Crustal Carbon Research Articles

Tracing magmatic footprints: Influence of a CO2-rich melt on the mineral assemblage of the San José del Guaviare Syenite, SE Colombia

A combined optical and electron microscopy (SEM, EDS, and EPMA) study was conducted on ten rock samples from the Proterozoic San José del Guaviare Syenite (SJGS), southeastern Colombia, to utilize minor and trace minerals in interpreting the magmatic evolution of these silica undersaturated rocks. The major minerals of the sampled syenites are nepheline, Fe-rich clinopyroxene, amphibole, alkali feldspar, and biotite. The minor minerals are titanite, calcite, cancrinite, and sodalite. Trace minerals are the niobium- or rare earth element-rich minerals pyrochlore, columbite, euxenite, britholite, ancylite-Ce and wöhlerite. Apatite, rhodochrosite, strontianite, fluorite, zircon, magnetite, ilmenite, and pyrite occur as traces as well. Crystallization started with primary (magmatic) calcite and Nb-rich minerals pyrochlore, columbite, euxenite, and a first generation of apatite, which occur as inclusions in foids, feldspars, and Fe-rich clinopyroxenes. Calcite is enriched in light rare earth elements and Sr, with low Mg concentrations, while primary apatite has high Sr concentrations. Both minerals have a composition typical for minerals crystallized in carbonatites. The presence of calcite and high Fe and low Ti clinopyroxene point to CO2-saturated conditions. During cooling, fluorbritholite-Ce formed as individual grains or by a fluid-enhanced apatite-britholite transformation. The formation of Fe-rich amphibole, often at the expense of Fe-rich clinopyroxene, reveals a decreasing influence of CO2 and temperature. Presumably, the transformation of orthoclase into microcline occurred simultaneously. Perthitic microcline as a second K-feldspar generation indicates slow cooling from high temperatures. A late stage of CO2-rich hydrothermal-metasomatic processes is suggested by the growth of secondary cancrinite, Sr-Mn carbonates and ancylite-Ce. The composition of primary and early crystallized calcite and apatite makes their origin as residues of an early segregated or independently formed mantle-derived carbonatitic melt more likely than crystallization from a CO2-rich syenitic melt. An origin from melted crustal carbonates is unlikely as well. Therefore, the presence of a carbonatitic melt at an early magmatic evolutive stage, as opposed to a non-carbonatitic melt at a late stage, seems possible for the SJGS rocks.

Forearc magmatism in the Pannonian basin recorded by metasomatised skarnoid xenoliths in Pliocene basalt (Novohrad-Gemer Volcanic Field, Slovakia)

Anorthite-diopside-spinel-forsterite xenoliths occur in basaltic lava flow in northern part of the Pannonian basin in the Novohrad-Gemer Volcanic Field (NGVF). Several lines of geochemical evidence suggest that the typical skarn assemblage originated by multiple carbonatitic and alkaline silicate metasomatism of pristine basalt to picrobasalt protolith rather than by the modification of crustal carbonates around shallow magma chambers. Interaction with mantle-derived magnesite or dolomite carbonatite melt triggered the crystallization of neoformed forsterite at the expense of primary clinopyroxene. The alkaline silicate metasomatism resulted in the replacement of primary diopside and augite by neoformed aegirine and aegirine-augite accompanied by amphiboles (richterite, katophorite, arfvedsonite). Weakly metasomatised Group 1 xenoliths retained their fore-arc magmatic geochemical signatures: flat REE patterns, low concentrations of high-field strength elements (Ti, P, Zr) and Ni compared to MORB, low ratios of highly-to-less incompatible elements (Ti/V, Zr/Y) relative to MORB, low Na2O, low total REE, and supra-subduction affinity of primary clinopyroxenes, being thus analogues to fore-arc basalts (FAB). In contrast, strongly metasomatised Group 2 xenoliths exhibit LREE-enriched patterns sub-parallel with those of host trachybasalts. Some Group 1 xenoliths also exhibit large positive Eu anomalies and strongly depleted signatures diagnostic of low-pressure plagioclase-pyroxene cumulates. One xenolith with complementary negative Eu, Sr and positive Rb anomalies has been identified as high-Mg basaltic andesite melt residuum. Sr and Nd isotopes plot along the OIB-MORB array. The Group 1 converges at initial 87Sr/86Sr and 143Nd/144Nd ratios of 0.7038–0.7041 and 0.5127–0.5128, respectively, thus recording a 140–190 Ma old mantle source. Thermobarometry data on metasomatically unmodified pyroxenes have been arranged along decompression cooling trend, propagating from ∼1230 °C and ∼ 1 GPa and ending at ∼1160 °C around atmospheric pressure. The maximum pressure indicates the melt generation zone ∼35 km deep, reflecting thin lithospheric mantle. The FABs identified for the first time in the AlCaPa megaunit corroborate the affinity of the NGVF lithospheric segment to Dinarides.

