Primordial Magma Ocean Research Articles

The Geological History of Water: From Earth’s Accretion to the Modern Deep Water Cycle

The abundance of water on Earth and its distribution between surficial and deep reservoirs are the outcome of 4.6 billion years of geological history involving various mechanisms of water in and outgassing. Here, we use the metaphor of a pipeline connecting Earth’s deep and surface water reservoirs. The net flux through this pipeline has changed over time due to contrasting Hadean, Archean, and modern geodynamic regimes. Most water was dissolved in the primordial magma ocean, entrapped in the solidifying mantle, and massively released by volcanism during the Hadean and Archaean. As Earth cooled, plate tectonics enabled water ingassing into the mantle, which appears to exceed outgassing under the modern tectonic regime, implying that Earth’s surface has been drying out and will continue to do so.

Hf-Nd isotopes from ultramafic and mafic rocks in the western Dharwar Craton, India, record early Archean mantle heterogeneity

Both primary mantle differentiation and early extraction of oceanic and continental crust can have resulted in a chemically heterogeneous Archean mantle. Evidence for heterogeneity of the Archean mantle is preserved in the correlated Hf and Nd isotopes in mafic (basaltic) magmatic rocks and their ‘decoupling’ in ultramafic rocks, especially in komatiites, from the Archean Dharwar Craton, India. The komatiites and komatiitic basalts from the western Dharwar Craton were emplaced at ~3.25 Ga and have large spread in initial radiogenic isotope compositions with initial ɛHf (ranging from +0.9 to +21.3) and initial ɛNd from +0.2 to +7.4 that do not correlate. In contrast, the ɛHfi and ɛNdi in contemporaneous mafic (ɛHfi = +5.9 ± 11.2; ɛNdi = +3.2 ± 7.6) and felsic (ɛHfi = +4.3 ± 4.6; ɛNdi = +2.7 ± 4.2) rocks in the same area correlate, as is typical of rocks that are the product of magmatic processes involving melting and magmatic differentiation. The high initial ɛHf and the lack of correlation (‘decoupling’) of ɛHf and ɛNd in the komatiites and komatiitic basalts indicate that the source had superchondritic Lu/Hf which is attributed to early mantle differentiation at great depth, possibly in the perovskite stability field. This finding indicates that the primordial mantle was mineralogically and chemically layered and this layering was the result of fractionation processes very early in Earth's history, probably during magma ocean solidification. During crystallization of a primordial magma ocean, fractionation of Mg-perovskite with minor Ca-perovskite component could have resulted in a melt with high Lu/Hf and nearly unfractionated Sm/Nd. The correlation of the initial Hf isotope composition of the komatiites and komatiitic basalts with their MgO content, and thus melting temperatures, indicates the presence of a highly fractionated component derived from the deep mantle. The coupling of ɛHf-ɛNd in mafic to felsic igneous rocks and the decoupling in contemporaneous ultramafic rocks is thus the result of mantle material originating from different depths involving different ancient fractionated reservoirs. The existence of a chemical and isotopically heterogeneous Archean mantle that supplied material to form the continental crust has to be accommodated in crustal growth and preservation models that commonly assume a nearly homogeneous and gradually depleting mantle reservoir as the source of the continental crust.

A new equation of state applied to planetary impacts

Observed FeO/MgO ratios in the Moon and Earth are inconsistent with simulations done with a single homogeneous silicate layer. In this paper we use a newly developed equation of state to perform smoothed particle hydrodynamics simulations on the lunar-forming impact, testing the effect of a primordial magma ocean on Earth. This is investigated using the impact parameters of both the canonical case, in which a Mars-sized impactor hits a non-rotating Earth at an oblate angle, and the fast-rotating case, in which a half-sized Mars impactor hits a fast-spinning Earth head-on. We find that the inclusion of a magma ocean results in a less massive Moon and leads to slightly more mixing. Additionally, we test how an icy Theia would affect the results and find that this reduces the probability of a successful Moon formation. Simulations of the fast-spinning case are found to be unable to form a massive-enough Moon.

