Undegassed Reservoir Research Articles

The survival of tracers of primordial mantle heterogeneity investigated through 142Nd/144Nd and 3He/4He isotope decoupling in the Gorgona Island lavas

While 142Nd/144Nd, 3He/4He, and 182W/184W isotope ratios are recognized and applied as tracers of primordial mantle heterogeneity, their distribution and reasons behind the extent of variability in the modern-day mantle remain unclear. Lavas associated with the Galapagos plume exhibit the steepest 3He/4He vs. μ182W isotope correlations and thus hold the potential to elucidate the processes involved in the preservation of isotope variability. Among the various eruptions suggested to be related to the Galapagos plume is Gorgona Island, which is geologically significant for their Phanerozoic komatiites. We report the first high precision 142Nd/144Nd isotope measurements for the Gorgona komatiites, as well as 3He/4He and major and trace element data for an array of lavas including komatiites, a picrite, a d-gabbro (“depleted”), and an e-basalt (“enriched”). The 142Nd/144Nd ratios of six komatiite samples are indistinguishable to within ±2.4 ppm (2σ) from the modern-day terrestrial mantle as sampled by most mid-ocean ridge and ocean island basalts. The 3He/4He values determined for the komatiites and a picrite are from 4.6 to 45.1 RA. Eight of the 3He/4He measurements are higher than the MORB average (8 ± 1 RA), and thus strongly point to a plume source sampling an undegassed reservoir. The RA value of 45.1 in particular ranks among the highest 3He/4He ratios measured to date at Gorgona Island and elsewhere, close to what is observed for Phanerozoic flood basalts from Baffin Island.Evaluation of the sources of the lavas using major and rare earth elements along with ε143Nd signatures suggest that the komatiites and picrites are derived from a depleted peridotite source. Meanwhile, the e-basalts are deduced to have been derived from a hybrid source consisting of roughly 8% pyroxenite to 92% peridotite. The inferred and calculated degree of pyroxenite in the sources of the different lava types align with the magnitude of 3He/4He ratios observed. Model calculations involving pyroxenite sources in the e-basalts, which are reported to exhibit 182W/184W isotope anomalies, attest to their survival, while analogous calculations with 142Nd/144Nd isotope ratios suggest homogenization occurring on a faster timescale. It is deduced that 182W/184W anomalies are likely to outlast mixing with recycled pyroxenite compared to 142Nd/144Nd and 3He/4He ratios. The decoupling of modern-day 142Nd/144Nd, 3He/4He, and 182W/184W isotope variabilities thus likely reflect the sensitivity each system has to different mantle processes that contribute to their preservation throughout time.

Long-term core–mantle interaction explains W-He isotope heterogeneities

The isotopic characteristics of ocean island basalts have long been used to infer the nature of their source and the long-term evolution of the Earth's mantle. Anticorrelation between tungsten and helium isotopic signatures is a particularly puzzling feature in those basalts, which no single process appears to explain. Traditionally, the high 3He/4He signature has been attributed to an undegassed reservoir in the deep mantle. Additional processes needed to obtain low 182W/184W often entail unobserved ancillary geochemical effects. It has been suggested, however, that the core feeds the lower mantle with primordial helium, obviating the need for an undegassed mantle reservoir. Independently, the tungsten-rich core has been suggested to impart the plume source with anomalous tungsten isotope signatures. We advance the idea that isotopic diffusion may simultaneously transport both tungsten and helium across the core-mantle boundary, with the striking implication that diffusion can naturally account for the observed isotopic trend. By modeling the long-term isotopic evolution of mantle domains, we demonstrate that this mechanism can account for more than sufficient isotopic ratios in plume-source material, which, after dynamical transport to the Earth's surface, are consistent with the present-day mantle W-He isotopic heterogeneities. No undegassed mantle reservoir is required, bearing significance on early Earth conditions such as the extent of magma oceans.

The Seven Sisters Hydrothermal System: First Record of Shallow Hybrid Mineralization Hosted in Mafic Volcaniclasts on the Arctic Mid-Ocean Ridge

