Indentation of a free‐falling lance penetrometer into a poroelastic seabed

A solution is developed for the build‐up, steady and post‐arrest dissipative pore fluid pressure fields that develop around a blunt penetrometer that self‐embeds from freefall into the seabed. Arrest from freefall considers deceleration under undrained conditions in a purely cohesive soil, with constant shear strength with depth. The resulting decelerating velocity field is controlled by soil strength, geometric bearing capacity factors, and inertial components. At low impact velocities the embedment process is controlled by soil strength, and at high velocities by inertia. With the deceleration defined, a solution is evaluated for a point normal dislocation penetrating in a poroelastic medium with a prescribed decelerating velocity. Dynamic steady pressures, P_D, develop relative to the penetrating tip geometry with their distribution conditioned by the non‐dimensional penetration rate, U_D, incorporating impacting penetration rate, consolidation coefficient and penetrometer radius, and the non‐dimensional strength, N_D, additionally incorporating undrained shear strength of the sediment. Pore pressures develop to a steady peak magnitude at the penetrometer tip, and drop as P_D=1/x_D with distance x_D behind the tip and along the shaft. Peak induced pressure magnitudes may be correlated with sediment permeabilities, post‐arrest dissipation rates may be correlated with consolidation coefficients, and depths of penetration may be correlated with shear strengths. Together, these records enable strength and transport parameters to be recovered from lance penetrometer data. Penetrometer data recorded off La Palma in the Canary Islands (J. Volcanol. Geotherm. Res. 2000; 101 :253) are used to recover permeabilities and consolidation coefficients from peak pressure and dissipation response, respectively. Copyright © 2004 John Wiley & Sons, Ltd.     

The Basal Onaping Intrusion in the North Range: Roof rocks of the Sudbury Igneous Complex

The 1.85 Ga Sudbury impact structure is one of the largest impact structures on Earth. Igneous bodies—the so‐called “Basal Onaping Intrusion”—occur at the contact between the Sudbury Igneous Complex (SIC) and the overlying Onaping Formation and occupy ~50% of this contact zone. The Basal Onaping Intrusion is presently considered part of the Onaping Formation, which is a complex series of breccias. Here, we present petrological and geochemical data from two drill cores and field data from the North Range of the Sudbury structure, which suggests that the Basal Onaping Intrusion is not part of the Onaping Formation. Our observations indicate that the Basal Onaping Intrusion crystallized from a melt and has a groundmass comprising a skeletal intergrowth of feldspar and quartz that points to simultaneous cooling of both components. Increasing grain size and decreasing amounts of clasts with increasing depth are general features of roof rocks of coherent impact melt rocks at other impact structures and the Basal Onaping Intrusion. Planar deformation features within quartz clasts of the Basal Onaping Intrusion are indicators for shock metamorphism and, together with the melt matrix, point to the Basal Onaping Intrusion as being an impact melt rock, by definition. Importantly, the contact between Granophyre of the SIC and Basal Onaping Intrusion is transitional and we suggest that the Basal Onaping Intrusion is what remains of the roof rocks of the SIC and, thus, is a unit of the SIC and not the Onaping Formation. We suggest henceforth that this unit be referred to as the “Upper Contact Unit” of the SIC.     

The Vicência meteorite fall: A new unshocked (S1) weakly metamorphosed (3.2) LL chondrite