The niobium and tantalum concentration in the mantle constrains the composition of Earth’s primordial magma ocean

The bulk silicate Earth (BSE), and all its sampleable reservoirs, have a subchondritic niobium-to-tantalum ratio (Nb/Ta). Because both elements are refractory, and Nb/Ta is fairly constant across chondrite groups, this can only be explained by a preferential sequestration of Nb relative to Ta in a hidden (unsampled) reservoir. Experiments have shown that Nb becomes more siderophile than Ta under very reducing conditions, leading the way for the accepted hypothesis that Earth's core could have stripped sufficient amounts of Nb during its formation to account for the subchondritic signature of the BSE. Consequently, this suggestion has been used as an argument that Earth accreted and differentiated, for most of its history, under very reducing conditions. Here, we present a series of metal-silicate partitioning experiments of Nb and Ta in a laser-heated diamond anvil cell, at pressure and temperature conditions directly comparable to those of core formation; we find that Nb is more siderophile than Ta under any conditions relevant to a deep magma ocean, confirming that BSE's missing Nb is in the core. However, multistage core formation modeling only allows for moderately reducing or oxidizing accretionary conditions, ruling out the need for very reducing conditions, which lead to an overdepletion of Nb from the mantle (and a low Nb/Ta ratio) that is incompatible with geochemical observations. Earth's primordial magma ocean cannot have contained less than 2% or more than 18% FeO since the onset of core formation.

Venusian Habitable Climate Scenarios: Modeling Venus Through Time and Applications to Slowly Rotating Venus‐Like Exoplanets

AbstractOne popular view of Venus' climate history describes a world that has spent much of its life with surface liquid water, plate tectonics, and a stable temperate climate. Part of the basis for this optimistic scenario is the high deuterium to hydrogen ratio from the Pioneer Venus mission that was interpreted to imply Venus had a shallow ocean's worth of water throughout much of its history. Another view is that Venus had a long‐lived (∼100 million years) primordial magma ocean with a CO2and steam atmosphere. Venus' long‐lived steam atmosphere would sufficient time to dissociate most of the water vapor, allow significant hydrogen escape, and oxidize the magma ocean. A third scenario is that Venus had surface water and habitable conditions early in its history for a short period of time (<1 Gyr), but that a moist/runaway greenhouse took effect because of a gradually warming Sun, leaving the planet desiccated ever since. Using a general circulation model, we demonstrate the viability of the first scenario using the few observational constraints available. We further speculate that large igneous provinces and the global resurfacing hundreds of millions of years ago played key roles in ending the clement period in its history and presenting the Venus we see today. The results have implications for what astronomers term “the habitable zone,” and if Venus‐like exoplanets exist with clement conditions akin to modern Earth, we propose to place them in what we term the “optimistic Venus zone.”

Differentiation mechanisms of the early Hadean mantle: Insights from combined 176Hf-142,143Nd signatures of Archean rocks from the Saglek Block

