— CR chondrites contain metal lumps (>300 μm) either attached to chondrule silicates or apparently isolated in the matrix. Here, laser ablation microanalysis of six metal lumps from a polished thin ...section of the Acfer 097 CR2 chondrite at 15 μm spatial resolution revealed zoning profiles for the volatile elements Cu and Ga. The mutual diffusivities of Cu and Ga were used to infer T ∼ 1473 ± 100 K from the correlation of Cu versus Ga. The cooling rates of the metal lumps were calculated to be 0.5–50 K h−1 for Tp ∼ 1473 ± 100 K, with a maximum possible range of 0.1–400 K h−1 for Tp ∼ 1200–1800 K, overlapping the range of cooling rates inferred from petrological studies of type I chondrules (10–1000 K h−1). Chondrule textures were established near the peak heating temperatures of chondrules (approximately 1900–2000 K), while the Cu and Ga diffusive profiles were established after solidification (T ∼ 1500 K), consistent with nonlinear cooling. Furthermore, one chondrule (N2) has a more complex zoning profile that is modeled as a three‐stage cooling history involving initial cooling at approximately 1 K h−1, followed by mild re‐heating (T ∼ 1700 K) that re‐evaporated Cu and Ga from the outer approximately 100 μm of the metal lump and then cooled again at approximately 5 K h−1. The thermal effects of parent body and other preaccretionary heating events on the Cu and Ga zoning profiles are examined. Although CR parent bodies have experienced aqueous alteration, the thermal effects of this process can neither produce nor erase the Cu and Ga diffusive profiles. Thus, metal lumps in CR chondrites record the solid‐state thermal history of chondrules as they travelled away from the chondrule‐forming region.
A Unique Piece of Mars Humayun, Munir
Science (American Association for the Advancement of Science),
02/2013, Volume:
339, Issue:
6121
Journal Article
Peer reviewed
Following the pioneering Mars Exploration Rovers, NASA's Curiosity rover is actively exploring the crustal rocks of Mars. Despite the exciting results returned by the rovers, there is no substitute ...for a hand sample of crustal rock. Because such samples will not be returned to Earth anytime soon, geochemists who want a piece of Mars in their labs must satisfy themselves with martian meteorites (1). These comprise a group of igneous rocks with telltale signs of martian alteration products (2) and have provided ground truth for the information returned by the rovers. Oddly, however, the hundred or so known martian meteorites are chemically unrepresentative of the martian crust determined by missions (3). On page 780 of this issue, Agee et al. (4) put an end to this conundrum with the finding of a new martian meteorite, Northwest Africa (NWA) 7034, a basaltic breccia unique among known martian meteorites with respect to age, oxygen isotopes, and petrology.
Walter et al. issue a number of critical comments on our report about the discovery of davemaoite to the end that they believe to show that our results do not provide compelling evidence for the ...presence of davemaoite in the type specimen and that the hosting diamond had formed in the lithosphere. Their claim is based on a misinterpretation of the diffraction data contained in the paper, an insufficient analysis of the compositional data that disregards the three-dimensional distribution of inclusions, and the arbitrary assumption that Earth's mantle shows no lateral variations in temperature, inconsistent with state-of-the-art assessments of mantle temperature variations and with their own published results.
