Abstracts for Virtual Thermo2020

November 10, 2020, Session 1

Talk 1

A geo-thermochronology study (LAFT-U/Pb and AHe) offshore West of Ireland – Age and exhumation of the North Porcupine High

Rémi Rateau1*, David Chew1

1 Department of Geology, Trinity College Dublin – Irish Centre for Research in Applied Geosciences

* remi.rateau@icrag-centre.org

The offshore basins and basement highs west of Ireland are part of the northeast Atlantic Margin and are the result of the protracted break-up of Pangea and opening of the North Atlantic during the Mesozoic to Cenozoic. We acquired new geo-thermochronological data (dual-dating LAFT-U/Pb, AHe and zircon U/Pb) from borehole cuttings and seabed dredges and modelled both new and legacy data (QTQt) to better understand the thermal history and evolution of this part of the margin.

One area of interest is the Northern Porcupine High (NPH) which is a source of sediments for both the hyper-extended Porcupine Basin and Rockall Basin. The new results show that protracted exhumation of the high was associated with pulsed Mesozoic rifting (1 to 3 km of total erosion in total) and that the SE margin of the NPH started to be buried during the Early Cretaceous while the summit was still being exhumed and intruded by rift-related magmatism with an associated geothermal gradient increase. However, the new data fail to constrain a base Eocene exhumation event (visible on seismic) because of conflicting track length and AHe ages and the presence of coeval magmatism that may have affected some samples.

Zircon and apatite LA-ICP-MS U/Pb dating of basement samples on the SE margin of the NPH revealed Neoproterozoic (Lower Dalradian Supergroup) metasediments affected by a Late Caledonian thermal event. This new basement geochronology data, combined with previous published ages for the basement samples at the summit of the NPH, confirms that the low magnetic anomaly associated with the NPH represents Dalradian basement and that the sharp and straight magnetic boundary to the south probably represents the offshore extension of the Fair Head-Clew Bay lineament (the Irish extension of the Scottish Highland Boundary Fault, a major Caledonian suture).

Talk 2

The rise and fall of the Acadian altiplano: Evidence for a Paleozoic orogenic plateau in New England, USA

Ian Hillebrand1*, Michael Williams1, Cong Li1, Haiying Gao1

1 Department of Geosciences, University of Massachusetts at Amherst

* ihillenbrand@umass.edu

High elevation orogenic plateaus are formed by a complex interplay of deep and surficial processes and influence a variety of Earth systems. However, limited exposures of plateau mid-crust are presently recognized, hindering understanding of these deeper processes. We present evidence for the existence of an orogenic plateau during and after the Devonian Acadian orogeny whose mid-crustal roots are exposed in the New England Appalachians. The four-dimensional crustal evolution of this paleo-plateau is constrained by the integration of new petrologic and geochronologic databases with geophysical imaging. Doubly thickened crust, widespread amphibolite to granulite-facies metamorphic conditions, a paleo-isobaric surface, and protracted mid-crustal anatexis indicate the development of a high elevation, low relief plateau by 380 Ma.40Ar/39Ar thermochronology shows a distinct thermochronologic signature with very slow cooling rates of 2-4°C/m.y. following peak metamorphic conditions. Thermochronologic data, trace element and Nd isotope geochemistry, and monazite petrochronology suggest a 50 m.y. lifespan of the plateau. Orogen parallel ductile flow and extrusion of gneiss domes resulted in plateau collapse, crustal thinning, and block-like exhumation at ca. 330-310 Ma. Thinning of the plateau crust may have led to the sharp 12-15 km step in Moho depth in western New England, possibly by reactivating the suture between Laurentia and accreted Gondwanan-derived terranes. 

Talk 3

New constraints on the timing of Carnegie Ridge subduction from low-temperature thermochronology (Ecuadorian Andes)

Audrey Margirier1,2*, Manfred Strecker1, Peter Reiners3, Ismael Casado1, Stuart Thompson3, Sarah W.M. George3, Alexandra Alvarado4

1 Institut für Erd-und Umweltwissenschaften, Universität Potsdam, Potsdam, Germany
2 Helmholtz Centre Potsdam, GFZ German Research Center for Geosciences, Potsdam, Germany
3 Department of Geosciences, University of Arizona, Tucson, United States of America
4 Instituto Geofísico, Escuela Politécnica Nacional, Quito, Ecuador

* margirier@uni-potsdam.de

The Cenozoic growth of the Andes has been strongly influenced by the subduction dynamics and the superposed effects of climate. Recent studies proposed that the Carnegie ridge subduction controlled the late Cenozoic tectonic activity in Ecuador with the formation of a crustal sliver escaping northward; however, the timing of the arrival of the ridge and its effect on topographic growth remain unclear. We provided new thermochronological data from the Western Cordillera to evaluate the possible role of ridge subduction in prompting the growth of the Ecuadorian Andes and to pin point the timing of the ridge subduction. Time-temperature inverse modeling of this thermochronological dataset evidenced two cooling phases. A first one following intrusions emplacement in the Western Cordillera and a second one initiating at ~6 Ma. We interpret the first cooling phase to be associated with magmatic cooling. The second cooling phase, synchronous with the last cooling phase evidenced in the Eastern Cordillera, is likely to be associated with exhumation of the Western Cordillera. Based on our thermal modeling and geological cross-sections we propose that recent crustal shortening and rock uplift triggered exhumation of Ecuadorian Andes at ~6 Ma. We suggest that the onset of Carnegie Ridge subduction at ~6 Ma increased the coupling at the subduction interface and promoted shortening and rock uplift.

