David L. Shuster

Records of an ancient Martian magnetic field in ALH84001

Although Mars does not presently appear to have a global dynamo magnetic field, strong crustal fields have recently been detected by the Mars Global Surveyor above surfaces formed ∼3 or more Ga. We present magnetic and textural studies of Martian meteorite ALH84001 demonstrating that 4 Ga carbonates containing magnetite and pyrrhotite carry a stable natural remanent magnetization. Because ^(40)Ar/^(39)Ar thermochronology demonstrates that most ALH84001 carbonates have probably been well below the Curie point of magnetite since near the time of their formation [Weiss et al., Earth Planet. Sci. Lett. (2002) this issue], their magnetization originated at 3.9–4.1 Ga on Mars. This magnetization is at least 500 million years (Myr) older than that known in any other planetary rock, and its strong intensity suggests that Mars had generated a geodynamo and global magnetic field within 450–650 Myr of its formation. The intensity of this field was roughly within an order of magnitude of that at the surface of the present-day Earth, sufficient for magnetotaxis by the bacteria whose magnetofossils have been reported in ALH84001 and possibly for the production of the strong crustal anomalies. Chromite in ALH84001 may retain even older records of Martian magnetic fields, possibly extending back to near the time of planetary formation.

Rapid Glacial Erosion at 1.8 Ma Revealed by ^4He/^3He Thermochronometry

Alpine glaciation and river incision control the topography of mountain ranges, but their relative contributions have been debated for years. Apatite 4He/3He thermochronometry tightly constrains the timing and rate of glacial erosion within one of the largest valleys in the southern Coast Mountains of British Columbia, Canada. Five proximate samples require accelerated denudation of the Klinaklini Valley initiating 1.8 ± 0.2 million years ago (Ma). At least 2 kilometers of overlying rock were removed from the valley at ≥5 millimeters per year, indicating that glacial valley deepening proceeded ≥6 times as fast as erosion rates before ∼1.8 Ma. This intense erosion may be related to a global transition to enhanced climate instability ∼1.9 Ma.

Early Lunar Magnetism

It is uncertain whether the Moon ever formed a metallic core or generated a core dynamo. The lunar crust and returned samples are magnetized, but the source of this magnetization could be meteoroid impacts rather than a dynamo. Here, we report magnetic measurements and 40Ar/39Ar thermochronological calculations for the oldest known unshocked lunar rock, troctolite 76535. These data imply that there was a long-lived field on the Moon of at least 1 microtesla ∼4.2 billion years ago. The early age, substantial intensity, and long lifetime of this field support the hypothesis of an ancient lunar core dynamo.

Radiation damage control on apatite (U-Th)/He dates from the Grand Canyon region, Colorado Plateau

Individual detrital apatite grains from the Esplanade, Coconino, and Moenkopi Formations in the Grand Canyon region of the Colorado Plateau yield (U-Th)/He dates from 104 to 5 Ma. The range of dates within each unit far exceeds analytical uncertainty, but correlates with both He concentration [He] and effective U concentration [eU]. These dates are all signifi cantly younger than the sandstone units, indicating partial to complete He loss following deposition. Recently published laboratory diffusion data suggest that He retentivity in apatite increases with radiation damage. Forward models predict that the consequences of this effect will be manifested most clearly as a correlation between (U-Th)/He dates and the [He] and [eU] in suites of apatites that (1) are characterized by a large span of [eU], and (2) had thermal histories in which suffi cient time elapsed for the apatite He diffusion kinetics to diverge prior to reheating and partial resetting. Apatites in the sedimentary units investigated fi t these cri teria. Using geologically reasonable deposition, burial, and unroofi ng histories, simulations that include the effect of radiation damage on apatite He retentivity can reproduce the observed distributions of apatite dates and correlations with parent and daughter concentrations. These results suggest that a span of (U-Th)/He dates positively correlated with [eU] may provide important information regarding a sample’s thermal history.

Quantifying the diffusion kinetics and spatial distributions of radiogenic ^4He in minerals containing proton-induced ^3He

Apatite, titanite and olivine samples were bombarded with a ~ 150 MeV proton beam to produce ~ 10^8 atoms/mg of spallation ^3He. High-precision stepped-heating experiments were then performed in which the artificial ^3He and, for apatite and titanite, the natural radiogenic ^4He were measured to characterize the diffusive behavior of each isotope. Helium-3 diffusion coefficients are in excellent agreement with concurrently and/or previously determined He diffusion coefficients for each mineral. Our results indicate that proton-induced ^3He is uniformly distributed and that radiation damage associated with a proton fluence of ~ 5 x 10^(14) protons/cm^2 does not cause noticeable changes in ^4He diffusion behavior in at least apatite and titanite. Proton-induced ^3He can therefore be used to establish He diffusion coefficients in minerals with insufficient natural helium for analysis or those in which the natural ^4He distribution is inhomogeneous. In addition,step-heating ^4He/^3He analysis of a mineral with a uniform synthetic ^3He concentration provides a means by which a natural ^4He distribution can be determined.

