fjällkedjan har en lång historia. Dess nuvarande utformning är ett resultat av att Iapetus-havet, en föregångare till dagens Atlanten, slöts och de tidigare kontinenterna Baltica och Laurentia kolliderade. Trots att fjällkedjan studerats flitigt sedan före sekelskiftet är det mycket som är okänt om de geologiska processerna som varit en del av bergens utveckling. För att bättre förstå fjällkedjans ut-veckling studeras bergarter från områden som varit särskilt kraftigt påverkade. Flera sådana områden påträffas i Jämtlandsfjällen. Denna studie har fokuserat på Helagsfjällets område, beläget i södra delen av Jämtlands län. Studier av det särskilt motståndskraftiga mineralet granat från områdets bergarter har tillsammans med dateringar av mineralet monazit givit ny information om områdets geologiska historia.Helagsfjällets geologi, liksom den välstuderade Åreskutan, utgörs av en enhet av granatförande gneisser, vilken överlagrar en undre enhet av lägre omvandlingsgrad. Detta är typiskt för den mellersta och lägre delen av det så kallade Sevekomplexet. Två generationer granater tyder på att minst två geologiska processer, skilda åt i tid, påverkat områdets bergarter. Den första av dessa granatgenerationer uppvisar bland annat inneslutningar vilka tolkas som bevarade delar av en tidigare smälta. Det är möjligt att denna granatgeneration är relaterad till den tidiga händelse som orsakat uppsmältning, av vilken spår kan studeras på flera platser i området. Tryck- och temperaturberäkningar visar att detta hände under tryck omkring 1 GPa, och temperaturer på över minst 600°C, kanske över 700°C. Den andra granat-generationen är mer svårtolkad. Tryck och temperatur var lägre, och likheter med granater observerade på andra platser tyder på att denna andra granattillväxt skedde i samband med skjuvning av den övre enheten, över den underliggande enheten. Försök att datera dessa två perioder av granattillväxt gav åldrar mellan 400 och 480 miljoner år. Liksom på Åreskutan tyder detta på en tektonisk historia som sträcker sig från Ordovicium till tidig Devon.

L’arc équatorien est caractérisé par un nombre particulièrement élevé de volcans quaternaires. Nos quatre-vingt-dix nouveaux âges K-Ar, réalisés pour une vingtaine d’édifices, montrent que son développement a commencé vers 1 Ma au nord et a migré vers le sud à partir de 600 ka. Ce déplacement pourrait être induit par un changement récent de la géométrie de la plaque Nazca en profondeur et l'activation des failles crustales. Les taux de production magmatique sont assez homogènes à l’échelle de l’arc, et varient entre 0,1 et 5,6 km³.ka⁻¹. Ces données montrent que les volcans se sont construits lors de phases d’activité brèves, séparées par des périodes de repos. Il n'y a pas de corrélation apparente entre l'âge des volcans et leur taux d'émission, mais la subduction de la ride de Carnegie pourrait favoriser la genèse du magma et expliquer la densité de volcans quaternaires en Équateur. Les taux d'érosion varient entre 0,01 et 0,14 km³.ka⁻¹. Les volcans les plus jeunes semblent être érodés par des processus d'altération chimique, tandis que les volcans plus anciens sont principalement démantelés par des processus physiques tels que l'abrasion glaciaire et l'activité tectonique. Enfin, nous montrons que la méthode de datation K-Ar réalisées sur la mésostase ou les esquilles de verre des ponces est bien adaptée pour dater les produits volcaniques quaternaires de l'arc équatorien. / The Ecuadorian arc presents a large number of Quaternary volcanoes. Our ninety new K-Ar ages, carried out on products from twenty volcanic edifices, show that the development of the Quaternary arc began about 1 Ma in northern Ecuador and migrated southward since 600 ka. This migration could be induced by recent change of the slab geometry at depth and by the activation of crustal faults. The magmatic productivity rates are rather homogeneous at the arc scale, and range between 0.1 and 5.6 km³.ka⁻¹. These data highlights that volcanoes grew during sporadic activity pulses separated by quiescence periods. There is no clear correlation between the age of the volcanoes and their emission rates, but the subduction of the Carnegie ridge could favor the magma genesis and explained the increase of the volcanic activity in northern Ecuador during the Quaternary. Erosion rates range between 0.01 and 0.14 km³.ka⁻¹. Youngest volcanic edifices seem to be eroded by a chemical alteration process, while the dismantling of older volcanoes is mostly influenced by physical processes such as glacial abrasion and tectonic activity. Finally, we show that K-Ar ages performed on groundmass or pumice glass shards can be successfully applied to Quaternary volcanic products from the Ecuadorian arc.

