Apatite (U–Th)/He evidence for exhumation along the central Transantarctic MountainsBailey J Nordin1 (bjn2122@columbia.edu), Stephen E. Cox2, Sidney R. Hemming2, Stuart N. Thomsom3, Peter W. Reiners3, Kathy J. Licht4

1. Department of Earth and Environmental Sciences, Columbia University, New York, NY; 2. Lamont-Doherty Earth Observatory, Palisades, NY;3. Department of Geosciences, University of Arizona, Tucson, AZ; 4. Department of Earth Sciences, Indiana University-Purdue University Indianapolis, IN

In addition to each co-author, it has been both a genuine pleasure to work with the following people: Toby Koffman, Cody Randel, Jennifer Castaneda, Samuel Kodama, Christine Siddoway, Uttam Chowdhury, and Lydia Bailey. Special thanks to Dallas Abbott and Mike Kaplan for mentorship and support through the Lamont summer intern program.

Acknowledgements

The Transantarctic Mountains (TAM) make up the western boundary of the West Antarctic rift system, forming the longest rift shoulder in the world. The uplift history of these mountains, with some of the world’s highest tip-to-trough relief, is enigmatic, thought to be caused in part by incision of the massive outlet glaciers which drain the East Antarctic Ice Sheet into the Ross Ice Shelf.

The TAM served as a nucleation site for Antarctic glaciation at the Eocene-Oligocene boundary ~34 Ma and are thought to have developed their unique topography through a mechanism of peak preservation by non-erosive, cold glaciers at high elevations and rapid-flowing, warm based ice which erodes and incises deep troughts further down. This prompts isostatic rebound and the exhumation of new bedrock in response to particularly erosive intervals.

Motivations: understanding uplift and glacial erosion in the Transantarctic Mountains

Methods: apatite (U-Th)/He thermochronometry

In the (U-Th)/He system, alpha particles produced by the decay of 238U, 235U, and 232Th come to rest as 4He in minerals. This 4He is lost by thermal diffusion, which is accelerated by elevated temperatures at depth. Once a rock uplifts and cools to a certain temperature, however, diffusion slows and He is retained. In our samples, these “closure temperatures” tell us the time at which a particular rock was last deeper than ~2km, assuming a typical geothermal gradient with temperatures of 50-70 oC.

Referenc

es

Conclusions and discussion

Future 4He/3He applications

[1] Shuster, D. L., & Farley, K. A. (2005). 4He/3He thermochronometry: theory, practice, and potential complications. Reviews in Mineralogy and Geochemistry, 58(1), 181-203.

[2] Fretwell, P., Pritchard, H. D., Vaughan, D. G., Bamber, J. L., Barrand, N. E., Bell, R., ... & Catania, G. A. (2013). Bedmap2: improved ice bed, surface and thickness datasets for Antarctica.

[3] Licht, K. J., & Hemming, S. R. (2017). Analysis of Antarctic glacigenic sediment provenance through geochemical and petrologic applications. Quaternary Science Reviews, 164, 1-24.

Old AHe ages, uplift at ~99 Ma (below): When the (U–Th)/He cooling ages from each sample are plotted against the elevations from which they originated, a cluster at ~99 Ma emerges, consistent with resetting at Cretaceous TAM uplift, though many samples exhibit scatter. A smaller exhumation event is hinted at ~55 Ma.

Wilkes subglacial

basin

Transantarctic Mountains

Since warming climates result in rapidly flowing ice with more erosive basal conditions, constraining the timing of glacial incision can illuminate ice sheet response to past warmth. Reconstructions of past ice flow also require knowledge of the way this topography has changed over time.

Uplift and thermochronometric response to incisive glacial erosion or lack thereof (above): Isostacy causes uplift and as apatite grains pass through partial retention zone (PRZ) they record changes in the thermal field through retention or diffusive loss of 4He at higher temperatures.

Samples further inland exhibit age scatter, no break in slope (above): Maximum, minimum, and mean elevations along the length of the Shackleton trough with AHe ages obtained for this study. The lack of ages 34 Ma and younger suggests minimal glacial erosion. Large age scatter present between grains from the same sample may result from time spent in the partial retention zone.

51-95 Ma

76-81 Ma68-83 Ma

94-99 Ma

48-201 Ma

174-201 Ma

Results: uplift with opening of West Antarctic rift basin, minimal glacial erosion

Ross ice shelf

4000

3000

2000

1000

0

-1000

26-34

Elev

atio

n (m

)

33-4421-34

28-3729-45

19-2224-25 31-39

EAST ANTARCTIC ICE SHEETWEST

ANTARCTIC ICE SHEET

20-29

Shackleton samples (AHe ages, shown in white, this study)Ross transect samples to be analyzed for 4He/3He, planned spring 2020Beardmore and Shackleton samples analyzed by Stuart Thomson for the EAGLE project (young AHe ages, shown in black, are from Thomson, in progress)

Swath profile of Shackleton glacier

Mount ButtersRed Raider Rampart

Bedmap2 topography of central Transantarctic

Mountains with AHe ages labeled in Ma

94-9948-201

51-9576-81 68-83

174-201

Elevations and apatite helium ages (left): Bedmap2 elevations from [2] illustrate the over-deepened outlet glaciers. Young ages nearest Ross suggest enhanced glacial erosion in the early history of the East Antarctic Ice Sheet (20-30 Ma), with old ages further back. High grade metamorphic and intrusive igneous samples from low elevations along the Ross ice shelf/TAM margin are prioritized for analysis in order to target enhanced glacial erosion.

Apatite (U-Th)/He or AHe ages therefore reflect a combination of the radiogenic ingrowth and time- and temperature-dependent diffusive loss of 4He.

Closure temperatures of different geo- and thermochronometers (right): modified from [3], shows the sensitivity of the apatite helium system to shifts in the thermal field of the upper crust, due to low closure temperatures.

Large resetting event with opening of rift basin in Mid-Cretaceous (99 Ma)

• Evidence of mafic magmatism in Jurassic with reconfiguration to form Gondwana

• Smaller exhumation pulse at ~55 Ma

Glacial incision in this area was not extensive enough to excavate younger cooling ages

• Upper portion of Shackleton trough lacks the 34 Ma and younger ages associated with glaciation,

found along Beardmore and Ross margins• Very slow erosion in much of East Antarctica

The age scatter characteristic of this landscape could be evidence of significant time spent in the partial

retention zone for 4He diffusion• Age dispersion often results from reheating and

gradual cooling without reaching closure temps

Max

Mean

Min

(a) (from Shuster and Farley, 2005) hypothetical 4He/3He ratio evolution spectra as a function of 3He release fraction in a given grain of natural granitic apatite. (curves, with Temperature-time paths shown in (b)) The example sample shown by white circles in (a) has a bulk (U-Th)/He age that could be explained by any of the three curves in (b), but the 4He/3He release spectrum can be used to distinguish them.

a)

b)

33-4127-35

95-129

73-127 58-84

73-9697-135

109-13999-135

34-5932-45

33-4653-8073-129