Alan D. Brandon
University of Houston
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Earth and Planetary Science Letters | 2000
Alan D. Brandon; J. E. Snow; Richard J. Walker; John W. Morgan; Timothy D. Mock
Abstract Abyssal peridotites are normally thought to be residues of melting of the mid-ocean ridge basalt (MORB) source and are presumably a record of processes affecting the upper mantle. Samples from a single section of abyssal peridotite from the Kane Transform area in the Atlantic Ocean were examined for 190Pt–186Os and 187Re–187Os systematics. They have uniform 186Os/188Os ratios with a mean of 0.1198353±7, identical to the mean of 0.1198340±12 for Os–Ir alloys and chromitites believed to be representative of the upper mantle. While the Pt/Os ratios of the upper mantle may be affected locally by magmatic processes, these data show that the Pt/Os ratio for the bulk upper mantle has not deviated by more than about ±30% from a chondritic Pt/Os ratio over 4.5 billion years. These observations are consistent with the addition of a chondritic late veneer after core separation as the primary control on the highly siderophile element budget of the terrestrial upper mantle. The 187Os/188Os of the samples range from 0.12267 to 0.12760 and correlate well with Pt and Pt/Os, but not Re/Os. These relationships may be explained by variable amounts of partial melting with changing DRe, reflecting in part garnet in the residue, with a model-dependent melting age between about 600 and 1700 Ma. A model where the correlation between Pt/Os and 187Os/188Os results from multiple ancient melting events, in mantle peridotites that were later juxtaposed by convection, is also consistent with these data. This melting event or events are evidently unrelated to recent melting under mid-ocean ridges, because recent melting would have disturbed the relationship between Pt/Os and 187Os/188Os. Instead, this section of abyssal peridotite may be a block of refractory mantle that remained isolated from the convecting portions of the upper mantle for 600 Ma to >1 Ga. Alternatively, Pt and Os may have been sequestered during more recent melting and possibly melt/rock reaction processes, thereby preserving an ancient melting history. If representative of other abyssal peridotites, then the rocks from this suite with subchondritic 187Os/188Os are not simple residues of recent MORB source melting at ridges, but instead have a more complex history. This suite of variably depleted samples projects to an undepleted present-day Pt/Os of about 2.2 and 187Os/188Os of about 0.128–0.129, consistent with estimates for the primitive upper mantle.
Science | 1996
Alan D. Brandon; Robert A. Creaser; Steven B. Shirey; Richard W. Carlson
Peridotite xenoliths from the Cascade arc in the United States and in the Japan arc have neodymium and osmium isotopic compositions that are consistent with addition of 5 to 15 percent of subducted material to the present-day depleted mantle. These observations suggest that osmium can be partitioned into oxidized and chlorine-rich slab-derived fluids or melts. These results place new constraints on the behavior of osmium (and possibly other platinum group elements) during subduction of oceanic crust by showing that osmium can be transported into the mantle wedge.
Geochimica et Cosmochimica Acta | 1996
Alan D. Brandon; David S. Draper
Type I spinel peridotite xenoliths from Simcoe Volcano, southern Washington (USA), are from lithospheric mantle approximately 65 km inboard from the axis of the subduction-related Cascade Range. Oxygen fugacities calculated from contents of Fe3+/ΣFe in Simcoe spinels, determined by Mossbauer spectroscopy, are up to 1.4 log units more oxidizing than the FMQ buffer. These are among the most oxidized mantle xenoliths reported, with fugacities substantially higher than those calculated for mantle beneath most of western North America. These results, together with those from amphibole-bearing spinel peridotites from Ichinomegata, Japan (Wood and Virgo, 1989), provide evidence that the mantle above subduction zones is more oxidized than is oceanic or ancient cratonic mantle. We suggest that oxidation was accomplished by an agent ranging in composition from solute-rich hydrous fluid to water-bearing silicate melt. A qualitative model relating extent of oxidation, duration of the oxidation process, and proportion of the available water (derived from subducting slabs) that oxidizes Fe in subarc mantle peridotite, suggests that such an agent can easily produce the observed extents of oxidation over timescales similar to the typical lifespans of subduction zones. For the Cascade arc with a duration of 50 Ma, the observed oxidation in the Simcoe peridotites can be achieved by reacting about 6–11 % of the available water with the mantle. These results demonstrate that water can make an efficient oxidizing agent, and because of the comparatively low ferric iron contents reported for mantle peridotites from other tectonic settings, oxidation of the mantle by water is mostly restricted to subduction zones where water is recycled from the surface and transferred into the mantle wedge.
