Ancient, highly heterogeneous mantle beneath Gakkel ridge, Arctic Ocean

The Earth's mantle beneath ocean ridges is widely thought to be depleted by previous melt extraction, but well homogenized by convective stirring. This inference of homogeneity has been complicated by the occurrence of portions enriched in incompatible elements. Here we show that some refractory abyssal peridotites from the ultraslow-spreading Gakkel ridge (Arctic Ocean) have very depleted 187Os/188Os ratios with model ages up to 2 billion years, implying the long-term preservation of refractory domains in the asthenospheric mantle rather than their erasure by mantle convection. The refractory domains would not be sampled by mid-ocean-ridge basalts because they contribute little to the genesis of magmas. We thus suggest that the upwelling mantle beneath mid-ocean ridges is highly heterogeneous, which makes it difficult to constrain its composition by mid-ocean-ridge basalts alone. Furthermore, the existence of ancient domains in oceanic mantle suggests that using osmium model ages to constrain the evolution of continental lithosphere should be approached with caution.     

Earth science: role of fO2 on fluid saturation in oceanic basalt

Assessing the conditions under which magmas become fluid-saturated has important bearings on the geochemical modelling of magmas because volatile exsolution may profoundly alter the behaviour of certain trace elements that are strongly partitioned in the coexisting fluid. Saal et al. report primitive melt inclusions from dredged oceanic basalts of the Siqueiros transform fault, from which they derive volatile abundances of the depleted mantle, based on the demonstration that magmas are not fluid-saturated at their eruption depth and so preserve the mantle signature in terms of their volatile contents. However, in their analysis, Saal et al. consider only fluid-melt equilibria, and do not take into account the homogeneous equilibria between fluid species, which, as we show here, may lead to a significant underestimation of the pressure depth of fluid saturation.     

Early differentiation and volatile accretion recorded in deep-mantle neon and xenon

The isotopes (129)Xe, produced from the radioactive decay of extinct (129)I, and (136)Xe, produced from extinct (244)Pu and extant (238)U, have provided important constraints on early mantle outgassing and volatile loss from Earth. The low ratios of radiogenic to non-radiogenic xenon ((129)Xe/(130)Xe) in ocean island basalts (OIBs) compared with mid-ocean-ridge basalts (MORBs) have been used as evidence for the existence of a relatively undegassed primitive deep-mantle reservoir. However, the low (129)Xe/(130)Xe ratios in OIBs have also been attributed to mixing between subducted atmospheric Xe and MORB Xe, which obviates the need for a less degassed deep-mantle reservoir. Here I present new noble gas (He, Ne, Ar, Xe) measurements from an Icelandic OIB that reveal differences in elemental abundances and (20)Ne/(22)Ne ratios between the Iceland mantle plume and the MORB source. These observations show that the lower (129)Xe/(130)Xe ratios in OIBs are due to a lower I/Xe ratio in the OIB mantle source and cannot be explained solely by mixing atmospheric Xe with MORB-type Xe. Because (129)I became extinct about 100 million years after the formation of the Solar System, OIB and MORB mantle sources must have differentiated by 4.45 billion years ago and subsequent mixing must have been limited. The Iceland plume source also has a higher proportion of Pu- to U-derived fission Xe, requiring the plume source to be less degassed than MORBs, a conclusion that is independent of noble gas concentrations and the partitioning behaviour of the noble gases with respect to their radiogenic parents. Overall, these results show that Earth's mantle accreted volatiles from at least two separate sources and that neither the Moon-forming impact nor 4.45 billion years of mantle convection has erased the signature of Earth's heterogeneous accretion and early differentiation.     

