Early history of the moon: Implications of U-Th-Pb and Rb-Sr systematics

Anorthosite 60015 contains the lowest initial Sr-87/Sr-86 ratio (0.69884 + or - 0.00004) yet reported for a lunar sample. The initial ratio is equal to that of the achondrite Angra dos Reis and slightly higher than the lowest measured Sr-87/Sr-86 ratio for an inclusion in the C3 carbonaceous chondrite Allende. The Pb-Pb ages of both Angra dos Reis and Allende are 4.62 x 10 to the 9th power years (4.62 billion years). Thus, the initial Sr-87/Sr-86 ratio found in lunar anorthosite 60015 strongly supports the hypothesis that the age of the moon is about 4.65 b.y. The U-238/Pb-204 value estimated for the source of the excess lead in orange soil 74220 is lower than the values estimated for the sources of KREEP (600-1000), high K (300-600) and low K (100-300) basalts

Physics and chemistry of magma oceans

Evidence for the existence of magma oceans is discussed in great detail, and among the many new items introduced were high-pressure phase equilibrium experiments, calculations of depth of impact-produced melting, models incorporating crystal growth rates with degree of crystallinity and convection, and models of hard turbulent convection.sponsored by Lunar and Planetary Institute, Lunar and Planetary Sample Team (LAPST), and NASA Johnson Space Center.edited by Carl B. Agee and John LonghiOrigin of the moon and lunar core formation / Hillgren, Valerie J. -- Superheat in Magma Oceans / Jakes, P. -- A new angle on lunar ferroan-suite differentiation / Jolliff, B.L. -- Magma Ocean: Mechanisms of Formation / Kaula, W.M. -- Fate of a Perched Crystal Layer in a Magma Ocean / Morse, S.A

Meteoritic material on the moon

Micrometeorites, ancient planetesimal debris from the early intense bombardment, and debris of recent, crater-forming projectiles are discussed and their amounts and compositions have been determined from trace element studies. The micrometeorite component is uniformly distrubuted over the entire lunar surface, but is seen most clearly in mare soils whereas, the ancient component is seen in highland breccias and soils. A few properties of the basin-forming objects are inferred from the trace element data. An attempt is made to reconstruct the bombardment history of the moon from the observation that only basin-forming objects fell on the moon after crustal differentiation. The apparent half-life of basin-forming bodies is close to the calculated value for earth-crossing planetesimals. It is shown that a gap in radiometric ages is expected between the Imbrium and Nectaris impacts, because all 7 basins formed in this interval lie on the farside or east limb

Conference on Planetary Volatiles

Initial and present volatile inventories and distributions in the Earth, other planets, meteorites, and comets; observational evidence on the time history of volatile transfer among reservoirs; and volatiles in planetary bodies, their mechanisms of transport, and their relation to thermal, chemical, geological and biological evolution were addressed

Conference on Planetary Volatiles

Initial and present volatile inventories and distributions in the Earth, other planets, meteorites, and comets; observational evidence on the time history of volatile transfer among reservoirs; and volatiles in planetary bodies, their mechanisms of transport, and their relation to thermal, chemical, geological and biological evolution are addressed

High-Precision 26A1-26Mg Systematics of Basaltic Achondrites, Chondrites and Ultramafic Achondrites

