Igor Tolstikhin

CO2 fluxes from mid-ocean ridges, arcs and plumes

Abstract Estimates of CO2 emissions at spreading centres, convergent margins, and plumes have been reviewed and upgraded using observed CO2/3He ratios in magmatic volatiles, 3He content estimates in the magmatic sources, and magma emplacement rates in the different tectonic settings. The effect of volatile fractionation during magma degassing, investigated using new rare gas and CO2 abundances determined simultaneously for a suite of Mid-Ocean Ridge (MOR) basalt glasses, is not the major factor controlling the spread of data, which mainly result from volatile heterogeneity in the mantle source. The computed C flux at ridges (2.2±0.9)×1012 mol/a, is essentially similar to previous estimates based on a more restricted data base. Variation of the C flux in the past can be simply scaled to that of spreading rate since the computed C depends mainly on the volatile content of the mantle source, which can be considered constant during the last 108 a. The flux of CO2 from arcs may be approximated using the CO2/3He ratios of volcanic gases at arcs and the magma emplacement rate, assuming that the 3He content of the mantle end-member is that of the MORB source. The resulting flux is ∼2.5×1012 mol/a, with approx. 80% of carbon being derived from the subducting plate. The flux of CO2 from plumes, based on time-averaged magma production rates and on estimated contributions of geochemical sources to plume magmatism, is ≤3×1012 mol/a. Significant enhancements of the CO2 flux from plumes might have occurred in the past during giant magma emplacements, depending on the duration of these events, although the time-integrated effect does not appear important. The global magmatic flux of CO2 into the atmosphere and the hydrosphere is found to be 6×1012 mol/a, with a range of (4–10)×1012 mol/a. Improvement on the precision of this estimate is linked to a better understanding of the volatile inventory at arcs on one hand, and on the dynamics of plumes and their mantle source contribution on the other hand.

Two terrestrial lead isotope paradoxes, forward transport modelling, core formation and the history of the continental crust

A combined solution for the two major terrestrial lead paradoxes has been sought. These are the ‘future’ paradox (upper crustal and upper mantle Pb isotope compositions plot in the future field in 207pb/204Pb vs. 206Pb/204Pb space) and the Th/U mantle paradox (The Th/U ratio of the upper mantle is ca. 2.6, whereas Pb isotopes indicate a value of 3.8). Constraints considered other than UThPb data include siderophile element concentrations of the mantle (which restrict core formation scenarios), W isotopes and the oldest lunar ages (bracketing the accretion and core formation time scale to 60–100 Ma), and noble gas systematics (requiring a two-layered mantle structure dating back to just after accretion). Further, Nd isotopes allow a test of the validity of crustal growth models used. The transport balance model used includes a continental crust divided into four parts: upper (high U/Pb) and lower (low U/Pb), as well as older and younger. The latter division is generated by erosion removing proportionally more younger than older crust. After 2 Ga ago erosion transfers U to the ocean floor in preference to Pb and Th, as a consequence of U solubility in an oxidizing environment. Within the constraints imposed on the model, the future paradox cannot be solved by postulating a delayed core formation. An additional low U/Pb reservoir required for this can be found in the continental crust. Solving the future paradox requires that particularly the older lower crust reservoir is conservative, which limits the amount of continental crust that can have,been recycled into the. mantle over Earths history. On the other hand, a solution of the mantle Th/U paradox requires a considerable amount of continent recycling, particularly in the last 1–2 Ga. A restricted family of crustal history scenarios allows a solution to both paradoxes. These are characterized by < I0% of the present amount of continental crust existing just after Earth accretion, rapid crustal growth, with relatively insignificant recycling into the mantle, during the Archaean, and increasing continent recycling in the Proterozoic, reaching ca. 60% of the rate of continent formation today. Scenarios in which delamination of lower crust accounts for over 5% of continent recycling do not provide solutions. The result portrays a non-steady-state Earth in which the net mass of continental crust is at present still growing at 2 × 1015 g/a, and the U content of the upper mantle is increasing.