This study synthesizes the deposition and tectonic evolution of the Tertiary deposits in southern Cache Valley, a narrow, north-trending valley in the northeastern Basin-and- Range Province. The surrounding mountains consist of Proterozoic and Paleozoic sedimentary rocks. Southern Cache Valley is an east-tilted half-graben. The oldest Tertiary sediments are on the west side of the basin, and the overall dip is to the east. The Late Miocene to Early Pliocene Salt Lake Formation (Tsl) accumulated above the thin (to absent) Early to Middle Eocene Wasatch Formation (Tw) and the newly identified Fowkes and Norwood Tuff equivalents (Tfn; late Middle Eocene to Middle Oligocene). The two post-Wasatch units consist of felsic tuff, tuffaceous sandstone, pebble to boulder conglomerate, limestone, and sandstone. Pebble counts, generalized measured sections, and detailed mapping permitted subdivision of the Tertiary deposits into mappable subunits. Numerous faults and N-trending folds are present in Tertiary deposits in the SW part of the area. The Cenozoic-Paleozoic contact is offset by normal faults. The faults with ENE trends offset both the Paleozoic and Tertiary rocks, and thus are younger than 5 .1 Ma. Thick gravels, rich in Paleozoic carbonates, dominate exposures in the east, near the East Cache fault zone. Few cobbles are present within the Tfn except near the base, whereas conglomerates rich in clasts of Paleozoic carbonates are intertongued with tuffaceous sediments throughout the Tsl, especially eastward. Up to 8,000 feet (3439 m) of the Tsl accumulated as the narrow Cache Valley basin formed during the Neogene by rapid eastward downfaulting.

Amino acid geochronology based on fossil molluscs provides a useful approach to determining the Quaternary history of Great Basin lakes. The Lahontan basin, Nevada, and the Chewaucan basin, Oregon, in the northwest corner of the Great Basin, both contained lakes during the Quaternary. The aim of this study is to improve the Quaternary geochronology in these two basins by measuring time-dependent changes in amino acids preserved in fossil molluscan shells. The abundance of D-alloisoleucine relative to Lisoleucine (All) characterizes the extent of racemization, which increases with age and Ul forms the basis of relative and correlated ages. An age-calibration curve for Vorticifex was developed using All ratios in shells from layers with radiocarbon-dated shells and with one thermoluminescence date in the Chewaucan basin. The All ratios from non-dated deposits were assigned ages from this calibration curve. The All ratios in 77 samples (-350 shells) of mainly Vorticifex were analyzed to improve the lake chronology in the Lahontan and Chewaucan basins. From the stratigraphic position, All ratios in the shells, and previously published radiometric ages, at least five and possibly six lake cycles were inferred in the Lahontan basin for the Quaternary period. Shells with highest All ratios ( -0.8) might correlate with the Rye Patch Alloformation, named for deep-lake sediments deposited in the Lahontan basin -630 ka. The next younger lake deposits are ascribed to the Eetza Alloformation. On the basis of the amino acid data, two and possibly three distinct lake expansions took place during the Eetza lacustrine episode, which lasted from -385 to 145 ka. Deposits of the Sehoo Alloformation ( -35 to 12 ka) can be differentiated from older deposits on the basis of All ratios in mollusc shells. Finally, a few shells with low All ratios near Pyramid Lake may indicate a minor lake expansion during the Holocene. Only two lake cycles were inferred from the amino acid data in the Chewaucan basin for the Quaternary period. Shells with the highest ratios correlated with the Eetza Alloformation and the shells with lowest ratios correlate with the Sehoo Alloformation. The amino acid data suggest that Lake Lahontan and Lake Bonneville experienced similar lake-level histories during the past -660 ka. The Sehoo Alloformation in the Lahontan basin broadly correlates with Bonneville Alloformation in the Bonneville basin based on All ratios and radiocarbon dates. The late and early aminozones within the Eetza Alloformation might correlate, respectively, with the Little Valley and Pokes Point Alloformations in the Bonneville basin.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18803号 / 理博第4061号 / 新制||理||1584(附属図書館) / 31754 / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)教授 平田 岳史, 教授 田上 高広, 教授 土`山 明 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

