scholarly journals In situ cosmogenic ¹⁰Be in pyroxene with an application to surface exposure dating

2021 ◽  
Author(s):  
◽  
Julia Anne Collins
Keyword(s):  
New Zealand ◽  
Production Rate ◽  
Production Rates ◽  
New Production ◽  
Radiocarbon Age ◽  
Exposure Dating ◽  
Ice Surface ◽  
Cross Calibration ◽  
Exposure Ages

<p>Cosmogenic nuclides are an important tool in quantifying many Earth-surface processes. Beryllium-10 (¹⁰Be) is commonly extracted out of the mineral quartz; however many landscapes lack quartz bearing rocks. In order to establish a new chronometer based on ¹⁰Be in pyroxene for use in New Zealand and Antarctica, it is necessary to verify cleaning protocols and determine a local production rate. In this study, I have tested and modified an existing pyroxene decontamination procedure in order to further develop the use of ¹⁰Be in pyroxene as a chronometer. This method successfully removes the meteoric component of ¹⁰Be in pyroxene, allowing only the concentration of in situ produced ¹⁰Be to be measured. Additionally, production rates for ¹⁰Be in pyroxene have been determined empirically for New Zealand using cross-calibration with measured ³He concentrations and an independent radiocarbon age of the Murimotu debris avalanche in the central North Island, New Zealand of 10.6 ± 1.1 ka. Theoretical ¹⁰Be pyroxene production rates were also determined, based on the composition of the Murimotu pyroxene. The best estimate for the 10Be pyroxene production rate is 3.4 ± 0.8 atoms g⁻¹ yr⁻¹ at sea-level high latitude, which was determined via cross-calibration with the radiocarbon age for the deposit. This work shows that production rates for ¹⁰Be in pyroxene are both empirically and theoretically 8-27% lower than in quartz. The ³He/¹⁰Be ratio in the Murimotu pyroxene is 34.5 ± 9.9; this is indistinguishable from global ³He-pyroxene/¹⁰Be-quartz production ratios.  In a case study surface exposure ages were determined for bedrock samples and cobble erratics collected in a vertical transect on Mount Gran, Antarctica, by applying the aforementioned ¹⁰Be pyroxene decontamination procedure and radiocarbon derived production rates. A chronology for ice surface lowering was obtained for the adjacent Mackay Glacier, indicating the ice surface lowered approximately 60 m during a relatively rapid episode of thinning which occurred between ~13.5 ka and 11 ka.  This thesis presents a successful test of decontamination procedures, new production rates, and an example application, showing the promise of ¹⁰Be in pyroxene as a chronometer. The development of ¹⁰Be in pyroxene allows environments without quartz-bearing rocks to be dated using this widely used nuclide. The pairing of ¹⁰Be with ³He in pyroxene would allow complex exposure histories to be determined, expanding the application.</p>

scholarly journals In situ cosmogenic ¹⁰Be in pyroxene with an application to surface exposure dating

2021 ◽  
Author(s):  
◽  
Julia Anne Collins
Keyword(s):  
New Zealand ◽  
Production Rate ◽  
Production Rates ◽  
New Production ◽  
Radiocarbon Age ◽  
Exposure Dating ◽  
Ice Surface ◽  
Cross Calibration ◽  
Exposure Ages

