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Dr. Sarah Chen
Stanford · NLP & interpretability
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Fairness + NLP projects overlap with her recent work.
Strong methods fit: model evaluation and interpretability.
Clear outreach angle for current trustworthy AI research.
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Ask how her lab is extending interpretability methods into fairness audits for real-world AI systems.
The K/Ar chronology of glauconite pellets is a long-used method for directly dating marine sedimentary deposits. Many papers have explored the processes that form glauconite and the factors that lead to greater reliability in the ages. Although K/Ar ages of glauconite are generally in agreement with other measures of stratigraphic age, there are examples of occurrences with ages too old and examples with ages too young. This paper seeks to build on the accumulated knowledge of glauconite, using synchrotron radiation to non-destructively characterize individual pellets and then consecutively measure the argon and potassium to obtain a K/Ar age. This strategy provides the advantage of measurements on a single aliquot while avoiding recoil loss of 40Ar in the nuclear reactor during irradiation for 40Ar/39Ar dating. We have used the glauconite reference material GL-O-1 to showcase several non-destructive methods for evaluating the maturity of individual pellets. In our argon measurements, we have found that the radiogenic argon concentration of large bulk samples underestimates the values for individual visually mature pellets, and we determined a K/Ar age of 101.0 ± 0.3 Ma (1σ SEM), M.S.W.D. 0.54 from 15 of 16 visually mature individual pellets. This age is 6% older than the reference value of 95.03 ± 1.11 Ma (1σ), and it is in good agreement with constraints from the U-Pb dating of volcanic minerals near the Albian–Cenomanian boundary.
Journal of Geophysical Research Solid Earth · 2023-09-01 · 6 citations
articleOpen access
Abstract There is a consensus that volcanism along the East African Rift System (EARS) is related to plume activities. However, because of our limited knowledge of the local lithospheric mantle, the dynamics of the plume are poorly constrained by magma chemistry. The Turkana Basin is one of the best places to study plume‐related volcanism because the lithospheric mantle there is unusually thin. New Ar‐Ar geochronology and geochemical data on lavas from western Turkana show that Eocene volcanics have relatively low 206 Pb/ 204 Pb (<19.1) and high εNd (>3.78). Their relatively high Ba/Rb (35–78) ratios suggest contributions from the shallow lithospheric mantle. Oligo‐Miocene Turkana volcanics have HIMU‐ and EMI‐ type enriched mantle signatures with overall lower Ba/Rb ratios, which is consistent with partial melting of plume material. Pliocene and younger Turkana volcanics have low Ba/Rb and Sr‐Nd‐Pb isotope ratios that resemble those of Ethiopian volcanics with elevated 3 He/ 4 He ratios. This temporal variation can be reconciled with a layered plume model where an outer layer of ancient recycled oceanic crust and sediment overlies more primitive lower mantle material. Beneath Ethiopia, the outer layer of the plume is either missing or punctured by the delamination of the thicker overlying lithospheric mantle at ca. 30 Ma, an event that would have facilitated the rapid upwelling of the inner portion of the plume and triggered the Ethiopian flood volcanism. The outer layer of the plume may be thicker in the southern EARS, which could explain the occurrence of young HIMU‐ and EMI‐type volcanics with primordial noble gas signatures.
High-resolution 40 Ar/ 39 Ar dating of Bingchang (BC) eruptions of Changbaishan-Tianchi volcano in NE China yields an oldest plateau age of 137.7 ka, well coinciding with the onset of the Penultimate Deglaciation (PDG). Subsequent eruptions occurred at 132.5-131.7 and 124.2 ka during the PDG and the early phase of the Last Interglacial. The BC tephra in marine sediments from the Japan Sea was deposited during the glacial/interglacial climatic transition. These findings suggest that the BC eruptions were likely triggered by depressurization of the volcano’s magma chamber through mountain glacial melting/retreat during the early phase of the PDG. The peak timing of the Penultimate Glacial Maximum thus derived falls between 142.7-137.7 ka, closely tied to the time of maximum global ice volume/sea level drop at ~140 ka. Since the BC tephra is widely dispersed in marine sediments in the Japan Sea, it will serve as a new well-dated stratigraphic marker for the region.