Rare gas isotopes and parent trace elements in ultrabasic-alkaline-carbonatite complexes, Kola Peninsula: identification of lower mantle plume component

During the Devonian magmatism (370 Ma ago) ∼20 ultrabasic-alkaline-carbonatite complexes (UACC) were formed in the Kola Peninsula (north-east of the Baltic Shield). In order to understand mantle and crust sources and processes having set these complexes, rare gases were studied in ∼300 rocks and mineral separates from 9 UACC, and concentrations of parent Li, K, U, and Th were measured in ∼70 samples. 4He/3He ratios in He released by fusion vary from pure radiogenic values ∼108 down to 6 × 104. The cosmogenic and extraterrestrial sources as well as the radiogenic production are unable to account for the extremely high abundances of 3He, up to 4 × 10−9 cc/g, indicating a mantle-derived fluid in the Kola rocks. In some samples helium extracted by crushing shows quite low 4He/3He = 3 × 104, well below the mean ratio in mid ocean ridge basalts (MORB), (8.9 ± 1.0) × 104, indicating the contribution of 3He-rich plume component. Magnetites are principal carriers of this component. Trapped 3He is extracted from these minerals at high temperatures 1100°C to 1600°C which may correspond to decrepitation or annealing primary fluid inclusions, whereas radiogenic 4He is manly released at a temperature range of 500°C to 1200°C, probably corresponding to activation of 4He sites degraded by U, Th decay. Similar 4He/3He ratios were observed in Oligocene flood basalts from the Ethiopian plume. According to a paleo-plate-tectonic reconstruction, 450 Ma ago the Baltica (including the Kola Peninsula) continent drifted not far from the present-day site of that plume. It appears that both magmatic provinces could relate to one and the same deep-seated mantle source. The neon isotopic compositions confirm the occurrence of a plume component since, within a conventional 20Ne/22Ne versus 21Ne/22Ne diagram, the regression line for Kola samples is indistinguishable from those typical of plumes, such as Loihi (Hawaii). 20Ne/22Ne ratios (up to 12.1) correlate well with 40Ar/36Ar ones, allowing to infer a source 40Ar/36Ar ratio of about 4000 for the mantle end-member, which is 10 times lower than that of the MORB source end-member. In (3He/22Ne)PRIM versus (4He/21Ne)RAD plot the Kola samples are within array established for plume and MORB samples; almost constant production ratio of (4He/21Ne)RAD ≅ 2 × 107 is translated via this array into (3He/22Ne)PRIM ∼ 10. The latter value approaches the solar ratio implying the non-fractionated solar-like rare gas pattern in a plume source. The Kola UACC show systematic variations in the respective contributions of in situ-produced radiogenic isotopes and mantle-derived isotopes. Since these complexes were essentially plutonic, we propose that the depth of emplacement exerted a primary control on the retention of both trapped and radiogenic species, which is consistent with geological observations. The available data allow to infer the following sequence of processes for the emplacement and evolution of Kola Devonian UACC: 1) Ascent of the plume from the lower mantle to the subcontinental lithosphere; the plume triggered mantle metasomatism not later than ∼700 to 400 Ma ago. 2) Metasomatism of the lithosphere (beneath the central part of the Kola Peninsula), including enrichment in volatile (e.g., He, Ne) and in incompatible (e.g., U, Th) elements. 3) Multistage intrusions of parental melts, their degassing, and crystallisation differentiation ∼370 Ma ago. 4) Postcrystallisation migration of fluids, including loss of radiogenic and of trapped helium. Based on model compositions of the principle terrestrial reservoirs we estimate the contributions (by mass) of the plume material, the upper mantle material, and the atmosphere (air-saturated groundwater), into the source of parent melt at ∼2%, 97.95%, and ∼0.05%, respectively.