Massive crustal carbon mobilization and emission driven by India underthrusting Asia

The active Himalayan-Tibetan orogen, where India underthrusts into Asia, is an important geological source of carbon dioxide (CO2) emission into Earth’s atmosphere. However, the extent to which Indian underthrusting could stimulate the mobilization of deeply-sourced carbon and its subsequent emission remains unknown. Here, we use a combination of field observations coupled with in-situ CO2 flux measurements and helium and carbon isotopic data, to study the controls on CO2 origins and fluxes in a 400-kilometre-long rift transecting northern Himalaya and southern Tibet. High diffuse CO2 fluxes sustained by pure crustal fluids are confined to rift segments in the northern Himalaya, while toward southern Tibet, CO2 fluxes become lower but mantle fluid inputs are identified. Such rift-related CO2 degassing profile suggests metamorphic decarbonation and release of carbon-bearing fluids enhanced by the underthrusting Indian lower crust, agreeing well with Himalayan metamorphism and orogen-parallel lithospheric extension. Deep CO2 fluxes from extensional tectonics in northern Himalaya and southern Tibet, primarily of crustal origins, are comparable to mantle CO2 fluxes from global mid-ocean ridges. Our findings demonstrate that geophysical and geo-tectonic responses to continental underthrusting could facilitate massive crustal carbon mobilization and emission, making active collisional orogens globally important carbon sources.

Carbon-rich polyphasic inclusions in postcollisional mafic magmatic rocks from the Dabie Shan, China: Implications for the carbon cycle in continental subduction zones

Subduction is a fundamental geodynamic process that transfers carbon from Earth’s surface into the mantle. However, current understanding of the migration mechanisms, final storage region, and species involved in carbon recycling from continental crust remains limited. Here, we investigated the compositions of polyphasic inclusions and Mg isotopes in postcollisional mafic magmatic rocks from the Dabie Shan region of China. The main rock-forming minerals contained two distinct types of polyphasic inclusions, which displayed systematic differences in daughter mineral/gaseous phase assemblages, including host-like silicates ± carbonates (magnesite, dolomite, and calcite) + CH4 and carbonates + talc ± SiO2 (aqueous) + CH4, respectively. These inclusions indicate that carbon-rich silicate melts and carbon-rich magmatic fluids were trapped by host minerals during magmatic processes. The abundant carbonates and CH4 in both types of inclusions suggest that the mantle source of these postcollisional mafic magmatic rocks was rich in carbon, most likely existing in the forms of CO2 and CH4. Moreover, the studied postcollisional mafic magmatic rocks have mantle-like Mg isotope compositions, with δ26Mg values ranging from −0.23‰ to −0.16‰. The combined observations of polyphase inclusions and Mg isotopes indicate that a substantial carbon-rich mantle domain arose from the metasomatism of silicate melts derived from subducted continental slabs that had dissolved a certain quantity of CO2 and CH4. We proposed that continental subduction is an efficient pathway for transporting crustal carbon into an orogenic subcontinental lithospheric mantle wedge, where the recycled carbon can be stored for >100 m.y. and eventually released to the surface during postcollisional magmatism.

Crustal carbonate build-up as a driver for Earth’s oxygenation

Oxygenation of Earth’s atmosphere and oceans played a pivotal role in the evolution of the surface environment and life. It is thought that the rise in oxygen over Earth’s history was driven by an increasing availability of the photosynthetic limiting nutrient phosphate combined with declining oxygen-consuming inputs from the mantle and crust. However, it has been difficult to assess whether these processes alone can explain Earth’s oxygenation history. Here we develop a theoretical framework for the long-term global oxygen, phosphorus and carbon cycles, incorporating potential trajectories for the emergence of continents, the degassing of mantle volatiles and the resulting increase in the size of the crustal carbonate reservoir. We find that we can adequately simulate the Earth’s oxygenation trajectory in both the atmosphere and oceans, alongside reasonable reconstructions of planetary temperature, atmospheric carbon dioxide concentration, phosphorus burial records and carbon isotope ratios. Importantly, this is only possible when we include the accumulation of carbonates in the crust, which permits ever-increasing carbon recycling rates through weathering and degassing. This carbonate build-up is a missing factor in models of Earth’s coupled climate, nutrient and oxygen evolution and is important for reconstructing Earth’s history and potential exoplanet biogeochemistry.

Mechanism of carbonate assimilation by intraplate basaltic magma and liquid immiscibility: example of Wangtian’e volcano (Changbaishan volcanic area, NE China)