Abstracts with programs - Geological Society of America · 2022-01-01
article
While the northern and southern locked sections of the San Andreas Fault are associated with large earthquakes, its central creeping section has not historically hosted rupture. However, new dating and biomarker maturity data on core from the San Andreas Fault Observatory at Depth (SAFOD) provide evidence that large paleoearthquakes have occurred geologically recently along the central section (Coffey et al., 2022). Diffusion experiments on these samples suggest that the ages are only partially reset. Thus, actual event ages must be younger than bulk apparent ages. To refine earthquake ages, we dated size separates of eight of the original samples using K/Ar geochronometry. Applying hydrodynamic settling, we split each sample into five fine size fractions less than 2 µm. Chemical composition and illite crystallinity were determined via benchtop and synchrotron x-ray diffraction. Ar concentrations from signal standardization and 40Ar/36Ar ratios were measured on a noble gas mass spectrometer to obtain the concentration of radiogenic 40Ar (40Ar*). The same aliquots were spiked with 41K, then put through column chromatography before isotope ratio measurement for precise determination of K concentration. Characterization of the 10-angstrom 001 peak of illite confirms that in samples with high thermal maturity and young bulk ages, the finest size fractions host the most authigenic illite, interpreted to form during fault slip (Schleicher et al., 2010). Consistent with this inverse relationship between size and authigenic fraction, size is inversely related to K content and directly related to age in these samples. Calculating a York regression between age and detrital illite abundance, we extrapolate the age at which there was 0% detrital and 100% authigenic illite — the formation age. Samples with high thermal maturity record age intercepts as young as 0.67 ± 1.8 Ma (SEM). Not only are the new ages of authigenic illite measured here younger than bulk sample ages, they are young enough to suggest that translation of this material from the locked southern San Andreas Fault into the creeping section did not occur. Therefore, our results imply that the evidence of earthquake heating identified in Coffey et al. (2022) was a consequence of large earthquakes propagating into the creeping section within the last million years.
Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.
Zenodo (CERN European Organization for Nuclear Research) · 2021-04-15
datasetOpen access
Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.
Creeping faults are typically not associated with large earthquakes. However, new K/Ar dating and biomarker maturity data on the San Andreas Fault Observatory at Depth (SAFOD) present evidence that large paleoearthquakes have occurred in the creeping section of the San Andreas Fault, California. K/Ar ages of bulk samples with evidence of coseismic heating range from 3.3 to 15.8 Ma, and argon diffusion experiments suggest that these ages are only partially reset and the actual event ages may be even younger. Thus, questions remain as to how we can refine such dates to reveal the precise age and location of these earthquakes. To refine the ages and more accurately assess seismic hazard, we date size separates of eight samples from different sections of the SAFOD core. Following Stokes’ Law, we split each sample into five size fractions using hydrodynamic settling: <0.2, 0.2-0.5, 0.5-0.8, 0.8-1.4, and 1.4-2 micrometers. The finest size fractions contain the most authigenic illite, which form during fault slip. We determined chemical composition and separated illite polytypes using x-ray diffraction, and also measured K/Ar ages on each sample. Preliminary results from two scaly black fault rock samples, previously shown to have hosted earthquakes, (3,193.69 m and 3,193.96 m along the core) support that the finest size fractions contain the greatest ratio of authigenic illite. With a York regression between age and detrital illite abundance, we place the authigenic illite ages at 1.08 ± 2.40 Ma and 0.88 ± 5.08 Ma for these two samples, and observe that the detrital illite matches the late Cretaceous age for the country rock. This new age estimate for the authigenic illite means that large earthquakes must have propagated into the creeping section within the last million years. Not only is it significantly younger than the bulk sample age, it is recent enough that translation of faulted material from the locked southern San Andreas fault into the creeping section cannot explain the record. Moving forward, we will expand our procedure to include isotope dilution for measuring K concentration and analyze the other samples previously measured for biomarker maturity and bulk K/Ar age. Resulting insights into the fault rock composition and the timing of past earthquakes will be crucial in assessing the region’s seismic hazard.
Zenodo (CERN European Organization for Nuclear Research) · 2021-04-15
datasetOpen access
Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.