Some plumes are thought to originate at the core‐mantle boundary, but geochemical evidence of core‐mantle interaction is limited to Os isotopes in samples from Hawaii, Gorgona (89 Ma), and ...Kostomuksha (2.7 Ga). The Os isotopes have been explained by physical entrainment of Earth's liquid outer core into mantle plumes. This model has come into conflict with geophysical estimates of the timing of core formation, high‐pressure experimental determinations of the solid metal‐liquid metal partition coefficients (D), and the absence of expected 182W anomalies. A new model is proposed where metallic liquid from the outer core is partially trapped in a compacting cumulate pile of Fe‐rich nonmetallic precipitates (FeO, FeS, Fe3Si, etc.) at the top of the core and undergoes fractional crystallization precipitating solid metal grains, followed by expulsion of the residual metallic liquid back to the outer core. The Os isotopic composition of the solids and liquids in the cumulate pile is modeled as a function of the residual liquid remaining and the emplacement age using 1 bar D values, with variable amounts of oxygen (0–10 wt %) as the light element. The precipitated solids evolve Os isotope compositions that match the trends for Hawaii (at an emplacement age of 3.5–4.5 Ga; 5%–10% oxygen) and Gorgona (emplacement age < 1.5 Ga; 0%–5% oxygen). The Fe‐rich matrix of the cumulate pile dilutes the precipitated solid metal decoupling the Fe/Mn ratio from Os and W isotopes. The advantages to using precipitated solid metal as the Os host include a lower platinum group element and Ni content to the mantle source region relative to excess iron, miniscule anomalies in 182W (<0.1 ɛ), and no effects for Pb isotopes, etc. A gradual thermomechanical erosion of the cumulate pile results in incorporation of this material into the base of the mantle, where mantle plumes subsequently entrain it. Fractional crystallization of metallic liquids within the CMB provides a consistent explanation of both Os isotope correlations, Os‐W isotope systematics, and Fe/Mn evidence for core‐mantle interaction over the entire Hawaiian source.
Key Points
Radiogenic and unradiogenic Os isotopes require a new physical model
A cumulate pile model of the CMB provides a suitable explanation
Other isotope systematics are consistent with this model
Abstract
Rocky bodies of the inner solar system display a systematic depletion of “moderately volatile elements” (MVEs) that correlates with the expected condensation temperature of their likely host ...materials under protoplanetary nebula conditions. In this paper, we present and test a new hypothesis in which open-system loss processes irreversibly remove vaporized MVEs from high nebula altitudes, leaving behind the more refractory solids residing much closer to the midplane. The MVEs irreversibly lost from the nebula through these open-system loss processes are then simply unavailable for condensation onto planetesimals forming even much later, after the nebula has cooled, overcoming a critical difficulty encountered by previous models of this type. We model open-system loss processes operating at high nebula altitudes, such as resulting from disk winds flowing out of the system entirely, or layered accretion directly onto the young Sun. We find that mass-loss rates higher than those found in typical T-Tauri disk winds, lasting short periods of time, are most satisfactory, pointing to multiple intense early outburst stages. Using our global nebula model, incorporating realistic particle growth and inward drift for solids, we constrain how much the MVE-depletion signature in the inner region is diluted by the drift of undepleted material from the outer nebula. We also find that a significant irreversible loss of the common rock-forming elements (Fe, Mg, Si) can occur, leading to a new explanation of another long-standing puzzle of the apparent “enhancement” in the relative abundance of highly refractory elements in chondrites.
Carbonaceous chondritic meteorites are primordial Solar System materials and a source of water delivery to Earth. Fluid flow on the parent bodies of these meteorites is known to have occurred very ...early in Solar System history (first <4 million years). We analyze short-lived uranium isotopes in carbonaceous chondrites, finding excesses of 234-uranium over 238-uranium and 238-uranium over 230-thorium. These indicate that the fluid-mobile uranium ion U
moved within the past few 100,000 years. In some meteorites, this time scale is less than the cosmic-ray exposure age, which measures when they were ejected from their parent body into space. Fluid flow occurred after melting of ice, potentially by impact heating, solar heating, or atmospheric ablation. We favor the impact heating hypothesis, which implies that the parent bodies still contain ice.
In this study, a data set of 60 elements in 319 mid‐oceanic ridge basalt (MORB) glasses representing 144 chemically distinct lava flows from the Mid‐Atlantic Ridge was acquired by laser ablation ...inductively coupled plasma mass spectrometry. Our new data are comparable in terms of the number of elements analyzed to several recent MORB data sets, albeit limited in geographic coverage to the North Mid‐Atlantic Ridge. This extensive data set was used to examine the elemental systematics of depleted (D)‐, normal (N)‐, and enriched (E)‐MORBs. We show that elemental ratios sensitive to fluid transport in subduction zones (e.g., Th/U, Nb/U, Ba/Th, and Ba/La) are constant and similar to their primitive mantle values in N‐ and E‐MORBs, but depleted in accordance with their compatibility in D‐MORBs. The absence of evidence for subduction zone processing indicates the need to reassess the relation between MORB enrichment and recycled materials. Additionally, we reexamined the ratios of chalcophile and siderophile elements to lithophile elements of comparable compatibility, which are important in assessing planetary accretion, core formation, crust‐mantle differentiation, and hydrothermal ore formation processes. MORBs show significant negative anomalies of As, Tl, Pb, and Bi, but not of Sb, relative to lithophile elements of similar compatibility, which cannot be accounted for by melt depletion alone. The depletions of As, Tl, Pb, and Bi in MORB are complementary to significant enrichments observed in the continental crust, indicating that they have been transferred to the continental crust via fluid mobility in arcs or obduction of seafloor hydrothermal ores.
