Principal investigator at the Center for Isotope Research, A.P.Karpinsky Geological Institute (VSEGEI). The main scientific interests of Dr. Belyatsky are in the study of geochronology and isotope geochemical features of the Proterozoic and Paleozoic alkaline-ultramafic intrusions with carbonatites of Kola-Karelian region and associated ore mineralization. He authored a number of research articles on behaviour of isotope systems during geological processes, crust-mantle interaction and evolution of this system from the isotope-geochemical point of view, on isotope dating of the ore-forming processes and searching of the ore-substance sources, on the formation and evolution of continental and oceanic crust during Earth history, on the modern magmatism and isotope signatures of its mantle sources (mantle xenoliths), which are published in the national and international journals. He supervised and took part in several international research projects, and involved in academic activities and State Program on geological mapping including Polar Regions of the World.
The Llandoverian black shales of the Prades Mountains, SW Catalonian Coastal Ranges, contain seve... more The Llandoverian black shales of the Prades Mountains, SW Catalonian Coastal Ranges, contain several metamorphosed stratiform sulfide deposits. The mineralized interval, up to 30 m in thickness, consists of interbedded sulfide-rich (mostly pyrrhotite) shales, feldspar-rich layers and apatite beds. The ore contains Zn, Cu, Pb, Au, Ag and PGE. Whole-rock trace-element analyses were performed by ICP-MS, and the results were normalized to NASC reference standard. The REE patterns show enrichment in Eu (La) and a strong depletion in Ce. This distribution is compatible with REE mostly inherited from seawater, but a significant hydrothermal component is inferred for Eu. Profiles of redox-sensitive trace elements show great V, Cr, Ni, Co, Mo and U enrichments with respect to NASC standard. Part of this enrichment could derive from a direct precipitation from seawater, favoured by the euxinic conditions of the Silurian basin. Nevertheless, V (up to 5444 ppm) and Cr (up to 640 ppm) contents would require additional sources. These elements could be scavenged from seawater by exhalative particles in a hydrothermal derived plume that finally accumulated on the seafloor. In contrast, high Ni, Co and Mo values could be of hydrothermal origin. Sm–Nd isotopic analyses of feldspar-rich layers yielded an isochron age of 437±57 Ma (Llandoverian). These results, as well as the fine-grained textures, the lack of evidences of replacement and the pre-deformational and pre-metamorphic character support the syngenetic origin of the mineralization. Trace-element geochemistry and Sm–Nd isotopes are consistent with a submarine-exhalative origin of the mineralization processes, and suggest that the feldspar-rich levels are metaexhalites.
This study is an attempt to unravel the tectono-metamorphic history of high-grade metamorphic roc... more This study is an attempt to unravel the tectono-metamorphic history of high-grade metamorphic rocks in the Eastern Erzgebirge region. Metamorphism has strongly disturbed the primary petrological genetic characteristics of the rocks. We compare geological, geochemical, and petrological data, and zircon populations as well as isotope and geochronological data Ž. Ž. for the major gneiss units of the Eastern Erzgebirge; 1 coarse-to medium-grained AInner Grey GneissB, 2 fine-grained Ž. Ž. AOuter Grey GneissB, and 3 ARed GneissB. The Inner and Outer Grey Gneiss units MP–MT overprinted have very similar geochemical and mineralogical compositions, but they contain different zircon populations. The Inner Grey Gneiss is Ž. found to be of primary igneous origin as documented by the presence of long-prismatic, oscillatory zoned zircons 540 Ma and relics of granitic textures. Geochemical and isotope data classify the igneous precursor as a S-type granite. In contrast, Outer Grey Gneiss samples are free of long-prismatic zircons and contain zircons with signs of mechanical rounding through sedimentary transport. Geochemical data indicate greywackes as main previous precursor. The most euhedral zircons are Ž. zoned and document Neoproterozoic ca. 575 Ma source rocks eroded to form these greywackes. U–Pb-SHRIMP measurements revealed three further ancient sources, which zircons survived in both the Inner and Outer Grey Gneiss: Ž. Ž. Ž. These results point to absence of Grenvillian type sources and derivation of the crust from the West African Craton. The granite magma of the Inner Grey Gneiss was probably derived through in situ melting of the Outer Grey Gneiss sedimentary protolith as indicated q Electronic supplements available on the journal's homepage: http:rrwww.elsevier.comrlocaterlithos 0024-4937r01r$-see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž. PII: S 0 0 2 4-4 9 3 7 0 0 0 0 0 6 6-9
