Middle Archean ocean ridge hydrothermal metamorphism and alteration recorded in the Cleaverville area, Pilbara Craton, Western Australia

A hydrothermally metamorphosed greenstone complex, capped by bedded cherts and banded iron formations (BIFs), is exposed in the Cleaverville area, Pilbara Craton, Western Australia. It has been interpreted as an accretionary complex characterized by both a duplex structure and an oceanic plate stratigraphy, and is shown to represent a 3.2 Ga upper oceanic crust. Three metamorphic zones are identified in the basaltic greenstones. The metamorphic grade increases from sub-greenschist facies (zones A and B) to greenschist facies (zone C) under low-pressure conditions. The boundaries between three mineral zones are subparallel to the bedding plane of overlying chert/BIF, and metamorphic temperature increases stratigraphically downward. The zones correspond to the thermal structure of ocean-floor metamorphism, at a mid-ocean ridge.
The uppermost greenstone in the study area is more pervasively altered and carbonatized than the modern upper oceanic crust. This indicates the enrichment of CO₂ in the metamorphic fluid by which widespread formation of carbonate occurred, compared with a narrow stability region of Ca-Al silicates. It is, therefore, suggested that the Archean hydrothermal alteration played a more important role in fixation of CO₂ than present-day ocean-ridge hydrothermal alteration, as an interaction between sea water and oceanic crust.     

Carbonatization of oceanic crust by the seafloor hydrothermal activity and its significance as a CO2 sink in the Early Archean

Early Archean (3.46 Ga) hydrothermally altered basaltic rocks exposed near Marble Bar, eastern Pilbara Craton, have been studied in order to reveal geological and geochemical natures of seafloor hydrothermal carbonatization and to estimate the CO₂ flux sunk into the altered oceanic crust by the carbonatization. The basaltic rocks are divided into basalt and dolerite, and the basalt is further subdivided into type I, having original igneous rock textures, and type II, lacking these textures due to strong hydrothermal alteration. Primary clinopyroxene phenocrysts are preserved in some part of the dolerite samples, and the alteration mineral assemblage of dolerite (chlorite + epidote + albite + quartz ± actinolite) indicates that the alteration condition was typical greenschist facies. In other samples, all primary minerals were completely replaced by secondary minerals, and the alteration mineral assemblage of the type I and type II basalts (chlorite + K-mica + quartz + carbonate minerals ± albite) is characterized by the presence of K-mica and carbonate minerals and the absence of Ca-Al silicate minerals such as epidote and actinolite, suggesting the alteration condition of high CO₂ fugacity. The difference of the alteration mineral assemblages between basalt and dolerite is probably attributed to the difference of water/rock ratio that, in turn, depends on their porosity.Carbonate minerals in the carbonatized basalt include calcite, ankerite, and siderite, but calcite is quite dominant. The δ¹³C values of the carbonate minerals are −0.3 ± 1.2‰ and mostly within the range of marine carbonate, indicating that the carbonate minerals were formed by seafloor hydrothermal alteration and that carbonate carbon in the altered basalt was derived from seawater. Whole-rock chemical composition of the basaltic rocks is essentially similar to that of modern mid-ocean ridge basalt (MORB) except for highly mobile elements such as K₂O, Rb, Sr, and Ba. Compared to the least altered dolerite, all altered basalt samples are enriched in K₂O, Rb, and Ba, and are depleted in Na₂O, reflecting the presence of K-mica replacing primary plagioclase. In addition, noticeable CO₂ enrichment is recognized in the basalt due to the ubiquitous presence of carbonate minerals, but there was essentially neither gain nor loss of CaO. This suggests that the CO₂ in the hydrothermal fluid (seawater) was trapped by using Ca originally contained in the basalt. The CaO/CO₂ ratios of the basalt are generally the same as that of pure calcite, indicating that Ca in the basalt was almost completely converted to calcite during the carbonatization, although Mg and Fe were mainly redistributed into noncarbonate minerals such as chlorite.The carbon flux into the Early Archean oceanic crust by the seafloor hydrothermal carbonatization is estimated to be 3.8 × 10¹³ mol/yr, based on the average carbon content of altered oceanic crust of 1.4 × 10^-3 mol/g, the alteration depth of 500 m, and the spreading rate of 1.8 × 10¹¹ cm²/yr. This flux is equivalent to or greater than the present-day total carbon flux. It is most likely that the seafloor hydrothermal carbonatization played an important role as a sink of atmospheric and oceanic CO₂ in the Early Archean.     