^4He/^3He thermochronometry

Using classical diffusion theory, we present a mathematical technique for the determination of ^4He concentration profiles in minerals. This approach should prove useful for constraining the low-temperature cooling histories of individual samples and for correcting (U–Th)/He ages for partial diffusive loss. The calculation assumes that the mineral of interest contains an artificially produced and uniform distribution of ^3He obtained by proton irradiation [Shuster et al., Earth Planet. Sci. Lett. 217 (2004) 19–32]. In minerals devoid of natural helium, this isotope allows measurement of He diffusion coefficients; in minerals with measurable radiogenic He, it permits determination of ^4He profiles arising during ingrowth and diffusion in nature. The ^4He profile can be extracted from stepwise degassing experiments in which the ^4He/^3He ratio is measured. The evolution of the ^4He/^3He ratio as a function of cumulative ^3He released can be compared with forward models to constrain the shape of the profile. Alternatively, we present a linear inversion that can be used to directly solve for the unknown ^4He distribution. The inversion incorporates a standard regularization technique to filter the influence of random measurement errors on the solution. Using either approach we show that stepwise degassing data can yield robust and high-resolution information on the ^4He profile. Profiles of radiogenic He are a sensitive function of the time–Temperate (t–T) path that a cooling sample experienced. Thus, by step heating a proton-irradiated sample it is possible to restrict the sample’s acceptable t–T paths. The sensitivity of this approach was explored by forward-modeling ^4He profiles resulting from a range of realistic t–T paths, using apatite as an example. Results indicate that ^4He profiles provide rich information on t–T paths, especially when the profiles are coupled with (U–Th)/He cooling ages on the same sample. Samples that experienced only moderate diffusive loss have ^4He concentration profiles that are rounded at the edge but uniform in the core of the diffusion domain. Such profiles can be identified by nearly invariant ^4He/^3He ratios after the first few to few tens of percent of ^3He have been extracted by step heating. We show how such data can be used to correct (U–Th)/He ages for partial diffusive loss.

High CO2 emissions through porous media: transport mechanisms and implications for flux measurement and fractionation

Abstract Diffuse emissions of CO2 are known to be large around some volcanoes and hydrothermal areas. Accumulation-chamber measurements of CO2 flux are increasingly used to estimate the total magmatic or metamorphic CO2 released from such areas. To assess the performance of accumulation chamber systems at fluxes one to three orders of magnitude higher than normally encountered in soil respiration studies, a test system was constructed in the laboratory where known fluxes could be maintained through dry sand. Steady-state gas concentration profiles and fractionation effects observed in the 30-cm sand column nearly match those predicted by the Stefan-Maxwell equations, indicating that the test system was functioning successfully as a uniform porous medium. Eight groups of investigators tested their accumulation chamber equipment, all configured with continuous infrared gas analyzers (IRGA), in this system. Over a flux range of ∼200–12,000 g m−2 day−1, 90% of their 203 flux measurements were 0–25% lower than the imposed flux with a mean difference of −12.5%. Although this difference would seem to be within the range of acceptability for many geologic investigations, some potential sources for larger errors were discovered. A steady-state pressure gradient of −20 Pa/m was measured in the sand column at a flux of 11,200 g m−2 day−1. The derived permeability (50 darcies) was used in the dusty-gas model (DGM) of transport to quantify various diffusive and viscous flux components. These calculations were used to demonstrate that accumulation chambers, in addition to reducing the underlying diffusive gradient, severely disrupt the steady-state pressure gradient. The resultant diversion of the net gas flow is probably responsible for the systematically low flux measurements. It was also shown that the fractionating effects of a viscous CO2 efflux against a diffusive influx of air will have a major impact on some important geochemical indicators, such as N2/Ar, δ15N–N2, and 4He/22Ne.