November 10, 2020, Session 2

Talk 1

Using continuous ramped heating to assess dispersion in (U-Th)/He ages from Transantarctic Mountain apatites

Hongcheng Guo1*, Peter K. Zeitler1, Bruce D. Idleman1, Annia K. Fayon2, Paul G. Fitzgerald3

1 Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015
2 Department of Earth Sciences, University of Minnesota, Minneapolis, MN 55455
3 Department of Earth Sciences, Syracuse University, Syracuse, NY 13210

* hog217@lehigh.edu

We performed continuous ramped heating (CRH) on apatite suites from the Transantarctic Mountains (TAM) that are known to have significant intra-sample dispersion in He ages. Examining > 100 apatite grains from a total of six samples, we confirmed prior results showing that measured AHe ages have at least three-fold intra-sample dispersion with no obvious relationships between ages and effective uranium (eU) concentration or grain size.

Apatites showing simple volume diffusion behavior, characterized by unimodal incremental 4He gas-release curves, have younger He ages. In contrast, older grains generally show anomalous gas release in the form of sharp spikes and / or extended gas-release at high temperatures (i.e., >= 800 °C). Considerable age dispersion still remains in well-behaved apatites and exceeds what grain size, radiation damage, and analytical uncertainty can explain, but this dispersion appears to be related to variations in 4He diffusion kinetics. The screened AHe ages from well-behaved apatites together with kinetic information obtained from these grains suggest that the sampled region experienced slow exhumation prior to rapid exhumation beginning ca.  ~35 Ma, an interpretation consistent with studies suggesting that increasing incision rates at this time was related to initiation of glaciation and the Eocene-Oligocene climate transition. We attempted to correct the anomalous apatite ages by simply removing anomalous gas-release components (spikes and high-temperature release), but this approach yields some ages that are too young for the samples’ geologic setting, suggesting that the factors that lead to anomalous laboratory release behavior can impact both the expected radiogenic component as well as those that are anomalous.

From our observations we conclude that: (1) CRH screening can serve as a routine tool for AHe dating and offers opportunities to reveal first-order kinetic variations; (2) model-dependent age correction might still be possible but would require some means of estimating the broad proportions of 4He components incorporated into grains before and after cooling, and (3) interpretation of highly dispersed AHe ages will require assessment of individual-grain diffusion kinetics beyond that predicted by radiation-damage models. We further infer that many apatite grains will contain imperfections of varying kinds that contribute significantly to kinetic variability beyond that associated with radiation damage.

Talk 2

Step-etch experiments on fission tracks in apatite: How much has been done and where are we going?

Murat T. Tamer1*, Richard A. Ketcham1

1 Jackson School of Geosciences, University of Texas at Austin

* tamer@jsg.utexas.edu

In step-etch experiments, fission tracks are etched and confined track lengths measured in a series of steps. For apatite, step-etch experiments were used to create the standard single-step etch protocols around three decades ago based the concept of maximum etchable length, by monitoring the increase of mean track lengths to reach an optimum etch time. In these early step-etch studies, different sets of tracks were measured in each etch step, due to practical limitations. Recent developments and innovations in hardware and software for fission track analysis have enabled a new generation of step-etch studies, where individual tracks are monitored throughout an experiment, instead of random track selection in each etching step. These have provided new insights affecting the fundamentals of fission-track dating and modelling, such as the need to revise the linear track etch model, a difference in etching behavior between fossil and induced tracks, and the failure to reach the maximum etchable length over the prescribed etch time in the current standard single-step etch protocols.

We conducted a new series of step-etch experiments on unannealed induced, lab-annealed induced, and fossil tracks in Durango apatite to decipher along-track etch rates (VT(x)). Additional etch-anneal-etch (EAE) experiments aimed to determine the bulk etch rate (VB). We estimate the average VB as ~0.02 µm/s in Durango apatite, and 98% of EAE experiment tracks had etching velocities under 0.06 µm/s.  Generally, etch rates along unannealed tracks did not fall to VB until the 25-30s etching step, even when considering only tracks that were visible within the first 10 s of etching. Using 0.06 µm/s as a threshold for under-etching, ~78% of induced and ~53% of fossil tracks in our data were under etched in Durango apatite using the 5.5M 20s 21°C etching protocol, once again considering only tracks that were visible after 10 s. This procedure eliminates analyst bias in track selection, as it only considers a group of tracks that have etched for well over 10 seconds, resulting in an overall increase in mean length.  Analyst selection (bias) by normal measurement after 20 s lowers this number to ~60% for induced tracks. We expect that different analysts with different track selection criteria would get different values, and that the combination of under-etching and variable criteria underlies the poor level of community reproducibility of these measurements.  A quantitative understanding of along-track etching, based on data such as this study, can provide an avenue to addressing this important problem, and optimizing etching procedures.

Discussion

TOPIC: Quantifying fission-track etching, and how it affects efficiency and reproducibility.

Led by: Rich Ketcham, Jackson School of Geosciences, University of Texas at Austin

Why do we etch apatite fission tracks the way we do?    Why do different lab groups etch with different recipes?  Does it even matter, if labs using the same etching get different results?  What are the advantages we seek, and what are the penalties we suffer?

A contribution, now in its public review period at https://gchron.copernicus.org/preprints/gchron-2020-31/, proposes that differences in how analysts select tracks to measure is actually more important for reproducibility than precisely how we etch.  However, it also proposes we don’t etch as much as we should, and that etching can be optimized to reveal more and better-defined confined tracks, while still retaining a quantitative link to experimental annealing data sets used for modeling.  Is this credible?

There will be a short presentation to kick things off, and then the floor will be open for discussion.  The more different perspectives we get, the better!