A precise and accurate chronology of events that shaped the early Solar System is crucial in understanding its formation. One of the high-resolution chronometers that can be used to establish a relative chronology is the short-lived 26A1-to-26Mg clock (t1/2 = 0.73 Myr). This study developed new Mg chemical separation techniques for complex meteoritic matrices that produces Mg purities > 99% with > 99% yields. Mg was analysed by pseudo-high resolution multiple collector inductively coupled plasma mass spectrometry. These techniques make it possible to measure the mass-independent abundance of 26Mg (d26Mg*) that is related to 26A1 decay to very high-precision (+_ 0.0025 to 0.0050 per1000). These new techniques were then applied to three research objectives. The first part of this study presents Mg isotope data for thirteen bulk basaltic achondrites from at least 3 different parent bodies, as well as mineral isochrons for the angrites Sahara 99555 and D'Orbingy and the ungrouped NWA 2976. Model 26A1-26Mg ages based on bulk rock d26Mg* excesses for basaltic magmatism range from 2.6-4.1 Myr, respectively, after formation of calcium-aluminium-rich inclusions (CAIs) and the mineral isochrons for the angrites Sahara 99555 and D'Orbigny, and the ungrouped NWA 2976 yield apparent crystallisation ages of 5.06+0:06-0:05 Myr and 4.86+0:10-0:09 Myr after CAI formation. The elevated initial d26Mg* of the mineral isochron of NWA 2976 (+0.0175+ _0.0034h) likely reflects thermal resetting during an impact event and slow cooling on its parent body. However, in the case of the angrites, the marginally elevated initial d26Mg* (+0.0068 -0.0058h) could reflect d26Mg* in-growth in a magma ocean prior to eruption and crystallisation or in an older igneous protolith with super-chondritic A1/Mg prior to impact melting and crystallisation of these angrites, or partial internal re-equilibration of Mg isotopes after crystallisation. 26A1-26Mg model ages and an olivine+pyroxene+whole rock isochron for the angrites Sahara 99555 and D'Orbigny are in good agreement with age constraints from 53Mn-53Cr and 182Hf-182W shortlived chronometers. This suggests that the 26A1-26Mg feldspar-controlled isochron ages for these angrites may be compromised by the partial resetting of feldspar Mg isotope systematics. However, even the 26A1-26Mg angrite model ages cannot be reconciled with Pb-Pb ages for Sahara 99555/D'Orbigny and CAIs, which are ca. 1.0 Myr too old (angrites) or too young (CAIs) for reasons that are not clear. This discrepancy might indicate that 26A1 was markedly lower (ca. 40%) in the planetesimal- and planet-forming regions of the proto-planetary disk as compared to CAIs, or that CAI Pb-Pb ages may not accurately date CAI formation. The second part of this thesis focuses on investigating the homogeneity of (26A1/27A1)0 and Mg isotopes in the proto-planetary disk and to test the validity of the short-lived 26A1-to-26Mg chronometer applied to meteorites. Nineteen chondrites representing nearly all major chondrite classes were analysed, including a step-leaching experiment on the CM2 chondrite Murchison. d26Mg* variations in leachates of Murchison representing acid soluble material are <_30 times smaller than reported for neutron-rich isotopes of Ti and Cr and do not reveal resolvable deficits in d26Mg* (-0.002 to +0.118h). Very small variations in d26Mg* anomalies in bulk chondrites (-0.006 to +0.019h) correlate with increasing 27A1/24Mg ratios and d50Ti, reflecting the variable presence of CAIs in some types of carbonaceous chondrites. Overall, the observed variations in d26Mg* are small and potential differences beyond those resulting from the presence of CAI-like material could not be detected. The results do not allow radical heterogeneity of 26A1 (>_+_ 30%) or measurable Mg nucleosynthetic heterogeneity (>_+_ 0.005h) to have existed on a planetesimal scale in the proto-planetary disk. The data imply that planets (i.e. chondrite parent bodies) accreted from material with initial (26Al/27A1)0 in the range of 2.1 to 6.7 x 10-5. The average stable Mg isotope composition of all analysed bulk chondrites is d25MgDSM-3 = -0.152 +_ 0.079 per1000(2 sd) and is indistinguishable from that of Earth's mantle. The third part of this study comprises a high-precision Mg isotope and mineral major and trace element study of 24 diogenites. Diogenites are ultramafic pyroxene and olivine cumulate rocks that are presumed to have resulted from magmatic differentiation on the howardite-eucritediogenite (HED) parent body. There are, however, no precise and independent age constraints on the formation of diogenites and, in particular, their age relationships to the basaltic eucrites. Mg isotope analysis of diogenites showed significant variability in d26Mg* anomalies that range from -0.0108 +_ 0.0018 to +0.0128 +_ 0.0018 per1000. These anomalies generally correlate with the mineral major and trace element chemistry and demonstrate active 26A1 decay during magmatic differentiation. Furthermore, it also suggests that diogenites are products of fractional crystallisation from a large scale magmatic system. Heating and melting of the HED parent body was driven by 26A1 decay and led to diogenite formation 0.7 to 1.3 Myr after CAIs depending on whether a heterogeneous or homogeneous (26Al/27A1)0 distribution is assumed between the proto-planetary disk and CAIs. These data show that diogenite formation pre-dates eucrite formation and indicate HED parent body accretion and core formation occurred within the first Myr of the Solar System. The lifetime of the magmatic evolution is less well constrained. The data suggest that the complete range of diogenites may have formed as quickly as ~ 0.2 Myr