Key Points
We present a new set of elemental analysis of 60 elements in 319 MORB glasses by LA‐ICP‐MS
A recycled crustal origin for the enrichment of MORBs is not supported by elemental ratios sensitive to processing in the subduction factory
All MORB glasses have identical depletions of As, Tl, Pb, and Bi, complementary to the continental crust acquired early in Earth history
Lower mantle “garbage can”Calcium silicate perovskite has finally been identified in a natural sample and now has the mineral name davemaoite. Tschauner et al. discovered the type mineral trapped at ...high pressure and temperature as a diamond inclusion (see the Perspective by Fei). Structural and chemical analysis of the mineral showed that it is able to host a wide variety of elements, not unlike fitting bulky objects into garbage can. Specifically, it has a large amount of trapped potassium. Davemaoite can thus host three of the major heat-producing elements (uranium and thorium were previously shown experimentally) affecting heat generation in Earth’s lower mantle. —BGCalcium silicate perovskite, CaSiO3, is arguably the most geochemically important phase in the lower mantle, because it concentrates elements that are incompatible in the upper mantle, including the heat-generating elements thorium and uranium, which have half-lives longer than the geologic history of Earth. We report CaSiO3-perovskite as an approved mineral (IMA2020-012a) with the name davemaoite. The natural specimen of davemaoite proves the existence of compositional heterogeneity within the lower mantle. Our observations indicate that davemaoite also hosts potassium in addition to uranium and thorium in its structure. Hence, the regional and global abundances of davemaoite influence the heat budget of the deep mantle, where the mineral is thermodynamically stable.
Ratios of first-row transition elements (FRTE), such as Fe/Mn and Zn/Fe, may be fractionated differently by partial melting of peridotite than by partial melting of recycled lithologies like ...eclogite, and therefore may be useful as indicators of the source lithologies of mantle-derived basalts. Interpretation of basalt source lithologies from FRTE ratios requires accurate assessment of FRTE partitioning behavior between peridotitic minerals and coexisting melts. We present experimental determinations of partition coefficients for several of the FRTE (Sc, Ti, V, Cr, Mn, Fe, Co, Zn) and Ga and Ge between basaltic melt and olivine, garnet, pyroxenes, and spinel at 3GPa. Because mineral/melt partitioning is sensitive to phase compositions, a key feature of these experiments is that the melts and minerals are known from previous experiments to be in equilibrium at the solidus of garnet peridotite at 3GPa. Therefore, these partition coefficients are directly applicable to near-solidus partial melting of the mantle at 3GPa. We use these partition coefficients to calculate compositions of model partial melts of peridotite and compare these to natural OIB. Model partial melts of peridotite have lower Fe/Mn (<62) and higher Co/Fe (>7*10−4) than many primitive OIB, which implies that some other source lithology participates in the formation of many OIB. Alternatively, these ratios may potentially be produced by garnet peridotite if the source contains ∼0.3% Fe2O3, consistent with observations from continental xenoliths. Zn/Fe is a less sensitive indicator of non-peridotite source lithology than either Fe/Mn or Co/Fe, as Zn/Fe in partial melts of peridotite overlaps with >75% of primitive OIB. Ga and Sc are fractionated significantly by residual garnet, and high Ga/Sc may indicate the presence of garnet in basalt source regions. When taking into account several FRTE ratios simultaneously, few OIB appear to be consistent with derivation solely from a reduced peridotitic source. The source either must have a modest non-peridotitic component, be Fe-enriched, or be slightly oxidized.
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