Kaare Lund Rasmussen

A New Seismic Velocity Model for the Moon from a Monte Carlo Inversion of the Apollo Lunar Seismic Data

A reanalysis of the Apollo lunar seismic data and the subsequent application of an inverse Monte Carlo method to P and S-wave arrival times has resulted in a more detailed lunar velocity structure than previously obtainable. The velocity is seen to increase from the surface down to the base of the crust at 45±5 km depth. The results furthermore indicate a constant velocity upper mantle extending to 560±15 km km depth, separated from a more complex high velocity middle mantle by an increase in velocity of 1.0 km/s. In addition, the moonquake locations have been improved. The shallow moonquakes are found to be located in the depth range 50–220 km. The majority of deep moonquakes are concentrated in the depth range 850–1000 km with an apparently rather sharp lower boundary.

The thermal evolution of IVA iron meteorites: evidence from metallographic cooling rates

Metallographic cooling rates of group IVA iron meteorites have been recalculated based on the most recent Ni diffusion coefficients and phase diagram. The cooling rates are revised upwards by a factor of ca. 15 relative to previous estimates. A large range in cooling rate is found in the low-Ni part of group IVA (Ni < 8.4 wt%), while the high-Ni part shows approximately constant cooling rates. Undercooling is observed only in the high-Ni IVA members. Some of the taenite lamellae in the high-Ni IVA irons, which were apparently affected by moderate undercooling, can, alternatively, be interpreted to have experienced a nonlinear cooling history. The variation in cooling rate of the entire group IVA spans two orders of magnitude (19–3400 K/My). This span is still so large that it constitutes severe problems for both a core origin model and a raisin-bread model but seemingly it does not contradict a model where the parent body is broken up and reassembled after core crystallization but prior to Widmanstatten pattern formation.

Megaregolith insulation and the duration of cooling to isotopic closure within differentiated asteroids and the Moon

Ages determined for extraterrestrial samples by the Sm-Nd and Rb-Sr techniques are commonly assumed to record igneous crystallization events, because in solid silicates, Nd and Sr diffuse at exceedingly slow rates. However, we find that for coarse-grained igneous cumulate rocks from the Moon or from a large, thoroughly brecciated asteroid, this assumption may not be reliable. The Moon and at least one asteroid (the parent body of the eucrite, diogenite, and howardite meteorites) appear to have been largely molten at or about the time they formed. We have modeled global cooling of the Moon and large (R = 40–250 km) asteroids, starting at or near the solidus. A crucial factor in determining the prevailing interval (Ic) of cooling between igenous crystallization and isotopic closure, for any given depth in the crust, is the extent to which the body is insulated by a regolith/megaregolith layer of porous, fragmental impact debris. Given plausible assumptions regarding the thicknesses of such layers on the Moon and the eucrite parent asteroid (and regarding the radius of the eucrite asteroid), our results indicate that deep-crustal regions tend to remain above the Nd and Sr isotopic closure temperature for intervals that are long in comparison to the precision of modern Nd- and Sr-based age measurements, and in comparison to suggested chronologic scenarios of global differentiation. Ic intervals of as long as 100 m.y. may be common among available samples of primordial, deep-crustal cumulates from both bodies. Chronologies for the gross solidification of the Moon and the eucrite asteroid should allow for the possibility that any single age for a coarse-grained “plutonic” or cumulate-textured rock might be many tens of millions of years younger than the igneous crystallization age.

