Alberto E. Saal

Vapour undersaturation in primitive mid-ocean-ridge basalt and the volatile content of Earth's upper mantle

The analysis of volatiles in magmatic systems can be used to constrain the volatile content of the Earths mantle and the influence that magmatic degassing has on the chemistry of the oceans and the atmosphere. But most volatile elements have very low solubilities in magmas at atmospheric pressure, and therefore virtually all erupted lavas are degassed and do not retain their primary volatile signatures. Here we report the undersaturated pre-eruptive volatile content for a suite of mid-ocean-ridge basalts from the Siqueiros intra-transform spreading centre. The undersaturation leads to correlations between volatiles and refractory trace elements that provide new constraints on volatile abundances and their behaviour in the upper mantle. Our data generate improved limits on the abundances of carbon dioxide, water, fluorine, sulphur and chlorine in the source of normal mid-ocean-ridge basalt. The incompatible behaviour of carbon dioxide, together with the CO2/Nb and CO2/Cl ratios, permit estimates of primitive carbon dioxide and chlorine to be made for degassed and chlorine-contaminated mid-ocean-ridge basalt magmas, and hence constrain degassing and contamination histories of mid-ocean ridges.

Volatile content of lunar volcanic glasses and the presence of water in the Moon’s interior

The Moon is generally thought to have formed and evolved through a single or a series of catastrophic heating events, during which most of the highly volatile elements were lost. Hydrogen, being the lightest element, is believed to have been completely lost during this period. Here we make use of considerable advances in secondary ion mass spectrometry to obtain improved limits on the indigenous volatile (CO2, H2O, F, S and Cl) contents of the most primitive basalts in the Moon—the lunar volcanic glasses. Although the pre-eruptive water content of the lunar volcanic glasses cannot be precisely constrained, numerical modelling of diffusive degassing of the very-low-Ti glasses provides a best estimate of 745 p.p.m. water, with a minimum of 260 p.p.m. at the 95 per cent confidence level. Our results indicate that, contrary to prevailing ideas, the bulk Moon might not be entirely depleted in highly volatile elements, including water. Thus, the presence of water must be considered in models constraining the Moon’s formation and its thermal and chemical evolution.

High Pre-Eruptive Water Contents Preserved in Lunar Melt Inclusions

Primitive magmatic melt inclusions from the Moon contain as much water as some terrestrial mid-ocean ridge magmas. The Moon has long been thought to be highly depleted in volatiles such as water, and indeed published direct measurements of water in lunar volcanic glasses have never exceeded 50 parts per million (ppm). Here, we report in situ measurements of water in lunar melt inclusions; these samples of primitive lunar magma, by virtue of being trapped within olivine crystals before volcanic eruption, did not experience posteruptive degassing. The lunar melt inclusions contain 615 to 1410 ppm water and high correlated amounts of fluorine (50 to 78 ppm), sulfur (612 to 877 ppm), and chlorine (1.5 to 3.0 ppm). These volatile contents are very similar to primitive terrestrial mid-ocean ridge basalts and indicate that some parts of the lunar interior contain as much water as Earth’s upper mantle.

Re–Os isotope evidence for the composition, formation and age of the lower continental crust

Knowledge of the composition of the lower continental crust is important for understanding the formation and evolution of the crust as a whole, and the petrogenesis of continental basalts. Here we present rhenium–osmium isotope data for two well characterized suites of lower-crustal xenoliths from North Queensland, Australia, which have average major- and trace-element compositions similar to estimates of the bulk lower continental crust,. Our data indicate that the lower crust has 1 to 2 times as much osmium, about half as much rhenium, and is less radiogenic than the upper continental crust. We interpret the rhenium–osmium isotope systematics to indicate that assimilation and fractional crystallization are important processes in the formation of the lower crust, and lead to dramatic changes in the osmium isotopic composition of basalts that pond and fractionate there. A consequence of this is that the rhenium–osmium isotopic system should not be relied on to yield accurate mantle extraction ages for continental rocks.

Hydrogen Isotopes in Lunar Volcanic Glasses and Melt Inclusions Reveal a Carbonaceous Chondrite Heritage

Common Water The Moon has been traditionally considered bone-dry, but in recent years a number of studies have shown that during mantle melting, the lunar mantle had as much water as Earths upper mantle. Saal et al. (p. 1317, published online 9 May; see the cover) measured the isotopic composition of hydrogen dissolved in volcanic glass and olivine-hosted melt inclusions recovered from the Moon by the Apollo 15 and 17 missions. Lunar magmatic water was indistinguishable from the bulk water in carbonaceous chondrites and similar to terrestrial water, implying a common origin for the water contained in the interiors of Earth and the Moon. Hydrogen isotope ratios in lunar samples imply a common origin for Earth’s and the Moon’s water. Water is perhaps the most important molecule in the solar system, and determining its origin and distribution in planetary interiors has important implications for understanding the evolution of planetary bodies. Here we report in situ measurements of the isotopic composition of hydrogen dissolved in primitive volcanic glass and olivine-hosted melt inclusions recovered from the Moon by the Apollo 15 and 17 missions. After consideration of cosmic-ray spallation and degassing processes, our results demonstrate that lunar magmatic water has an isotopic composition that is indistinguishable from that of the bulk water in carbonaceous chondrites and similar to that of terrestrial water, implying a common origin for the water contained in the interiors of Earth and the Moon.