We document the discovery of an active, shallow, seafloor hydrothermal system (known as the Seven Sisters Vent Field) hosted in mafic volcaniclasts at a mid-ocean ridge setting. The vent field is located at the southern part of the Arctic mid-ocean ridge where it lies on top of a flat-topped volcano at ~130 m depth. Up to 200 °C phase-separating fluids vent from summit depressions in the volcano, and from pinnacle-like edifices on top of large hydrothermal mounds. The hydrothermal mineralization at Seven Sisters manifests as a replacement of mafic volcaniclasts, as direct intraclast precipitation from the hydrothermal fluid, and as elemental sulfur deposition within orifices. Barite is ubiquitous, and is sequentially replaced by pyrite, which is the first sulfide to form, followed by Zn-Cu-Pb-Ag bearing sulfides, sulfosalts, and silica. The mineralized rocks at Seven Sisters contain highly anomalous concentrations of ‘epithermal suite’ elements such as Tl, As, Sb and Hg, with secondary alteration assemblages including silica and dickite. Vent fluids have a pH of ~5 and are Ba and metal depleted. Relatively high dissolved Si (~7.6 mmol/L Si) combined with low (0.2–0.4) Fe/Mn suggest high-temperature reactions at ~150 bar. A δ13C value of −5.4‰ in CO2 dominated fluids denotes magmatic degassing from a relatively undegassed reservoir. Furthermore, low CH4 and H2 (<0.026 mmol/kg and <0.009 mmol/kg, respectively) and 3He/4He of ~8.3 R/Racorr support a MORB-like, sediment-free fluid signature from an upper mantle source. Sulfide and secondary alteration mineralogy, fluid and gas chemistry, as well as δ34S and 87Sr/86Sr values in barite and pyrite indicate that mineralization at Seven Sisters is sustained by the input of magmatic fluids with minimal seawater contribution. 226Ra/Ba radiometric dating of the barite suggests that this hydrothermal system has been active for at least 4670 ± 60 yr.

Krypton storage capacity of the Earth's lower mantle

Noble gases are important geochemical tracers allowing reconstructing global volatile cycles in Earth's reservoirs. To constrain these fundamental processes, precise data on their partitioning behavior at deep Earth conditions are needed. Such data are only available at moderate pressures up to 25 GPa due to experimental challenges. We have investigated the possibility of noble gas storage in the Earth's lower mantle up to 115 GPa. We studied the incorporation of krypton in the second most abundant lower mantle mineral (Mg1-x,Fex)O (ferropericlase) as well as in liquid metal-alloys by performing experiments up to 115 GPa and 3700 K using the laser-heated diamond anvil cell coupled to post-mortem EMPA analysis and X-ray absorption spectroscopy. The results reveal that, at these extreme conditions, up to 3 wt.% of krypton can be stored in (Mg1-x,Fex)O and 3000 ppm in the Fe-rich liquid metal. For both phases the storage capacities increase with pressure (between 40 GPa and 60 GPa) at a constant high temperature of 2300 K. Fpc has never been considered as a NG host, despite being the second most abundant mineral in the Earth's LM. Using recent accurate compressibility data, we demonstrate that a substitution of krypton into the anion site of (Mg1-x,Fex)O in form of neutral oxygen Schottky defects at diluted lower mantle conditions is possible. This noble gas incorporation mechanism is in agreement with a previous study on bridgmanite. We show that (Mg1-x,Fex)O exhibits higher noble gas storage capacities than bridgmanite through the lower mantle using lattice strain modeling and including experimental solubility and thermoelastic data for neon, argon, krypton and xenon. We also demonstrate that both phases exhibit the highest solubilities for argon and krypton. We used the solubility data from lattice strain modeling to predict noble gas abundances stored in the solid lower mantle after magma ocean crystallization. The modeled abundances show apparent similarities with estimates for the deep noble gas reservoir that are based on either 3He abundances in ocean island basalts or radiogenic 40Ar abundances in the bulk Earth. This strongly indicates that the crystalline lower mantle may play an important role as deep noble gas storage reservoir. We propose, based on considerations on noble gas replenishment from the lower mantle to the atmosphere, that the lower mantle can only contribute to a small fraction of the present-day atmospheric noble gases. This suggests that the lower mantle is an un-degassed reservoir.