Thermal and shock history of mesosiderites and their large parent asteroid

To elucidate the geological evolution of the mesosiderite stony-iron meteorites and their 3.4–3.8 Gy 40Ar39Ar ages, we have investigated their shock and thermal histories. We have studied shock metamorphism in sixteen mesosiderites and find that none have been shocked to more than 10 GPa (shock stage S1–S2). Three mesosiderites contain a tiny fraction (≈0.1 % overall) of mineral fragments that were shocked to shock stage S3–S6 levels. The uniformity of shock features within all fragments shows that these fragments were not shocked in situ. Our shock data for mesosiderites, the absence of evidence of 3.6–3.9 Gy impact melt, the shock history of impact-heated ordinary chondrites, and the difficulty in quantitatively removing Ar in an impact event, all suggest that the mesosiderite parent body did not suffer a major impact event 3.6–3.9 Gy ago. Metallographic cooling rates of ≈0.03°C/My at 400°C were estimated from taenite lamellae in four mesosiderites using the latest diffusion coefficients and FeNiP phase diagram. Cooling rates of 0.01°C / My at 425–325°C were estimated from published compositional data for kamacite grains in four mesosiderites. These two techniques and four other semiquantitative, metallographic cooling rate indicators show that the mesosiderites cooled slower than any iron meteorite. We infer that cooling rates at 400°C were ∼0.02–0.03°C/My and certainly less than 0.5°C/My. The inferred cooling rate is too slow to allow Ar closure before 4 Gy. All of the shock, thermal, and age data for mesosiderites are consistent with slow cooling at depth < 1 My after metal and silicate were mixed around 4.4 Gy ago. Thermal models indicate that the mesosiderites probably cooled in an asteroid some 200–400 km in radius.

Nitrate in the Greenland ice sheet in the years following the 1908 tunguska event

Abstract The Tunguska event on 30 June 1908 has been subjected to much speculation within different fields of research. Publication of the results of the 1961 expedition to the Tunguska area ( Florensky, 1963 ) supports that a cometary impact caused the event. Based on this interpretation, calculations of the impactor energy release and explosion height have been reported by Ben-Menahem (1975) , and velocity, mass, and density of the impactor by Petrov and Stulov (1975) . Park (1978) and Turco et al., 1981 , Turco et al., 1982 , used these numbers to calculate a production of ca. 30 × 106 tons of NO during atmospheric transit. This paper presents a high-resolution study of nitrate concentration in the Greenland ice sheet in ca. 10 years covering the Tunguska event. No signs of excess nitrate are found in three ice cores from two different sites in Greenland in the years following the Tunguska event. By comparing these results with results for other aerosols generally found in the ice, the lack of excess NO3− following the Tunguska event can be interpreted as indicating that the impactor nitrate production calculated by Park (1978) and Turco et al., 1981 , Turco et al., 1982 are 1–2 orders of magnitude too high. To explain this it is suggested, from other lines of reasoning, that the impactor density determined by Petrov and Stulov (1975) probably is too low.

Cooling rates and parent bodies of iron meteorites from group IIICD, IAB, and IVB

New metallographic cooling rates have been calculated for the iron meteorites Carlton (IIICD), Toluca (IAB), and Odessa (IAB) based on the central Ni concentration versus taenite width method. New kamacite bandwidth cooling rates have been calculated for the members of group IVB, the ungrouped iron meteorite Chinga, and the high-Ni members of IIICD and IAB. The cooling rates determined are up to several orders of magnitude faster than those previously reported. A difference of about a factor of 60 is found between the kamacite bandwidth cooling rate and the central Ni versus taenite width cooling rate of the same meteorite, Carlton. This discrepancy is interpreted as resulting from extensive regolith accretion on the parent body at the time of Widmanstatten pattern formation. No systematic trend is found in the cooling rates as a function of bulk meteorite Ni of the magmatic group IVB. Combined with the strong inter-element correlations found in group IVB [15], the constant cooling rate suggests a core origin of this group. No systematic cooling rate variation is found in neither of the non-magmatic groups IIICD or IAB. Combined with the weak or missing inter-element correlations in these groups, the results of the present work support the megaregolith melt pool scenario suggested by Wasson et al. [1] for the origin of the non-magmatic groups IIICD and IAB.