The balance of CO2 during abundant basaltic magma production is an important factor of volcanic hazards and climate. In particular, this can be explored based on CO2-rich mantle-derived magmas or carbonate assimilation by basaltic melts. To reconstruct the origin of Fe-rich carbonates hosted by Cenozoic basalts from Wangtian’e volcano (northeast China), we studied elemental compositions of melt, crystalline and fluid inclusions in magmatic minerals as well as the oxygen and carbon isotope compositions of the plagioclase and carbonates from basalts. The crystallization of basaltic magmas occurred in shallow chamber (∼4 km) at temperatures of 1,180°C–1,200°C and a pressure of 0.1 ± 0.01 GPa. Stable Fe-rich carbonates occur in the Wangtian’e tholeiite basalts as groundmass minerals, crystalline inclusions in plagioclase and globules in melt inclusions, which suggests that they crystallized from a ferrocarbonate melt. The values of δ18О and δ13С in the minerals analyzed by laser fluorination method are in line with the sedimentary source of Fe-rich carbonates, indicating assimilation and partial decomposition of carbonate phases. The parent ferrocarbonate melt could be produced during interactions between the basaltic magma and the crustal marbles. The phase diagram and thermodynamic calculations show that the ferrocarbonate melt is stable at a temperature of 1,200°C and a pressure of 0.1 GPa. Our thermodynamic calculations show that carbonate melt containing 73 wt% FeCO3, 24 wt% MgCO3 and 3 wt% CaCO3 is in thermodynamic equilibrium with silicate melt in agreement with our natural observations. The proposed mechanism is crustal carbonate sediment assimilation by the intraplate basaltic magma resulting in the melt immiscibility, production of the ferrocarbonate melt and the following Fe-rich carbonate mineral crystallization during magma residence and cooling.

Magma-carbonate interactions drive CO2 production and metal enrichment in shallow dikes and sills at volcanic arcs

Abstract The contribution of CO2 from crustal carbonates into arc magmas is debated, as is its role in the long-term C cycle. To better understand the contributions and mechanisms that drive CO2 production in arc magmas, we examined in detail basaltic dike and sill contacts with carbonate in the Jurassic Bonanza arc on Vancouver Island, Canada. We discovered discrete boundary melts that formed along dike and sill margins in contact with limestone, which display unique Ca, U, and Sr enrichments, Si depletion, and 87Sr/86Sr that approaches host limestone values (~0.708). Binary mixing modeling indicates ~20%–25% limestone assimilation into basalt formed the boundary melts. Contrasting viscosities between boundary and interior melts hinder mixing and chemical homogenization but appear to promote uphill diffusion and metal enrichment within systems that cool in minutes to days. While shallow dikes and sills may be volumetrically minor in an arc magma system, the open flow of magma and large surface area in channels greatly enhances magma-carbonate interactions, and ultimately CO2 production, likely over that of more common and voluminous plutons.

Age and Geochemistry of High Arctic Large Igneous Province Tholeiitic Magmatism in NW Axel Heiberg Island, Canada

AbstractThe Cretaceous High Arctic Large Igneous Province (HALIP) in Canada involved extrusion of continental flood basalts (CFBs) at 130–120 Ma and 100‐95 Ma and emplacement of an extensive sill and dike network that intersected the Carboniferous to Paleogene Sverdrup Basin. In this paper, we present new 40Ar/39Ar ages, major and trace elements, and Sr‐Nd‐Pb isotope ratios for HALIP lava, dikes, and sills from Bukken Fiord, NW Axel Heiberg Island, Canadian Arctic Islands. Our best constrained 40Ar/39Ar ages yield a weighted average of 124.1 ± 1 (2σ) Ma, coincident with the first pulse of tholeiitic CFB magmatism in the Arctic‐wide HALIP as exemplified by Isachsen Formation flood basalts on Axel Heiberg Island. The Bukken Fiord samples are plagioclase and clinopyroxene‐phyric tholeiitic basalts, are relatively evolved (3.2–6.5 wt% MgO), and share similar major and trace element compositions to typical HALIP tholeiites. Initial 143Nd/144Nd ranges from 0.51260 to 0.51291 and initial 87Sr/86Sr ranges from 0.70362 to 0.70776, while measured 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb range from 18.614 to 19.199, 15.534 to 15.630, and 38.404 to 39.054, respectively. The most primitive sample in this study has Sr‐Nd‐Pb isotope signatures that suggest an enriched plume‐derived mantle source for HALIP tholeiites. Most samples, however, possess relatively radiogenic isotope signatures that can be explained by moderate degrees of assimilation of Sverdrup Basin sedimentary rocks. Magma‐crust interaction in the HALIP plumbing system was likely widespread and may have increased the environmental impact of the HALIP, particularly if crustal carbon was volatilized.

GEOCHEMICAL AND Sr-Nd ISOTOPE SYSTEMATICS OF THE LATE PERMIAN–EARLY TRIASSIC TRAPS FROM THE KUZNETSK BASIN: MAGMA SOURCES AND CORRELATION WITH THE NORIL’SK REGION VOLCANICS