Positive 142Nd anomalies in Eoarchean rocks provide evidence for early (4.40 ± 0.03 Ga; Morino et al., 2017) depletion of Earth's mantle. This model age lies within errors of the Pb-Pb “age of the Earth” (Connelly and Bizzarro, 2016), and is similar to model ages inferred for crystallization of the lunar mantle (McLeod et al., 2014), implying that this large-scale event may reflect crystallization of a magma ocean following the Moon-forming impact. However, differentiation mechanisms responsible for the formation of this early depleted mantle reservoir and the depth at which it formed cannot be constrained from the Sm-Nd isotope system alone, because the magnitude of Sm/Nd fractionation during partial melting or fractional crystallization shows little dependence on pressure-controlled changes in mantle mineralogy. In contrast, the Lu-Hf isotope system is highly dependent on mineralogy, notably the presence or absence of garnet, an upper mantle phase, and thus may be used to constrain the pressure of fractionation. This study provides the first 176Lu-176Hf isotopic results on mafic and ultramafic rocks belonging to the Eoarchean (Nulliak) and Mesoarchean suites of the Saglek Block (northern Labrador, 3.2–3.9 Ga). The 176Lu-176Hf dating confirms the distinction between these two groups of rocks and provides ages consistent with those obtained from 147Sm-143Nd dating. The whole rock 176Lu-176Hf errorchrons yield ages and initial epsilon values of 3766 ± 140 Ma, ε176Hfi = 6.0 ± 2.5 and 3023 ± 390 Ma, ε176Hfi = −0.3 ± 2.5 for the Nulliak suite and the Mesoarchean suite respectively. The time-integrated 176Lu/177Hf for the sources of the Nulliak and the Mesoarchean suites considering a time of differentiation at 4.40 ± 0.03 Ga are estimated to be 0.047 ± 0.005 and 0.033 ± 0.005, respectively. For the Mesoarchean samples, the combined 146,147Sm-142,143Nd and 176Lu-176Hf data are consistent with a near-chondritic mantle source. On the other hand, Nulliak ultramafic rocks were derived from a mantle reservoir with superchondritic Lu/Hf and Sm/Nd. The Nulliak parent reservoir, however, does not plot on the ε176Hf-ε143Nd mantle array defined by modern oceanic basalts. Instead, the Nulliak source more likely belongs to a distinct array defined by Eo- and Meso-Archean komatiites. These results are interpreted in the framework of a simple model of crystallization of a primordial magma ocean. It appears that the fractionation observed in the mantle source of Nulliak was most likely generated by crystallization of a garnet-bearing assemblage in the shallow mantle, above the transition zone rather than by perovskite fractionation in the lower mantle. To preserve this depleted reservoir from the rest of the hot and vigorously convecting mantle, the Nulliak mantle source may have been isolated either at the top of the mantle in a buoyant lithosphere or near the core-mantle boundary, with the latter setting being more consistent with the komatiitic nature of the erupted rocks. Given that the garnet signature argues for differentiation of the Nulliak source at relatively shallow depth (few hundred kilometers), its isolation in the deep mantle would require a cumulate overturn following crystallization of the magma ocean.

Thermodynamics of the MgO‐FeO‐SiO2system up to 140 GPa: Application to the crystallization of Earth's magma ocean

AbstractAt the end of Earth's accretion and after the core‐mantle segregation, the existence of a basal magma ocean at the top of the core‐mantle boundary (CMB) depends on the physical properties of mantle materials at relevant pressure and temperature. Present‐day deep mantle structures such as ultralow‐velocity zones and low‐shear velocity provinces might be directly linked to the still ongoing crystallization of a primordial magma ocean. We provide the first steps toward a self‐consistent thermodynamic model of magma ocean crystallization at high pressure. We build a solid‐liquid thermodynamic database for silicates in the MgO‐FeO‐SiO2system from 20 GPa to 140 GPa. We use already published chemical potentials for solids, liquid MgO, and SiO2. We derive standard state chemical potential for liquid FeO and mixing relations from various indirect observations. Using this database, we compute the ternary phase diagram in the MgO‐FeO‐SiO2system as a function of temperature and pressure. We confirm that the melt is lighter than the solid of same composition for all mantle conditions but at thermodynamic equilibrium, the iron‐rich liquid is denser than the solid in the deep mantle. We compute a whole fractional crystallization sequence of the mantle and show that an iron‐rich and fusible layer should be left above the CMB at the end of the crystallization.