Kimberlite ascent by assimilation-fuelled buoyancy

Kimberlite magmas have the deepest origin of all terrestrial magmas and are exclusively associated with cratons. During ascent, they travel through about 150 kilometres of cratonic mantle lithosphere and entrain seemingly prohibitive loads (more than 25 per cent by volume) of mantle-derived xenoliths and xenocrysts (including diamond). Kimberlite magmas also reputedly have higher ascent rates than other xenolith-bearing magmas. Exsolution of dissolved volatiles (carbon dioxide and water) is thought to be essential to provide sufficient buoyancy for the rapid ascent of these dense, crystal-rich magmas. The cause and nature of such exsolution, however, remains elusive and is rarely specified. Here we use a series of high-temperature experiments to demonstrate a mechanism for the spontaneous, efficient and continuous production of this volatile phase. This mechanism requires parental melts of kimberlite to originate as carbonatite-like melts. In transit through the mantle lithosphere, these silica-undersaturated melts assimilate mantle minerals, especially orthopyroxene, driving the melt to more silicic compositions, and causing a marked drop in carbon dioxide solubility. The solubility drop manifests itself immediately in a continuous and vigorous exsolution of a fluid phase, thereby reducing magma density, increasing buoyancy, and driving the rapid and accelerating ascent of the increasingly kimberlitic magma. Our model provides an explanation for continuous ascent of magmas laden with high volumes of dense mantle cargo, an explanation for the chemical diversity of kimberlite, and a connection between kimberlites and cratons.     

Neon isotopes constrain convection and volatile origin in the Earth's mantle

Identifying the origin of primordial volatiles in the Earth's mantle provides a critical test between models that advocate magma-ocean equilibration with an early massive solar-nebula atmosphere and those that require subduction of volatiles implanted in late accreting material. Here we show that neon isotopes in the convecting mantle, resolved in magmatic CO2 well gases, are consistent with a volatile source related to solar corpuscular irradiation of accreting material. This contrasts with recent results that indicated a solar-nebula origin for neon in mantle plume material, which is thought to be sampling the deep mantle. Neon isotope heterogeneity in different mantle sources suggests that models in which the plume source supplies the convecting mantle with its volatile inventory require revision. Although higher than accepted noble gas concentrations in the convecting mantle may reduce the need for a deep mantle volatile flux, any such flux must be dominated by the neon (and helium) isotopic signature of late accreting material.     

Spectroscopic evidence for a lava fountain driven by previously accumulated magmatic gas

Lava fountains are spectacular continuous gas jets, propelling lava fragments to heights of several hundred metres, which occasionally occur during eruptions of low-viscosity magmas. Whether they are generated by the effervescent disruption of fast-rising bubbly melt or by the separate ascent of a bubble foam layer accumulated at depth still remains a matter of debate. No field measurement has yet allowed firm discrimination between these two models. A key insight into the origin of lava fountains may be gained by measuring the chemical composition of the driving gas phase. This composition should differ markedly depending on whether the magma degassing occurs before or during eruption. Here we report the analysis of magmatic gas during a powerful (250-600 m high) lava fountain, measured with Fourier transform infrared spectroscopy on Mount Etna, Sicily. The abundances of volcanic gas species, determined from absorption spectra of lava radiation, reveal a fountain gas having higher CO2/S and S/Cl ratios than other etnean emissions, and which cannot derive from syn-eruptive bulk degassing of Etna basalt. Instead, its composition suggests violent emptying of a gas bubble layer previously accumulated at about 1.5 km depth below the erupting crater.     

300-Myr-old magmatic CO2 in natural gas reservoirs of the west Texas Permian basin

Except in regions of recent crustal extension, the dominant origin of carbon dioxide in fluids in sedimentary basins has been assumed to be from crustal organic matter or mineral reactions. Here we show, by contrast, that Rayleigh fractionation caused by partial degassing of a magma body can explain the CO2/3He ratios and delta13C(CO2) values observed in CO2-rich natural gases in the west Texas Val Verde basin and also the mantle 3He/22Ne ratios observed in other basin systems. Regional changes in CO2/3He and CO2/CH4 ratios can be explained if the CO2 input pre-dates methane generation in the basin, which occurred about 280 Myr ago. Uplift to the north of the Val Verde basin between 310 and 280 Myr ago appears to be the only tectonic event with appropriate timing and location to be the source of the magmatic CO2. Our identification of magmatic CO2 in a foreland basin indicates that the origin of CO2 in other mid-continent basin systems should be re-evaluated. Also, the inferred closed-system preservation of natural gas in a trapping structure for approximately 300 Myr is far longer than the residence time predicted by diffusion models.     

