Linda T. Elkins-Tanton

Effect of solid flow above a subducting slab on water distribution and melting at convergent plate boundaries

[1] Hydrous fluids derived by dehydration of the downgoing slab at convergent plate boundaries are thought to provoke wet melting in the wedge above the downgoing plate. We have investigated the distribution of hydrous fluid and subsequent melt in the wedge using two-dimensional models that include solid mantle flow and associated temperature distributions along with buoyant fluid migration and melting. Solid mantle flow deflects hydrous fluid from their buoyant vertical migration through the wedge. Melting therefore does not occur directly above the region where hydrous fluids are released from the slab. A melting front develops where hydrous fluids first encounter mantle material hot enough to melt. Wet melting is influenced by solid flow through the advection of fertile mantle material into the wet melting region and the removal of depleted material. The region of maximum melting occurs where the maximum flux of water from slab mineral dehydration reaches the wet melting region. The extent of melting (F) and melt production rates increase with increasing convergence rate and grain size due to increased temperatures along the melting front and to increased fractions of water reaching the melting front, respectively. The position of isotherms above the wet solidus varies with increasing slab dip and thereby also influences F and melt production rates. Applying the understanding of wet melting from this study to geochemical studies of the Aleutians may help elucidate the processes influencing fluid migration and melt production in that region. Estimates of the timescale of fluid migration, seismic velocity variation, and attenuation are also investigated.

Recent hotspot volcanism on venus from VIRTIS emissivity data

Hotspots on Venus The surface of Venus shows clear signs of volcanism, but are there active volcanoes on Venus today? The answer to this question will bear on our understanding of the planets climate evolution and interior dynamics. Using surface thermal emissivity data returned by the Venus Express spacecraft, Smrekar et al. (p. 605, published online 8 April) looked at three hotspots on Venus. These places were identified by analogy with terrestrial hotspots like Hawaii, which are believed to overlie mantle plumes and to be the most likely sites for current volcanic activity. Lava flows at the three hotspots have anomalously high thermal emissions when compared with their surroundings. Low emissivity is generally interpreted as the result of surface alteration by the corrosive atmosphere of Venus. High emissivity implies that not much alteration took place and thus that the hotspots must represent recently active volcanoes younger than 2.5 million years. Satellite observations suggest that Venus is a geologically active planet. The questions of whether Venus is geologically active and how the planet has resurfaced over the past billion years have major implications for interior dynamics and climate change. Nine “hotspots”—areas analogous to Hawaii, with volcanism, broad topographic rises, and large positive gravity anomalies suggesting mantle plumes at depth—have been identified as possibly active. This study used variations in the thermal emissivity of the surface observed by the Visible and Infrared Thermal Imaging Spectrometer on the European Space Agency’s Venus Express spacecraft to identify compositional differences in lava flows at three hotspots. The anomalies are interpreted as a lack of surface weathering. We estimate the flows to be younger than 2.5 million years and probably much younger, about 250,000 years or less, indicating that Venus is actively resurfacing.

Continental magmatism caused by lithospheric delamination

Ductile delamination of continental lower lithosphere via Rayleigh-Taylor instabilities can produce continental magmatism with a range of majorand trace-element compositions and volatile contents. Here I investigate the process of delamination, resulting uppermantle flow patterns, topographic expression, and, most significantly, the potential for the delaminating material to dehydrate as it sinks. The delamination process can produce a heterogeneous, locally hydrous upper mantle, as well as short-duration eruptive episodes of hydrous, alkali-rich magmas in the absence of subduction, subsidence during eruption, and shallow, dry melting under cratonic lithosphere. Delamination has been inferred from increases in crustal heatflow and seismic tomography in specific regions, from rapid regional uplift, and from the appearance of signature high-potassium magmas. A dense lower-lithospheric region may develop through melt injection and transformation into eclogitic phase assemblages, through thickening and cooling of a lithospheric root, or through accumulation of mafic phases in magma chambers. Lower-crustal and mantle compositions that result from arc magmatism are likely to exceed the mantle density by 50–250 kg/m, corresponding to ~1%–5% density contrast. Density contrasts as small as 1% are sufficient to drive gravitational Rayleigh-Taylor instabilities.