A helium stratified and ingassed lower mantle: resolving the helium paradoxes

It is generally believed a variation of 3He/4He isotopic ratios in the mantle is due to only the decay of U and Th, which produces 4He as well as heat. Here we show that not only 3He/4He isotopic ratios but also helium contents can be fractionated by thermal diffusion in the lower mantle. The driving force for that fractionation is the adiabatic or convective temperature gradient, which always produces elemental and isotopic fractionation along temperature gradient by thermal diffusion with higher light/heavy isotopic ratio in the hot end. Our theoretical model and calculations indicate that the lower mantle is helium stratified, caused by thermal diffusion due to ~ 400 °C temperature contrast across the lower mantle. The highest 3He/4He isotopic ratios and lowest He contents are in the lowermost mantle, which is a consequence of thermal-diffusion fractionation rather than the lower mantle is a primordial and undegassed reservoir. Therefore, oceanic-island basalts derived from the deepest lower mantle with high 3He/4He isotopic ratios and less He contents—the long-standing helium paradox, is solved by our model. Because vigorous convection in the upper mantle had resulted in disordered or disorganized thermal-diffusion effects in He, Mid-ocean ridge basalts unaffected by mantle plume have a relatively homogenous and lower 3He/4He isotopic compositions. Our model also predicts that 3He/4He isotopic ratios in the deepest lower mantle of early Earth could be even higher than that of Jupiter, the initial He isotopic ratio in our solar system, because the temperature contrast across the lower mantle in the early Earth is the largest and less 4He had been produced by the decay of U and Th. Moreover, the early helium-stratified lower mantle owned the lowest He contents due to over-degassing caused by the largest temperature contrast. Consequently, succeeding evolution of the lower mantle is a He ingassed process due to secular cooling of the deepest mantle. This explains why significant amount of He produced by the decay of U and Th in the lower mantle were not released, another long-standing heat–helium paradox.

Helium not in store

The ratio of helium isotopes in some oceanic volcanoes seemed to demand a reservoir of virgin primordial gas in the Earth's mantle. In fact, that might not be necessary — a relief for other geophysical models. Degassing of the Earth's mantle through magmatism results in the irreversible loss of helium to space. As virtually all Earth's 3He is ‘primordial’, high 3He/4He ratios in ocean island basalts are generally taken as evidence for a primitive, unmelted reservoir in the deep Earth. A new global data compilation now shows that ocean island basalts displaying the strongest primordial signal are chemically and isotopically most like mid-ocean-ridge basalts, indicating a common melting history for continent and ocean crust formation via incomplete degassing. This suggests that there is no primordial undegassed reservoir of helium in the mantle, reconciling a long-standing paradox in mantle dynamics.

Role of recycled oceanic crust in the potassium and argon budget of the Earth: Toward a resolution of the “missing argon” problem

The K content of the Silicate Earth is commonly estimated at ∼250 ppm on the basis of K/U ratios of ∼12,000–13,000 in mid‐ocean ridge basalts and continental crust, and assuming a chondritic U content for the Earth. Given this K content, only ∼50% of the 40Ar generated over Earth history from the decay of 40K currently resides in the atmosphere. Because the upper mantle is thoroughly degassed and contains little 40Ar, the “missing” Ar has long been taken as evidence for a gas‐rich (and therefore convectively isolated) lower mantle. An alternative solution of the “missing Ar” problem is that the K content of the Silicate Earth is substantially lower than 250 ppm. Previous estimates have failed to take into account the composition of recycled oceanic crust, which could comprise up to 6–9% of the mantle by mass and contain up to 40% of the Earth's U. Because K is more soluble than U in subduction‐generated hydrous fluids, recycled, dehydrated oceanic crust is characterized by extremely low K/U ratios. This is evidenced in the low K/U ratios of eclogites (∼1000), and in HIMU‐type basalts (∼4000), which are thought to derive from sources containing significant quantities of recycled crust. Ocean island basalts, which are generated from mantle containing varying amounts and types of recycled material, possess highly variable K/U ratios, but average K/U ratios from individual island chains are negatively correlated with 206Pb/204Pb, indicating that the low K/U in some ocean‐island basalts is a long‐lived source feature. Inclusion of a recycled crust reservoir in K and U mass balance calculations results in an estimate of the Bulk Silicate Earth K/U of ∼7000–9000, or a K content of ∼150–195 ppm. As a result, much of the “missing” 40Ar thought to be stored in the deep mantle may not exist, with ∼60–90% of the Earth's 40Ar residing in the atmosphere or continental crust. The remaining 40Ar can be accommodated in the convecting mantle without isolation of a deep, undegassed reservoir.

A Tale of Two Planets

Most of Earth's volcanism occurs at plate boundaries—mid-ocean ridges and island arcs. Most of the 3He and other noble gases also are expelled from the interior at these locations. A small amount, compared to the global budget, of magma (<10%) and 3He («1%) escapes at mid-plate volcanoes, the socalled “hotspots.” Locations of hotspots are controlled by stresses and cracks, not by deep, narrow mantle upwellings. Most, or all, of Earth's volatile inventory appears to have been brought in by a late veneer. The high 3He content and high He/Ne and He/Ar ratios of mid-ocean ridge basalts, plus the excess 129Xe and 136Xe, suggest that the ridge basalt reservoir is relatively undegassed and that there is no primordial undegassed reservoir. The two prevailing views of the origin of Earth are summarized in the context of this information.