Explosive volcanism on the ultraslow-spreading Gakkel ridge, Arctic Ocean

Roughly 60% of the Earth's outer surface is composed of oceanic crust formed by volcanic processes at mid-ocean ridges. Although only a small fraction of this vast volcanic terrain has been visually surveyed or sampled, the available evidence suggests that explosive eruptions are rare on mid-ocean ridges, particularly at depths below the critical point for seawater (3,000 m). A pyroclastic deposit has never been observed on the sea floor below 3,000 m, presumably because the volatile content of mid-ocean-ridge basalts is generally too low to produce the gas fractions required for fragmenting a magma at such high hydrostatic pressure. We employed new deep submergence technologies during an International Polar Year expedition to the Gakkel ridge in the Arctic Basin at 85 degrees E, to acquire photographic and video images of 'zero-age' volcanic terrain on this remote, ice-covered ridge. Here we present images revealing that the axial valley at 4,000 m water depth is blanketed with unconsolidated pyroclastic deposits, including bubble wall fragments (limu o Pele), covering a large (>10 km(2)) area. At least 13.5 wt% CO(2) is necessary to fragment magma at these depths, which is about tenfold the highest values previously measured in a mid-ocean-ridge basalt. These observations raise important questions about the accumulation and discharge of magmatic volatiles at ultraslow spreading rates on the Gakkel ridge and demonstrate that large-scale pyroclastic activity is possible along even the deepest portions of the global mid-ocean ridge volcanic system.     

Carbon isotopic composition of mantle xenoliths in alkali basalt from Damaping, Hebei

The carbon isotopic composition of CO-2, CO and CH-4 extracted by pyrolysis from spinel lherzolite and black pyroxenolite xenoliths in alkali basalt from Damaping has been measured with GC_MS. The results show that, at heating temperatures from 400 to 1 140℃, the δ 13C of CO-2 and CO is -22‰--27‰, and that of CH-4 -30‰--50‰. The data imply that the magma which was generated in the upper mantle by partial melting in the area has experienced the multistage degassing process, and the δ 13C values represent the carbon isotopic composition of CO-2, CO and CH-4 remaining in it, respectively.     

二氧化碳成因、成藏主控因素及脱气模式研究综述

高纯度的CO_2气藏不仅具有重要的资源意义,同时也有很重要的地质意义,CO_2气藏成因及主控因素研究已成为世界范围内地球科学领域广为关注的重要课题。对CO_2气藏的成因、鉴别方法、成藏主控等进行了系统的论述。结果表明:CO_2气藏的成因可以分为无机成因和有机成因,且以前者为主;CO_2气藏成因的判别主要是综合多种地球化学指标,结合天然气成藏地质条件来分析;高地温场、深大断裂和岩浆活动是CO_2气运聚成藏最重要、最直接的三大主控因素;幔源CO_2进入沉积盆地中具有4种脱气模式,即沿岩石圈断裂直接脱气模式、热流底辟体脱气模式、壳内岩浆房-基底断裂组合脱气模式和火山岩吸附气后期脱气模式。但在CO_2对油气运移聚集的影响、CO_2释放机理、脱气模式等方面的研究尚存一定的争议,将是未来主要的研究方向。     

相山矿田斑岩型铀矿床地球化学特征及成矿机制探讨

相山矿田北部多数铀矿床在时空和成因上与超浅成侵入相的花岗斑岩密切相关,可归属为斑岩型铀矿床.以横涧—岗上英、沙洲矿床为例,通过稀土元素、微量元素及C,O同位素地球化学特征研究,探讨铀成矿物质来源、成矿流体演化及成矿机制.研究表明,相山矿田火山岩系具同源性,是陆壳物质熔融的产物,火山岩浆在通向地表途中受到高度分馏的结晶作...     

40Ar retention in the terrestrial planets

The solid Earth is widely believed to have lost its original gases through a combination of early catastrophic release and regulated output over geologic time. In principle, the abundance of 40Ar in the atmosphere represents the time-integrated loss of gases from the interior, thought to occur through partial melting in the mantle followed by melt ascent to the surface and gas exsolution. Here we present data that reveal two major difficulties with this simple magmatic degassing scenario--argon seems to be compatible in the major phases of the terrestrial planets, and argon diffusion in these phases is slow at upper-mantle conditions. These results challenge the common belief that the upper mantle is nearly degassed of 40Ar, and they call into question the suitability of 40Ar as a monitor of planetary degassing. An alternative to magmatism is needed to release argon to the atmosphere, with one possibility being hydration of oceanic lithosphere consisting of relatively argon-rich olivine and orthopyroxene.     

