Mafic-ultramafic magmatism of the Early Precambrian (from the Archean to Paleoproterozoic)

Compositional evolution of the Archean mafic-ultramafic volcanics is considered in comparison with evolution of the Paleoproterozoic volcanism using available data on the Baltic shield, Pilbara (Australia) and Superior (Canada) cratons, and the Isua greenstone belt (Greenland). The Archean volcanics of mantle origin are of two major types, represented (a) by komatiite-basaltic complexes (komatiites, komatiitic and tholeiitic basalts) and (b) by geochemical analogs of boninites (GAB) and siliceous high-Mg series (SHMS) of volcanic rocks. As is established, the komatiitic and GAB volcanism ceased in the terminal Archean, whereas the SHMS rocks prevailed in the Paleoproterozoic to become extinct about 2 Ga ago in connection with transition to the Phanerozoic type of tectonomagmatic activity. Geochemical trends of mafic-ultramafic associations occurring in the considered cratons are not uniform, being of particular character to certain extent. With transition from the Paleo- to Neoarchean, rock associations of both types reveal a minor increase in Ti and Fe contents. Comparatively high Fe₂O_3tot TiO₂, and P₂O₅ concentrations (maximal ones in the Archean), which are characteristic of the Neoarchean (2.75–2.70 Ga) basalts from the Superior and Pilbara cratons or the Baltic shield, represent a result of relatively high-Ti intracratonic magmatic activity that commenced in that period practically for the first time in the Earth history. This magmatic activity of the Neoarchean was not as intense as the high-Mg basaltic volcanism, and the absolute maximum in concentrations of the above components was attained only 2.2–1.9 Ga ago, at the time of appearance in abundance of Fe-Ti picrites and basalts typical of the Phanerozoic intraplate magmatism. The Archean volcanic complexes demonstrate gradual secular increase in concentrations of incompatible elements (LREE inclusive) and growth of Nb/Th ratio that apparently reflected the progressing influence of mantle plumes. In the early Paleoproterozoic (2.5–2.35 Ga), values of that ratio considerably declined in the SHMS rocks and then quickly grew in the Middle Paleoproterozoic volcanics (2.2–1.9 Ga) to attain finally the values typical of the Phanerozoic magmas associated in origin with mantle plumes. The ?_Nd(T) parameter was decreasing with time from positive values in the Paleoarchean to negative ones in the SHMS rocks of the Paleoproterozoic most likely in response to grown proportion of ancient crustal material in magmatic melts. Since the mid-Paleoproterozoic, the ?_Nd(T) values turn in general into positive again reflecting change in the character of magmatic activity: the SHMS melts gave place at that time to the Fe-Ti picrite-basaltic magmas. The primary crust of the Earth was presumably of sialic composition and originated during solidification from the bottom upward of the global magma ocean a few hundreds kilometers deep, when most fusible components migrated up to the surface to form there the granitic crust. Geological history of the Earth commenced at the appearance time of granite-greenstone terranes and granulite belts separating them, the first large tectonic structures formed under influence of raising mantle superplumes.     

Coronitic textures in the ferrogabbroids of the Elet’ozero intrusive complex (northern Karelia,Russia) as evidence for the existence of Fe-rich melt. 1. Types of coronas

Fe–Ti oxides (magnetite, Ti-magnetite, ilmenite, and associated high-Al spinel) in the ferrogabbroids of the Middle Paleoproterozoic Elet’ozero syenite–gabbro intrusion are intercumulus minerals usually surrounded by coronitic rims of two types. The first type usually represents multilayer amphibole–biotite ± olivine coronas along contacts of Fe–Ti oxides with cumulus moderate-Ca plagioclase and more rarely, clinopyroxene. Two-layer rim is developed in contact with high-Ca plagioclase; the inner rim consists of pargasite and spinel, while the outer rim is made up of sadanagaite and spinel. The second type is represented by two-stage coronitic textures developed along boundaries of olivine and Fe–Ti oxide clusters with plagioclase. Initially, the olivine was surrounded by orthopyroxene rim, while Fe–Ti oxides were rimmed by pargasite with thin ingrowths of high-Al spinel (hercynite). At the next stage, the entire cluster was fringed by a common symplectite reaction rim, the composition of which also depended on the composition of plagioclase matrix: the spinel–sadanagaite rim was formed in contact with high-Ca plagioclase, while pargasite–muscovite–scapolite rim was formed in contact with moderate-Ca plagioclase. The formation of the outer rims occurred after hydration of the inner parts of coronas around olivine and oxides within the clusters. It is suggested that the Fe–Ti oxides and surrounding coronitic rims were microsystems formed by crystallization of drops of residual hydrous Fe-rich liquid.     