CO2 transfer between the upper mantle and the atmosphere: temporary storage in the lower continental crust

The CO₂ atmospheric content has shown large variations over geological times. High contents (up to one order of magnitude more than present-day values) ultimately correspond to discrete episodes of mantle degassing, either juvenile, or subduction-related (carbon recycling). A number of arguments (e.g. the continuous volume increase of carbonate-bearing sediments with time) suggest that, throughout the Earth's history, juvenile CO₂ has formed a major contribution to the global carbon budget of the Earth.
The absence of a direct relationship between major volcanic episodes and the average CO₂ atmospheric content suggests that volcanoes might not be the only way by which mantle CO₂ is transported to the surface. It is proposed that large quantities of juvenile CO₂ could temporarily be stored in the lower continental crust during major episodes of granulite formation. These are primarily caused by magmatic underplating and they result in a vertical accretion of the crust by accumulation of CO₂-bearing, mantle-derived magmas. Most of the CO₂ migrates through the crust during post-metamorphic evolution and isostatic restoration of the normal continental thickness. However, large quantities of CO₂ can still be present in some areas, notably as high-density fluids enclosed in minerals.     

Carbon sources in arc volcanism, with implications for the carbon cycle

New CO₂/³ He data from the East Sunda Arc (Indonesia) confirm earlier observations that arc volcanic gases have higher CO₂/³ He ratios than MOR environments.On average, > 80% of arc volcanic CO₂ is recycled, exogene carbon. Addition of a few percent of carbonate-bearing sediments to the mantle wedge explains much of the carbon abundance andcarbon isotopic data of arc gases, but can not explain the He isotope observations. The CO₂/³He in arc volcanoes is not strongly dependent on the composition of modem trough sediments (e.g. deep sea clays vs carbonate-rich sequences), and calcite veins in the hydrothermally altered subducted slab may provide a contribution to the recycled carbon flux of, arcs. The sum of globally deep-subducted sediment and slab carbon exceeds the estimated arc CO₂ flux, and approximately 3.5 teramole of carbon may return annually to the mantle in convergent zones. The modem combined processes of MOR volcanism, slab alteration, and subduction volcanism do not produce a substantial carbon flux into the exosphere, and rate-changes in ocean floor spreading are unlikely to cause major changes in atmospheric CO₂ as a result of changes in the volcanic CO₂ fluxes. Intense pulses of flood basalt volcanism, however, may alter the CO₂ contents of the atmosphere over the course of a millenium or so, and influence global climate.     

Gas Measurement of Fluid Inclusions from the Hishikari Epithermal Gold Deposit, Southern Kyushu, Japan, Using Laser Raman Microprobe

Abstract. Laser Raman microprobe analysis was performed on the fluid inclusions from the Honko-Sanjin zone in the Hishikari epithermal gold deposit, southern Kyushu, Japan. Gas concentrations of fluid inclusions through the zone were below detection limits (e.g., 5 mmole/kg H₂O for CO₂), with an exception at shallow portion in which the CO₂/N₂ mole ratio was determined to be 5.3. Boiling of hydrothermal solutions probably separated gases from ore fluids at the deep portion of the deposit, and migration of gases to shallow portion resulted in CO₂-rich steam-heated water and related acid alteration.     

