Person:

Mukhopadhyay, Sujoy

To better understand the composition, characteristics of helium diffusion, and size distribution of interplanetary dust particles (IDPs) responsible for the long-term retention of extraterrestrial ³He, we carried out leaching, stepped heating, and sieving experiments on pelagic clays that varied in age from 0.5 Ma to similar to 90 Myr. The leaching experiments suggest that the host phase(s) of ³He in geologically old sediments are neither organic matter nor refractory phases, such as diamond, graphite, Al₂O₃, and SiC, but are consistent with extraterrestrial silicates, Fe-Ni sulfides, and possibly magnetite. Stepped heating experiments demonstrate that the ³He release profiles from the magnetic and non-magnetic components of the pelagic clays are remarkably similar. Because helium diffusion is likely to be controlled by mineral chemistry and structure, the stepped heating results suggest a single carrier that may be magnetite, or more probably a phase associated with magnetite. Furthermore, the stepped outgassing experiments indicate that about 20% of the ³He will be lost through diffusion at seafloor temperatures after 50 Myrs, while sedimentary rocks exposed on the Earth's surface for the same amount of time would lose up to 60%. The absolute magnitude of the 3 He loss is, however, likely to depend upon the ³He concentration profile within the IDPs, which is not well known. Contrary to previous suggestions that micrometeorites in the size range of 50-100 mu m in diameter are responsible for the extraterrestrial ³He in geologically old sediments [Stuart, F.M., Harrop, P.J., Knott, S., Turner, G., 1999. Laser extraction of helium isotopes from Antarctic micrometeorites: source of He and implications for the flux of extraterrestrial ³He flux to earth. Geochim. Cosmochim. Acta 63,2653-2665 ], our sieving experiment demonstrates that at most 20% of the ³He is carried by particles greater than 50 mu m in diameter. The size-distribution of the ³He-bearing particles implies that extraterrestrial ³He in sediments record the IDP flux rather than the micrometeorite flux.

Cosmogenic He-3 was measured in apatite, titanite, and zircon and cosmogenic Ne-21 in quartz at 13 depth intervals in a 2.7-m long drill core in a Miocene ignimbrite from the Altiplano of Bolivia. All three He-3 depth profiles as well as the Ne-21 profile attenuate exponentially with depth, indicating that both of these isotopes are cosmogenic in origin with no significant contribution from other sources. The attenuation lengthscale for He-3 production of Lambda=180 +/- 11 g/cm(2) is consistent with expectations for neutron spallation, and is identical to that found for the cosmogenic Ne-21 in quartz. By normalizing the measured He-3 concentrations to Ne-21 and using the independently known cosmogenic Ne-21 production rate, the apparent cosmogenic He-3 production rates in apatite, titanite, and zircon were respectively found to be 112, 97, and 87 atoms/g/yr at sea-level and high latitude. The formal uncertainty on these estimates is similar to 20% (2 sigma), and arises in equal parts from uncertainties in the measured He-3/Ne-21 ratios and the uncertainty in the Ne-21 production rate. However an additional factor affecting the apparent He-3 production rate in these phases arises from the long stopping range of spalled He-3 and tritium (which decays to He-3). Because all three accessory phases have higher mean atomic number than major rock-forming minerals, they will have lower He-3 production rates than their surroundings. As a consequence the long stopping ranges will cause a net implantation of He-3 and therefore higher apparent production rates than would apply for purely in-situ production. Thus these apparent production rates apply only to the specific grain sizes analyzed. Analysis of sieved zircon aliquots suggests that a factor of 2 increase in grain size (from similar to 50 to similar to 100 mu m cross-section) yields a 10% decrease in apparent production rate. While this effect warrants further study, the grain sizes analyzed here are typical of the accessory phases commonly encountered, so the apparent rates provide an appropriate starting place for surface exposure dating using He-3 in these minerals.