Coronitic textures in the ferrogabbros of the Elet’ozero intrusive complex (Northern Karelia,Russia) as evidence for the existence of Fe-rich melt. 2. Origin of Fe-rich liquid

The study of coronitic textures in ferrogabbros and data on rhythmic layering of the Elet’ozero Massif supports the existence of a specific low-temperature Fe-rich liquid in nature. This liquid was formed during solidification of intrusion owing to the local multiple accumulation of Fe and Ti contents in a parental Fe-rich Fe–Ti basaltic melt. According to obtained data, this occurred on micro- and macroscale: 1) in the interglanular (intercumulus) space of the crystallization zone where intercumulus melt becomes rich in Fe and Ti owing to the crystallization of cumulus silicate minerals and is transformed into Fe-rich liquid, which concentrates residual components of an intergranular melt; 2) during formation of rhythmic layering when Fe-rich residual melt is accumulated before the upper part of the moving front of solidification; when Fe content reaches a certain limit, the melt is also transformed in a separate Fe-rich liquid, the interlayers of which form the upper (lowest temperature) members of rhythms. It was concluded that the emergence of a Fe-rich melt is related to its specific structure, which is formed when the Fe content reaches certain critical values in a liquid. Thus, this liquid is not a product of immiscible splitting of a melt, but represents a peculiar phenomenon. The preservation of primary textures and structures of the rocks is supposedly related to the lyophobic properties of surfaces, i.e., “repulsion” of nonwetting liquid by facets of cumulus crystals, especially plagioclase. Owing to this, the drops and even horizons of heavy Fe-rich liquid are retained in situ of their formation.     

Satellite Radiothermovision on Synoptic and Climatically Significant Scales

This paper is focused on the development of a methodological basis for the authors’ approach to the processing of large volumes of satellite radiothermal data, which is known as satellite radiothermovision. A closed scheme for calculating the latent heat flux (and other integral characteristics of the dynamics of geophysical fields) through arbitrary contours (boundaries) has been constructed and mathematically described. The opportunity for working with static, as well as movable and deformable boundaries of arbitrary shape, has been provided. The computational scheme was tested using the example of calculations of the atmospheric advection of the latent heat from the North Atlantics to the Arctic in 2014. Preliminary analysis of the results showed a high potential of the approach when applying it to the study of a wide range of synoptic and climatically significant atmospheric processes of the Earth. Some areas for the further development of the satellite radiothermovision approach are briefly discussed. It is noted that expanding the analysis of the available satellite data to as much data as possible is of considerable importance. Among the immediate prospects is the analysis of large arrays of data already accumulated and processed in terms of the satellite radiothermovision ideology, which are partially presented and continuously updated on a specialized geoportal.     

Differential Radiothermal Methods for Satellite Retrieval of Atmospheric Humidity Profile

The existing radiometric methods for measuring the profile of the water-vapor concentration from spacecrafts are analyzed. Questions of the sensitivity of radiometric measurements to a deviation in the water-vapor concentration from the standard profile are considered. It is shown that the traditional measurements near absorption line 183 GHz have a low sensitivity to a change in the water-vapor concentration in the lower troposphere. A new differential method of humidity measurement in the lower troposphere has been proposed based on measurements near the 22-GHz line. The advantages of differential measurements near 22 GHz when compared to similar measurements in the vicinity of 183 GHz are confirmed by the results of computer simulation.     