The study provides whole-rock geochemical and Rb-Sr and Sm-Nd isotope examinations of Permian-Triassic volcanics (basalts, basaltic andesites) and sills (trachydolerites, monzodiorites) from the Kuznetsk Basin, which is part of the Siberian Large Igneous Province. It is shown that the volcanic section of the Kuznetsk Basin consists of the Lower (I and II units: TiO2 = 1.6–1.9 wt.%, Gd/Yb = 2.0–2.1) and Upper (III–VIII units: TiO2 = 1.4–1.8 wt.%, Gd/Yb = 1.7–1.9) Sequences. The Mayzass sill is geochemically correlated with the Unit II basaltic andesite of the Lower Sequence, and the Syrkashev sill is geochemically akin to the Upper Sequence. The Kuznetsk traps originated from the partial melting of a Sr-Nd isotopically heterogeneous subduction-modified lithospheric mantle. They are geochemically comparable to low-Ti basalts of the low–middle Nadezhdinsky suite from the Noril’sk region, but differ in Sr-Nd isotope composition. The Sr-Nd isotope signatures of the uncontaminated lavas and Syrkashev sill rocks (εNd(T) = (+4.6)–(+2.4), (87Sr/86Sr)T = 0.7047–0.7054) are inherited from the ancient lithospheric mantle and do not represent the contribution of the Siberian plume. The Mayzass sill rocks (εNd(T) = (+2.2)–(+2.1); (87Sr/86Sr)T = 0.7055–0.7063)are derived from a parental melt contaminated with crustal carbonates.

DIAMOND CRYSTALLIZATION AND PHASE COMPOSITION IN THE FeNi – GRAPHITE – CaCO3 SYSTEM AT 5.5 Gpa

An experimental simulation of diamond crystallization in the system FeNi - graphite - calcium carbonate at a pressure of 5.5 GPa and a temperature of 1400℃ was carried out. Two sample assembly configurations were used. In the first one – the starting materials were put layer by layer, and in the second one - the components were mixed. It has been established that calcium carbonate, when interacting with the FeNi-melt, decomposes with the formation of Ca,Fe oxides and the release of CO2. Magnetite may be present as an accessory phase. Due to the formation of solid reaction products (Ca,Fe oxides) during layer-by-layer filling of the growth volume, the presence of calcium carbonate between graphite and FeNi-melt prevents diamond crystallization in the graphite layer and carbon transport to diamond seed crystals. When the components are mixed in the growth volume, diamond synthesis and growth onto seed crystals occur. The phenomenon of segregation of diamond crystals together with calcium carbonate and oxide phases, the products of the reaction in the bulk of the metal, has been discovered. Aliphatic, cyclic, and oxygenated hydrocarbons, including heavy compounds (C13-C17), CO2, H2O, nitrogen- and sulfonated compounds, were identified in the fluid phase captured by diamonds in the form of inclusions during growth. The composition of the fluid phase in the studied diamonds is more oxidized compared to the composition of fluid inclusions in diamonds grown in the FeNi – graphite system without carbonate. The results obtained correlate with the data on natural diamonds, among which there are crystals with “essentially carbon dioxide” compositions of fluid inclusions, which indicates the possible participation of crustal carbonate matter in the processes of diamond formation during subduction into the deep mantle.

Recycled Carbonate‐Bearing Silicate Sediments in the Sources of Circum‐Mediterranean K‐Rich Lavas: Evidence From Mg‐Zn Isotopic Decoupling

AbstractThe high flux of subaerial volcanic CO2 emissions around the circum‐Mediterranean region requires the involvement of an unusually carbon‐rich reservoir, but the origin of which is still unclear. Here, we aim to resolve this problem by analyzing Mg and Zn isotopes for the widely distributed mafic potassic to ultrapotassic lavas in this region. These K‐rich lavas have lower δ26Mg but similar δ66Zn compared to mid‐ocean ridge basalts (MORB). No known magmatic processes can explain the isotopic data, which must therefore be characteristics of the mantle sources. Recycled carbonate sediments are capable of explaining the low δ26Mg, but they typically also have high δ66Zn. Thus, the low δ26Mg but unfractionated δ66Zn of these K‐rich lavas define “Mg‐Zn isotopic decoupling” which has not yet been observed for other types of mantle‐derived lavas. The carbonate‐bearing silicate sediments analyzed here possess low δ26Mg and MORB‐like δ66Zn, which can account for the Mg‐Zn isotopic decoupling. Therefore, the nature of recycled materials (carbonates vs. carbonate‐bearing silicate sediments) in the mantle can be distinguished by the coupling or decoupling of Mg and Zn isotopes of mantle‐derived magmas. The input flux of carbon from the sediments to the lithospheric mantle is estimated to be ∼8.1 Mt/yr, and ∼22.4 Mt/yr of CO2 emissions are predicted, which fit well with the observed output flux of 20.1 ± 13.4 Mt/yr. Our results demonstrate that recycled crustal carbon stored in the lithospheric mantle is an important source for the extensive subaerial volcanic CO2 emissions in the circum‐Mediterranean region.

