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Cosmogenic nuclide surface exposure dating and erosion rate measurements in basaltic landscapes rely primarily on measurement of 3He in olivine or pyroxene. However, geochemical investigations using 3He have been impossible in the substantial fraction of basalts that lack separable olivine or pyroxene crystals, or where such crystals were present, but have been chemically weathered. Fine-textured basalts often contain small grains of ilmenite, a weathering-resistant mineral that is a target for cosmogenic 3He production with good He retention and straightforward mineral separation, but with a poorly constrained production rate. Here we empirically calibrate the cosmogenic 3He production rate in ilmenite by measuring 3He concentrations in basalts with fine-grained (~20 lm cross-section) ilmenite and co-existing pyroxene or olivine from the Columbia River and Snake River Plain basalt provinces in the western United States. The concentration ratio of ilmenite to pyroxene and olivine is 0.78 ± 0.02, yielding an apparent cosmogenic 3He production rate of 93.6 ± 7.7 atom g-1 yr-1 that is 20–30% greater than expected from prior theoretical and empirical estimates for compositionally similar minerals. The production rate discrepancy arises from the high energy with which cosmic ray spallation reactions emit tritium and 3He and the associated long stopping distances that cause them to redistribute within a rock. Fine-grained phases with low cosmogenic 3He production rates, like ilmenite, will have anomalously high production rates owing to net implantation of 3He from the surrounding, higher 3He production rate, matrix. Semi-quantitative modeling indicates implantation of spallation 3He increases with decreasing ilmenite grain size, leading to production rates that exceed those in a large grain by ~10% when grain radii are <150 lm. The modeling predicts that for the ilmenite grain size in our samples, implantation causes production rates to be ~20% greater than expected for a large grain, and within uncertainty resolves the discrepancy between our calibrated production rate, theory, and rates from previous work. The redistribution effect is maximized when the host rock and crystals differ substantially in mean atomic number, as they do between whole-rock basalt and ilmenite.
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Technical note: A software framework for calculating compositionally dependent in situ 14 C production rates
Abstract. Over the last 30 years, in situ cosmogenic nuclides (CNs) have revolutionizedsurficial processes and Quaternary geologic studies. Commonly measured CNsextracted from common mineral quartz have long half-lives (e.g.,10Be, 26Al) and have been applied over timescales from a fewhundred years to millions of years. However, their long half-lives alsorender them largely insensitive to complex histories of burial and exposure ofless than ca. 100 kyr. On the other hand, in situ cosmogenic 14C (in situ 14C) isalso produced in quartz, yet its 5.7 kyr half-life renders it very sensitiveto complex exposure histories during the last ∼25 ka, aparticularly unique and powerful tool when analyzed in concert withlong-lived nuclides. In situ 14C measurements are currently limited torelatively coarse-grained (typically sand-sized or larger, crushed or sieved tosand) quartz-bearing rock types, but while such rocks are common, they arenot ubiquitous. The ability to extract and interpret in situ 14C fromquartz-poor and fine-grained rocks would thus open its unique applicationsto a broader array of landscape elements and environments. As a first step toward this goal, a robust means of interpreting in situ 14Cconcentrations derived from rocks and minerals spanning wider compositionaland textural ranges will be crucial. We have thus developed aMATLAB®-based software framework to quantifyspallogenic production of in situ 14C from a broad range of silicate rock andmineral compositions, including rocks too fine grained to achieve purequartz separates. As expected from prior work, production from oxygendominates the overall in situ 14C signal, accounting for >90 %of production for common silicate minerals and six different rock types atsea level and high latitudes (SLHL). This work confirms that Si, Al, and Mgare important targets but also predicts greater production from Na thanfrom those elements. The compositionally dependent production rates for rockand mineral compositions investigated here are typically lower than that ofquartz, although that predicted for albite is comparable to quartz,reflecting the significance of production from Na. Predicted productionrates drop as compositions become more mafic (particularly Fe-rich). This framework should thus be a useful tool in efforts to broaden the utility ofin situ 14C to quartz-poor and fine-grained rock types, but futureimprovements in measured and modeled excitation functions would bebeneficial.
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- Award ID(s):
- 2300559
- PAR ID:
- 10515244
- Publisher / Repository:
- Copernicus Publications
- Date Published:
- Journal Name:
- Geochronology
- Volume:
- 5
- Issue:
- 1
- ISSN:
- 2628-3719
- Page Range / eLocation ID:
- 21 to 33
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Trona, nahcolite, and other Na-carbonate evaporite minerals in lakes are often closely associated with active volcanism, suggesting that the excess alkalinity required for their formation may arise from fluid-rock interactions involving hydrothermal waters that contain magmatic CO2. Paradoxically, the world’s largest Na-carbonate occurrence, contained within the Eocene Green River Formation in Wyoming, was not associated with nearby active magmatism. Magmatism was active ~200 km southeast in the Colorado Mineral Belt however, suggesting that a river draining this area could have supplied excess alkalinity to Eocene lakes. Sedimentologic studies in southwestern Wyoming, along the course of the hypothesized Aspen paleoriver, document fluvial and deltaic sandstone with generally northwest-directed paleocurrent indicators. Sandstone framework grain compositions and detrital zircon ages are consistent with derivation from the Colorado Mineral Belt and its host rocks. These results provide the first confirmation of a fluvial connection to downstream Eocene lakes, and indicate that lake deposits may offer a unique perspective on upstream magmatic and hydrothermal histories.more » « less
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Abstract. In situ cosmogenic 14C (in situ 14C) in quartz provides a recently developed tool to date exposure of bedrock surfaces of up to ∼ 25 000 years. From outcrops located in east-central Sweden, we tested the accuracy of in situ 14C dating against (i) a relative sea level (RSL) curve constructed from radiocarbon dating of organic material in isolation basins and (ii) the timing of local deglaciation constructed from a clay varve chronology complemented with traditional radiocarbon dating. Five samples of granitoid bedrock were taken along an elevation transect extending southwestwards from the coast of the Baltic Sea near Forsmark. Because these samples derive from bedrock outcrops positioned below the highest postglacial shoreline, they target the timing of progressive landscape emergence above sea level. In contrast, in situ 14C concentrations in an additional five samples taken from granitoid outcrops above the highest postglacial shoreline, located 100 km west of Forsmark, should reflect local deglaciation ages. The 10 in situ 14C measurements provide robust age constraints that, within uncertainties, compare favourably with the RSL curve and the local deglaciation chronology. These data demonstrate the utility of in situ 14C to accurately date ice sheet deglaciation, and durations of postglacial exposure, in regions where cosmogenic 10Be and 26Al routinely return complex exposure results.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/gchron-5-21-2023