Synthetic Fluid Inclusions in the Systems NaCl-H2O and NaCl-CO2-H2O: Dissolution Temperatures of Halite

Abstract. This study examined the effect of CO₂ on NaCl solubility in hydrothermal fluid, with the synthetic fluid inclusion technique. Fluid inclusions of 30–40 wt% NaCl and 5 mol % CO₂ were synthesized, and their halite dissolution temperatures, T_m(halite), were measured. The solubilities of NaCl in CO₂-bearing aqueous fluid were obtained at 160–320C under vapor-saturated pressures. The T_m(halite) value in aqueous fluid with 5 mol % CO₂ obtained in this study agrees with that of Schmidt et al. (1995), showing that 5 mol % CO₂ reduces the solubility of NaCl by about 1 wt%.
Calculation of magnetite solubility suggests that 5–10 mol % CO2 decreases magnetite solubility by 4.5–8.9 % relative to the magnetite solubility in CO2-free solution. Therefore, an increase of CO2 content in ore-forming solutions may cause deposition of iron minerals and produce ore deposits.     

Magmatic and hydrothermal processes in the Bouvet Triple Junction Region (South Atlantic)

Results of electron microprobe and microthermometric studies of samples collected from the Bouvet Triple Junction Region (BTJR) during a joint Russian-Italian geological expedition on the R/V Academician Nikolaj Strakhov (1994) have revealed new data on the composition of basaltic magmas and oceanic hydrothermal fluids connected with magmatic processes. Detailed analysis of basaltic glasses shows that the modem Mid-Atlantic Ridge (MAR) rift valley is composed of normal mid-ocean ridge basalts with low concentrations of K₂ O and TiO_z (N-MORB), while its flanks are more enriched with these components approaching E-MORB. A marked influence of the Bouvet hot spot volcanism on magma generation on the South-West Indian Ridge (SWIR) near Bouvet Island is observed. Basaltic melts in this area belong to alkalic and transitional series and have maximum contents of K₂O, TiO₂, H₂O.
Microthermometric analyses of fluid inclusions in the samples from the BTJR have revealed major differences in the oceanic hydrothermal fluid systems on the MAR and near SWIR, which depends on the peculiarities of magma. In the area of the MAR (with dry melts) only H₂O solution inclusions in quartz were found; thus, seawater is probably the only primary source of hydrothermal fluids (NaCl + MgCl₂+ H₂O; T = 170–200°C). In the SWIR area (with the high content of water in melts) syngenetic liquid CO₂ and H₂O solution inclusions in quartz indicate the influence of the magmatic fluid component on the ore-forming water/carbon dioxide solutions (NaCl + CaC1₂+ H₂O + CO₂; T = 200–310 °C; P = 900–1700 bar).     

Fluid-mineral equilibria in prehnite-pumpellyite to greenschist facies metabasites near Flin Flon, Manitoba, Canada: implications for petrogenetic grids

A sequence of regional metamorphic isograds indicating a range from prehnite-pumpellyite to lower amphibolite facies was mapped in metabasites near Flin Flon, Manitoba. The lowest grade rocks contain prehnite + pumpellyite and are cut by younger brittle faults containing epidote + chlorite + calcite. Isobaric temperature- X _CO2 and pressure-temperature (constant X _CO2) diagrams were calculated to quantify the effects of CO₂ in the metamorphic fluid on the stability of prehnite-pumpellyite facies minerals in metabasites containing excess quartz and chlorite. Prehnite and, to a lesser extent, pumpellyite are stable only in fluids with X _co2 <0.002. For X _co2>0.002, epidote + chlorite + calcite assemblages are stable. Our calculated phase relations are consistent with regional metamorphism in the Flin Flon area in the presence of an H₂O-rich fluid and a more CO₂-rich fluid in the later fault zones. We believe that the potential effects of small amounts of CO₂ in the metamorphic fluid should be assessed when considering the pressure-temperature implications of mineral assemblages in low-grade metabasites.     