Insights into early Earth from the Pt–Re–Os isotope and highly siderophile element abundance systematics of Barberton komatiites

Highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundance and Pt–Re–Os isotopic data are reported for well-preserved komatiites from the Komati and Weltevreden Formations of the Barberton Greenstone Belt in South Africa. The Re–Os data for whole-rock samples and olivine and chromite separates define isochrons with ages of 3484±38 and 3263±12Ma for the Komati and Weltevreden systems, respectively. The respective initial 187Os/188Os=0.10335±15 (γ187Os=+0.34±0.15) and 0.10442±4 (γ187Os=−0.14±0.04) are well within the range defined by chondritic meteorites. When considered together with the Re–Os data for late Archean komatiite systems, these data indicate that the mantle sources of most Archean komatiites evolved with essentially uniform long-term Re/Os that is well within the chondritic range. By contrast, the initial 186Os/188Os=0.1198283±9 (ε186Os=−0.12±0.08) and 0.1198330±8 (ε186Os=+0.22±0.07) for the Komati and Weltevreden systems, respectively, are outside of known chondritic evolution paths, indicating that the mantle sources of these two komatiite systems evolved with fractionated time-integrated Pt/Os. The new 186,187Os isotopic data for these early Archean komatiite systems, combined with published 142,143Nd and 176Hf isotopic data for these systems, are consistent with formation and long-term isolation of deep-seated mantle domains with fractionated time-integrated Sm/Nd, Lu/Hf, and Pt/Os ratios, at ca. 4400Ma. These domains may have been generated as a result of late-stage crystallization of a primordial magma ocean involving Mg-perovskite, Ca-perovskite and Pt-alloys acting as the fractionating phases. The inferred fractionated mantle domains were sampled by the early Archean komatiites, but were largely mixed away by 2.7Ga, as evidenced by uniform time-integrated Sm/Nd, Lu/Hf, and Pt/Os ratios inferred for the sources of most late Archean komatiite systems. The calculated total Pt+Pd abundances present in the sources of the early Archean komatiite systems fall only 7–14% short of those present in estimates for the modern primitive mantle. These are also within the range of the total Pt+Pd abundances present in the sources of late Archean komatiite systems, indicating little change in the HSE abundances in the Archean mantle between 3.5 and 2.7Ga. The new HSE data for the early Archean komatiite systems may implicate late accretion of HSE to the mantle prior to completion of crystallization of a final terrestrial magma ocean, followed by sluggish mixing of diverse, post-magma ocean domains characterized by variably fractionated lithophile element and HSE abundances.

Insights into early Earth from Barberton komatiites: Evidence from lithophile isotope and trace element systematics

Major, minor, and lithophile trace element abundances and Nd and Hf isotope systematics are reported for two sets of remarkably fresh, by Archean standards, samples of komatiitic lavas from the 3.48Ga Komati and the 3.27Ga Weltevreden Formations of the Barberton Greenstone Belt (BGB) in South Africa. These data are used to place new constraints on the thermal history of the early Archean mantle, on the timing of its differentiation, and on the origin and chemical nature of early mantle reservoirs and their evolution through time.Projected moderate to strong depletions of highly incompatible lithophile trace elements and water in the mantle sources of both komatiite systems, combined with the partitioning behavior of V during lava differentiation, are consistent with anhydrous conditions during generation of the komatiite magmas. Komati and Weltevreden lavas are inferred to have erupted with temperatures of ∼1600°C, and, thus, represent the hottest known lavas on Earth. The calculated mantle potential temperatures of ∼1800°C for both komatiite systems are 150–200°C higher than those of contemporary ambient mantle. Combined, these observations are consistent with the origin of these BGB komatiite magmas in mantle plumes in the lower mantle.New Sm–Nd and Lu–Hf isotopic data allow precise determination of initial ε143Nd=+0.46±0.10 and +0.50±0.11 and initial ε176Hf=+1.9±0.3 and +4.7±0.8 for the Komati and the Weltevreden system komatiites, respectively. These positive initial values reflect prior fractionation of Sm/Nd and Lu/Hf in the mantle early in Earth history. Conversely, μ142Nd values are 0.0±2.4 and +2.2±4.1 for the Komati and the Weltevreden systems, respectively. These values overlap, within uncertainties, those of modern terrestrial rocks, thus, limiting the magnitudes of possible Sm/Nd fractionations generated by early Earth processes in the sources of these rocks. Combined 142,143Nd and Hf isotope and lithophile trace element systematics are consistent with formation and long-term isolation of deep-seated mantle domains with fractionated Sm/Nd and Lu/Hf at ca. 4400Ma. These domains were likely generated as a result of crystallization of a primordial magma ocean, with Mg-perovskite and minor Ca-perovskite acting as fractionating phases. The inferred mantle domains were evidently mixed away by 2.7Ga on the scale of mantle reservoirs sampled by late Archean komatiite lavas emplaced worldwide.