Degassing of reduced carbon from planetary basalts.

Degassing of planetary interiors through surface volcanism plays an important role in the evolution of planetary bodies and atmospheres. On Earth, carbon dioxide and water are the primary volatile species in magmas. However, little is known about the speciation and degassing of carbon in magmas formed on other planets (i.e., Moon, Mars, Mercury), where the mantle oxidation state [oxygen fugacity (fO2)] is different from that of the Earth. Using experiments on a lunar basalt composition, we confirm that carbon dissolves as carbonate at an fO2 higher than -0.55 relative to the iron wustite oxygen buffer (IW-0.55), whereas at a lower fO2, we discover that carbon is present mainly as iron pentacarbonyl and in smaller amounts as methane in the melt. The transition of carbon speciation in mantle-derived melts at fO2 less than IW-0.55 is associated with a decrease in carbon solubility by a factor of 2. Thus, the fO2 controls carbon speciation and solubility in mantle-derived melts even more than previous data indicate, and the degassing of reduced carbon from Fe-rich basalts on planetary bodies would produce methane-bearing, CO-rich early atmospheres with a strong greenhouse potential.

Insights into the dynamics of mantle plumes from uranium-series geochemistry

The long-standing paradigm that hotspot volcanoes such as Hawaii or Iceland represent the surface expression of mantle plumes—hot, buoyant upwelling regions beneath the Earth’s lithosphere—has recently been the focus of controversy. Whether mantle plumes exist or not is pivotal for our understanding of the thermal, dynamic and compositional evolution of the Earth’s mantle. Here we show that uranium-series disequilibria measured in hotspot lavas indicate that hotspots are indeed associated with hot and buoyant upwellings and that weaker (low buoyancy flux) hotspots such as Iceland and the Azores are characterized by lower excess temperatures than stronger hotspots such as Hawaii. This direct link between buoyancy flux and mantle temperature is evidence for the existence of mantle plumes.

Insights into the Galápagos plume from uranium‐series isotopes of recently erupted basalts

Uranium-series isotopes (238U-230Th-226Ra-210Pb), major element, trace element, and Sr-Nd isotopic data are presented for recent (<60 years old) Galapagos archipelago basalts. Volcanic rocks from all centers studied (Fernandina, Cerro Azul, Sierra Negra, and Wolf Volcano) display 230Th excesses (4%–15%) and steep rare earth element (REE) patterns indicative of residual garnet during partial melting of their mantle source. Rare earth element modeling suggests that only a few percent of garnet is involved. Correlations between (238U/232Th), radiogenic isotopes and Nb/Zr ratio suggest that the U/Th ratio of these Galapagos volcanic rocks is primarily controlled by geochemical source variations and not fractionation during partial melting. The lowest (230Th/238U) ratio is not observed at Fernandina (the supposed center of the plume) but at the more geochemically “depleted” Wolf Volcano, further to the north. Small radium excesses are observed for all samples with (226Ra/230Th) ranging from 1.107 to 1.614. The 226Ra-230Th disequilibria do not correlate with other uranium-series parent-daughter nuclide pairs or geochemical data, suggesting modification at shallow levels on timescales relevant to the half-life of 226Ra (1600 years). The combination of 226Ra and 210Pb excesses is inconsistent with interaction of magma with cumulate material unless decoupling of 210Pb (or an intermediate daughter, such as 222Rn) occurs prior to modification of Ra-Th disequilibria. An intriguing correlation of (210Pb/226Ra)0 with Nb/Zr and radiogenic isotopes requires further investigation but suggests possible control via magmatic degassing and accumulation that may somehow be related to source heterogeneities.