Mass Transfer, Oxygen Isotopic Variation and Gold Precipitation in Epithermal System: A Case Study of the Hishikari Deposit, Southern Kyushu, Japan

Abstract: Transportation of various kinds of elements occurred in wall rocks (Quaternary andesites) during the hydrothermal alteration accompanied by the Hishikari epithermal gold mineralization. For example, K₂O and MgO contents of wall rocks decrease away from the gold-quartz veins, while (CaO+Na₂O) content increases, and SiO₂ content is variable near the veins. Hydrothermal alteration zoning and bulk compositional variations in wall rocks suggest that the mixing of hydrothermal solution and acidic groundwater took place an important role as the cause for the hydrothermal alteration and bulk compositional variations. The relationship between dissolved silica concentration and temperature of hydrothermal solution mixed with groundwater is obtained based on precipitation kinetics-fluid flow–mixing model, and the computed results are compared with the distribution of SiO₂ minerals (quartz and cristobalite) in the hydrothermal alteration zones. This comparison suggests that the most reasonable flow rate of fluids migrating through hydrothermal alteration zones, and A/M (A: surface area of rocks interacting with fluid, M: mass of fluid) are estimated to be ca. 10^-4.2 m/sec, and ca. 0.10 m²/kg, respectively. The mixing of two fluids (hydrothermal solution and acidic groundwater) can also explain δ¹⁸O zoning in the altered country rocks, hydrothermal alteration zoning from K-feldspar through K-mica to kaolinite from the center (veins) to margin, and deposition of gold.     

碳酸盐化榴辉岩的岩石学研究进展

碳循环进入地幔中主要是通过大洋板块俯冲作用完成的,再通过火山去气作用释放出来,以维持大气中CO2的平衡。洋壳主要由玄武岩组成,一般经热液改造后的洋壳含有一定数量的碳酸盐(质量分数大约为3％),而这些以脉或角砾形式存在的碳酸盐是碳沉淀的一个巨大储库。这些碳酸盐化的玄武岩在俯冲带背景下经历高压变质作用,相应地形成了碳酸盐化榴辉岩。碳酸盐化榴辉岩的部分熔融形成的熔体和气体对于弧岩浆的生成、CO2去气都有非常重要的作用,从而对整个碳循环研究具有重要的意义。文中较全面地总结了近年来,在不同的超高压变质地区发现的碳酸盐化榴辉岩的岩石学和野外产状特征,对比经热液改造的玄武岩 碳酸盐体系的高温高压岩石学研究取得的进展,明确了在俯冲带变质过程中碳酸盐与榴辉岩质硅酸盐体系是如何发生变化的,并对于NCFMASH+CO2体系的热力学相平衡计算的研究进展展开了讨论,对于探讨碳的深部循环过程具有重要意义。     

Oceanic crust production and climate during the last 100 Myr

In order to evaluate the possible influence of oceanic crust production on climatic changes during the past 100Myr variations in total oceanic crust for this period including production at mid-ocean ridges, oceanic plateaus, and back-arc basins were calculated using the most recent and accurate time-scales. The rates presented here differ from those of Larson (1991a, b) on Cenozoic fluctuations and show that (1) maximum production values occurred during the Cenomanian, Palaeocene, and late Oligocene-early Miocene and (2) minimum values occurred in Campanian-Maastrichtian, late Eocene, and middle Miocene. Significantly, variations of oceanic crust production correspond with variations in the δ¹⁸O of deep-water benthic foraminifera: maximum values of oceanic crust production correspond with minimum values of δ¹⁸O, and minimum production values with maximum values of δ¹⁸O. This latter synchronism suggests that changes in land-sea relationships and atmospheric CO₂ related to major fluctuations in oceanic crust production were the main cause of mid-Cretaceous warming and Late Cretaceous cooling, and of climatic quasi-cycles having a periodicity of 33–38 million years over the last 100 Myr. This is the first report showing variations of ocean crust production synchronized with the Cenozoic climate changes.     