OCEAN PLANET OR THICK ATMOSPHERE: ON THE MASS-RADIUS RELATIONSHIP FOR SOLID EXOPLANETS WITH MASSIVE ATMOSPHERES

The bulk composition of an exoplanet is commonly inferred from its average density. For small planets, however, the average density is not unique within the range of compositions. Variations of a number of important planetary parameters—whicharedifficultorimpossibletoconstrainfrommeasurementsalone—produceplanetswiththesame averagedensitiesbutwidelyvaryingbulkcompositions.Wefindthataddingagasenvelopeequivalentto0.1%Y10% of the mass of a solid planet causes the radius to increase 5%Y60% above its gas-free value. A planet with a given mass and radius might have substantial water ice content (a so-called ocean planet), or alternatively alarge rocky iron coreandsomeHand/orHe.Forexample,awidevarietyof compositionscanexplaintheobservedradiusof GJ436b, althoughallmodelsrequiresomeH/He.Weconcludethattheidentificationof waterworldsbasedonthemass-radius relationship alone is impossible unless a significant gas layer can be ruled out by other means. Subject headingg planets and satellites: general — planetary systems — stars: individual (GJ 436) Online material: color figures

Stochastic Late Accretion to Earth, the Moon, and Mars

For the Love of Iron Iron-loving elements such as Re, Os, Ir, Pt, Rh, Pd, and Au must have been delivered to the upper mantle of Earth, Mars, and the Moon after formation of the planetary cores, because, before that, these elements tended to bond with the cores metallic iron, stripping them from the planetary upper layers. Using Monte Carlo models, Bottke et al. (p. 1527) show that the relative abundances of iron-loving elements on Earth, Mars, and the Moon can be explained if most of the impacting planetesimals that delivered the elements had sizes extending up to several thousand kilometers. In these circumstances, most of the iron-loving elements would arrive in a small number of random impacts, the most massive of which hit Earth but not the Moon. Some of these impacts may also have altered Earths obliquity, produced the Moons orbital inclination, and delivered water to the Moons mantle. Random impacts led to the late delivery of highly siderophile elements (noble metals) during the growth of these bodies. Core formation should have stripped the terrestrial, lunar, and martian mantles of highly siderophile elements (HSEs). Instead, each world has disparate, yet elevated HSE abundances. Late accretion may offer a solution, provided that ≥0.5% Earth masses of broadly chondritic planetesimals reach Earth’s mantle and that ~10 and ~1200 times less mass goes to Mars and the Moon, respectively. We show that leftover planetesimal populations dominated by massive projectiles can explain these additions, with our inferred size distribution matching those derived from the inner asteroid belt, ancient martian impact basins, and planetary accretion models. The largest late terrestrial impactors, at 2500 to 3000 kilometers in diameter, potentially modified Earth’s obliquity by ~10°, whereas those for the Moon, at ~250 to 300 kilometers, may have delivered water to its mantle.

Ranges of Atmospheric Mass and Composition of Super-Earth Exoplanets

Terrestrial-likeexoplanets may obtainatmospheres from three primary sources:capture of nebulargases,degassing during accretion, and degassing from subsequent tectonic activity. Here we model degassing during accretion to estimate the range of atmospheric mass and composition on exoplanets ranging from 1 to 30 Earth masses. We use bulk compositions drawn from primitive and differentiated meteorite compositions. Degassing alone can create a widerangeof massesof planetaryatmospheres,rangingfromlessthan1%of theplanet’stotalmassupto � 6percent bymass (mass%) of hydrogen, � 20 mass% of water, and/or � 5mass% of carbon compounds. Hydrogen-rich atmospheres can be outgassed as a result of oxidizing metallic iron with water, and excess water and carbon can produce atmospheres through simple degassing. As a byproduct of our atmospheric outgassing models we find that modest initial water contents (10 mass% of the planet and above) create planets with deep surface liquid water oceans soon after accretion is complete. Subject headingg accretion, accretion disks — planets and satellites: formation — solar system: formation Online material: color figures