Magmatic systems of large continental igneous provinces

Large igneous provinces (LIPs) formed by mantle superplume events have irreversibly changed their composition in the geological evolution of the Earth from high-Mg melts (during Archean and early Paleoproterozoic) to Phanerozoic-type geochemically enriched Fe-Ti basalts and picrites at 2.3 Ga. We propose that this upheaval could be related to the change in the source and nature of the mantle superplumes of different generations. The first generation plumes were derived from the depleted mantle, whereas the second generation (thermochemical) originated from the core-mantle boundary (CMB). This study mainly focuses on the second (Phanerozoic) type of LIPs, as exemplified by the mid-Paleoproterozoic Jatulian–Ludicovian LIP in the Fennoscandian Shield, the Permian–Triassic Siberian LIP, and the late Cenozoic flood basalts of Syria. The latter LIP contains mantle xenoliths represented by green and black series. These xenoliths are fragments of cooled upper margins of the mantle plume heads, above zones of adiabatic melting, and provide information about composition of the plume material and processes in the plume head. Based on the previous studies on the composition of the mantle xenoliths in within-plate basalts around the world, it is inferred that the heads of the mantle (thermochemical) plumes are made up of moderately depleted spinel peridotites (mainly lherzolites) and geochemically-enriched intergranular fluid/melt. Further, it is presumed that the plume heads intrude the mafic lower crust and reach up to the bottom of the upper crust at depths ～20 km. The generation of two major types of mantle-derived magmas (alkali and tholeiitic basalts) was previously attributed to the processes related to different PT-parameters in the adiabatic melting zone whereas this study relates to the fluid regime in the plume heads. It is also suggested that a newly-formed melt can occur on different sides of a critical plane of silica undersaturation and can acquire either alkalic or tholeiitic composition depending on the concentration and composition of the fluids. The presence of melt-pockets in the peridotite matrix indicates fluid migration to the rocks of cooled upper margin of the plume head from the lower portion. This process causes secondary melting in this zone and the generation of melts of the black series and differentiated trachytic magmas.     

Pillow lavas of the Sierra Leone test site,Mid-Atlantic Ridge, 5°–7°N: Sr-Nd isotope systematics,geochemistry, and petrology

Based on detailed petrological, geochemical, and isotope-geochemical study, fragments of fresh pillow lavas with chilled glass margins dredged at the Sierra-Leone test site in the axial MAR rift zone between 5° and 7°N correspond to MORB tholeiites, which are not primitive mantle melts but were differentiated in intermediate magmatic (intrusive) chambers. Small-scale geochemical and Sr-Nd isotope heterogeneities were established for the first time in the basalts and their glasses. It was shown that some samples show significant nonsystematic differences in the ⁸⁷Sr/⁸⁶Sr ratio between the basalts and their chilled glasses and less significant difference in ?_Nd; higher Sr ratios can be observed both in the glasses and basalts of the same lava fragments. No significant correlation is observed between the isotope characteristics of the samples and their geochemistry; it was also shown that seawater did not affect the Sr and Nd isotope composition of the chilled glasses of the studied pillow lavas. It is suggested that such differences in isotope ratios are related to a small-scale heterogeneity of the melts owing to incomplete homogenization during their rapid ascent to the surface. The heterogeneity of the basaltic melts is explained by their partial contamination by the older plutonic rocks (especially gabbroids) of the lower oceanic crust, through which they ascended to the ocean floor surface. The wider scatter of the Sr isotopic ratios relative to Nd is related to the presence of xenocrysts of calcic plagioclase; correspondingly, the absence of a Nd mineral carrier in the rocks results in less distinct Nd isotope variations. It was shown that all of the studied basalts define a single trend along the mantle correlation array in the Sr-Nd isotope diagram. The causes of this phenomenon remain unclear.     

Petrology of the Early Paleoproterozoic Burakovsky complex,southern Karelia

The Early Paleoproterozoic Burakovsky complex contains Europe’s largest layered mafic-ultramafic pluton and giant Avdeevo gabbronorite dike. The pluton consists of two independent bodies of different age (the Aganozero and Shalozero-Burakovsky bodies), each having its own internal structure and contacting each other in their apical parts. Both bodies have a similar rock sequence including five differentiated zones (from bottom upward, based on the predominant cumulus assemblages): ultrabasic, pyroxenite, gabbronorite, pigeonite gabbronorite, and magnetite gabbronorite-diorite. Being generally similar to each other owing to a common differentiation trend of similar melts, these bodies differ notably in the character of their cumulus stratigraphy and, to a lesser extent, in mineral and geochemical composition. The pluton is distinguished by the presence of marker horizons—singular interlayers of high-temperature mafic cumulates emplaced in the sequence of lower-temperature gabbroid. Their origin is believed to have been associated with the influxes of fresh magma portions into the crystallizing magma chambers. Based on petrological, geochemical, and mineralogical data, the Aganozero and Shalozero-Burakovsky bodies, as well as additional intrusive phases, were derived from the compositionally similar but not identical melts of siliceous high-Mg (boninite like) series. The Avdeevo dike (2436 ± 46 Ma at ?_Nd(T) = ?1.5) extends along the southeastern contact of the Shalozero-Burakovsky body and was formed almost simultaneously with it. The dike is made up of pigeonite gabbronorites, which are almost identical to the rocks of the Pigeonite gabbronorite zone of the Burakovsky pluton in geochemistry and mineral composition. It was concluded that the Burakovsky Complex was a long-lived igneous center, whose origin was related to the activity of an Early Paleoproterozoic mantle superplume.