Interior ice elevations of the West Antarctic Ice Sheet (WAIS) during the last glaciation, which can serve as benchmarks for ice-sheet models, are largely unconstrained. Here we report past ice elevation data from the Ohio Range, located near the WAIS divide and the onset region of the Mercer Ice Stream. Cosmogenic exposure ages of glacial erratics that record a WAIS highstand similar to 125 m above the present surface date to similar to 11.5 ka. The deglacial chronology prohibits an interior WAIS contribution to meltwater pulse 1A. Our observational data of ice elevation changes compare well with predictions of a thermomechanical ice-sheet model that incorporates very low basal shear stress downstream of the present day grounding line. We conclude that ice streams in the Ross Sea Embayment had thin, low-slope profiles during the last glaciation and interior WAIS ice elevations during this period were several hundred meters lower than previous reconstructions.

To constrain the spatial distribution of erosion rates in the Waimea river watershed, on the western side of the island of Kauai, Hawaii, we calculate the frequency distribution of cosmogenic He-3 concentrations ([He-3],) from helium isotopic measurements in olivine grains from a single sample of river sediment. Helium measurements were made in 26 aliquots of similar to 30 olivine grains each. The average [He-3], from the 26 aliquots was used to estimate a basin-wide average erosion rate of 0.056 mm/yr, a value that is similar to erosion rates obtained from geochemical analyses of river sediments from tectonically stable landforms. However, forward models of cosmogenic nuclide production and sediment generation rates are inconsistent with the hypothesis that the observed [He-3], frequency distribution is the result of a homogeneous, basin wide, erosion rate. Instead, a distribution of erosion rates, from similar to 0 to 4 mm/yr, may account for the observed frequency distribution. The distribution of erosion rates can be modeled by both non-linear slope- and curvature-dependent erosion rates with power law exponents ranging from 2.0 to 2.5. However, the spatial distribution of cosmogenic nuclides for slope- and curvature-dependent erosion rates are distinct, and we propose strategies to test further the extent to which erosion rates are controlled by the macroscale topographic features. These results demonstrate that the observed frequency distribution of cosmogenic nuclide concentrations in river sediments, combined with numerical modeling of erosion rates, can provide constraints on both the spatial variability of erosion rates in a drainage basin and the form of parameterized erosion laws.

Radioactive decay of uranium and thorium produces He-4, whereas He-3 in the Earth's mantle is not produced by radioactive decay and was only incorporated during accretion-that is, it is primordial(1). He-3/He-4 ratios in many ocean-island basalts (OIBs) that erupt at hotspot volcanoes, such as Hawaii and Iceland, can be up to sixfold higher than in mid-ocean ridge basalts (MORBs). This is inferred to be the result of outgassing by melt production at mid-ocean ridges in conjunction with radiogenic ingrowth of He-4, which has led to a volatile-depleted upper mantle (MORB source) with low He-3 concentrations and low He-3/He-4 ratios(2-6). Consequently, high He-3/He-4 ratios in OIBs are conventionally viewed as evidence for an undegassed, primitive mantle source, which is sampled by hot, buoyantly upwelling deep-mantle plumes(3,6,7). However, this conventional model provides no viable explanation of why helium concentrations and elemental ratios of He/Ne and He/Ar in OIBs are an order of magnitude lower than in MORBs. This has been described as the 'helium concentration paradox'(8) and has contributed to a long-standing controversy about the structure and dynamics of the Earth's mantle. Here we show that the helium concentration paradox, as well as the full range of noble-gas concentrations observed in MORB and OIB glasses, can self-consistently be explained by disequilibrium open-system degassing of the erupting magma. We show that a higher CO2 content in OIBs than in MORBs leads to more extensive degassing of helium in OIB magmas and that noble gases in OIB lavas can be derived from a largely undegassed primitive mantle source.