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The Barsele area is located in the "Gold Line", southwest of the volcanogenic massive sulphide (VMS) Skellefte district which hosts more than 85 known VMS-deposits where some are gold-bearing. The Barsele area consists of the Central, Avan and Skiråsen intrusion hosted gold deposits, the Norra gold-bearing VMS-deposit and the Risberget zone. This study includes a geochronological study of two coherent volcanic rocks from the Norra zone using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry and Scanning Electron Microscope images of zircons and also includes petrographic microscopy of thin sections and lithogeochemical data used in classification diagrams.  This study presents two new LA-ICP-MS zircon U-Pb ages for VMS-hosting volcanic rocks from the Barsele area: o   1956.8 ± 6.47 Ma (n: 17, MSWD: 0.99), sample A from a hydrothermally altered volcanic rock  o   1952.2 ± 14.5 Ma (n: 6, MSWD: 1.1), sample B from a rhyolite-dacite  The new U-Pb age constraints presented in this study indicate that the volcanic rocks of the Norra zone are older than the volcanic rocks in the Skellefte Group. The results presented in this report together with the earlier dated c. 1.96 Ga metadacite suggests that the volcanic rocks of the Barsele area are located at a lower chronostratigraphic level than the Skellefte Group, i.e., at the chronostratigraphically lower Bothnian Supergroup. Additionally, the results presented here conclude that two VMS-events can be delineated by geochronological data which further opens up for the question if the Barsele area belong to an older arc system (e.g., the Knaften arc) or were part of an arc system evolving continuously between 1.95-1.88 Ga (e.g., the Knaften-Skellefte arc). / Barsele, Västerbottens län, norra Sverige, ligger i ett område som kallas för Guld linjen och är beläget sydväst om Skelleftefältet där mer än 85 kända förekomster av massiva sulfidmalmer (VMS), vissa guldförande, förekommer. Barseleområdet är uppdelat i Central, Avan och Skiråsen zonen, Norra zonen och Risberget zonen där Norra zonen är en guldförande VMS-fyndighet. Denna studie inkluderar en geokronologisk studie av zirkoner från två vulkaniter från Norra zonen som har analyserats med Laser Ablation Inductively Coupled Plasma Mass Spectrometry och avbildats med svepelektronmikroskop samt petrografisk studie av två tunnslip och klassificeringsdiagram baserade på litogeokemisk data. Denna studie presenterar två nya LA-ICP-MS zirkon U-Pb åldrar för VMS-förande vulkaniter i  Barseleområdet:  o   1956.8 ± 6.47 Ma (n: 17, MSWD: 0.99), prov A från en hydrotermalt omvandlad vulkanisk bergart o   1952.2 ± 14.5 Ma (n: 6, MSWD: 1.1), prov B från en ryolit-dacit De två nya  U-Pb zirkon åldersbestämningarna som presenteras i denna studie indikerar att de vulkaniska bergarterna i Norra zonen är äldre än de vulkaniska bergarterna som tillhör Skelleftegruppen. Vidare antyder de presenterade åldrarna i denna studie tillsammans med den tidigare genomförda dateringen av en metadacit (cirka 1.96 Ga) på att de vulkaniska bergarterna i Barseleområdet kan tillhöra suprakrustala bergarter tillhörande Bottniska bassängen. Barseleområdet föreslås därför förekomma vid en lägre kronostratigrafisk nivå jämfört med Skelleftegruppen, det vill säga vid samma kronostratigrafisk nivå som Bottniska Supergruppen. Utifrån de resultat som presenteras här kan slutsatsen att två VMS-områden kan avgränsas av geokronologiska data som ytterligare öppnar för frågan om Barsele-området tillhör ett äldre bågsystem (t.ex. Knaften-bågen) eller var en del av ett bågsystem som utvecklades mellan 1.95-1.88 Ga (t.ex. Knaften-Skelleftebågen).

Thesis advisor: Ethan Baxter / Thesis advisor: Mark Behn / Subduction zones are areas of significant mass transfer between Earth’s crust and mantle. The dehydration of water-rich minerals such as serpentinite and lawsonite introduces water and volatiles into the subduction interface, that then travel to the mantle wedge above. The chemical composition, speciation, and redox effect of these fluids carry important implications for arc volcanism and the mobility of economically significant elements. This study uses Fe and Zn isotopic variation in eclogite-breccias from the Monviso ophiolite, combined with Sm-Nd garnet geochronology, to study the composition and redox effects of fluids that were produced during eclogite and blueschist facies metamorphism, and the timescales over which the brecciation and fluid flux events happened. Fe and Zn isotopic measurements were made on a series of four breccia matrix generations (M1-M4), generated during the progressive brecciation of the original Fe-Ti gabbros and the influx of both internally and externally derived fluids. The ∂56Fe and ∂66Zn data display a bi-modal distribution, with early matrix crystallization (M1-3) imparting progressively lighter ∂66Zn values while the ∂56Fe remains relatively unchanged. The last stage of metasomatic rind formation (M4) is associated with a decrease in both Fe and Zn isotopic values and a particularly significant shift in the Fe isotopes. This distribution suggests that early brecciation (M1-3) resulted from small-scale internal fluid flow that did not have a measurable effect on the isotopic composition and redox state of the system. By contrast, late metasomatic rind formation (M4) was facilitated by the flow of large amounts of external fluids with a strongly negative Fe and Zn isotope signature that affected the redox state of the mafic slab and may be responsible for transferring oxidized material into the mantle wedge. Dating of the M4 matrix generation yielded an age of 41.31± 0.60 Ma. A compilation of age data from Monviso suggests peak metamorphism and initial brecciation (M1 formation) likely occurred at ~45 Ma, the formation of the M4 matrix representing the end of eclogite-facies retrogression and brecciation at ~41 Ma, and final blueschist and greenschist retrogression at ~38-35Ma, yielding timescale of ~4Ma for the entire history of brecciation and fluid flux associated with the Monviso eclogite breccias. / Thesis (MS) — Boston College, 2022. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

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