The heterogeneity of the Mexican lithospheric mantle: Clues from noble gas and CO2 isotopes in fluid inclusions

The abundance of mantle-derived rocks and lavas, in combination with its tectonic evolution, render Mexico a perfect laboratory to investigate the chemical and the isotopic heterogeneity of the lithospheric mantle. New data on the composition of noble gases and CO2in Mexican mantle xenoliths and lavas is reported. Our samples consist of six ultramafic nodules from the Durango Volcanic Field (DVF) and the San Quintin Volcanic Field (SQVF), monogenetic complexes belonging to the Mexican Basin and Range province; and four lavas from the Sierra Chichinautzin (SCN), a Quaternary monogenetic volcanic field located in the Mexican volcanic arc. Ne and Ar isotopes in fluid inclusions reveal mixing between atmospheric and MORB-like fluids (e.g.,40Ar/36Ar < 1,200). DVF and SQVF nodules record low40Ar/36Ar and4He/20Ne that confirm the existence of recycled atmospheric-derived noble gases in the local mantle. The averages of the Rc/Ra ratios (3He/4He corrected for atmospheric contamination) measured in Mexican localities are within the MORB-like range: DVF= 8.39 ± 0.24 Ra, SQVF = 7.43 ± 0.19 Ra and SCN lavas = 7.15 ± 0.33 Ra (1σ). With the aim of assessing the isotopic variability of the Mexican lithospheric mantle, the above results were compared with similar data previously obtained from ultramafic nodules found in the Ventura Espiritu Santo Volcanic Field (VESVF), another Quaternary monogenetic volcanic complex belonging the Basin and Range. The higher3He/4He ratios in DVF relative to those reported for the VESVF and the SQVF are explained as reflecting different ages of mantle refertilization, triggered by the retreating of the Farallon slab (∼40 Ma ago) and associated delamination slab processes. We propose that the DVF mantle was refertilized more recently (<10 Ma ago) than the mantle beneath the SQVF and VESVF (∼40–20 Ma ago). On the other hand, He-Ne-Ar compositions of SCN olivines share similarities with VESVF xenoliths, suggesting a relatively homogeneous lithospheric mantle in central Mexico. Finally, DVF and the SCN samples exhibit δ13C values within the MORB range (comparable to other values previously reported in fluid inclusions and fumaroles from Popocatépetl, Colima—Ceboruco volcanoes). While we explain the MORB-like carbon signatures of the DVF samples as the result of the above-mentioned refertilization process, the SCN signatures likely reflect either (i) trapping of isotopically fractionated CO2derived from magmatic degassing or (ii) a mantle source unaffected by subduction-related crustal carbon recycling.

The carbon budget of crustal reworking during continental collision: Clues from nanorocks and fluid inclusions

The source of volatiles in the continental crust is a long-standing issue. In addition to controlling the amount of melt generated during anatexis, H2O and CO2 budgets of the middle and lower siliciclastic crust are also of great importance for carbonate precipitation, ore concentration, orogenic degassing and carbon storage. Here we focus on two case studies of partially melted metamorphic rocks of crustal affinity– the Ivrea Zone in the Western Alps (0.8 GPa) and the Central Maine Terrane (1.8 GPa), USA. These terrains contain fluid inclusions and carbon-bearing nanogranitoids (former melt inclusions) from which the H2O and CO2 content has been estimated via in-situ analyses. Thermodynamic modelling is used to quantify the amount of internally derived, mineral-bound bulk rock CO2 necessary to reproduce the volatile contents of these melt inclusions. The minimum amount of bulk rock CO2 present at peak metamorphic conditions is estimated at 400 ppm for the Ivrea Zone and 3000 ppm for the Central Maine Terrane. This suggests that the flux of carbon associated with the burial of siliciclastic sediments in the lower crust during the Phanerozoic is 0.2–4.4 Mt. C/yr. These values, as well as the nature of the source of the deep crustal carbon might have changed with time, with periods dominated by internal reworking rather than external inputs. The protracted growth and differentiation of the continental crust through the reworking of supracrustal materials in continental collision settings is a key element of carbon storage processes. The stability of the continental crust through time provides an ultimate, long-lasting reservoir of carbon.

Deep CO2 and N2 emissions from Peruvian hot springs: Stable isotopic constraints on volatile cycling in a flat-slab subduction zone