Earth and Planetary Science Letters | 1999
Alan D. Brandon; Marc D. Norman; Richard J. Walker; John W. Morgan
Primitive Hawaiian picrites have 187Os/188Os as high as ∼0.145 and are more radiogenic than the depleted upper mantle, reflecting a time-integrated suprachondritic Re/Os ratio. The high Re/Os may be explained either by an ancient recycled crustal component or an evolved outer core component in the Hawaiian plume. New high precision 186Os/188Os measurements for these picrites, combined with previous analyses, show that the Hawaiian plume source has 186Os/188Os that range from chondritic mantle values of ∼0.119834 to more radiogenic values as high as ∼0.119848. The higher 186Os/188Os reflects long-term suprachondritic Pt/Os and is coupled with higher 187Os/188Os in all but the Koolau picrites. The latter have near-chondritic 186Os/188Os but with radiogenic 187Os/188Os. The Pt/Re of crustal materials that may make up ancient recycled slabs ranges from ∼0.1 to 33. Recycled slab material with such Pt/Re ratios evolved for 1–3 Ga and added to the plume source may explain the Koolau Os isotopic compositions. Pt/Re ratios of 88–100, however, are required for the ancient recycled crust to generate the coupled enrichments of 186Os/188Os and 187Os/188Os in picrites from Loihi and Hualalai. These high Pt/Re ratios do not occur in any known crustal materials, but they are consistent with the observed partition coefficients for Os>Re>Pt, during metal crystallization from an initially chondritic molten core (Pt/Re ∼21–24). Such partitioning may have produced an evolved outer core with suprachondritic Pt/Os, Re/Os and Pt/Re, resulting in the production of suprachondritic 186Os/188Os and 187Os/188Os over time. Small amounts of outer core metal (≤1.2%) mixed into the Hawaiian plume source can explain the coupled 186Os/188Os and 187Os/188Os enrichment in some of the Hawaiian picrites. In addition, the most radiogenic 186Os/188Os in the Hawaiian picrites is correlated with higher 3He/4He, consistent with an undegassed, and likely, lower mantle source. These data provide compelling geochemical evidence that the Hawaiian plume was generated at the core-mantle boundary.
Science | 2007
Vickie C. Bennett; Alan D. Brandon; Allen P. Nutman
The oldest rocks—3.85 billion years old—from southwest Greenland have coupled neodymium-142 excesses (from decay of now-extinct samarium-146; half-life, 103 million years) and neodymium-143 excesses (from decay of samarium-147; half-life, 106 billion years), relative to chondritic meteorites, that directly date the formation of chemically distinct silicate reservoirs in the first 30 million to 75 million years of Earth history. The differences in 142Nd signatures of coeval rocks from the two most extensive crustal relicts more than 3.6 billion years old, in Western Australia and southwest Greenland, reveal early-formed large-scale chemical heterogeneities in Earths mantle that persisted for at least the first billion years of Earth history. Temporal variations in 142Nd signatures track the subsequent incomplete remixing of very-early-formed mantle chemical domains.