Helium concentration and isotopic composition were measured in a suite of samples across the Permian–Triassic boundary at Opal Creek, Canada, to determine whether high extraterrestrial helium concentrations are associated with a possible extinction-inducing impact event at this time. No extraterrestrial [super]3He was detected, implying that neither fullerene-hosted nor IDP-hosted He is present at or near the boundary. This observation is consistent with similar studies of some Permian–Triassic sections, but contrasts sharply with reports of both fullerene- and IDP-hosted extraterrestrial [super]3He at other sections. Step-heat experiments indicate rapid diffusion of extraterrestrial helium from sediments heated to temperatures above ~70 °C. Given the likelihood of burial and associated heating in Permian–Triassic age rocks, the initially unexpected absence of IDP-hosted [sup]3He likely indicates thermally induced diffusive loss. Indeed one of the key sections (Graphite Peak, Antarctica) from which extraterrestrial [super]3He has been reported at and near the Permian–Triassic boundary has been sufficiently heated that the reported preservation of extraterrestrial helium, in both IDPs and fullerenes, is inexplicable. Recent contamination provides a plausible explanation for extraterrestrial [super]3He in these samples. While no extraterrestrial [sup]3He was detected at Opal Creek, there is a sharp increase in nucleogenic [super]3He very close to or at the Permian–Triassic boundary. This presumably arises from the major lithologic change at this time, from cherts in the Permian to shales and siltstones in the Triassic. Increased nucleogenic [super]3He is associated with increases in both lithium and organic carbon content into the Triassic. Either the production rate or the retention of this [super]3He is higher in the shales and siltstones than in the cherts. Care must be taken to eliminate such artifacts before interpreting changes in [super]3He concentration in terms of fluctuations in the delivery of [super]3He from space.

Long-term (10(5) years) fault slip rates test the scale of discrepancy between infrequent paleoseismicity and relatively rapid geodetic rates of dextral shear in the Eastern California Shear Zone (ECSZ). The Calico fault is one of a family of dextral faults that traverse the Mojave Desert portion of the ECSZ. Its slip rate is determined from matching and dating incised Pleistocene alluvial fan deposits and surfaces displaced by fault slip. A high-resolution topographic base acquired via airborne laser swath mapping aids in identification and mapping of deformed geomorphic features. The oldest geomorphically preserved alluvial fan, unit B, is displaced 900 +/- 200 m from its source at Sheep Springs Wash in the northern Rodman Mountains. This fan deposit contains the first preserved occurrence of basalt clasts derived from the Pipkin lava field and overlies Quaternary conglomerate deposits lacking these clasts. The Ar-40/Ar-39 dating of two flows from this field yields consistent ages of 770 +/- 40 ka and 735 +/- 9 ka. An age of 650 +/- 100 ka is assigned to this fan deposit based on these ages and on the oldest cosmogenic He-3 exposure date of 653 +/- 20 ka on a basalt boulder from the surface of unit B. This assigned age and offset together yield a mid-Pleistocene to present average slip rate of 1.4 +/- 0.4 mm/yr. A younger fan surface, unit K, records 100 +/- 10 m of dextral displacement and preserves original depositional morphology of its surface. Granitic boulders and pavement samples from this surface yield an average age of 56.4 +/- 7.7 ka after taking into account minimal cosmogenic inheritance of granitic clasts. The displaced and dated K fans yield a slip rate of 1.8 +/- 0.3 mm/yr. Distributed deformation of the region surrounding the fault trace, if active, could increase the overall displacement rate to 2.1 +/- 0.5 mm/yr. Acceleration of slip rate from an average of 1.4 mm/yr prior to similar to 50 ka to 1.8 mm/yr since similar to 50 ka is possible, though a single time-averaged slip rate of 1.6 +/- 0.2 mm/yr satisfies the data. These rates are faster than any other paleoseismic or long-term slip rate yet determined for other dextral faults in the Mojave Desert and imply that fault slip rates and earthquake productivity are heterogeneous across this portion of the ECSZ. Total displacement across the Calico fault diminishes northward as shear is distributed into folding and sinistral faults in the Calico Mountains. This pattern is consistent with an approximately threefold drop in geologic slip rate as the Calico fault steps over onto the Blackwater fault and demonstrates the significance of fault interaction for understanding the pattern of present-day strain accumulation in the ECSZ.