Gas-rich hot springs throughout the Peruvian Andes contain a surprising contribution of mantle and crustal volatiles (CO2 and N2) despite being located along a volcanic gap associated with modern flat-slab subduction. Similar mantle and crustal volatile degassing is observed in springs from the backarc region of the northern Altiplano Plateau which experienced flat-slab subduction in the Oligocene. We constrain the sources of deeply-derived volatiles using C, N, O and H stable isotope analyses alongside previously reported helium isotope (3He/4He) and gas abundance data. δ18O and δ2H values of spring waters are consistent with regional meteoric water, with deviations towards higher values due to fluid mixing and high-temperature fluid-rock interaction. δ15N values of N2 gas range from −0.5 to +4.4‰ (vs AIR), indicating contributions of mantle and crustal (sedimentary) nitrogen. δ13C values of dissolved inorganic carbon (DIC) and CO2 gas from bubbling springs are used to model initial δ13C values before degassing, with resulting values ranging from −13.6 to −0.3‰ (vs VPDB) and an average value of −6.9‰. DIC concentrations range from 1 to 57 mmol/kg, of which carbonate dissolution accounts for only a small fraction (average of 15%). The remaining carbon (1 to 44 mmol/kg) is derived from a mixture of deep CO2 sources with endmember δ13C values of −7‰ and −14‰ which, in light of relationships with 3He/4He ratios, are interpreted to be the subcontinental lithospheric mantle (SCLM) and metamorphic crustal carbon, respectively. These results are consistent with previous interpretations that slab-to-lithosphere fluid transfer is mobilizing volatiles from the SCLM to the continental crust above the flat slab and confirms that mantle CO2 is migrating with mantle helium. Metamorphic CO2 and N2 are mobilized from the continental crust and entrained by mantle fluids on their way to the surface. In the backarc region, similar geochemical patterns suggest that volatiles are released by dehydration and/or partial melting of previously hydrated lithosphere. We estimate a CO2 flux of ~3 × 108 mol yr−1 (34 t d−1) from 45 springs with available discharge values. If scaled to all reported thermal springs in Peru, then the total CO2 emissions from Peruvian thermal springs may approach 2 × 109 mol yr−1 (203 t d−1), or ~ 0.2% of global emissions from subaerial volcanism. Regional CO2 emissions may be 10 to 100 times greater when considering CO2 diffusely lost along fault zones or temporarily dissolved near-surface aquifers. These results demonstrate that flat-slab subduction leads to an efficient and unexpected transfer of mantle and crustal volatiles to Earth's surface, and more generally, that deep volatile fluxes in subduction zones are not limited to active volcanism.

Carbonate dissolution and replacement by odinite and saponite in the Lafayette nakhlite: Part of the CO2-CH4 cycle on Mars?

New mineralogical studies of Lafayette reveal that it contains a notably variable abundance of martian carbonate. Four percent was identified in mesostasis (3.2%) and olivine-hosted (0.8%) fractures in one polished section, but only 0.2% of both textural types in another. The Lafayette carbonates are Mg0.0-2.0Cc13.2-38.6Sd17.7-81.9Rh3.1-42.9. They have undergone variable but extensive amounts of dissolution and replacement as the nakhlite secondary fluid evolved, associated with the precipitation of ferric saponite in olivine fractures and a serpentine-like phyllosilicate in the mesostasis. The mesostasis carbonate has undergone the highest degree of corrosion and replacement. TEM analysis has shown the presence of Fe-(hydr)oxide (likely ferrihydrite) nanoparticles on olivine-hosted carbonates which can be linked to the cessation of more extensive carbonate dissolution at those sites. The mesostasis serpentine-like mineral has been described here on the basis of WDS and EDX analyses, HRTEM and Fe-K XANES, as odinite, a ferric, 0.7 nm d001-spacings phyllosilicate mineral with a characteristic 1:1 serpentine-like structure. The carbonate dissolution stage and then formation of Fe-(hydr)oxide nanoparticles occurred under circumneutral-alkaline conditions (7) < pH < 10. This range of pH is also where the general dissolution mechanism switched from a proton-promoted, to a water hydrolysis reaction associated with a reduction in the dissolution rates. As dissolution rates were reduced and the fluid had cooled to ≤50 °C, the precipitation of the ferric saponite and odinite, a phyllosilicate associated with temperatures of ∼25 °C, dominated over the carbonate dissolution. The extensive dissolution of such crustal carbonate across the upper martian crust, producing bicarbonate and carbon dioxide, and the coupled formation of ferric phyllosilicates, would lead to the formation of CH4 in substantial amounts via a Fischer-Tropsch type reaction. The results of our study illustrate a process to explain the relatively low abundance of detected carbonate on Mars and a likely source for some of the methane on Mars.

Geochemical, isotopic and U-Pb geochronological investigation of the late Cretaceous Karaçayır carbonatite (Sivas, Turkey): Insights into mantle sources within a post-collisional tectonic setting

The Karaçayır carbonatite is located within the northeastern edge of the Central Anatolian Crystalline Complex, and represents one of the significant rare earth element (REE) mineralizations in Turkey. The carbonatite intrudes the associated syenite as dikes and veins, and is mainly composed of medium- to coarse-grained calcite, and lesser amounts of apatite, mica, amphibole, pyroxene, and magnetite. This study reports new and combined geochemical, radiogenic (Sr, Pb), stable (O and C), and for the first time B and Nd isotopic ratios and in-situ U-Pb age data from the Karaçayır carbonatite. The carbonate separates from the Karaçayır carbonatite are characterized by high Ba (30 to 484 ppm), Sr (613 to 9106 ppm) and total rare earth element (TREE) concentrations (172 to 2892 ppm), which are consistent with geochemical signatures for mantle-derived carbonate melts. The combined δ18OV-SMOW (+10.7 to +19.0 ‰), δ13CV-PDB (−2.9 to +1.8 ‰) and δ11B (−13.9 to −2.9 ‰) data for a majority of the carbonate separates investigated here in contrast indicate that they are the result of open system behavior involving contamination by the country rock (marble), and/or sub-solidus, low temperature alteration. Based on the combined stable and radiogenic isotopic signatures for the least altered/contaminated samples, the carbonatite magma at Karaçayır most likely represents a low-degree partial melt of an isotopically heterogeneous (metasomatized) mantle source containing EM (enriched mantle)-like components and recycled crustal carbon; the latter finding is consistent with those recorded for young (<300 Ma old) carbonatites worldwide. A weighted mean 206Pb/238U age of 69.3 ± 1.7 Ma is obtained for apatite from the Karaçayır carbonatite, and indicates its emplacement within a post-collisional tectonic setting during the late Cretaceous.