Evidence for Archean ocean crust with low high field strength element signature from diamondiferous eclogite xenoliths

Late Archean (2.57 Ga) diamond-bearing eclogite xenoliths from Udachnaya, Siberia, exhibit geochemical characteristics including variation in oxygen isotope values, and correlations of δ¹⁸O with major elements and radiogenic isotopes which can be explained by an origin as subducted oceanic crust. Trace element analyses of constituent garnet and clinopyroxene by Laser-ICPMS are used to reconstruct whole-rock trace element compositions, which indicate that the eclogites have very low high field strength element (HFSE) concentrations and Zr/Hf and Nb/Ta ratios most similar to modern island arcs or ultradepleted mantle. Although hydrothermal alteration on the Archean sea floor had enough geochemical effect to allow the recognition of its effects in the eclogites and thus diagnose them as former oceanic crust, it was not severe enough to erase many other geochemical features of the original igneous rocks, particularly the relatively immobile HFSEs. Correlations of the trace element patterns with oxygen isotopes show that some, generally Mg-richer, eclogites originated as lavas, whereas others have lower δ¹⁸O and higher Sr and Eu contents indicating an origin as plagioclase-bearing intrusive rocks formed in magma chambers within the ocean crust. Major and trace element correlations demonstrate that the eclogites are residues after partial melting during the subduction process, and that their present compositions were enriched in MgO by this process. The original lava compositions were picritic, but not komatiitic, whereas the intrusives had lower, basaltic MgO contents. The HFSE signature of the eclogites may indicate that ocean floor basalts of the time were relatively close to island arcs and recycled material, which would be consistent with a larger number of smaller oceanic plates. Their composition appears to indicate that komatiitic ocean crust compositions were restricted to the early Archean which is not known to be represented among the eclogite xenolith population.     

Volatile fluxes from volcanoes

Volatile fluxes from Mid Ocean Ridge (MOR) and subaerial volcanism have been estimated or re-evaluated using several natural tracers-³He, ²¹⁰Po, SO₂-and chemical ratios of volatile species in lavas and volcanic gases.
These estimates confirm the net predominance of anthropogenic fluxes over volcanic fluxes for CO₂, SO₂ and trace metals.
They also suggest that, while most of the volatiles transferred during MOR volcanism come from the mantle, volatiles stored at the surface of the Earth supply an appreciable fraction of subaerial fluxes and can be the dominant source for some of them.
The surface inventory of volatile species cannot result from steady-state degassing with constant rate and needs much greater fluxes in the past or other volatile supply processes. This inventory is the result of several of the following processes: capture of the solar nebula and its subsequent partial escape, impact degassing of accreting bodies, and, from Archean to present mantle, degasssing through volcanism and associated phenomena, with recycling into the mantle through subduction.     

Ionic association in H2O–CO2 fluids at mid-crustal conditions

The equilibrium constant, K _a, of the association reaction to form ion pairs from charged solute species in supercritical solutions can be calculated from a model based on published equations. Log K _a at constant pressure is a linear function of the inverse in the dielectric constant of the fluid times temperature. The dielectric properties of H₂O and CO₂ at supercritical pressures and temperatures can also be evaluated using the Kirkwood equation. Using Looyenga mixing rules, the dielectric constant of H₂O–CO₂ mixtures can be obtained and the change in log K _a with addition of CO₂ in aqueous solutions evaluated. These changes in log K _a with addition of CO₂ are consistent with measured changes of log K _a with addition of Ar in supercritical H₂O–Ar solutions.
Log K _a of KCl and NaCl increase to an increasing extent as the mole fraction of CO₂ increases in H₂O–CO₂ solutions. For instance, at 2 kbar and constant temperature between 400 and 600° C, log K _a of KCl increases by about two orders of magnitude whilst that of NaCl increases by over four orders of magnitude as the CO₂ mole fraction increases from 0.0 to 0.35. Such changes in log K _a will have dramatic effects on the solubility of minerals in CO₂-rich environments.     