Isotopic composition of the earth's helium and the problem of the motive forces of tectogenesis

The isotopic composition of helium (3He4He = R) contained in underground fluids was studied in different geotectonic provinces. Provincial mean values (R) were estimated by a routine statistic procedure. The R-values were found to depend on the age of tectono-magnetic activity (t): as t increases, R decreases. Rmin of ∼2 · 10−8 is observed in the ancient (pre-Baikalian) structures of continents. Much higher R-values (≈1.2 · 10−5) are characteristic for the oceanic rift zones. Rmax of ∼2 · 10−5 is established in Iceland which is considered as a “hot spot”. The source of these types of He isotopic composition appear to be continental crust, depleted mantle reservoir and undepleted mantle. Regional characteristics of the R distribution are similar to those of a terrestrial heat flow (q). Correlation of the values of R and q observed at each station of He sampling was revealed at the 95% probability level. Quasi-synchronous changes of q and R with geologic time reflect two (geophysical and geochemical) aspects of one and the same general geologic process — tectogenesis. The triple relationship of q, R and t demonstrates that this process is related to the influx of thermal energy and silicate material by the heat-mass flow from the mantle into the lithosphere. The silicate composition of this flow is confirmed by the correlation between R and 87Sr86Sr. In the course of geologic time these evidences of the mantle heat-mass flow are gradually reduced because of cooling of the newly formed blocks of the lithosphere, the dissipation of the mantle He into space, and the generation of the radiogenic isotopes.

Geodynamics, magmatism, and degassing of the Earth

Abstract In this study terrestrial degassing processes are identified, formalized both on local and global scales, and combined in a single model of evolution of the upper mantle (which constitutes 1 3 of the silicate Earth), the crust, and the atmosphere; unknown parameters of these processes are estimated by means of numerical modelling. The model envisages: early intensive convection and fractionation in the upper mantle resulting in sharp degassing of its matter; later, relatively slow convection in this reservoir, accompanied by partial melting of silicate matter in rift and subduction zones; uplift of melts into the oceanic and continental crust and their degassing; early formation and subsequent recycling of the continental crust and its degassing due to intracrustal magmatism and metamorphism; and accumulation of gases (excepting He) in the atmosphere. Magmatic processes are considered the most important for terrestrial degassing. Two stages of these processes—i.e., partitioning of the gases between melted and solid portions and partial degassing of melt—yield a pronounced “U”-shaped fractionation of noble gases. Helium and neon (due to high solubility and, accordingly, conservation in partially degassed melts) as well as xenon (due to a high partition coefficient and, accordingly, conservation in residual portions) are released from partially melted matter much more slowly than argon. To reconcile calculated and observed noble gas data a high rate of recycling of mantle matter must be provided in the past, 74 × 1017 g/y during the first 0.1 AE; about 90% of 36Ar had been released into the atmosphere at that time. Later, this rate decreased sharply, down to its contemporary value of about 0.6 × 1017 g/y; less than 0.3% of 36Ar is still conserved in the upper mantle; the degassing process resulted in a pronounced “U”-shaped fractionation of noble gases in this reservoir: ( 20 Ne 36 Ar M /( 20 Ne 36 Ar AIR = 3 and ( 130 Xe 36 Ar ) M /( 130 Xe 36 Ar ) AIR = 7 . 4 He 3 He and 40 Ar 36 Ar in the upper mantle both depend on the radioactive decay of parent elements and the rate of mantle degassing; note that the “noble gas radiogenic-degassing clock” was the only one functional during early stages of the evolution of the Earth. The complementary reservoir, continental crust, was formed very early, its mass attaining the contemporary value at about 4 Ga. Self-consistent concentrations of radioactive elements in the silicate Earth (upper mantle + crust) are considerably higher than recent estimates: a concentration of K equal to 0.048% is determined by 40Ar amounts in the mantle, the crust, and (mainly) in the atmosphere; correspondingly, U becomes 0.038 ppm and Th, 0.152 ppm, contemporary ratios of K U = 13,000 and solTh U = 4 being assumed for all silicate reservoirs. The total contemporary fluxes (atoms/cm2 sec) of 3He, 4He, and 40ArRAD are equal to 4.3, 2.0 × 106 and 0.66 × 106, respectively. Practically the whole 3He flux and 1 4 of the 4He flux are produced by upper mantle degassing. The residence times for K and 40ArRAD in the upper mantle are 1.1 and 4.1 Ga, respectively, resulting in an enormously high value for its K-Ar “age,” 4.4 Ga; the continental crust residence times for both species, K and Ar, are high, and the crustal mean K-Ar age is 1.6 Ga.