Nature | 2007
Vinciane Debaille; Alan D. Brandon; Qing-Zhu Yin; B. Jacobsen
Resolving early silicate differentiation timescales is crucial for understanding the chemical evolution and thermal histories of terrestrial planets. Planetary-scale magma oceans are thought to have formed during early stages of differentiation, but the longevity of such magma oceans is poorly constrained. In Mars, the absence of vigorous convection and plate tectonics has limited the scale of compositional mixing within its interior, thus preserving the early stages of planetary differentiation. The SNC (Shergotty–Nakhla–Chassigny) meteorites from Mars retain ‘memory’ of these events. Here we apply the short-lived 146Sm–142Nd and the long-lived 147Sm–143Nd chronometers to a suite of shergottites to unravel the history of early silicate differentiation in Mars. Our data are best explained by progressive crystallization of a magma ocean with a duration of ∼100 million years after core formation. This prolonged solidification requires the existence of a primitive thick atmosphere on Mars that reduces the cooling rate of the interior.
Earth and Planetary Science Letters | 2003
Alan D. Brandon; Richard J. Walker; Igor S. Puchtel; Harry Becker; Munir Humayun; Sidonie Revillon
The presence of coupled enrichments in 186 Os/ 188 Os and 187 Os/ 188 Os in some mantle-derived materials reflects longterm elevation of Pt/Os and Re/Os relative to the primitive upper mantle. New Os data for the 89 Ma Gorgona Island, Colombia komatiites indicate that these lavas are also variably enriched in 186 Os and 187 Os, with 186 Os/ 188 Os ranging between 0.1198397 7 22 and 0.1198470 7 38, and with QOs correspondingly ranging from +0.15 to +4.4. These data define a linear trend that converges with the previously reported linear trend generated from data for modern Hawaiian picritic lavas and a sample from the ca. 251 Ma Siberian plume, to a common component with a 186 Os/ 188 Os of approximately 0.119870 and QOs of +17.5. The convergence of these data to this Os isotopic composition may imply a single ubiquitous source in the Earth’s interior that mixes with a variety of different mantle compositions distinguished by variations in QOs. The 187 Os- and 186 Os-enriched component may have been generated via early crystallization of the solid inner core and consequent increases in Pt/Os and Re/Os in the liquid outer core, with time leading to suprachondritic 186 Os/ 188 Os and QOs in the outer core. The presence of Os from the outer core in certain portions of the mantle would require a mechanism that could transfer Os from the outer core to the lower mantle, and thence to the surface. If this is the process that generated the isotopic enrichments in the mantle sources of these plume-derived systems, then the current understanding of solid metal^liquid metal partitioning of Pt, Re and Os requires that crystallization of the inner core began prior to 3.5 Ga. Thus, the Os isotopic data reported here provide a new source of data to better constrain the timing of inner core formation, complementing magnetic field paleointensity measurements as data sources that constrain models based on secular cooling of the Earth. Published by Elsevier Science B.V.
Science | 2010
Thomas J. Lapen; M. Righter; Alan D. Brandon; Vinciane Debaille; Brian L. Beard; J. T. Shafer; A. H. Peslier
Less Old Martian Meteorite The oldest Martian meteorite known, ALH84001, was thought to be a remnant of primordial martian crust formed during solidification of an early magma ocean. Using isotope data, Lapen et al. (p. 347) revised the crystallization age of this meteorite from 4.51 billion years to 4.09 billion years ago, meaning that this rock cannot be a fragment of primordial crust that escaped the period of intense bombardment that occurred between 4.25 and 4.10 billion years ago. The revised age also suggests that magmatism was ongoing in Mars for a large part of its history and that ALH84001 was actually formed during the heavy bombardment period, just before the martian core dynamo stopped and the planetary magnetic field was lost. The oldest known martian meteorite is younger than previously thought, precluding it from sampling primeval martian crust. Martian meteorite ALH84001 (ALH) is the oldest known igneous rock from Mars and has been used to constrain its early history. Lutetium-hafnium (Lu-Hf) isotope data for ALH indicate an igneous age of 4.091 ± 0.030 billion years, nearly coeval with an interval of heavy bombardment and cessation of the martian core dynamo and magnetic field. The calculated Lu/Hf and Sm/Nd (samarium/neodymium) ratios of the ALH parental magma source indicate that it must have undergone extensive igneous processing associated with the crystallization of a deep magma ocean. This same mantle source region also produced the shergottite magmas (dated 150 to 570 million years ago), possibly indicating uniform igneous processes in Mars for nearly 4 billion years.