早前寒武纪洋壳的地质记录及其板块构造意义

元古宙蛇绿岩的不断发现表明威尔逊旋回至少在早元古代已经明显起作用，部分太古宙绿岩带由不同的构造单元拼合而成，并发育不同于显生宙蛇绿岩的大洋壳岩石组合，地幔柱在早太古代构造演化过程中起重要作用，是板块构造的重要补充机制，地球早期热产量较高，可能是造成板块规模较小，洋壳较厚，板块运动速度较快的原因，并以缓倾角俯冲为特征。     

Crustal zircons and mantle sulfides: Archean to Triassic events in the lithosphere beneath south-eastern Sicily

Three types of zircon coexist in an unusual lower crustal xenolith from the Valle Guffari diatreme (Hyblean Plateau, Sicily): igneous Type 1 (near-euhedral, weakly zoned; Ce/Ce > 1); partially recrystallised Type 2 (ovoid, structureless; weak Ce anomaly); hydrothermal Type 3 (sugary, spongy-textured, probably related to F-rich aqueous fluids). U–Pb dating by LAM-ICPMS, supported by in situ Hf-isotope analysis, suggests that both Type 1 and Type 2 zircons were originally Archean (ca 2.7 Ga), though many of these grains have experienced severe Pb loss. The U–Pb ages of the hydrothermal zircons cluster around 246 Ma, interpreted as the timing of the hydrothermal event. Their ε_Hf (+ 8.5 to − 1.2) indicates the mixing of old crustal components and material from a juvenile source.

In situ Os-isotope analyses of sulfides hosted in peridotite xenoliths from Valle Guffari show Paleoproterozoic–Archean T_RD minimum ages, corresponding to the age of the oldest zircon grains in the crustal xenolith. Other peaks of T_RD ages suggest that multiple metasomatic events have affected the lithospheric mantle.

These observations suggest that the lower crust and the upper part of the lithospheric mantle beneath the Hyblean Plateau represent the northernmost portion of the African Plate. These two units have coexisted since at least late Archean time, and have remained linked through several episodes of crustal modification, including the Permo-Triassic hydrothermal event, which was probably related to the onset of rifting in the Ionian Basin.     

现代海底热液成矿作用

从现代块状硫化物矿床成矿特征对比角度,总结分析了世界现代海底喷流的块状硫化物成矿堆积,综述了现代海底块状硫化物成矿主要形成于洋壳和岛弧环境的实际观察结果,突出强调了洋壳环境和岛弧或陆壳环境两种成矿环境对成矿类型分类的意义。对上地幔部分熔融岩浆来源与地壳物质可能带人、火山喷发岩浆系列的演化和对热液成矿作用的控制进行了讨论,对比分析了岩浆流体对成矿的重要贡献和控制作用,以及成矿热液循环体系形成的条件和模式。     

Subduction geodynamics in Archean and formation of diamond-bearing lithospheric keels and early continental crust of cratons

The diamond-bearing mantle keels underlying Archean cratons are a unique phenomenon of Early Precambrian geology. The common stable assemblage of the Archean TTG early continental crust and underlying subcontinental lithospheric mantle clearly shows their coupled tectogenesis, which was not repeated in younger geological epochs. One of the least studied aspects of this phenomenon is concerned with the eclogitic xenoliths carried up by kimberlite pipes together with mantle-derived nodules. The eclogitic xenoliths reveal evidence for their subduction-related origin, but the Archean crustal counterparts of such xenoliths remained unknown for a long time, and the question of their crustal source and relationships to the formation of early continental crust remained open. The Archean crustal eclogites recently found in the Belomorian Belt of the Baltic Shield are compared in this paper with eclogitic xenoliths from kimberlites in the context of the formation of both Archean subcontinental lithospheric mantle (SCLM) and early continental crust. The crustal eclogites from the Belomorian Belt are identical in mineral and chemical compositions to the eclogite nodules (group B), including their diamond-bearing varieties. The eclogite protoliths are comparable in composition with the primary melts of the Meso- and Neoarchean oceanic crust, which was formed at a potential temperature of the upper mantle which exceeded its present-day temperature by 150–250 K. The reconstructed pathways of the Archean oceanic crust plunging in the upper mantle suggest that the Archean mantle was hotter than in the modern convergence settings. The proposed geodynamic model assumes coupled formation of the Archean diamond-bearing SCLM and growth of early continental crust as a phenomenon related to the specific geodynamics of that time controlled by a higher terrestrial heat flow.     

