Henning Haack

Mg isotope evidence for contemporaneous formation of chondrules and refractory inclusions

Primitive or undifferentiated meteorites (chondrites) date back to the origin of the Solar System, and thus preserve a record of the physical and chemical processes that occurred during the earliest evolution of the accretion disk surrounding the young Sun. The oldest Solar System materials present within these meteorites are millimetre- to centimetre-sized calcium-aluminium-rich inclusions (CAIs) and ferromagnesian silicate spherules (chondrules), which probably originated by thermal processing of pre-existing nebula solids. Chondrules are currently believed to have formed ∼2–3 million years (Myr) after CAIs (refs 5–10)—a timescale inconsistent with the dynamical lifespan of small particles in the early Solar System. Here, we report the presence of excess 26Mg resulting from in situ decay of the short-lived 26Al nuclide in CAIs and chondrules from the Allende meteorite. Six CAIs define an isochron corresponding to an initial 26Al/27Al ratio of (5.25 ± 0.10) × 10-5, and individual model ages with uncertainties as low as ± 30,000 years, suggesting that these objects possibly formed over a period as short as 50,000 years. In contrast, the chondrules record a range of initial 26Al/27Al ratios from (5.66 ± 0.80) to (1.36 ± 0.52) × 10-5, indicating that Allende chondrule formation began contemporaneously with the formation of CAIs, and continued for at least 1.4 Myr. Chondrule formation processes recorded by Allende and other chondrites may have persisted for at least 2–3 Myr in the young Solar System.

Icelandic analogs to Martian flood lavas

We report on new field observations from Icelandic lava flows that have the same surface morphology as many Martian flood lava flows. The Martian flood lavas are characterized by a platy-ridged surface morphology whose formation is not well understood. The examples on Mars include some of the most pristine lava on the planet and flows >1500 km long. The surfaces of the flows are characterized by (1) ridges tens of meters tall and wide and hundreds of meters long, (2) plates hundreds of meters to kilometers across that are bounded by ridges, (3) smooth surfaces broken into polygons several meters across and bowed up slightly in the center, (4) parallel grooves 1–10 km long cut into the flow surface by flow past obstacles, and (5) inflated pahoehoe margins. The Icelandic examples we examined (the 1783–1784 Laki Flow Field, the Burfells Lava Flow Field by Lake Myvatn, and a lava flow from Krafla Volcano) have all these surface characteristics. When examined in detail, we find that the surfaces of the Icelandic examples are composed primarily of disrupted pahoehoe. In some cases the breccia consists of simple slabs of pahoehoe lava; in other cases it is a thick layer dominated by contorted fragments of pahoehoe lobes. Our field observations lead us to conclude that these breccias are formed by the disruption of an initial pahoehoe surface by a large flux of liquid lava within the flow. In the case of Laki, the lava flux was provided by surges in the erupted effusion rate. At Burfells it appears that the rapid flow came from the sudden breaching of the margins of a large ponded lava flow. Using the observations from Iceland, we have improved our earlier thermal modeling of the Martian flood lavas. We now conclude that these platy-ridged lava flows may have been quite thermally efficient, allowing the flow to extend for >100 km under a disrupted crust that was carried on top of the flow.

Rapid Timescales for Accretion and Melting of Differentiated Planetesimals Inferred from 26Al-26Mg Chronometry

Constraining the timescales for the assembly and differentiation of planetary bodies in our young solar system is essential for a complete understanding of planet-forming processes. This is best achieved through the study of the daughter products of extinct radionuclides with short half-lives, as they provide unsurpassed time resolution as compared to long-lived chronometers. Here we report high-precision Mg isotope measurements of bulk samples of basalt, gabbro, and pyroxenite meteorites obtained by multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). All samples from the eucrite and mesosiderite parent bodies (EPB and MPB) with suprachondritic Al/Mg ratios have resolvable 26Mg excesses compared to matrix-matched samples from the Earth, the Moon, Mars, and chondrites. Basaltic magmatism on the EPB and MPB thus occurred during the life span of the now-extinct 26Al nuclide. Initial 26Al/27Al values range from (1.26 ± 0.37) × 10-6 to (5.12 ± 0.81) × 10-6 at the time of magmatism on the EPB and MPB, and are among the highest 26Al abundances reported for igneous meteorites. These results indicate that widespread silicate melting and differentiation of rocky bodies occurred within 3 million years of solar system formation, when 26Al and 60Fe were extant enough to induce planetesimal melting. Finally, thermal modeling constrains the accretion of these differentiated asteroids to within 1 million years of solar system formation, that is, prior to the accretion of chondrite parent bodies.

Early history of Earth's crust-mantle system inferred from hafnium isotopes in chondrites.