Metamorphic devolatilization of subducted marine sediments and the transport of volatiles into the Earth's mantle

Volatiles, most notably CO2, are recycled back into the Earth's interior at subduction zones. The amount of CO2 emitted from arc volcanism appears to be less than that subducted, which implies that a significant amount of CO2 either is released before reaching the depth at which arc magmas are generated or is subducted to deeper depths. Few high-pressure experimental studies have addressed this problem and therefore metamorphic decarbonation in subduction zones remains largely unquantified, despite its importance to arc magmatism, palaeoatmospheric CO2 concentrations and the global carbon cycle. Here we present computed phase equilibria to quantify the evolution of CO2 and H2O through the subduction-zone metamorphism of carbonate-bearing marine sediments (which are considered to be a major source for CO2 released by arc volcanoes). Our analysis indicates that siliceous limestones undergo negligible devolatilization under subduction-zone conditions. Along high-temperature geotherms clay-rich marls completely devolatilize before reaching the depths at which arc magmatism is generated, but along low-temperature geotherms, they undergo virtually no devolatilization. And from 80 to 180 km depth, little devolatilization occurs for all carbonate-bearing marine sediments. Infiltration of H2O-rich fluids therefore seems essential to promote subarc decarbonation of most marine sediments. In the absence of such infiltration, volatiles retained within marine sediments may explain the apparent discrepancy between subducted and volcanic volatile fluxes and represent a mechanism for return of carbon to the Earth's mantle.     

Recycled dehydrated lithosphere observed in plume-influenced mid-ocean-ridge basalt

A substantial uncertainty in the Earth's global geochemical water cycle is the amount of water that enters the deep mantle through the subduction and recycling of hydrated oceanic lithosphere. Here we address the question of recycling of water into the deep mantle by characterizing the volatile contents of different mantle components as sampled by ocean island basalts and mid-ocean-ridge basalts. Although all mantle plume (ocean island) basalts seem to contain more water than mid-ocean-ridge basalts, we demonstrate that basalts associated with mantle plume components containing subducted lithosphere--'enriched-mantle' or 'EM-type' basalts--contain less water than those associated with a common mantle source. We interpret this depletion as indicating that water is extracted from the lithosphere during the subduction process, with greater than 92 per cent efficiency.     

A discontinuity in mantle composition beneath the southwest Indian ridge

The composition of mid-ocean-ridge basalt is known to correlate with attributes such as ridge topography and seismic velocity in the underlying mantle, and these correlations have been interpreted to reflect variations in the average extent and mean pressures of melting during mantle upwelling. In this respect, the eastern extremity of the southwest Indian ridge is of special interest, as its mean depth of 4.7 km (ref. 4), high upper-mantle seismic wave velocities and thin oceanic crust of 4-5 km (ref. 6) suggest the presence of unusually cold mantle beneath the region. Here we show that basaltic glasses dredged in this zone, when compared to other sections of the global mid-ocean-ridge system, have higher Na(8.0), Sr and Al2O3 compositions, very low CaO/Al2O3 ratios relative to TiO2 and depleted heavy rare-earth element distributions. This signature cannot simply be ascribed to low-degree melting of a typical mid-ocean-ridge source mantle, as different geochemical indicators of the extent of melting are mutually inconsistent. Instead, we propose that the mantle beneath approximately 1,000 km of the southwest Indian ridge axis has a complex history involving extensive earlier melting events and interaction with partial melts of a more fertile source.     

Carbon dioxide degassing flux from two geothermal fields in Tibet,China

Over geological time scales,Earth degassing has a significant impact on atmospheric carbon dioxide(CO2) concentrations,which are an important component of global carbon cycle models.In Tibet,structural conditions and associated widespread geothermal systems lead to carbon dioxide degassing during geothermal water migration.We characterized the hydrochemical conditions of two geothermal fields on the Tibetan Plateau.The chemical composition of geothermal waters was controlled by K-feldspar and albite.Geother...     