<p>Cosmogenic nuclides are an important tool in quantifying many Earth-surface processes. Beryllium-10 (¹⁰Be) is commonly extracted out of the mineral quartz; however many landscapes lack quartz bearing rocks. In order to establish a new chronometer based on ¹⁰Be in pyroxene for use in New Zealand and Antarctica, it is necessary to verify cleaning protocols and determine a local production rate. In this study, I have tested and modified an existing pyroxene decontamination procedure in order to further develop the use of ¹⁰Be in pyroxene as a chronometer. This method successfully removes the meteoric component of ¹⁰Be in pyroxene, allowing only the concentration of in situ produced ¹⁰Be to be measured. Additionally, production rates for ¹⁰Be in pyroxene have been determined empirically for New Zealand using cross-calibration with measured ³He concentrations and an independent radiocarbon age of the Murimotu debris avalanche in the central North Island, New Zealand of 10.6 ± 1.1 ka. Theoretical ¹⁰Be pyroxene production rates were also determined, based on the composition of the Murimotu pyroxene. The best estimate for the 10Be pyroxene production rate is 3.4 ± 0.8 atoms g⁻¹ yr⁻¹ at sea-level high latitude, which was determined via cross-calibration with the radiocarbon age for the deposit. This work shows that production rates for ¹⁰Be in pyroxene are both empirically and theoretically 8-27% lower than in quartz. The ³He/¹⁰Be ratio in the Murimotu pyroxene is 34.5 ± 9.9; this is indistinguishable from global ³He-pyroxene/¹⁰Be-quartz production ratios.  In a case study surface exposure ages were determined for bedrock samples and cobble erratics collected in a vertical transect on Mount Gran, Antarctica, by applying the aforementioned ¹⁰Be pyroxene decontamination procedure and radiocarbon derived production rates. A chronology for ice surface lowering was obtained for the adjacent Mackay Glacier, indicating the ice surface lowered approximately 60 m during a relatively rapid episode of thinning which occurred between ~13.5 ka and 11 ka.  This thesis presents a successful test of decontamination procedures, new production rates, and an example application, showing the promise of ¹⁰Be in pyroxene as a chronometer. The development of ¹⁰Be in pyroxene allows environments without quartz-bearing rocks to be dated using this widely used nuclide. The pairing of ¹⁰Be with ³He in pyroxene would allow complex exposure histories to be determined, expanding the application.</p>

Yet another in-situ cosmogenic 10-Be local production rate for the British Isles : Llyn Arenig Fach, North Wales

2020 ◽  
Author(s):  
David Fink ◽  
Philip Hughes ◽  
Reka Fulop ◽  
Klaus Wilcken ◽  
Patrick Adams ◽  
...  
Keyword(s):  
Production Rate ◽  
British Isles ◽  
Production Rates ◽  
Site Specific ◽  
Current Collection ◽  
North Wales ◽  
Minimum Age ◽  
Exposure Ages ◽  
Analytical Errors

&lt;p&gt;Cosmogenic production rates (PRs) are the essential conversion factor between AMS cosmogenic concentrations and absolute exposure ages. The accuracy of cosmogenic glacial chronologies and reliability in their comparison to other plaeoclimate systems &amp;#160;is largely contingent on the precision and accuracy of the adopted production rate. This is particularly critical in determining past glacial geochronologies at the scale of millennial temporal resolution. Most PR calibrations are carried out at deglaciation sites where radiocarbon provides the independent chronometric control usually based on calibrated 14C ages in basal sediments or varves&amp;#160; from lake or bog cores which is assumed to represent the minimum age for glacial retreat. Under these conditions PRs should be considered as maximum-limiting values. Given that today most AMS facilities can deliver 10-Be, 26-Al and 36-Cl data with analytical errors less than 2%, the accuracy of a PR for a given scaling method (ie transfer function of the site-specific production rate to a reference sea-level high latitude (SLHL) PR) remains largely dependent &amp;#160;on the error in the independent chronology and accuracy of AMS standards. The history over the past 20 years of the ever-changing value of &amp;#160;SLHL 10-Be cosmogenic spallation PRs &amp;#160;with a continual decreasing value from initial estimates of about 7 atoms/g/a to the current&amp;#160; &amp;#8216;accepted &amp;#8216; value of ~4 atoms/g/a,&amp;#160;&amp;#160; is an interesting story in itself and demonstrates the complexity in such determinations. &amp;#160;&lt;/p&gt;&lt;p&gt;Today there are both global (average) SLHL PRs and also regional-specific PR values (referenced to SLHL). For the British Isles, there are a number of 10-Be &amp;#8216;British Isles&amp;#8217; choices that, for the Lm scaling scheme, range between 3.92&amp;#177;0.11 &amp;#160;atoms/g/a &amp;#160;(Putnam et al., QG, v50, 2019) to 4.41&amp;#177;0.25 atoms/g/a (Small et al., JQS, v30, 2015). This range in 10-Be spallation PRs has recently raised some debate and challenges for the assumed extent and timing of the local-LGM and demise of the British Ice Sheet. This work provides a new &amp;#160;British Isles site specific 10-Be PR from the &amp;#160;Arenig Mountains in North Wales. We have measured 10-Be concentrations in 13 selected moraine boulders that are tentatively mapped as outer and inner Younger Dryas deglacial deposits hugging a cirque lake,&amp;#160; Llyn Arenig Fach,&amp;#160; just below the head wall&amp;#160; at &amp;#160;Arenig Fach.&amp;#160;&amp;#160; Radiocarbon dating of basal sediments from a number of intermorainal core bogs has provided independent age control.&amp;#160; We will present our results and compare them to the current collection of other British Isles 10-Be production rates. &amp;#160;&lt;/p&gt;