The 176Lu to 176Hf decay series has been widely used to understand the nature of Earths early crust–mantle system. The interpretation, however, of Lu–Hf isotope data requires accurate knowledge of the radioactive decay constant of 176Lu (λ176Lu), as well as bulk-Earth reference parameters. A recent calibration of the λ176Lu value calls for the presence of highly unradiogenic hafnium in terrestrial zircons with ages greater than 3.9 Gyr, implying widespread continental crust extraction from an isotopically enriched mantle source more than 4.3 Gyr ago, but does not provide evidence for a complementary depleted mantle reservoir. Here we report Lu–Hf isotope measurements of different Solar System objects including chondrites and basaltic eucrites. The chondrites define a Lu–Hf isochron with an initial 176Hf/177Hf ratio of 0.279628 ± 0.000047, corresponding to λ176Lu = 1.983 ± 0.033 × 10-11 yr-1 using an age of 4.56 Gyr for the chondrite-forming event. This λ176Lu value indicates that Earths oldest minerals were derived from melts of a mantle source with a time-integrated history of depletion rather than enrichment. The depletion event must have occurred no later than 320 Myr after planetary accretion, consistent with timing inferred from extinct radionuclides.

Catastrophic fragmentation of asteroids: Evidence from meteorites

Abstract Meteorites are impact-derived fragments from ≈ 85 parent bodies. For seven of these bodies, the meteorites record evidence suggesting that they may have been catastrophically fragmented. We identify three types of catastrophic events: (a) impact and reassembly events > 4.4 Gy ago, involving molten or very hot parent bodies(> 1200°C); this affected the parent bodies of the ureilites, Shallowater, and the mesosiderites. In each case, the fragments cooled rapidly (≈ 1–1000°C day −1 ) and then reassembled, (b) Later impacts involving cold bodies which, in some cases, reassembled; this occurred on the H and L ordinary chondrite parent bodies. The L parent body probably suffered another catastrophic event about 500 My ago. (c) Recent impacts of cold, multi-kilometer-sized bodies that generated meter-sized meteoroids; this occurred on the parent bodies of the IIIAB irons (650 My ago), the IVA irons (400 My ago), and the H ordinary chondrite (7 My ago).

Chemical fractionations in Group IIIAB iron meteorites: Origin by dendritic crystallization of an asteroidal core

Abstract We have studied the crystallization history of the asteroidal core that produced nearly two hundred iron meteorites of group IIIAB, the largest group of irons. By critically reassessing the accuracy of the published distribution coefficients between solid and liquid metal for seven elements and their dependence on the concentrations of S and P using the formulation of Jones and Malvin (1990), we have developed a nonideal fractional crystallization model that reproduces all of the major features of the overall chemical trends in group IIIAB. In particular, we can match the variations of Ni, Ga, Ge, Ir, Au, Co, and P, including the Ga and Ge reversals at the IIIA-IIIB boundary. Previous authors were not able to model the Ga and Ge reversals simultaneously and used different initial S and P concentrations. Our models and the S concentration of IIIAB irons suggest that the apparent distribution coefficient for S was much higher than its equilibrium value (≤0.01) and that it increased during crystallization. In our preferred model the apparent distribution coefficient for S increases from 0.6 to 0.8 while the S concentration of the liquid increases from 6 to 13 wt%. We infer that light S-rich liquid accumulates preferentially at the top of the core and in structural traps formed by the advancing solid. This process may be aided by the formation of immiscible S-rich liquid in a boundary layer adjacent to the crystallizing solid. Our comparison of the chemical trends in the Cape York irons discovered by Esbensen et al. (1982) with the overall group IIIAB trends indicates that the Cape York irons are not abnormal IIIAB irons that formed by an atypical solid-liquid mixing event, as these authors suggest. We find a positive correlation between the divergence of the Cape York and IIIAB trends on element-Ni plots and the scatter of the IIIAB irons from the overall IIIAB trends. This correlation and the irregularity of the histogram of Ir concentrations in group IIIAB suggest that group IIIAB contains several Cape York-like sequences and could not have crystallized from a single well-mixed magma. Instead we suggest that the initially homogeneous magma was subdivided into numerous magma chambers by km-sized dendrites that grew down from the core-mantle boundary at an early stage. Although the Cape York irons crystallized from a single magma chamber, we cannot model their compositional trends with closed-system fractional crystallization, possibly because of magma exchange between chambers or formation of S-rich boundary layers.