Early Habitability and Crustal Decarbonation of a Stagnant‐Lid Venus

AbstractLittle is known about the early evolution of Venus and a potential habitable period during the first 1 billion years. In particular, it remains unclear whether or not plate tectonics and an active carbonate‐silicate cycle were present. In the presence of liquid water but without plate tectonics, weathering would have been limited to freshly produced basaltic crust, with an early carbon cycle restricted to the crust and atmosphere. With the evaporation of surface water, weathering would cease. With ongoing volcanism, carbonate sediments would be buried and sink downwards. Thereby, carbonates would heat up until they become unstable and the crust would become depleted in carbonates. With supply to the atmosphere the surface temperature rises further, the depth below which decarbonation occurs decreases, causing the release of even more . We assess the habitable period of an early stagnant‐lid Venus by employing a coupled interior‐atmosphere evolution model accounting for degassing, weathering, carbonate burial, and crustal decarbonation. We find that if initial surface conditions allow for liquid water, weathering can keep the planet habitable for up to 900 Myr, followed by evaporation of water and rapid crustal carbonate depletion. For the atmospheric of stagnant‐lid exoplanets, we predict a bimodal distribution, depending on whether or not these planets experienced a runaway greenhouse in their history. Planets with high atmospheric could be associated with crustal carbonate depletion as a consequence of a runaway greenhouse, whereas planets with low atmospheric would indicate active silicate weathering and thereby a habitable climate.

Recycled crustal carbon in the depleted mantle source of El Hierro volcano, Canary Islands

The Canary Islands, in the eastern Atlantic, are among the most enigmatic Oceanic Island provinces on Earth, as the mantle source feeding its volcanism exhibits wide spatial heterogeneity and a multiplicity of sources. Multi-isotope whole-rock studies have long revealed the presence of a recycled oceanic crust/lithosphere component in the mantle source. However, noble gas systematics have been more challenging to interpret, and the available carbon isotope data is limited and cannot support/dismiss this interpretation. Here, we present the very first isotopic characterisation of CO2 and noble gases (He-Ne-Ar) in fluid inclusions (FI) in minerals hosted in mantle xenoliths from El Hierro, the youngest and westernmost island of the Canary volcanic archipelago. Six fresh xenoliths from El Julan cliff valley were analysed (3 spinel lherzolites and 3 spinel harzburgites). We find carbon isotopic compositions of CO2 in FI (δ13C) ranging from −2.38 to −1.23‰ in pyroxenes and from −0.19 to +0.96‰ in olivines. These unusually positive δ13C values, well above the typical mantle range (−8‰ < δ13C < −4‰), prove, for the first time, the presence of a recycled crustal carbon component in the local source mantle. We interpret this 13C-rich component as inherited from a mantle metasomatism event driven by fluids carrying carbon from C. In contrast, our El Hierro xenoliths identify a depleted mantle-like He signature, with an average Rc/Ra ratio (3He/4He normalised to air ratio and corrected for atmospheric contamination) of 7.45 ± 0.26 Ra. The involvement of depleted mantle-like fluids, variably admixed with air-derived components (possibly recycled via paleo-subduction event(s)), is corroborated by Ne-Ar isotopic compositions. The depleted mantle-like He signature suggests instead the involvement of a primordial He source in the local lithospheric mantle and indicates a marginal role played by past subduction events in modifying the local mantle He budget. When put in the context of previous 3He/4He measurements in FI and surface gases along the Canary archipelago, our results confirm an overall west-to-east decrease of Rc/Ra ratios, which may be interpreted as due to increasing contributions from the African sub-continental mantle, the addition of radiogenic 4He during magma migration in the oceanic crust (whose thickness increases eastward) and/or magma ageing.