Rolling hills on the core–mantle boundary

Recent results suggest that an iron-rich oxide may have fractionally crystallized from a primordial magma ocean and settled on the core–mantle boundary (CMB). Based on experimental results, the presence of only a few percent of Fe-rich oxide could slow seismic waves down by several percent. This heavy layer can become highly undulating as predicted from dynamic modeling but can remain as a distinct structure with uniform velocity reductions. Here, we use the large USArray seismic network to search for such structures. Strong constraints on D″ are provided by the core-phase SKS where it bifurcates, containing a short segment of P-wave diffractions (Pd) when crossing the CMB, called SKSd. Synthetics from models with moderate velocity drops (less than 10%) involving a layer with variable thickness, perhaps a composite of sharp small structures, with strong variation in thickness can explain both the observed SKSd waveforms and large scatter in differential times between SKKS and SKS. A smooth 3D image is obtained from inverting SKSd waveforms displaying rolling-hills with elongated dome-like structures sitting on the CMB. The most prominent one has an 80-km height, ∼8° length, and ∼4° width, thus adding still more structural complexity to the lower mantle. We suggest that these results can be explained by a dynamically-stabilized material containing small amounts (∼5%) iron-rich (Mg,Fe)O providing a self-consistent physical interpretation.

Chronological evidence that the Moon is either young or did not have a global magma ocean

Chemical evolution of planetary bodies, ranging from asteroids to the large rocky planets, is thought to begin with differentiation through solidification of magma oceans many hundreds of kilometres in depth. The Earth's Moon is the archetypical example of this type of differentiation. Evidence for a lunar magma ocean is derived largely from the widespread distribution, compositional and mineralogical characteristics, and ancient ages inferred for the ferroan anorthosite (FAN) suite of lunar crustal rocks. The FANs are considered to be primary lunar flotation-cumulate crust that crystallized in the latter stages of magma ocean solidification. According to this theory, FANs represent the oldest lunar crustal rock type. Attempts to date this rock suite have yielded ambiguous results, however, because individual isochron measurements are typically incompatible with the geochemical make-up of the samples, and have not been confirmed by additional isotopic systems. By making improvements to the standard isotopic techniques, we report here the age of crystallization of FAN 60025 using the (207)Pb-(206)Pb, (147)Sm-(143)Nd and (146)Sm-(142)Nd isotopic systems to be 4,360 ± 3 million years. This extraordinarily young age requires that either the Moon solidified significantly later than most previous estimates or the long-held assumption that FANs are flotation cumulates of a primordial magma ocean is incorrect. If the latter is correct, then much of the lunar crust may have been produced by non-magma-ocean processes, such as serial magmatism.

The scientific rationale for the C1XS X-ray spectrometer on India's Chandrayaan-1 mission to the moon

The UK-built Chandrayaan-1 X-ray Spectrometer (C1XS) will fly as an ESA instrument on India's Chandrayaan-1 mission to the Moon, launched in October 2008. C1XS builds on experience gained with the earlier D-CIXS instrument on SMART-1, but will be a scientifically much more capable instrument. Here we describe the scientific objectives of this instrument, which include mapping the abundances of the major rock-forming elements (principally Mg, Al, Si, Ti, Ca and Fe) in the lunar crust. These data will aid in determining whether regional compositional differences (e.g., the Mg/Fe ratio) are consistent with models of lunar crustal evolution. C1XS data will also permit geochemical studies of smaller scale features, such as the ejecta blankets and central peaks of large impact craters, and individual lava flows and pyroclastic deposits. These objectives all bear on important, and currently unresolved, questions in lunar science, including the structure and evolution of any primordial magma ocean, as revealed by vertical and lateral geochemical variations in the crust, and the composition of the lunar mantle, which will further constrain theories of the Moon's origin, thermal history and internal structure.