Microstructural and geochemical constraints on the evolution of deep arc lithosphere

Mantle xenoliths from the Sierra Nevada, California, USA, sampled a vertical column (60–120 km) of lithosphere that formed during Mesozoic continental arc magmatism. This lithosphere experienced an anticlockwise P-T-t path resulting in rapid cooling that effectively “quenched in” features inherited from earlier high-temperature conditions. Here we combine new mineral chemistry data (water, trace element, and major element concentrations) with mineral crystallographic preferred orientations (CPOs) to investigate the relationship between melt infiltration and deformation. The peridotites record a refertilization trend with increasing depth, starting from shallow, coarse-protogranular, less-melt-infiltrated spinel peridotite with strong, orthorhombic olivine CPO to deep, fine-porphyroclastic, highly melt-infiltrated garnet peridotite with weak, axial-[010] olivine CPO. In contrast to the observed axial-[010] CPOs, subgrain boundary orientations and misorientation axes suggest the dominant activation of the (001)[100] slip system, suggesting deformation under moderately hydrous conditions. After accounting for effects of subsolidus cooling, we see coherent trends between mineral trace element abundance and water content, indicating that melt infiltration led to an increase in water content of the peridotites. However, measured olivine and pyroxene water contents in all peridotites (5–10 and 30–500 wt ppm, respectively) are lower than that required to promote dominant (001)[100] slip system observed in both natural and experimental samples. These results suggest that deformation occurred earlier along the P-T path, probably during or shortly after hydrous melt infiltration. Subsequent rapid cooling at 90 Ma led to water loss from olivine (owing to decreased solubility at low temperature), leaving behind a deep arc lithosphere that remained viscously coupled to the Farallon slab until the opening of the slab window in the late Cenozoic.

Submarine Basaltic Glasses from the Galapagos Archipelago: Determining the Volatile Budget of the Mantle Plume

We report new major, trace and volatile element contents (H_2O, CO_2, F, S, Cl), and new Sr, Nd, Pb and He isotopes on submarine glasses from the Galapagos Archipelago from several dredging expeditions. Four groups are distinguishable on the basis of composition and geographical distribution: the Fernandina group (^3He/^4He > 22 R_A), which is similar to the less degassed primitive mantle; the Sierra Negra group (enriched Pb and Sr isotopes, ^3He/^4He = 8–20 R_A), produced by mixing the Floreana (HIMU-type) and Fernandina end-members; the Pinta group (high Δ7/4, Δ8/4 and Th/La ratios, ^3He/^4He = 6–9 R_A), an enriched mantle (EM)-type mantle indicative of recycled material in the source; and the depleted mantle (DM) group, characterized by an isotopic composition similar to mid-ocean ridge basalts (MORB). Only a single submarine glass with the isotopic composition of the Floreana end-member has been identified in the sample suite. Degassing has significantly lowered the glass CO_2 content with little effect on the H_2O concentration. Volatile data for oceanic basalts reveal that CO_2–H_2O gas–melt equilibration at eruption depth is common in ocean island basalts (OIB) and rare in MORB, suggesting different ratios of melt transport to bubble formation and gas–melt equilibration. The Galapagos glasses range from sulfide saturated to undersaturated, and a subset of samples indicate that S degasses at pressures ≤ 400 bars. Assimilation of hydrothermally altered material affected the volatile contents of a number of samples in the groups. Once shallow-level processes have been accounted for, we evaluate the volatile contents in the different Galapagos mantle sources. Ratios between volatile and refractory elements with similar incompatibilities are used to estimate the volatile budget of the Galapagos mantle plume. Most of the glasses from the Fernandina, Sierra Negra and Pinta groups have high volatile/refractory element ratios, whereas a few pristine DM group lavas have ratios similar to those measured in MORB. The volatile/refractory element ratios are consistent with previous reports for the high ^3He/^4He, HIMU and MORB components. The values measured for the Pinta group, however, are higher than those found in other OIB associated with the presence of recycled material (EM-type). Our data suggest that mixing between the different mantle components is pervasive throughout the archipelago, which acts to normalize the volatile data between the groups. The Fernandina component can be modeled by a 6–20% mixture of the high ^3He/^4He primitive mantle component with the MORB source, assuming a two-layered mantle and using existing estimates of helium concentrations. The resulting estimated volatile content and H/C mass ratio for the high ^3He/^4He primitive mantle are consistent with previous estimates, but calculated C/3He ratios are lower than the canonical ratio. This indicates the following: (1) the estimates require ∼20–50 times higher C or lower ^3He contents, which is difficult to reconcile with the measured volatile/refractory ratios in oceanic basalts; (2) the C/^3He ratio is not constant throughout the mantle; (3) an impact erosion model, rather than a two-layered mantle model, is more consistent with the relatively constant C/3He ratios observed in oceanic basalts, although it is unclear how representative oceanic basalts are of the lower mantle. The high volatile content of the high ^3He/^4He component will affect mantle dynamics and melt migration during plume–ridge interaction as this component would be predicted to be less viscous than the ambient mantle. The lower viscosity material would have an enhanced vertical upwelling, which could explain the buoyancy flux of the Galapagos plume without the need for a temperature anomaly. A lower viscosity, high ^3He/^4He component could also provide an explanation for the lack of high ^3He/^4He in Galapagos Spreading Center lavas erupting in the vicinity of the Galapagos plume.