Widespread mixing and burial of Earth/'s Hadean crust by asteroid impacts

The history of the Hadean Earth (∼4.0–4.5 billion years ago) is poorly understood because few known rocks are older than ∼3.8 billion years old. The main constraints from this era come from ancient submillimetre zircon grains. Some of these zircons date back to ∼4.4 billion years ago when the Moon, and presumably the Earth, was being pummelled by an enormous flux of extraterrestrial bodies. The magnitude and exact timing of these early terrestrial impacts, and their effects on crustal growth and evolution, are unknown. Here we provide a new bombardment model of the Hadean Earth that has been calibrated using existing lunar and terrestrial data. We find that the surface of the Hadean Earth was widely reprocessed by impacts through mixing and burial by impact-generated melt. This model may explain the age distribution of Hadean zircons and the absence of early terrestrial rocks. Existing oceans would have repeatedly boiled away into steam atmospheres as a result of large collisions as late as about 4 billion years ago.

Magnetism on the Angrite Parent Body and the Early Differentiation of Planetesimals

Angrites are among the oldest known pristine basaltic meteorites and record the earliest stages of planet formation and differentiation. Our paleomagnetic analysis of three angrites found that they record a past magnetic field of ∼10 microteslas on the angrite parent body extending from 4564 to at least 4558 million years ago. Because the angrite paleomagnetic record extends beyond the expected lifetime of the early circumstellar disk, these paleofields were probably generated internally on the angrite parent body, possibly by an early dynamo in a rapidly formed metallic core.

Acid rain and ozone depletion from pulsed Siberian Traps magmatism

The Siberian Traps flood basalts have been invoked as a trigger for the catastrophic end-Permian mass extinction. Widespread aberrant plant remains across the Permian-Triassic boundary provide evidence that atmospheric stress contributed to the collapse in terrestrial diversity. We used detailed estimates of magmatic degassing from the Siberian Traps to complete the first three-dimensional global climate modeling of atmospheric chemistry during eruption of a large igneous province. Our results show that both strongly acidic rain and global ozone collapse are possible transient consequences of episodic pyroclastic volcanism and heating of volatile-rich Siberian country rocks. We suggest that in conjunction with abrupt warming from greenhouse gas emissions, these repeated, rapidly applied atmospheric stresses directly linked Siberian magmatism to end-Permian ecological failure on land. Our comprehensive modeling supplies the first picture of the global distribution and severity of acid rain and ozone depletion, providing testable predictions for the geography of end-Permian environmental proxies.

Evidence for deep melting of hydrous metasomatized mantle: Pliocene high-potassium magmas from the Sierra Nevadas

[1] Phase equilibrium experiments have been conducted on a primitive Pliocene olivine leucitite (WC-1) from the central Sierra Nevada, California. The near-liquidus phase relations were determined from 1.2 to 3.4 GPa and at temperatures from 1350° to 1460°C in a piston-cylinder apparatus. The composition with ∼2% H 2 O is multiply saturated with olivine and clinopyroxene at approximately 3.1 GPa and 1460°C and with 6% water in the coexisting melt phlogopite is stable. These results indicate that the magma was derived from a hydrous source at greater than 100 km depth. Xenoliths carried by other young Pliocene lavas in the vicinity of WC-1 have yielded temperatures of equilibrium from 700° to 900°C, with one outlier at 1060°C. These xenoliths are consistent with the hypothesis that the lower lithosphere under the Sierra Nevada delaminated just prior to the Pliocene, and fluid-metasomatized mantle melted to produce the high-potassium Pliocene lavas. We suggest that subduction-derived fluids drive a reaction that consumes garnet + orthopyroxene to create clinopyroxene + phlogopite, and that the high-potassium Sierran magmas are created by melting phlogopite-clinopyroxene metasomatized peridotite.