Evidence for anomalously large degree of polymerization in Mg2SiO4 glass and melt

Ab-initio molecular dynamics simulation of forsterite (Mg2SiO4) melt at 2273 K shows the presence of nearly 40% of the Si atoms as (Si2O7)6- dimers. This result is directly corroborated by the 29Si nuclear magnetic resonance spectrum of bulk Mg2SiO4 glass, prepared by container-less levitation techniques. The presence of a large excess of bridging O atoms associated with the (Si2O7)6- dimers in forsterite glass and melt is in sharp contrast with their complete absence in crystalline forsterite. Such structural differences between the crystal and the melt can have important implications in understanding the dynamics of crystallization and segregation in a primordial magma ocean and the continuing chemical differentiation of the Earth.

A new model of lunar crust: asymmetry in crustal composition and evolution

Abstract Earlier models of lunar crustal formation as a simple flotation of ferroan anorthosites (FAN) do not account for the diverse crustal composition revealed by feldspathic lunar meteorites and granulites in the Apollo samples. Based on the integrated results of recent studies of lunar meteorites and global chemical and mineralogical maps, we propose a novel asymmetric crust model with a ferroan, noritic, nearside crust and a magnesian, troctolitic farside crust. Asymmetric crystallization of a primordial magma ocean can be one possibility to produce a crust with an asymmetric composition. A post-magma-ocean origin for a portion of the lunar crust is also possible and would account for the positive eNd value for FAN and phase equilibria. The formation of giant basins, such as the South Pole-Aitken (SPA) basin may have significant effects on resurfacing of the early lunar crust. Thus, the observed surface composition of the feldspathic highland terrane (FHT) represents the combined results of magma ocean crystallization, post-magma-ocean magmatism and resurfacing by basin formation. The Mg/(Mg+Fe) ratios, rock types, and mineral compositions of the FHT and the South Pole-Aitken basin Terrane (SPAT) obtained from the KAGUYA mission, coupled with further mineralogical and isotopic studies of lunar meteorites, will facilitate an assessment of the feasibility of the proposed crust model and improve understanding of lunar crustal genesis and evolution.

地球中心核と水―地球形成時の水素分配反応―

Because of the dissolution of lighter elements such as sulfur and carbon, the Earth's outer core is about 10 percent less dense than molten iron at the relevant pressure and temperature conditions. To determine whether hydrogen can account for a major part of the density deficit, and is therefore an important constituent of the molten iron outer core, the hydrogen concentration in molten iron has been measured using a specially developed method. From these measurements, metal-magma partitioning of hydrogen in the Earth's primordial magma ocean was determined as a function of temperature and pressure where the reaction occurred. It was found. that if the pressure at the bottom of the magma ocean was above 7 to 8 GPa, most of the H2O absorbed in this ocean should have reacted with the iron to be transported into the core. This seems almost inevitable after the giant impact that formed the moon, although details of the metal-magma reaction process after the impact are not well constrained.

On lunar asymmetries 2. Origin and distribution of mare basalts and mascons

[1] A novel mechanism is proposed for the emplacement of mare basalts and the formation of mascons that does not involve melting or remelting of the source region. The source magmas are the remnants of the primordial lunar magma ocean and are rich in incompatible elements, including radioactive isotopes; potassium, rare earth elements, and phosphorus (KREEP); and volatiles. Exsolution of volatiles as the magmas cooled and crystallized (a process called second boiling) produced overpressures which drove the dense basalts upward to the surface, creating the mascons. This process occurred preferentially beneath nearside mare basins, where the crust is thinnest and lithostatic pressure is lowest. The delay between impact and magmatic extrusion reflects the time needed for the volatile-rich magma to cool to its second boiling point. Radioactive heating buffered the magmas against cooling and solidification, allowing for an extended period of magmatism.