Near-equilibrium isotope fractionation during planetesimal evaporation

Silicon and Mg in differentiated rocky bodies exhibit heavy isotope enrichments that have been attributed to evaporation of partially or entirely molten planetesimals. We evaluate the mechanisms of planetesimal evaporation in the early solar system and the conditions that controlled attendant isotope fractionations. Energy balance at the surface of a body accreted within ~1 Myr of CAI formation and heated from within by 26Al decay results in internal temperatures exceeding the silicate solidus, producing a transient magma ocean with a thin surface boundary layer of order < 1 meter that would be subject to foundering. Bodies that are massive enough to form magma oceans by radioisotope decay (ge 0.1%) can retain hot rock vapor even in the absence of ambient nebular gas. We find that a steady-state rock vapor forms within minutes to hours and results from a balance between rates of magma evaporation and atmospheric escape. Vapor pressure buildup adjacent to the surfaces of the evaporating magmas would have inevitably led to an approach to equilibrium isotope partitioning between the vapor phase and the silicate melt. Numerical simulations of this near-equilibrium evaporation process for a body with a radius of ~ 700 km yield a steady-state far-field vapor pressure corresponding to 95% saturation. Approaches to equilibrium isotope fractionation between vapor and melt should have been the norm during planet formation due to the formation of steady-state rock vapor atmospheres and/or the presence of protostellar gas. We model the Si and Mg isotopic composition of bulk Earth and show that the best fit is for a carbonaceous chondrite-like source material with about 12% loss of Mg and 15% loss of Si resulting from near-equilibrium evaporation into the solar protostellar disk of hydrogen gas on timescales of 10,000 to 100,000 years.Comment: 35 pages, 15 figure

Plates, planets, and phase changes: 50 years of petrology

Three advances of the previous half-century fundamentally altered petrology, along with the rest of the Earth sciences. Planetary exploration, plate tectonics, and a plethora of new tools all changed the way we understand, and the way we explore, our natural world. And yet the same large questions in petrology remain the same large questions. We now have more information and understanding, but we still wish to know the following. How do we account for the variety of rock types that are found? What does the variety and distribution of these materials in time and space tell us? Have there been secular changes to these patterns, and are there future implications? This review examines these bigger questions in the context of our new understandings and suggests the extent to which these questions have been answered. We now do know how the early evolution of planets can proceed from examples other than Earth, how the broad rock cycle of the present plate tectonic regime of Earth works, how the lithosphere atmosphere hydrosphere and biosphere have some connections to each other, and how our resources depend on all these things. We have learned that small planets, whose early histories have not been erased, go through a wholesale igneous processing essentially coeval with their formation. By inference, this also happened to Earth. The early differentiation on a small planet produces observable basaltic rock types—and produces little else besides a residue and a planetary core. In contrast, the larger Earth’s preservation of its original differentiation products has been eroded by continued activity which still involves extensive basaltic volcanism with further reprocessing through plate tectonic cycles to form continents and cratons. We also now have a good understanding of the pressure-induced phase changes that are responsible for the Earth’s mantle’s seismic layered structure. It is unclear the extent to which this layered seismic structure corresponds to chemical layering as well as to mineralogical layering. Earth’s transition zone, lower, and upper mantles may not have the same composition. It is possible that still larger exoplanets might be expected to develop additional modes of activity with emphasis on additional phase changes producing more internal layering and differentiation