岩浆脱气动力学研究进展

岩浆脱气动力学是地球动力学的一个重要方面。介绍了H2 O和CO2 在岩浆中溶解、气泡成核、气泡生长与上升以及岩浆喷发等过程的研究成果。由前人的研究结果可知 ,压力降低使得气体在岩浆中过饱和是气泡形成的主要原因 ;气体过饱和度增大加速气体的释放和气泡的生长 ,进而导致岩浆的浮力增大 ,岩浆上升速度加快 ;气泡膨胀和加速作用发生在岩浆爆裂之前 ,并且是引起爆裂的真正原因。此外 ,CO2 的溶解度较小和岩浆中CO2 含量较高是气泡中CO2 含量高的主要原因。同时也指出 ,文献中的模型与岩浆喷发的实际情况相比 ,体系成分过于简单 ,还有待进一步完善。对岩浆脱气动力学研究作了客观评价和展望。     

Explosive volcanism may not be an inevitable consequence of magma fragmentation

The fragmentation of magma, containing abundant gas bubbles, is thought to be the defining characteristic of explosive eruptions. When viscous stresses associated with the growth of bubbles and the flow of the ascending magma exceed the strength of the melt, the magma breaks into disconnected fragments suspended within an expanding gas phase. Although repeated effusive and explosive eruptions for individual volcanoes are common, the dynamics governing the transition between explosive and effusive eruptions remain unclear. Magmas for both types of eruptions originate from sources with similar volatile content, yet effusive lavas erupt considerably more degassed than their explosive counterparts. One mechanism for degassing during magma ascent, consistent with observations, is the generation of intermittent permeable fracture networks generated by non-explosive fragmentation near the conduit walls. Here we show that such fragmentation can occur by viscous shear in both effusive and explosive eruptions. Moreover, we suggest that such fragmentation may be important for magma degassing and the inhibition of explosive behaviour. This implies that, contrary to conventional views, explosive volcanism is not an inevitable consequence of magma fragmentation.     

Seawater subduction controls the heavy noble gas composition of the mantle

The relationship between solar volatiles and those now in the Earth's atmosphere and mantle reservoirs provides insight into the processes controlling the acquisition of volatiles during planetary accretion and their subsequent evolution. Whereas the light noble gases (helium and neon) in the Earth's mantle preserve a solar-like isotopic composition, heavy noble gases (argon, krypton and xenon) have an isotopic composition very similar to that of the modern atmosphere, with radiogenic and (in the case of xenon) solar contributions. Mantle noble gases in a magmatic CO2 natural gas field have been previously corrected for shallow atmosphere/groundwater and crustal additions. Here we analyse new data from this field and show that the elemental composition of non-radiogenic heavy noble gases in the mantle is remarkably similar to that of sea water. We challenge the popular concept of a noble gas 'subduction barrier'--the convecting mantle noble gas isotopic and elemental composition is explained by subduction of sediment and seawater-dominated pore fluids. This accounts for approximately 100% of the non-radiogenic argon and krypton and 80% of the xenon. Approximately 50% of the convecting mantle water concentration can then be explained by this mechanism. Enhanced recycling of subducted material to the mantle plume source region then accounts for the lower ratio of radiogenic to non-radiogenic heavy noble gas isotopes and higher water content of plume-derived basalts.     

Non-equilibrium degassing and a primordial source for helium in ocean-island volcanism

Radioactive decay of uranium and thorium produces 4He, whereas 3He in the Earth's mantle is not produced by radioactive decay and was only incorporated during accretion-that is, it is primordial. 3He/4He ratios in many ocean-island basalts (OIBs) that erupt at hotspot volcanoes, such as Hawaii and Iceland, can be up to sixfold higher than in mid-ocean ridge basalts (MORBs). This is inferred to be the result of outgassing by melt production at mid-ocean ridges in conjunction with radiogenic ingrowth of 4He, which has led to a volatile-depleted upper mantle (MORB source) with low 3He concentrations and low 3He/4He ratios. Consequently, high 3He/4He ratios in OIBs are conventionally viewed as evidence for an undegassed, primitive mantle source, which is sampled by hot, buoyantly upwelling deep-mantle plumes. However, this conventional model provides no viable explanation of why helium concentrations and elemental ratios of He/Ne and He/Ar in OIBs are an order of magnitude lower than in MORBs. This has been described as the 'helium concentration paradox' and has contributed to a long-standing controversy about the structure and dynamics of the Earth's mantle. Here we show that the helium concentration paradox, as well as the full range of noble-gas concentrations observed in MORB and OIB glasses, can self-consistently be explained by disequilibrium open-system degassing of the erupting magma. We show that a higher CO2 content in OIBs than in MORBs leads to more extensive degassing of helium in OIB magmas and that noble gases in OIB lavas can be derived from a largely undegassed primitive mantle source.