Juvenile helium in ancient rocks: I. 3He excess in amphiboles from 2.8 Ga charnockite series—Crust-mantle fluid in intracrustal magmatic processes

Abstract The 3 He 4 He in materials derived from the mantle (⩾10 × 10−6) is decreasing in the course of time and approaches the radiogenic value (~10−8) during ≅1 Ga. Some minerals, however, trapped volatiles, including He, and preserve them well during a much longer time; correspondingly, the isotope composition of trapped He in the minerals may shed light on the relation between crustal and mantle He in ancient fluids. Concentrations of U, Th, and K and noble gas (He and Ar) isotopes were measured in crustal charnockite series rocks and rock-forming minerals from the Vezha Tundra Complex, Kola Peninsula. The U-Pb zircon age of the Complex is 2.8 Ga. Amphibole was found to be the principal concentrator of He: 4He concentrations attain 670 × 10−6cm3/g, greater than that expected from U and Th decay, implying a significant contribution from a trapped He component. The measured 3 He 4 He ratios (≅0.2 × 10−6) are considerably higher than the radiogenic value, and the 3He concentrations are up to 25 times higher than those which could be produced from nuclear processes. Therefore, mixing of crustal and mantle volatiles appears to be responsible for the observed 3 He 4 He ratios. Partial degassing of the melt in the course of crystallization differentiation was revealed by the increase of 4 He 40 Ar RAD from ≅2.5 in hypersthene to ⪢ 10 in amphiboles, formed close to the end of the process. A similarity of U-Pb and K-Ar ages suggests that the rocks have not been altered by relatively recent metamorphic processes and have conserved He, trapped since crystallization. Distribution of U, Th, 4He, and 3H implies a considerable release of HeRAD by the rocks and minerals whereas HeTRAP appears to be preserved.

Helium isotopes, tectonics and heat flow in the Northern Caucasus

109 new measurements of 3He/4He ≡ R in subsurface fluids of the Northern Caucasus coupled with the data obtained previously allow regional regularities in the distribution of helium isotopic composition to be examined. Cis-Caucasian foredeeps show the lowest radiogenic R-values. The average Rav-value is slightly higher in gases of the Scythian plate beyond the Stavropol arch. Within the arch, elevated R = (1.6–4.5) × 10−7 indicates an input of mantle-derived helium. This input is even more evident to the south of Starvropol arch, in the Caucasian Mineral Water area, where the ≈8 Ma old laccolithes occur and R-values approach (5–11) × 10−7. The highest R-values, up to (0.7–0.9) × 10−5, are observed further to the south, in the central segment of the Greater Caucasus, where recent volcanism is manifested. Enhanced R-values do not correlate with the crustal thickness but reflect degassing of magmatic reservoirs including those yet unknown. According to the recent Sr-Nd-O data, the young volcanic rocks are of mantle affinity but they are contaminated by a crustal component. The average Rav-values in fluids and 87Sr/86Sr ratios in host magmatic rocks show an inverse correlation suggesting mixing of crustal and mantle materials. R-values vary inversely with apatite fission-track ages of crystalline basement rocks. The ages increase westward of the Elbrus volcano, most likely recording the thermal degradation of the Greater Caucasus since the pre-Cainozoic magmatic activity. A direct correlation between Rav-values and background conductive heat flow densities implies that discharge of the mantle melts into the crust is the common cause of the geochemical, geochronological and geothermal regularities observed. Elevated R-values are generally observed in CO2-bearing fluids, low values are typical of CH4 gases, a few N2-rich gases display highly variable R. Relationships between the major gas constituents and noble gas isotopes are discussed. Fractionation, loss, and gain of these species are considered as the processes controlling the compositions of underground fluids.