Determination of the cooling rates and nucleation histories of eight group IVA iron meteorites using local bulk Ni and P variation

Abstract Cooling rates and nucleation histories of six low-Ni and two high-Ni members of group IVA iron meteorites were calculated by a mid-taenite concentration-taenite lamella width method that included the effects of local bulk Ni and P variation. The local bulk Ni is determined experimentally as described in K. L. Rasmussen [ Icarus 45 , 564–576 (1981)] . The local bulk P parameter, included for the first time in the present work, is estimated from the phase diagram during the simulation. Two parent bodies are suggested for group IVA. The body containing the high-Ni members had a cooling rate (∼2°K/My) lower than earlier cooling rate determinations on IVA members. The variable (by a factor of 4) cooling rates found for the low-Ni members imply a raisin origin. The nucleation histories of the meteorites are interpreted as reflecting the very early shock histories of the meteorite parent bodies.

Radiocarbon wiggle-dating of elm declines in northwest Denmark and their significance

Four elm declines were found in a pollen diagram from a small lake in northwest Denmark. Matching consecutive radiocarbon dates with the dendro-chronological calibration curve indicated a reservoir effect of 120 years, and dates for the four elm declines were obtained (4530, 4130, 3870, 3410 cal B.C.). The occurrence of apophytes (native plants encouraged by human activities) and increased vegetation diversity during the four elm declines indicates human disturbance. The first and second elm declines coincide with traces of early agriculture in northern Germany. The third and fourth elm declines are contemporary with the transitions to the Early Neolithic and the Middle Neolithic Funnel Beaker Culture in Denmark. The possible influence of outbreaks of elm disease is discussed.

Cooling rates of IIIAB iron meteorites

Metallographic cooling rates have been determined for 12 iron meteorites of group IIIAB using the central taenite Ni content versus taenite width method. Experimentally determined values of local bulk Ni and P are included as input parameters in the calculations as outlined in Rasmussen (1982a, Icarus 52, 444–453). The calculations are based on the recently revised diffusivities and phase diagram recommended by Saikumar and Goldstein (1988, Geochim. Cosmochin. Acta 52, 715–726). The present results show no signs of undercooling in any of the meteorites investigated. The cooling rates are found to be constant through the group, supporting a core origin of group IIIAB. The average cooling rate for the group is found to be 49 K/my, corresponding to a parent body radius of 25 ± 7 km. An interpretation of the nucleation history reflecting the early impact history of the parent body is tentatively put forward.

Magmatic activity on the IVA parent body: Evidence from silicate-bearing iron meteorites

Abstract Four of the magmatic IVA iron meteorites contain tridymite or clinobronzite-orthobronzitetridymite which are quite unlike silicate assemblages in other iron meteorites. The textures, the bulk chemistry, and the zoning preserved in the pyroxenes strongly suggest that they are igneous cumulates. The pyroxenes have extremely low Fe/Mn-ratios (less than 20) and low contents of REEs and other incompatible elements. These cumulates crystallized from magmas of unusual composition, with some similarity to terrestrial boninites, at the protobronzite-tfdymite cotectic in the olivine-plagioclase-silica system. A liquidus temperature in the range 1400-1350°C was inferred for the Steinbach meteorite from the estimated distribution coefficient for Cr in pyroxene (DCrsolid/liquid ∼ 0.66). The low levels of incompatible elements show that less than 1% of the residual liquid was trapped in the cumulates. During cooling at subsolidus temperatures, most of the protobronzite transformed to orthobronzite and the rest inverted to a fine inter-growth of clino- and orthobronzite. In addition, the igneous zoning of Ca, Fe, Mg, and Mn was modified by diffusion, whereas Ti, Al, and Cr were not or only slightly affected. The silica-saturated magmas could not have evolved in an olivine-rich mantle. We assume that the magmas became incorporated in the metal core, possibly due to solidification shrinkage of the metal. We propose that the IVA parent magmas were formed by high degrees of partial melting (>40%) of a chondritic precursor along the olivine-pyroxene peritectic reaction curve in the olivine-plagioclase-silica system at low pressures. The precursor may have been depleted in incompatible elements by a preceding melting episode. The partial melts were then separated from the olivine residue and subsequently reduced to account for the low Fe Mn - ratios and the unusually high Si content.