Clementine Observations of the Aristarchus Region of the Moon

Multispectral and topographic data acquired by the Clementine spacecraft provide information on the composition and geologic history of the Aristarchus region of the moon. Altimetry profiles show the Aristarchus plateau dipping about 1 degrees to the north-northwest and rising about 2 kilometers above the surrounding lavas of Oceanus Procellarum to the south. Dark, reddish pyroclastic glass covers the plateau to average depths of 10 to 30 meters, as determined from the estimated excavation depths of 100- to 1000-meter-diameter craters that have exposed materials below the pyroclastics. These craters and the walls of sinuous rilles also show that mare basalts underlie the pyroclastics across much of the plateau. Near-infrared images of Aristarchus crater reveal olivine-rich materials and two kilometer-sized outcrops of anorthosite in the central peaks. The anorthosite could be either a derivative of local magnesium-suite magmatism or a remnant of the ferroan anorthosite crust that formed over the primordial magma ocean.

The ferroan‐anorthositic suite, the extent of primordial lunar melting, and the bulk composition of the Moon

The geochemical bimodality among compositionally pristine lunar rocks is strong evidence that a major component of the Moon's crust is fundamentally unrelated to the plagioclase flotation crust that accumulated over the primordial magma ocean. The ferroan anorthositic suite (FAS) has all the characteristics expected for a series of magma ocean plagioclase flotation cumulates, but we estimate, based on mass balance for Al2O3, that ∼ 30 wt% of the crust, and 20 wt% of the crust's total Al2O3 (or roughly 15% of the bulk‐Moon Al2O3 content, assumed to be 3–4 wt%), was emplaced by postmagma ocean, Mg‐suite “serial” magmatism. Remelting of plagioclase‐bearing magma ocean cumulates cannot plausibly account for the “late” emplacement of Al2O3, because the only plagioclase‐rich cumulates produced by a deep, ultramafic magma ocean would have mg ratios too low to engender the Mg‐suite. Conceivably, remelting of garnet‐rich magma ocean cumulates supplied the late Al2O3, but only if the bulk‐Moon Al2O3 content is substantially higher than generally assumed. High pressures in the magma ocean acted to enhance the py (pyroxene/(pyroxene+olivine)) ratio of the MO cumulates, and the Al2O3 content of whatever pyroxene formed. Remelting of pyroxene‐rich MO cumulates could conceivably have supplied the late Mg‐suite Al2O3 to the crust. The source region would have to encompass roughly 40 wt% of the mantle. Another possible source for late Al2O3 addition is a large portion of the deepest mantle that avoided more than a slight degree of partial melting, and thus remained Al‐rich, during formation of the magma ocean. The source region would have to encompass 20–30 wt% of the mantle. Ascent of diapiric masses from the lowermost mantle would likely coincide with (trigger, or be triggered by) a large‐scale overturn of the gravitationally unstable MO cumulate pile, with concomitant pressure‐release melting, in which case the Mg‐suite parental melts would be derived from a mix of deepest MO cumulates and the even deeper primitive mantle. The popular giant impact hypothesis of lunar origin implies an extremely hot beginning for the Moon, suggesting that the deep interior was either molten or depleted in Al2O3 (by differentiation in hot protomoons) as it formed. Origin of ∼ 30 wt% of the Moon's crust by postmagma ocean, serial magmatism is more consistent with coaccretion models that form the Moon without a giant, hot impact. In any scenario with a large fraction of the crust originating by partial remelting of deep magma ocean cumulates, mass balance for Al2O3 implies that the bulk‐Moon normative py ratio must be greater than roughly 0.5, i.e., significantly higher than generally estimated. In this respect, the Moon appears more compositionally similar to chondritic silicates than to the Earth's upper mantle.