Geology | 2015
Alan D. Rooney; Justin V. Strauss; Alan D. Brandon; Francis A. Macdonald
The snowball Earth hypothesis predicts globally synchronous glaciations that persisted on a multimillion year time scale. Geochronological tests of this hypothesis have been limited by a dearth of reliable age constraints bracketing these events on multiple cratons. Here we present four new Re-Os geochronology age constraints on Sturtian (717–660 Ma) and Marinoan (635 Ma termination) glacial deposits from three different paleocontinents. A 752.7 ± 5.5 Ma age from the base of the Callison Lake Formation in Yukon, Canada, confirms nonglacial sedimentation on the western margin of Laurentia between ca. 753 and 717 Ma. Coupled with a new 727.3 ± 4.9 Ma age directly below the glacigenic deposits of the Grand Conglomerate on the Congo craton (Africa), these data refute the notion of a global ca. 740 Ma Kaigas glaciation. A 659.0 ± 4.5 Ma age directly above the Maikhan-Uul diamictite in Mongolia confirms previous constraints on a long duration for the 717–660 Ma Sturtian glacial epoch and a relatively short nonglacial interlude. In addition, we provide the first direct radiometric age constraint for the termination of the Marinoan glaciation in Laurentia with an age of 632.3 ± 5.9 Ma from the basal Sheepbed Formation of northwest Canada, which is identical, within uncertainty, to U-Pb zircon ages from China, Australia, and Namibia. Together, these data unite Re-Os and U-Pb geochronological constraints and provide a refined temporal framework for Cryogenian Earth history.
Geochimica et Cosmochimica Acta | 2000
Alan D. Brandon; Richard J. Walker; John W. Morgan; Gordon G. Goles
Variations in the short-lived systems of 182Hf-182W and 146Sm-142Nd in the SNC meteorites indicate an early isolation of, and subsequent inefficient mixing between, mantle reservoirs in Mars. Correlations of eW and e142Nd with initial γOs are consistent with the Re-Os isotopic systematics of these meteorites being set during the earliest differentiation history of Mars. Contamination by a juvenile Martian crust may have affected Zagami Os isotopic systematics but successful contamination models combining Nd and Os systematics, are inconsistent with such a process affecting the isotopic compositions of the shergottite lherzolites (EETA 77005, LEW 88516, Y 793605). At least two long-lived mantle reservoirs, and possibly three, are required to explain the observed systematics. One mantle reservoir (NC Group), represented by Nakhla and Chassigny, has a projected present day γOs of −5.4 ± 2.6. Another mantle reservoir represented by the shergottite lherzolites and possibly Zagami, has a present day γOs of about +4. This represents a 2 to 3% enrichment in Re/Os relative to the primitive mantle estimated for the Earth (+1.6). A third mantle reservoir may be represented by DaG 476, having a nearly chondritic γOs coupled with very high e143Nd of around +40. The isotopic systematics of these reservoirs may be linked to development of cumulate crystal piles in a Martian magma ocean and variable amounts of late stage intercumulus melt (following Borg et al., 1997). In this model, fractional crystallization of olivine and possibly other phases with slightly subchondritic Re/Os, from a solidifying magma ocean, resulted in a lower Re/Os ratio in the NC Group source cumulates, and a resultant low γOs. Later cumulates or evolved melts crystallized with higher Re/Os ratios to produce the shergottite mantle reservoir(s), and hence, consequent higher γOs. Crystallization of the Martian magma ocean followed earliest core formation, as indicated by the correlation of eW with e142Nd and initial γOs.