Temporal evolution of δ44/40Ca and 87Sr/86Sr of carbonatites: Implications for crustal recycling through time

Carbonatites are igneous rocks composed of over 50% modal carbonate minerals and their exact origin and possible mode(s) of formation are still debated. We report stable Ca isotopic compositions (δ44/40CaSRM915a) of globally distributed carbonatites (n = 46) from 22 different locations ranging in age from 2.61 Ga to recent along with their major, trace element concentrations and 87Sr/86Sr ratios. The δ44/40CaSRM915a values for carbonatites older than 300 Ma, except for four samples from the Neoproterozoic south Indian carbonatites and the Phalaborwa complex of South Africa, display limited variation (~0.18‰, n = 12) with δ44/40Ca values overlapping (given their associated analytical uncertainties) with the estimated δ44/40CaSRM915a value of the bulk silicate Earth (BSE, 0.94 ± 0.10‰). The 87Sr/86Sr(t) of these samples (0.70149–0.70394) are consistent with derivation from a depleted mantle source with a time-integrated Rb/Sr ratio of ~0.017. In contrast, carbonatites with emplacement ages younger than 300 Ma display much wider variations in δ44/40Ca values (0.63‰, n = 29) with sixteen samples displaying values lower than that for the BSE and radiogenic 87Sr/86Sr compositions (0.70313–0.71044). The low δ44/40Ca values for the younger carbonatites are consistent with the presence of recycled crustal carbonates, with compositions similar to those of 100–200 Ma and 400–500 Ma old carbonates, in their mantle source, which is also consistent with their radiogenic 87Sr/86Sr ratios. Model calculations, based on δ44/40Ca and 87Sr/86Sr(t) ratios for mantle peridotite and eclogite sources and marine carbonates of different ages, suggest that the δ44/40Ca and 87Sr/86Sr for most young carbonatites (<300 Ma) examined here can be explained by the presence of <10 wt% recycled crustal carbonates in their mantle source.The contrasting Ca and Sr isotope compositions between younger (<300 Ma) and older (>300 Ma) carbonatites may reflect changing geodynamic conditions. The prevalent Ca and Sr isotope signatures attributed to crustal recycling that is observed in carbonatites younger than 300 Ma most likely reflect two global-scale processes: (i) an increased amount of subduction flux due to high convergence rates subsequent to the break-up of the Pangea supercontinent, and (ii) greater amount of plume activity during the last 300 million years of Earth history. Additionally, the low δ44/40Ca values of younger carbonatites could have been accentuated by subduction of marine carbonates (aragonitic) with relatively low δ44/40Ca. In contrast to the younger carbonatites, Ca and Sr isotope compositions for the carbonatites older than 300 Ma investigated here suggest that crustal recycling was much more sporadic in the Precambrian.

The composition of fluids stored in the central Mexican lithospheric mantle: Inferences from noble gases and CO2 in mantle xenoliths

We present the first isotopic (noble gases and CO2) characterization of fluid inclusions coupled to Raman microspectroscopy analyses in mantle xenoliths from Central Mexico, a geodynamically complex area where the Basin and Range extension was superimposed on the Farallon subduction (terminated at 28 Ma). To characterize the isotopic signature of the Central Mexican lithospheric mantle, we focus on fluid inclusions entrapped in mantle xenoliths found in deposits of the Joya Honda maar (JH), a Quaternary monogenetic volcano belonging to the Ventura Espiritu Santo Volcanic Field (VESVF) in the state of San Luis Potosí (central Mexico). Thirteen ultramafic plagioclase-free xenoliths were selected, all exhibiting a paragenesis Ol > Opx > Cpx > > Sp, and being classified as spinel-lherzolites and harzburgites. All xenoliths bring textural evidence of interstitial glass veins bearing dendritic trails of secondary melt and fluid inclusions (composed of silicate glass ± CO2 ± Mg-Ca carbonates ± pyrite). These are related to pervasive mantle metasomatism driven by carbonate-rich silicate melt. The Ar and Ne systematics reflect mixing between MORB-like upper mantle and atmospheric fluids, the latter interpreted as reflecting a recycled air component possibly inherited from the Farallon plate subduction. The 3He/4He ratios vary between 7.13 and 7.68 Ra, within the MORB range (7–9 Ra), and the 4He/40Ar* ratios (0.4–3.11) are similarly close to the expected range of the fertile mantle (1–5). Taken together, these pieces of evidence suggest that (i) either the mantle He budget was scarcely modified by the Farallon plate subduction, and/or (ii) that any (large) crustal contribution was masked by a later metasomatism/refertilization episode, possibly during the subsequent Basin and Range extension. A silicate melt-driven metasomatism/refertilization (revealed by the association between glass veins and fluid inclusions) is consistent with calculated helium residence time for the Mexican lithospheric mantle (20 to 60 Ma) that overlaps the timing of the above geodynamic events. We propose that, after the refertilization event (e.g., over the last ~20 Ma), the lithospheric mantle has evolved in a steady-state, becoming slightly more radiogenic. We also estimated 3He fluxes (0.027–0.080 mol/g), 4He production rates (340–1000 mol/yr), and mantle CO2 fluxes (3.93 × 107 mol/yr to 1.18 × 108 mol/yr) using the helium isotopic values measured in JH mantle xenoliths. Finally, the JH xenoliths exhibit CO2/3He ratios comparable to those of the upper mantle (from 3.38 × 108 to 3.82 × 109) but more positive δ13C values (between −1.0 and − 2.7‰), supporting the involvement of a crustal carbonate component. We propose that the metasomatic silicate melts recycled a crustal carbonate component, inherited by the Farallon plate subduction.