Tracing and quantifying magmatic carbon discharge in cold groundwaters: Lessons learned from Mammoth Mountain, USA

Abstract A major campaign to quantify the magmatic carbon discharge in cold groundwaters around Mammoth Mountain volcano in eastern California was carried out from 1996 to 1999. The total water flow from all sampled cold springs was ≥1.8×10 7 m 3 /yr draining an area that receives an estimated 2.5×10 7 m 3 /yr of recharge, suggesting that sample coverage of the groundwater system was essentially complete. Some of the waters contain magmatic helium with 3 He/ 4 He ratios as high as 4.5 times the atmospheric ratio, and a magmatic component in the dissolved inorganic carbon (DIC) can be identified in virtually every feature sampled. Many waters have a 14 C of 0–5 pmC, a δ 13 C near −5‰, and contain high concentrations (20–50 mmol/l) of CO 2(aq) ; but are otherwise dilute (specific conductance=100–300 μS/cm) with low pH values between 5 and 6. Such waters have previously escaped notice at Mammoth Mountain, and possibly at many other volcanoes, because CO 2 is rapidly lost to the air as the water flows away from the springs, leaving neutral pH waters containing only 1–3 mmol/l HCO 3 − . The total discharge of magmatic carbon in the cold groundwater system at Mammoth Mountain is ∼20 000 t/yr (as CO 2 ), ranging seasonally from about 30 to 90 t/day. Several types of evidence show that this high discharge of magmatic DIC arose in part because of shallow dike intrusion in 1989, but also demonstrate that a long-term discharge possibly half this magnitude (∼10 000 t/yr) predated that intrusion. To sustain a 10 000 t/yr DIC discharge would require a magma intrusion rate of 0.057 km 3 per century, assuming complete degassing of magma with 0.65 wt% CO 2 and a density of 2.7 t/m 3 . The geochemical data also identify a small (

A long-lived lunar core dynamo.

Magnetic Moon It has long been suspected that the Moon once had a core-dynamo magnetic field. Shea et al. (p. 453) describe a lunar basalt brought back by Apollo 11 that records evidence for a strong dynamo on the Moon 3.7 billion years ago. This study, together with a previous study of different lunar rock, implies that a lunar core dynamo existed between 4.2 and 3.7 billion years ago, which extends the known lifetime of the lunar dynamo by 500 million years. Analysis of a lunar basalt sample suggests that a lunar core dynamo existed between 4.2 and 3.7 billion years ago. Paleomagnetic measurements indicate that a core dynamo probably existed on the Moon 4.2 billion years ago. However, the subsequent history of the lunar core dynamo is unknown. Here we report paleomagnetic, petrologic, and 40Ar/39Ar thermochronometry measurements on the 3.7-billion-year-old mare basalt sample 10020. This sample contains a high-coercivity magnetization acquired in a stable field of at least ~12 microteslas. These data extend the known lifetime of the lunar dynamo by 500 million years. Such a long-lived lunar dynamo probably required a power source other than thermochemical convection from secular cooling of the lunar interior. The inferred strong intensity of the lunar paleofield presents a challenge to current dynamo theory.

An ancient core dynamo in asteroid Vesta.

Vesta to the Core Vesta is one of the largest bodies in the main asteroid belt. Unlike most other asteroids, which are fragments of once larger bodies, Vesta is thought to have survived as a protoplanet since its formation at the beginning of the solar system (see the Perspective by Binzel, published online 20 September). Based on data obtained with the Gamma Ray and Neutron Detector aboard the Dawn spacecraft, Prettyman et al. (p. 242, published online 20 September) show that Vestas reputed volatile-poor regolith contains substantial amounts of hydrogen delivered by carbonaceous chondrite impactors. Observations of pitted terrain on Vesta obtained by Dawns Framing Camera and analyzed by Denevi et al. (p. 246, published online 20 September), provide evidence for degassing of volatiles and hence the presence of hydrated materials. Finally, paleomagnetic studies by Fu et al. (p. 238) on a meteorite originating from Vesta suggest that magnetic fields existed on the surface of the asteroid 3.7 billion years ago, supporting the past existence of a magnetic core dynamo. Paleomagnetic studies of a meteorite from asteroid Vesta reveal remanent magnetization produced by an ancient core dynamo. The asteroid Vesta is the smallest known planetary body that has experienced large-scale igneous differentiation. However, it has been previously uncertain whether Vesta and similarly sized planetesimals formed advecting metallic cores and dynamo magnetic fields. Here we show that remanent magnetization in the eucrite meteorite Allan Hills A81001 formed during cooling on Vesta 3.69 billion years ago in a surface magnetic field of at least 2 microteslas. This field most likely originated from crustal remanence produced by an earlier dynamo, suggesting that Vesta formed an advecting liquid metallic core. Furthermore, the inferred present-day crustal fields can account for the lack of solar wind ion-generated space weathering effects on Vesta.