Cosmogenic Be-10 surface exposure data from the Sub-Antarctic Kerguelen Archipelago

2020 ◽  
Author(s):  
Henriette Linge ◽  
Jostein Bakke ◽  
Talin Tuestad ◽  
Philip Deline ◽  
Ludovic Ravanel ◽  
...  
Keyword(s):  
Hydrothermal Fluids ◽  
Large Igneous Province ◽  
Geochemical Composition ◽  
Surface Exposure Dating ◽  
Exposure Dating ◽  
Ice Cap ◽  
Wide Range ◽  
Exposure Ages ◽  
Kerguelen Archipelago

&lt;p&gt;The Kerguelen archipelago (around 49&amp;#176;S 69&amp;#176;E) is the emerged part of the Kerguelen Plateau, a large igneous province in the southwestern Indian Ocean. Information on past climatic and environmental conditions in the region is vital for understanding the past behaviour of the southern westerly winds. The cross-disciplinary project SOUTHSPERE seeks to investigate past variations in this weather system through reconstruction of temporal and spatial glacier variability from lake records and glacial landforms N and NE of the Cook Ice Cap. Reliable and accurate chronological control is crucial in this context.&lt;/p&gt;&lt;p&gt;Surface exposure dating of glacial geomorphological features S and SE of the Cook Ice Cap has previously been done using in situcosmogenic Cl-36 [1, 2]. Solifluction and gelifraction processes appear very active in our field area, as do aeolian erosion. Also, highly variable geochemical composition of the basalts and associated intrusions, as well as the degree and type of metamorphosis, lead to strong lithology-dependant weathering and erosion rates, as evident from differential weathering reliefs on cm and m scales. The very active surface environment constitutes a challenge for obtaining accurate surface exposure ages.&lt;/p&gt;&lt;p&gt;In the NW part of the archipelago, basaltic lava units altered by meteoric-hydrothermal fluids contain a wide variety of secondary silicate and carbonate minerals [3]. In settings where quartz-filled geodes and fractures in the basalt are located in favourable positions on bedrock and boulder surfaces, analysis of Be-10 in euhedral and microcrystalline quartz offers a means of validating in situ Cl-36 surface exposure ages. Moreover, multi-nuclide analysis would open up for a wide range of process and landscape development studies on this young archipelago. Percolation of hydrothermal fluids in fractures and geodes is probably related to the intrusion of younger (15-5 Ma) subvolcanic rocks [see 3 and references therein]. A meteoric source of the fluids would imply that the secondary silicates contain meteoric Be-10. As meteoric production is greater than in situ production, this may represent a problem for utilising in situ Be-10 for surface exposure dating. If secondary silicate formation occurred early, rather than late in the intrusive phase, complete radioactive decay of the meteoric Be-10 component is expected prior to surface exposure.&lt;/p&gt;&lt;p&gt;110 rock samples were collected for surface exposure dating with in situ cosmogenic nuclides during a field campaign in November and December 2019. Here we present the first Be-10 data from rock surfaces of glacially transported boulders and exposed bedrock.&lt;/p&gt;&lt;p&gt;[1] Jomelli et al. 2017. Quaternary Science Reviews 162, 128-144.&lt;/p&gt;&lt;p&gt;[2] Jomelli et al. 2018. Quaternary Science Reviews 183, 110-123.&lt;/p&gt;&lt;p&gt;[3] Renac et al. 2010. European Journal of Mineralogy 22, 215-234.&lt;/p&gt;