现代海底热液成矿作用研究现状及发展方向

现代海底热水成矿作用研究的重大进展表现在两个方面：（１）大批活动的和窒息的热液活动区和硫化物矿床在洋脊、岛弧、弧后盆地及板内火山活动中心等海底环境相继发现。在沉积物饥饿洋脊，矿床规模较小，Ｃｕ－Ｚｎ为主，沉积物覆盖洋脊，矿床规模巨大，Ｐｂ－Ｚｎ为主。弧后扩张或弧间裂陷盆地，形成Ｐｂ－Ｚｎ→Ｚｎ－Ｐｂ－Ｃｕ→Ｃｕ－Ｚｎ矿床谱系。岛弧环境硫化物矿床不具规模，板内火山活动中心以氧化物－硫化物矿化为特色。（２）现代海底热水成矿作用观察和研究为古代ＶＭＳ矿床成因研究提供了重要信息，对现有成矿理论产生重要影响。现代成矿观念强调：①海底成矿作用虽可产生于不同环境，但均与张裂断陷事件密切相关。矿床规模和分布特点受张裂速率制约；②成矿物质主体来源于热水循环的火山－沉积岩和下伏基底物质；③硫化物堆积发生于丘堤－烟囱联合构成的机构和结壳下部，通过开放空间的硫化物充填和先成矿石淋滤迁移来实现。④热液流体呈双扩散对流循环。现代海底热水成矿作用的未来研究方向可概括为强度方向和广度方向。广度研究将加大力度去发现新的矿床，强度研究将采用地球物理方法并配以必要的钻探，深入揭示矿床的三维结构和热液体系及成矿机制。     

Evolution of the early Earth: Constraints from Nd---Nd isotopic systematics

Combined ¹⁴⁷Sm---¹⁴³Nd and the now extinct [τ(1/2)₁₄₆=103×10⁶ yr] ¹⁴⁶Sm---¹⁴²Nd isotopic systematics are reported for early Archean gneisses from Greenland (Amîtsoq and Akilia associations), and Canada (Acasta gneiss). Using both field relationships and high resolution U---Pb SHRIMP ion-probe ages, it has been possible to identify the most ancient rocks in these terrains for isotopic analyses. Preliminary ¹⁴²Nd analyses of a still limited number of samples have failed to identify terrestrial ¹⁴²Nd anomalies. Effects, if present, are limited to < 10 ppm and we have thus been unable to confirm the +33±4 ppm ε₁₄₂ value claimed by Harper and Jacobsen (1992a, b) for a single sample. From the lack of ¹⁴⁶Sm---¹⁴²Nd effects we infer that large-scale fractionation events that may have occurred in the first 200 Ma of Earth history did not leave a significant nor widespread imprint on the early Archean mantle or crust. If a terrestrial magma ocean, with associated LREE fractionation, formed as a result of planetary accretion, then it had a lifetime of at most 250 m.y. before being remixed into the Earth's mantle.

The samples analysed in this study have a range of ε₁₄₃ values including highly positive values of up to +4.2. This requires that the earliest known Archean crust was differentiated from a reservoir that was strongly depleted in the LREE as compared with chondritic compositions. In the early Archean it is proposed that the depletions in LREE are a consequence of extraction of a limited fraction of the Earth's continental crust ( < 10%) from the upper 200 km of the mantle. A three reservoir model, consisting of the continental crust, depleted mantle and a more primitive mantle reservoir can be extended to account for both the present-day, as well as the evolving Nd isotopic composition of the Earth's crust and mantle. In contrast to previous models, the rate of growth of the continental crust is used as an input parameter to constrain the concomitant growth and evolution of the depleted mantle reservoir. Recycling of large volumes of bulk continental crust into the mantle is not considered to be an important process, nor is the existence of an additional major enriched component in the early Archean mantle.