Core crystallization and silicate-metal mixing in the parent body of the IVA iron and stony-iron meteorites

Abstract We have analyzed metallic and silicate phases in the IVA iron meteorites and two related stony irons, Steinbach and Sao Joao Nepomuceno. Analyses of bulk metal phases in the two stony irons using INAA show that they plot as close to the chemical trends within group IVA as most IVA irons, indicating a common source. Our fractional crystallization models for the IVA chemical trends suggest that the irons crystallized from a metallic melt that initially contained 2.5 ± 1 wt% S. After S concentrations in the liquid reached 6 wt%, liquid trapping during crystallization increased the apparent distribution coefficient for S, as in group IIIAB. Compositions of the metal fractions in Steinbach and Sao Joao Nepomuceno match the calculated solid compositions after 50 ± 10% and 80 ± 10%, respectively, of the metallic melt had crystallized. We confidently conclude that the IVA irons and metal in the two stony irons were derived from the core of a single asteroid that fractionally crystallized. The wide range of metallographic cooling rates of IVA irons cannot result from crystallization in isolated pools in one or more bodies, as some authors have argued. Large depletions of Ga, Ge, and other moderately volatile elements in group IVA are unlikely to result from planetary processes; they may have been inherited from chondritic precursor material. The two IVA stony irons contain up to 60 vol% of a unique, coarse-grained mixture of tridymite, orthobronzite, and clinobronzite. Silicate-metal textures resemble those in rounded-olivine pallasites; both may result from the depression of cumulate silicates into underlying molten S-rich metal. Two IVA irons contain rare plate-like, silica crystals up to 10 mm long, but these occurrences seem unrelated to the stony-iron silicates. Because of the difficulty in forming the stony irons in an isolated, slowly cooling asteroid, we infer that they may have formed during the breakup and reassembly event invoked by Haack et al. (1995) to account for the fast cooling of Steinbach from 1200°C.

Asteroid core crystallization by inward dendritic growth

To better understand the geology of metallic asteroids and the crystallization of planetary cores, we have studied the crystallization of the cores of iron meteorite parent bodies. Available data for the relative magnitudes of the adiabatic gradient and the liquidas gradient across the core indicate that for the low pressures of asteroidal cores (less than a few hundred megapascals), crystallization commences at the base of the mantle and continues as dominantly inward growth. A consequence of the rejection of S from the crystallizing solid is that a light S-rich layer forms below the crystallizing front. This inhibits concentric front growth and promotes formation of tree-like crystals called dendrites. The mode of crystallization was therefore completely different from that of Earth. Dendrites in asteroidal cores may have grown to lengths of hundreds of meters or perhaps even as large as the core radius thereby dividing the core into separate magma chambers. Supporting evidence comes from the Cape York iron meteorites and others from the same core: the surface area of the solid/liquid interface was very large; the direction of crystallization was horizontal; and the liquid was not perfectly mixed throughout crystallization. We argue that the Cape York irons did not form as a result of a rare event; they are typical products of core crystallization. Troilite, which has different optical and mechanical properties from Fe-Ni, formed in pockets between dendrites. The distribution of troilite may therefore provide a visual record of the crystallization history of asteroidal cores.

Thermal histories of IVA stony-iron and iron meteorites: Evidence for asteroid fragmentation and reaccretion

We have investigated the thermal history of the IVA iron and stony-iron meteorites to help resolve the apparent conflict between their metallographic cooling rates, which are highly diverse, and their chemical trends, which favor crystallization in a single core. Transmission electron microscopy of the disordered clinobronzite in the stony-iron, Steinbach, using electron diffraction and high resolution imaging techniques indicates that this meteorite was rapidly cooled at ≈ 100°C/hr through 1200°C. The IVA irons cooled much slower in the range 1200–1000°C: absence of dendrites in large troilite nodules indicate cooling rates of <300°C/y. We infer that the parent asteroid was catastrophically fragmented and reaccreted when the core had cooled to 1200°C and was 95% crystallized. We argue that radiative heat losses from the debris cloud would have been minor due to its high opacity, small size (only a few asteroid diameters), and short reaccretion times (∼ a few hours). We calculate that global heating effects were also minor (ΔT < 300°C for a body with a diameter of < 400 km) and that the mean temperature of the IVA parent body before and after the impact was 450–700°C. We infer that Steinbach cooled rapidly from 1200°C at the edge of a core fragment by thermal equilibration with cooler silicates during and after reaccretion. Metallographic cooling rates of IVA irons and stony-irons for the temperature range 600–350°C (Rasmussen et al., 1995) strongly support this model and indicate that the IVA meteorites are derived from only a few core fragments. The large range of these cooling rates (20–3000°C/My) and the decrease in the metallographic cooling rates of high-Ni IVA irons with falling temperature probably reflect the diversity of thermal environments in the reaccreted asteroid, the low thermal conductivity of fragmental silicates, and the limited sintering of this fragmental material.

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.