The Vicência meteorite, a stone of 1.547 kg, fell on September 21, 2013, at the village Borracha, near the city of Vicência, Pernambuco, Brazil. It was recovered immediately after the fall, and our consortium study showed it to be an unshocked (S1) LL3.2 ordinary chondrite. The LL group classification is based on the bulk density (3.13 g cm^?3); the chondrule mean apparent diameter (0.9 mm); the bulk oxygen isotopic composition (δ¹⁷O = 3.768 ± 0.042‰, δ¹⁸O = 5.359 ± 0.042‰, Δ¹⁷O = 0.981 ± 0.020‰); the content of metallic Fe,Ni (1.8 vol%); the Co content of kamacite (1.73 wt%); the bulk contents of the siderophile elements Ir and Co versus Au; and the ratios of metallic Fe⁰/total iron (0.105) versus total Fe/Mg (1.164), and of Ni/Mg (0.057) versus total Fe/Mg. The petrologic type 3.2 classification is indicated by the beautifully developed chondritic texture, the standard deviation (~0.09) versus mean Cr₂O₃ content (~0.14 wt%) of ferroan olivine, the TL sensitivity and the peak temperature and peak width at half maximum, the cathodoluminescence properties of chondrules, the content of trapped ¹³²Xe_tr (0.317 × 10^?8cm³STP g^?1), and the Raman spectra for organic material in the matrix. The cosmic ray exposure age is ~72 Ma, which is at the upper end of the age distribution of LL group chondrites. The meteorite is unusual in that it contains relatively large, up to nearly 100 μm in size, secondary fayalite grains, defined as olivine with Fa_>75, large enough to allow in situ measurement of oxygen and Mn‐Cr isotope systematics with SIMS. Its oxygen isotopes plot along a mass‐dependent fractionation line with a slope of ~0.5 and Δ¹⁷O of 4.0 ± 0.3‰, and are similar to those of secondary fayalite and magnetite in the unequilibrated chondrites EET 90161, MET 96503, and Ngawi. These data suggest that secondary fayalite in Vicência was in equilibrium with a fluid with a Δ¹⁷O of ~4‰, consistent with the composition of the fluid in equilibrium with secondary magnetite and fayalite in other unequilibrated ordinary chondrites. Secondary fayalite and the chondrule olivine phenocrysts in Vicência are not in isotopic equilibrium, consistent with low‐temperature formation of fayalite during aqueous alteration on the LL parent body. That alteration, as dated by the ⁵³Mn‐⁵³Cr chronology age of secondary fayalite, took place $4.0{}_{?1.1}^{+1.4}$ Ma after formation of CV CAIs when anchored to the quenched angrite D'Orbigny.     

Oral histories in meteoritics and planetary science – XXII: John T. Wasson

In this interview, John Wasson (Fig.  1 ) describes his childhood and undergraduate years in Arkansas and his desire to pursue nuclear chemistry as a graduate student at MIT. Upon graduation, John spent time in Munich (Technische Hochschule), the Air Force Labs in Cambridge, MA, and a sabbatical at the University of Bern where he developed his interests in meteorites. Upon obtaining his faculty position at UCLA, John established a neutron activation laboratory and began a long series of projects on the bulk compositions of iron meteorites and chondrites. He developed the chemical classification scheme for iron meteorites, gathered a huge set of iron meteorite compositional data with resultant insights into their formation, and documented the refractory and moderately volatile element trends that characterize the chondrites and chondrules. He also spent several years studying field relations and compositions of layered tektites from Southeast Asia, proposing an origin by radiant heating from a mega‐Tunguska explosion. Recently, John has explored oxygen isotope patterns in meteorites and their constituents believing the oxygen isotope results to be some of the most important discoveries in cosmochemistry. John also describes the role of postdoctoral colleagues and their important work, his efforts in the reorganization and modernization of the Meteoritical Society, his contributions in reshaping the journal Meteoritics, and how, with UCLA colleagues, he organized two meetings of the society. John Wasson earned the Leonard Medal of the Meteoritical Society in 1992 and the J. Lawrence Smith Medal of the National Academy in 2003.

Figure 1 Open in figure viewer PowerPoint John T. Wasson.

DS

John, thank you for letting me document your oral history. Let us start with my normal opening question, how did you get interested in meteorites?

JW

My Ph.D. research was in nuclear chemistry at MIT. Until late in my studies I thought I could be a nuclear chemist using the classical scientific method. That is, you gather data on a topic that seems interesting, you look for patterns in the data, and you write an interpretative paper that explains the data. I had learned, though, by going to Gordon Conferences, that this was not the way nuclear chemistry was being done. Nuclear chemists measured gamma ray energies as accurately as they could, they tried to fit these into energy levels diagrams, and then the nuclear physicists took over and interpreted the data. The nuclear physicists looked for the patterns in the energy‐level diagrams and made the models. That was not what I had in mind. But while I was at MIT, I heard lectures by Harold Urey, Hans Suess, and James Arnold. These were people whose backgrounds were not that different from mine and all three extolled the virtues of working on meteorites, and how you could learn neat things about how the solar system worked. That's a strength of MIT, exposure to neat ideas, and I credit the institution for doing this. So that was it. I was hooked.

DS

You have talked to us about how you became interested in meteorites, let's go back and talk about your precollege years.

     

The scientific value of the Carte du Ciel

Work began on the Astrographic Catalogue and Carte du Ciel in 1887 and continued until 1962. Derek Jones discusses this innovatory sky survey and its continuing value.