Chronology of Taylor Glacier Advances in Arena Valley, Antarctica, Using in Situ Cosmogenic 3He and 10Be

1993 ◽  
Vol 39 (1) ◽  
pp. 11-23 ◽  
Author(s):  
Edward J. Brook ◽  
Mark D. Kurz ◽  
Robert P. Ackert ◽  
George H. Denton ◽  
Erik T. Brown ◽  
...  
Keyword(s):  
Age Distribution ◽  
Early Pleistocene ◽  
Ice Surface ◽  
Victoria Land ◽  
A New Technique ◽  
Taylor Glacier ◽  
Exposure Ages ◽  
Lower Limits ◽  
Age Estimates

AbstractIn situ produced cosmogenic nuclides provide a new technique for constraining exposure ages of glacial deposits. In situ3He and 10Be in quartz sandstone boulders from Arena Valley, southern Victoria Land, Antarctica, provide chronological constraints for a sequence of moraines ("Taylor II-IVb" moraines) related to expansions of Taylor Glacier and the East Antarctic Ice Sheet. Mean 3He ages are 113,000 ± 45,000 yr, 208,000 ± 67,000 yr, 335,000 ± 187,000 yr, and 1.2 ± 0.2 myr, for Taylor II,III,IVa, and IVb moraines, respectively (mean ± 1σ). Corresponding mean 10Be ages for Taylor II and IVb moraines are 117,000 ± 51,000 yr and 2.1 ± 0.1 myr. For the older deposits the 3He ages are probably lower limits due to diffusive loss. Although the exposure ages appear consistent with the few previous age estimates, particularly with an isotope stage 5 age for Taylor II, each moraine exhibits a broad age distribution. The distribution probably results from a variety of factors, which may include prior exposure to cosmic rays, 3He loss, erosion, postdepositional boulder movement, and radiogenic production of 3He. Nonetheless, the exposure ages provide direct chronological constraints for the moraine sequence, and suggest a maximum thickening of Taylor Glacier relative to the present ice surface of ∼500 m since the late Pliocene-early Pleistocene.

scholarly journals Mid-Holocene thinning of David Glacier, Antarctica: Chronology and Controls

2020 ◽  
Author(s):  
Jamey Stutz ◽  
Andrew Mackintosh ◽  
Kevin Norton ◽  
Ross Whitmore ◽  
Carlo Baroni ◽  
...  
Keyword(s):  
Numerical Modelling ◽  
Flow Line ◽  
Ice Sheet ◽  
Outlet Glacier ◽  
Exposure Dating ◽  
Ice Surface ◽  
Geological Past ◽  
Climate Interactions ◽  
Glacier Flow ◽  
Exposure Ages

Abstract. Quantitative satellite observations provide a comprehensive assessment of ice sheet mass loss over the last four decades, but limited insights into long-term drivers of ice sheet change. Geological records can extend the observational record and aid our understanding of ice sheet–climate interactions. Here we present the first millennial-scale reconstruction of David Glacier, the largest East Antarctic outlet glacier in Victoria Land. We use surface exposure dating of glacial erratics deposited on nunataks to reconstruct changes in ice surface elevation through time. We then use numerical modelling experiments to determine the drivers of glacial thinning. Thinning profiles derived from 45 10Be and 3He surface exposure ages show that David Glacier experienced rapid thinning up to 2 m/yr during the mid-Holocene (~ 6,500 years ago). Thinning stabilised at 6 kyr, suggesting initial formation of the Drygalski Ice Tongue at this time. Our work, along with terrestrial cosmogenic nuclide records from adjacent glaciers, shows simultaneous glacier thinning in this sector of the Transantarctic Mountains occurred ~ 3 kyr after the retreat of marine-based grounded ice in the western Ross Embayment. The timing and rapidity of the reconstructed thinning at David Glacier is similar to reconstructions in the Amundsen and Weddell embayments. In order to identify the potential causes of these rapid changes along the David Glacier, we use a glacier flow line model designed for calving glaciers and compare modelled results against our geological data. We show that glacier thinning and marine-based grounding line retreat is initiated by interactions between enhanced sub-ice shelf melting and reduced lateral buttressing, leading to Marine Ice Sheet Instability. Such rapid glacier thinning events are not captured in continental or sector-scale numerical modelling reconstructions for this period. Together, our chronology and modelling suggest a ~ 2,000-year period of dynamic thinning in the recent geological past.