Apollo 15 yellow-brown volcanic glass: Chemistry and petrogenetic relations to green volcanic glass and olivine-normative mare basalts

Apollo 15 yellow-brown glass is one of twenty-five, high-Mg, primary magmas emplaced on the lunar surface in pyroclastic eruptions. Forty spherules of this glass were individually analyzed by electron microprobe and INAA for major- and trace-elements ( e.g. Sc, V, Cr, Co, La, Ce, Nd, Sm, Eu, Ho, Tm, Yb, Lu, Hf, Ta, and U). The abundances demonstrate that this primary magma was produced by partial melting of differentiated cumulates in the lunar mantle. Models are developed to explain the possible source-regions of several Apollo 15 and Apollo 12 low-Ti mare magmas as being products of hybridization involving three ancient differentiated components of a primordial lunar magma ocean: (a) early olivine ± orthopyroxene cumulates; (b) late-stage clinopyroxene + pigeonite + ilmenite + plagioclase cumulates; and (c) late-stage inter-cumulus liquid. Isotopic constraints on Sm-Nd fractionation and least-squares tests of potential mixing models require models of Apollo 15 yellow-brown glass, and Apollo 12 and 15 olivine mare basalts, involving low F ~3% cumulate remelting of sources in which olivine and orthopyroxene are residual phases. Models of the Apollo 15 green glass composition indicate higher (~4–7%) degrees of melting, but with only a slight preference for orthopyroxene-rich relative to orthopyroxene -free sources. The modeled dependence on residual orthopyroxene in mare source regions places source depths at ~400 km or greater in order to maintain olivine and orthopyroxene on the liquidus. Hybridization at such depths would likely result from convective overturn of the primordial magma ocean and suggests that separate fractionating masses were positioned at various depths. Our models for developing hybridized source regions of primary low-Ti mare magmas do not require selective contamination of primary liquids. The preferred model for Apollo 15 yellow-brown glass is presented as the commingling of 95% early cumulate olivine + orthopyroxene, 1.9% clinopyroxene, 0.27% pigeonite, 1.7% plagioclase, 0.02% ilmenite, and 0.56% trapped liquid. Respective proportions for the Apollo 15 olivine mare basalt source model are 98.5%, 0.37%, 0%, 0.79%, 0.16%, and 0.13%; and the proportions used for the preferred Apollo 12 olivine mare basalt model are 97.6%, 0.73%, 0.24%, 0.77%, 0.42%, and 0.21%. Our preferred model for the green glass source composition ( F = 0.07) requires the commingling of 97.4% early olivine + orthopyroxene, 0.18% clinopyroxene, 1.32% pigeonite, 0.85% plagioclase, and 0.28% trapped liquid.

Metal‐silicate fractionation in the growing Earth: Energy source for the terrestrial magma ocean

Based on the homogeneous accretion model of planets, thermal evolution of the growing earth has been studied. After the surface of the growing earth starts melting owing to the blanketing effect of the primordial atmosphere, gravitational differentiation between metallic and silicate materials takes place in its upper region: the differentiation would give rise to a large amount of gravitational energy release as well as layered structure inside the proto‐earth. A part of released energy would be transported outward by convection. Considering the effects of gravitational fractionation and convection, we have pursued numerically the evolution of the growing earth. Our results show that gravitational differentiation can supply energy large enough to keep the whole silicate layer in the molten state and that half of the released energy is conveyed outward by convective heat transport. In the growing stage, the temperature distribution in the silicate layer always lies between liquidus and solidus temperatures except in an uppermost thin zone. The thermal structure obtained favors a notion of the primordial magma ocean and of chemical differentiation of mantle. The structure of the growing earth which is composed of the central protocore, the metallic layer, and the silicate layer (magma ocean) would become gravitationally unstable and overturn to form a metallic core. Energy released by this overturn would not affect the structure of the upper magma ocean too much. But undifferentiated primordial materials which come from the protocore would add to mantle materials through the overturn and this would help explain the isotopic mantle heterogeneity presently observed.