Core growth and siderophile element depletion of the mantle during homogeneous Earth accretion

Abstract A simple mechanism is put forward to explain the abundances of siderophile elements in the Earths mantle. Within the frame of a homogeneous accretion hypothesis, the model involves repeated equilibrium fractionation, in a portion of the mantle, between solid and liquid silicate and metal phases. Fractionation events are followed by segregation of metal phases into the core and extensive mixing of different materials (newly accreted matter, terrestrial unfractionated matter, and silicate mantle material which has undergone fractionation) within the mantle. The time scale of a fractionation and mantle mixing event is very short compared to that of accretion. The process leads to a decrease of the metal fraction in the mantle with time. Therefore, the modelling was done in two stages. In the first, envisaged during accretion, metal fractions around 0.1 were considered. A second stage is set after the completion of accretion and metal fractions modelled are around 0.001. The depletion in the mantle by such a two-stage process was calculated for seven involatile siderophile elements, characterized by different partitioning between liquid metal, solid metal, liquid silicate, and solid silicate. In the first stage, the mantle is depleted equally in all siderophile elements irrespective of differences in partition coefficients: Abundances are controlled solely by the mass balance of metal and silicate phases within the fractionation events. In the second stage, abundances of the moderately siderophile elements (W, Co, Ni) are not changed, while strongly siderophile elements (Re, Ir, Au) are further depleted. Calculated depletions for five of the seven elements agree well with the observed concentrations (Cl and Al normalized), as follows: W = 0.053, Co = 0.074, Re = 0.0047, Ir = 0.0062, and Au = 0.0056. Results for Ni = 0.028 and Mo = 0.0046 are by factor 2.5 and 5 below the observed concentrations, respectively. An implication of the model is that the (mainly convective) remixing flux within the silicate mantle during accretion was about ten times larger than the metal flux into the core. The silicate mantle is depleted in Pb by a factor 2.5 during core formation. The already large increase in μ value caused by volatilization of Pb is thus augmented. With the probable time scale of accretion taken as around 100 Ma, this means that the discrepancy between meteoritic and terrestrial lead isotope systematics need not be a serious paradox.

3H-3He dating: A case for mixing of young and old groundwaters

Abstract 3 He 4 He and 20 Ne 4 He ratios were measured in shallow underground waters (opened by water-supplying wells) of the Large Vud-Javr intramountain artesian basin in the Khibiny alkaline massif, the Kola Peninsula. The ratios vary from 1.321 × 10−6 to 2.065 × 10−6 and from 1.412 to 2.941, respectively, and a well-defined correlation is observed between them. Both these ratios in aquifers are known to be time-dependent, the former increases with time due to accumulation of 3He, produced in waters by 3H β-decay; the latter decreases due to migration of helium from water-bearing rocks into the waters. The correlation is interpreted as a result of the mixing of two different types of waters. The approximation line enables us to estimate the isotopic ratios for the endmembers participating in the mixing and the mean residence time (τ) of tritigenic helium-3 in the water: 1. (1) 3 He 4 He = 3.655 × 10 −6 , 20 Ne 4 He = 4.03 , and taking into consideration 3H concentrations in the well waters, 3H = 31.1 TU (practically the same for all samples), τ = 15.8 ± 1.5 years for the young water; 2. (2) 3 He 4 He = 0.20 × 10 −6 , 20 Ne 4 He = 0.18 and T = 0.11 Ma for the old one, the contribution of the old water being less than 10%. In one well a considerable contribution of modern-day meteoric water, about 16%, is observed.

The evolution of matter : from the big bang to the present day earth

This book explains how matter in the Universe developed from the primordial production of light elements within minutes of the Big Bang, and from subsequent stellar processes that continue to create heavier elements at the expense of lighter ones. It also describes the evolution of interstellar matter and its differentiation during the accretion of the planets and the history of the Earth. Much emphasis is placed on isotopic data. Variations in the stable isotope compositions of many elements help us to understand the underlying chemical and physical processes of differentiation. Radioactive isotopes, and their radiogenic daughter isotopes, allow the time and duration of numerous natural processes to be constrained. Unlike many books on geochemistry, this volume follows the chemical history of matter from the very beginning to the present, demonstrating connections in space and time. It provides solid links from cosmochemistry to the geochemistry of the Earth, in the context of astrophysical and planetary processes. The book presents comprehensive descriptions of the various isotope systematics and fractionation processes occurring naturally in the Universe, using simple equations and helpful tables of data. With a glossary of terms and over 900 references, the text is accessible to readers from a variety of disciplines, whilst providing a guide to more detailed and advanced resources. This volume is should prove to be a valuable reference for researchers and advanced students studying the chemical evolution of the Earth, the solar system and the wider Universe.