Radiocarbon Dating of Quaternary Loess Deposits, Banks Peninsula, Canterbury, New Zealand

1977 ◽  
Vol 7 (2) ◽  
pp. 177-196 ◽  
Author(s):  
K.M. Goh ◽  
B.P.J. Molloy ◽  
T.A. Rafter
Keyword(s):  
Organic Matter ◽  
New Zealand ◽  
Late Glacial ◽  
Radiocarbon Age ◽  
Chemical Treatments ◽  
Before And After ◽  
Loess Deposits ◽  
Physical And Chemical ◽  
Apparent Conflict

Quaternary loess deposits containing charcoal and in situ organic matter constituents from Banks Peninsula, New Zealand, were subjected to various physical and chemical treatments before radiocarbon assay. A stepwide procedure was used in which each component was radiocarbon dated before and after the treatments were applied. The criterion adopted for judging the effectiveness of a treatment is an increase in the radiocarbon age of loess layer. On this basis the oldest and therefore the most reliable date was obtained from the intra-loess charcoal and its humic acid extract. By comparison, the radiocarbon ages of organic matter constituents of the whole or partitioned loess were much younger, and are considered to represent only average ages for the particular layers concerned. No advantage was gained by dating the carbon-enriched clay-humus fraction as opposed to whole loess samples. There is an apparent conflict between our dates and current interpretation of loess stratigraphy and chronology in the South Island of New Zealand, which requires further investigation. This uncertainty apart, the upper layers of multiple loess deposits are clearly much older than earlier reports indicate, and it follows that the correlation of these deposits with late-glacial events both within and beyond New Zealand is even more tenuous and unreliable than previously thought to be the case.

scholarly journals The glacial history of Tongariro and Ruapehu volcanoes, New Zealand

2021 ◽  
Author(s):  
◽  
Shaun Eaves
Keyword(s):  
New Zealand ◽  
Production Rate ◽  
Late Quaternary ◽  
Lateral Moraine ◽  
Glacial Cycle ◽  
Mountain Glacier ◽  
Last Glacial ◽  
Glacier Fluctuations ◽  
Exposure Ages ◽  
The Last Glacial

<p>Understanding the drivers and mechanisms of past, natural changes in Earth’s climate is a fundamental goal of palaeoclimate science. Recent advances in cosmogenic surface exposure dating and numerical glacier modelling have greatly improved the utility of geological glacial records for palaeoclimatic reconstruction. Here, I apply these techniques to investigate the timing and magnitude of late Quaternary mountain glacier fluctuations on Tongariro massif and Mt. Ruapehu volcanoes in central North Island, New Zealand (39°S).  First, I constrain the local cosmogenic ³He production rate, in order to compare my subsequent ³He moraine chronologies with other well-dated palaeoclimate records. I present a new radiocarbon age for a large debris avalanche event on the northwest slopes of Mt. Ruapehu that occurred at 10.4-10.6 cal. ka BP. Cosmogenic ³He concentrations in surficial boulders deposited during this event are consistent with that predicted by a global compilation of similar production rate calibrations. Thus, I conclude that this globally compiled production rate is suitable for cosmogenic ³He exposure age calculations in New Zealand.  Exposure ages from moraine boulders on both volcanoes constrain the timing of two periods of glaciation during the last glacial cycle, when the termini of valley glaciers reached c. 1200 m asl. The most recent of these events occurred between c. 31-17 ka, which corresponds with the global Last Glacial Maximum. During this period, the local equilibrium line altitude was depressed by c. 800-1100 m. Numerical model simulations of the glaciers, using a coupled energy balance/ice flow model, suggest that local atmospheric temperature was 4-7 °C colder than present. This palaeotemperature estimate is not greatly impacted by post-glacial topographic change on these active volcanoes. Surface exposure ages from a degraded lateral moraine on Tongariro massif indicate that an earlier period of glaciation, of similar extent to that at the LGM, culminated during Marine Isotope Stage 4.  During the last glacial-interglacial transition (c. 18-11 ka), glacial retreat on Mt. Ruapehu was interrupted by a re-advance during the late-glacial (c. 15-11 ka). Exposure ages for this event exhibit some scatter, likely due to surface processes. Accounting for these processes with a topographic diffusion model yields a best-estimate age of 14-13 ka, corresponding to the Lateglacial reversal in New Zealand. Glacier model experiments indicate this re-advance resulted from a temperature lowering of 2.5-3.4 °C relative to present. Comparison with other proxy records suggests that this cooling was most pronounced during summer. Due to its lower elevation, it is unlikely that glaciers were present on Tongariro massif at this time.  The results of this research provide the first direct age constraint and quantitative palaeoclimate reconstructions for late Quaternary glacier fluctuations in central North Island, New Zealand. The timing and magnitude of these changes are in good agreement with glacial records from the Southern Alps and South America. This suggests that glaciers in the southern mid-latitudes were responding to common climatic forcings at orbital- and millennial-timescales, during the last glacial cycle.</p>