A model to explain the various paradoxes associated with mantle noble gas geochemistry.

As a result of an energetic accretion, the Earth is a volatile-poor and strongly differentiated planet. The volatile elements can be accounted for by a late veneer ( approximately 1% of total mass of the Earth). The incompatible elements are strongly concentrated into the exosphere (atmosphere, oceans, sediments, and crust) and upper mantle. Recent geochemical models invoke a large primordial undegassed reservoir with chondritic abundances of uranium and helium, which is clearly at odds with mass and energy balance calculations. The basic assumption behind these models is that excess "primordial" 3He is responsible for 3He/4He ratios higher than the average for midocean ridge basalts. The evidence however favors depletion of 3He and excessive depletion of 4He and, therefore, favors a refractory, residual (low U, Th) source Petrological processes such as melt-crystal and melt-gas separation fractionate helium from U and Th and, with time, generate inhomogeneities in the 3He/4He ratio. A self-consistent model for noble gases involves a gas-poor planet with trapping of CO2 and noble gases in the shallow mantle. Such trapped gases are released by later tectonic and magmatic processes. Most of the mantle was depleted and degassed during the accretion process. High 3He/4He gases are viewed as products of ancient gas exsolution stored in low U environments, rather than products of primordial reservoirs.

The helium paradoxes.

The ratio 3He/4He (R) plays a central role in models of mantle evolution that propose an undegassed lower mantle, rich in the primordial isotope 3He. A large primordial volatile-rich reservoir, a feature of recent models, is inconsistent with high-temperature accretion and with estimates of crustal and bulk Earth chemistry. High R can alternatively reflect high integrated 3He/(U+Th) ratios or low 4He abundances, as expected in refractory portions of the upper mantle. I show that high R materials are gas-poor and are deficient in radiogenic 4He compared with midocean ridge basalts. The seemingly primitive (i.e., high R) signatures in "hotspot" magmas may be secondary, derived from CO2-rich gases, or residual peridotite, a result of differential partitioning of U and He into magmas. A shallow and low 3He source explains the spatial variability and the temporal trends of R in ocean islands and is consistent with a volatile-poor planet. A shallow origin for the "primitive" He signature in ocean island basalts, such as at Loihi, reconciles the paradoxical juxtaposition of crustal, seawater, and atmospheric signatures with inferred "primitive" characteristics. High 238U/204Pb components in ocean island basalts are generally attributed to recycled altered oceanic crust. The low 238U/3He component may be in the associated depleted refractory mantle. High 3He/4He ratios are due to low 4He, not excess 3He, and do not imply or require a deep or primordial or undegassed reservoir. 40Ar in the atmosphere also argues against such models.

Thermal and rare gas evolution of the mantle

Present-day seismic evidence implies that the 660 km-deep seismic velocity jump is associated with neither an internal thermal boundary layer nor a strong internal barrier to flow between the upper and lower mantle. However, the generally preferred geochemical paradigm for mantle rare-gas evolution concludes that the lower mantle has remained an isolated and undegassed reservoir throughout at least the past 4.35 Ga of Earth history, as ∼50% of the 40Ar produced by 40K-decay appears to still reside within the mantle. Here we reexplore this problem assuming that present-day differentiation processes have operated throughout Earth history. Present mantle overturn rates are slow (∼5–10 Ga to pass a mantle volume through the mid-ocean ridge crucible). If slab subduction has been the mantle's primary heat-loss mechanism, then a simple boundary layer argument suggests that paleo-subduction and mantle overturn rates were proportional to the heat loss-squared, and thus more than twenty times faster in the Archean than at the present day. Nevertheless, simple models of Ar evolution within a convecting mantle demonstrate that whole-mantle convection can retain 25–60% of the 40Ar produced during Earth evolution without the need for postulating an isolated and undegassed lower mantle. These models suggest that the 40Ar constraint should be reinterpreted to be a constraint that ∼50% of the mantle (and crust?!) has been undegassed since the 40Ar was produced within it.

Helium-3 from the Mantle: Primordial Signal or Cosmic Dust?

Helium-3 in hotspot magmas has been used as unambiguous evidence for the existence of a primordial, undegassed reservoir deep in the Earth's mantle. However, a large amount of helium-3 is delivered to the Earth's surface by interplanetary dust particles (IDPs). Recycling of deep-sea sediments containing these particles to the mantle, and eventual incorporation in magma, can explain the high helium-3/helium-4 ratios of hotspot magmas. Basafts with high helium-3/helium-4 ratios may represent degassing of helium introduced by ancient (probably 1.5 to 2.0 billion years old) pelagic sediments rather than degassing of primordial lower mantle material brought to the surface in plumes. Influx of IDPs can also explain the neon and siderophile compositions of mantle samples.