scholarly journals Exposure dating of detrital magnetite using 3He enabled by microCT and calibration of the cosmogenic 3He production rate in magnetite

2021 ◽  
Author(s):  
Florian Hofmann ◽  
Emily H. G. Cooperdock ◽  
A. Joshua West ◽  
Dominic Hildebrandt ◽  
Kathrin Strößner ◽  
...  
Keyword(s):  
Production Rate ◽  
Trace Element Analysis ◽  
Micro Computed Tomography ◽  
Element Analysis ◽  
Exposure Dating ◽  
Exposure Age ◽  
Accuracy And Precision ◽  
Microct Imaging ◽  
Exposure Ages

Abstract. We test whether X-ray micro computed tomography (microCT) imaging can be used as a tool for screening magnetite grains to improve the accuracy and precision of cosmogenic 3He exposure dating. We extracted magnetite from a soil developed on a fanglomerate at Whitewater, California, which was offset by the Banning Strand of the San Andreas Fault. This study shows that microCT screening can distinguish between inclusion-free magnetite and magnetite with fluid or common solid inclusions. Such inclusions can produce bulk 3He concentrations that are significantly in excess of expected cosmogenic production. We present Li concentrations, major and trace element analysis, and magnetite (U-Th)/He cooling ages of samples in order to model the contribution from radiogenic, nucleogenic, and cosmogenic thermal neutron production of 3He. We show that mineral inclusions in magnetite can produce 3He concentrations of up to four times that of the cosmogenic 3He component, leading to erroneous exposure ages. Therefore, grains with inclusions must be avoided in order to facilitate accurate and precise magnetite 3He exposure dating. Around 30 % of all grains were found to be without inclusions, as detectable by microCT, with the largest proportion of suitable grains in the grain size range of 400–800 µm. While grains with inclusions have 3He concentrations far in excess of the values expected from existing 10Be and 26Al data in quartz at the Whitewater site, magnetite grains without inclusions have concentrations close to the predicted depth profile. We measured 3He concentrations in aliquots without inclusions and corrected them for Li-produced components. By comparing these data to the known exposure age of 53.5 ka, we calibrate a magnetite 3He SLHL production rate of 116 ± 13 at g−1 a−1. We suggest that the microCT screening approach can be used to improve the quality of cosmogenic 3He measurements of magnetite and other opaque mineral phases for exposure age and detrital studies.

Timing and Extent of Late Quaternary Glaciation in the Ahuriri River Valley, Southern Alps, New Zealand

2021 ◽  
Author(s):  
Levan Tielidze ◽  
Shaun Eaves ◽  
Kevin Norton ◽  
Andrew Mackintosh
Keyword(s):  
New Zealand ◽  
Large Scale ◽  
Late Quaternary ◽  
Southern Alps ◽  
River Valley ◽  
Entire Study ◽  
Geomorphological Mapping ◽  
Exposure Dating ◽  
Quaternary Glaciation ◽  
Exposure Ages

&lt;p&gt;Some valleys in South Island, New Zealand already have a number of well-dated glacier records. However, understanding of the precise timing of old glacial events in many valleys still remains poor. For this purpose, the cosmogenic &lt;sup&gt;10&lt;/sup&gt;Be surface exposure dating technique was used to constrain the timing and extent of late Quaternary glaciation in the Ahuriri River valley, Southern Alps, New Zealand. The 33 &lt;sup&gt;10&lt;/sup&gt;Be surface-exposure ages from two different moraine complexes range from 16.6&amp;#177;0.4 ka to 19.7&amp;#177;0.5 ka suggesting rapid glacier recession (~17 km) during the last deglaciation.&lt;/p&gt;&lt;p&gt;Field observation and geomorphological mapping were also used to investigate the extent and drivers of glaciation in this valley. For the final step, we created detail and comprehensive map of the glacial geomorphology in an area covered by palaeo Ahuriri Glacier, in the central Southern Alps. Geomorphological mapping from high-resolution aerial imagery, large scale topographical maps, average resolution DEM, and several field investigations allowed us to produce the 1:38,000 scale map for the entire study site covering an area of about 532 km&lt;sup&gt;2&lt;/sup&gt;.&lt;/p&gt;&lt;p&gt;This newly created map along with the new &lt;sup&gt;10&lt;/sup&gt;Be surface exposure dataset will help us in better understanding of past glacier-climate interactions in the Southern Alps and in the Southern Hemisphere in general.&lt;/p&gt;

scholarly journals Exposure dating of detrital magnetite using <sup>3</sup>He enabled by microCT and calibration of the cosmogenic <sup>3</sup>He production rate in magnetite

Geochronology ◽  
2021 ◽  
Vol 3 (2) ◽  
pp. 395-414 ◽  
Author(s):  
Florian Hofmann ◽  
Emily H. G. Cooperdock ◽  
A. Joshua West ◽  
Dominic Hildebrandt ◽  
Kathrin Strößner ◽  
...  
Keyword(s):  
Production Rate ◽  
Micro Computed Tomography ◽  
Exposure Dating ◽  
San Andreas ◽  
Exposure Age ◽  
Mineral Phases ◽  
Accuracy And Precision ◽  
Microct Imaging ◽  
Exposure Ages

Abstract. We test whether X-ray micro-computed tomography (microCT) imaging can be used as a tool for screening magnetite grains to improve the accuracy and precision of cosmogenic 3He exposure dating. We extracted detrital magnetite from a soil developed on a fanglomerate at Whitewater, California, which was offset by the Banning strand of the San Andreas Fault. This study shows that microCT screening can distinguish between inclusion-free magnetite and magnetite with fluid or common solid inclusions. Such inclusions can produce bulk 3He concentrations that are significantly in excess of the expected spallation production. We present Li concentrations, major and trace element analyses, and estimated magnetite (U–Th) / He cooling ages of samples in order to model the contribution from fissiogenic, nucleogenic, and cosmogenic thermal neutron production of 3He. We show that mineral inclusions in magnetite can produce 3He concentrations of up to 4 times that of the spallation component, leading to erroneous exposure ages. Therefore, grains with inclusions must be avoided in order to facilitate accurate and precise magnetite 3He exposure dating. Around 30 % of all grains were found to be without inclusions, as detectable by microCT, with the largest proportion of suitable grains in the grain size range of 400–800 µm. While grains with inclusions have 3He concentrations far in excess of the values expected from existing 10Be and 26Al data in quartz at the Whitewater site, magnetite grains without inclusions have concentrations close to the predicted depth profile. We measured 3He concentrations in aliquots without inclusions and corrected them for Li-produced components. By comparing these data to the known exposure age of 53.5 ± 2.2 ka, we calibrate a production rate for magnetite 3He at sea level and high latitude (SLHL) of 116 ± 13 at g−1 a−1. We suggest that this microCT screening approach can be used to improve the quality of cosmogenic 3He measurements of magnetite and other opaque mineral phases for exposure age and detrital studies.

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