Mantle neon and atmospheric contamination

The apparent distinction between atmospheric and mantle 20Ne 22Ne ratios may provide a technique to quantify air contamination in mantle-derived materials. In the absence of mantle nuclear reactions, which produce either 20Ne or 22Ne in substantial quantities, it is likely that the entire mantle is characterized by a single, uniform 20Ne 22Ne ratio; a value of around 12.5 is suggested by analyses of MORBs, OIBs, diamonds and xenoliths. If this premise is correct, then any measured 20Ne 22Ne ratios in mantle samples that are lower than this must result from addition of an air component, with 20Ne 22Ne= 9.8 . This is most likely a syn- or post-eruptive contaminant. The degree of air contamination inferred from 20Ne 22Ne ratios is generally small for diamonds, but is increasingly significant for MORBs and OIBs; many OIB's may carry > 90% air neon. We calculated “air-neon corrected” 21Ne 22Ne and 40Ar 36Ar ratios for the highly degassed MORB mantle and for the less degassed (high 3He 4He ) “plume” reservoir. The inferred MORB composition is indistinguishable from measurements of some gas-rich glasses. The calculated plume composition is similar to the least air-like measurements from ocean islands, but is less air-like than has been proposed previously. This plume composition is not consistent with a completely undegassed reservoir. From these corrected mantle compositions, we calculated the relative time-integrated rare gas abundances in the mantle, using a simple evolutionary model, which simultaneously considers the isotopic compositions of He, Ne, Ar and Xe. The model shows that both MORB and plume reservoirs have evolved with nearly solar elemental abundances. This provides strong support to suggestions based on Ne isotopes that the Earth accreted with gases nearly solar in composition. Importantly, the inferred mantle Ne/Ar ratios are much higher than atmospheric, which is consistent with simultaneous fractionation of both the atmospheric neon isotope ratio and the Ne/Ar ratio by massive hydrodynamic escape. Mixing between MORB and plume reservoirs (with our calculated elemental and isotopic abundances), plus varying amounts of added air, can account for the rare gases in nearly all mantle-derived rocks.

Rare gases in Samoan xenoliths

The rare gas isotopic compositions of residual harzburgite xenoliths from Savai'i (SAV locality) and an unnamed seamount south of the Samoan chain (PPT locality) provide important constraints on the rare gas evolution of the mantle and atmosphere. Despite heterogeneous trace element compositions, the rare gas characteristics of the xenoliths from each of the two localities are strikingly similar. SAV and PPT xenoliths have 3He/ 4He ratios of11.1 ± 0.5 R A and21.6 ± 1 R A, respectively; this range is comparable to the 3He/ 4He ratios in Samoan lavas and clearly demonstrates that they have trapped gases from a relatively undegassed reservoir. The neon results are not consistent with mixing between MORB and a plume source with an atmospheric signature. Rather, the neon isotopes reflect either a variably degassed mantle (with a relative order of degassing of Loihi < PPT < Reunion < SAV < MORB), or mixing between the Loihi source and MORB. The data supports the conclusions of Honda et al. that the 20Ne/ 22Ne ratio in the mantle more closely resembles the solar ratio than the atmospheric one. 40Ar/ 36Ar ratios in the least contaminated samples range from 4,000 to 12,000 with the highest values in the 22 R A PPT xenoliths. There is no evidence for atmospheric 40Ar/ 36Ar ratios in the mantle source of these samples, which indicates that the lower mantle may have 40Ar/ 36Ar ratios in excess of 5,000. Xenon isotopic anomalies in 129Xe and 136Xe are as high as 6%, or about half of the maximum MORB excess and are consistent with the less degassed nature of the Samoan mantle source. These results contradict previous suggestions that the high 3He/ 4He mantle has a near-atmospheric heavy rare gas isotopic composition.

Constraints on evolution of Earth's mantle from rare gas systematics

Analyses of the isotopic composition of He, Ar and Xe in a suite of glasses from the mid-ocean ridges and from the island of Hawaii show that the Hawaiian samples have systematically lower 4He/3He, 40Ar/36Ar and 129Xe/130Xe ratios than the mid-ocean ridge basalts. We interpret this result to imply the existence of an undegassed mantle reservoir. Given the isotopic variations, and the half lives of 129I and 40K (parent isotopes of 129Xe and 40Ar), the undegassed reservoir must have been separated from the MORB source reservoir at least 4,400 Myr ago. The most reasonable explanation for the data is therefore the existence of a two-layered mantle.