We investigated the isotope composition (O, C, Sr, Nd, Pb) in mineral separates of the two Precam... more We investigated the isotope composition (O, C, Sr, Nd, Pb) in mineral separates of the two Precambrian carbonatite complexes Tiksheozero (1.98 Ga) and Siilinjärvi (2.61 Ga) from the Karelian–Kola region in order to obtain information on Precambrian mantle heterogeneity. All isotope systems yield a large range of variations. The combination of cathodoluminescence imaging with stable and radiogenic isotopes on the same samples and mineral separates indicates various processes that caused shifts in isotope systems. Primary isotope signatures are preserved in most calcites (O, C, Sr, Pb), apatites (O, Sr, Nd), amphiboles (O), magnetites (O), and whole rocks (Sr, Nd). The primary igneous C and O isotope composition is different for both complexes (Tiksheozero: δ 13 C = − 5.0‰, δ 18 O = 6.9‰; Siilinjärvi: δ 13 C = −3.7‰, δ 18 O = 7.4‰) but very uniform and requires homogenization of both carbon and oxygen in the carbonatite melt. The lowest Sr isotope ratios of our carbonates and apatites from the Archaean Siilinjärvi (0.70137) and the Palaeoproterozoic Tiksheozero (0.70228) complexes are in the range of bulk silicate earth (BSE). Positive ε Nd values of the two carbonatites point to very early Archaean enrichment of Sm/Nd in the Fennoscandian mantle. No HIMU components could be detected in the two complexes, whereas Tiksheozero carbonatites give the first indication of Palaeoproterozoic U depletion for Fennoscandia. Sub-solidus exchange processes with water during emplacement and cooling of carbonatites caused an increase in the oxygen isotope composition of some carbonates and probably also an increase of their 87 Sr/ 86 Sr ratio. A larger increase of initial Sr isotope ratios was found in carbonatized silicic rocks compared to carbonatite bodies. The Svecofennian metamorphic overprint (1.9– 1.7 Ga) caused reset of Rb/Sr (mainly mica) and Pb/Pb (mainly apatite) isochron systems.
Zircon grains from two Precambrian carbonatites from Fennoscandia (Siilinjärvi and Tiksheozero) w... more Zircon grains from two Precambrian carbonatites from Fennoscandia (Siilinjärvi and Tiksheozero) were studied by in-situ geochemical and isotope methods. Zircon domains which preserved primary mantle signatures were identified by a combination of microscopic investigations of thin sections of the carbonatite rocks and separated zircon grains (optical microscopy, optical microscopy combined with cathodoluminescence (OM–CL), and scanning electron microscopy). All studied samples show evidence for alteration processes caused by infiltration of late-stage carbonatite melt/s or fluid/s. This led to different changes in the geochemistry and isotope composition of altered zircon domains. For the 2.6 Ga old Siilinjärvi carbonatite complex, zircon grains underwent solid state recrystallization mainly at their rims. Zircon cores often preserved primary mantle signatures and register high HREE/LREE ratios, undisturbed U–Pb ages, ε Hf values close to CHUR (chondritic uniform reservoir), and δ 18 O values typical for the mantle. The solid state recrystallization of zircon occurred mainly in contact with a late-stage carbonatite melt or fluid and led to (i) diffusion driven loss of HREE, Th, and U, (ii) partial disturbance of the U–Pb system, (iii) a small shift of the δ 18 O toward lower values, and (iv) higher CL intensities in such zircon domains. Contrarily, zircons from the 2.0 Ga old Tiksheozero complex underwent a coupled dissolution– reprecipitation process. Dissolved–reprecipitated zircon domains typically have a distinctive patchy texture in BSE images and sometimes contained abundant micro-inclusions of calcite, phlogopite, apatite, and baddeleyite. The EDS spectra documented that these non-luminescent zircon regions have much higher concentrations of Ca (and sometimes Fe) indicating an " impure " zircon composition. In addition, many trace elements are highly enriched in such " impure " zircon. Enrichment of Th (and to a lower degree of U) resulted in high Th/U ratios (>1) and disturbance of the U/Pb ages. Hf and O isotope values varied widely even within a single zircon grain. Isotope signatures attest a complex crystallization history of carbonatites and can best be interpreted as melt mixing processes of different carbonatite melt pulses. The non-luminescent reprecipitated zircon domains in Tiksheozero are often rimmed by zircon with high CL-intensity recrystallized in a solid state as well as by new zircon overgrowths. These zircon regions consist of pure zircon without elevated concentrations of Ca and Fe. Solid state recrystallization was accompanied by annealing that caused volume loss resulting in fractures that are now frequently filled with calcite (often containing a minor proportion of apatite) and phlogopite which sampled the withdrawn elements from the recrystallized zircon. Co-crystallizing calcite may thus lead to HREE depletion in newly grown zircon. It is expected that interaction with carbonatite melts can lead to similar changes in zircon grains from other sub-continental mantle rocks. Then, the primary geochemical and isotope systems of zircon may be changed despite its chemical and physical robustness. Infiltration of late-stage carbonatite melts seems to be a common process within many carbonatitic rocks that may hamper to determine the primary geochemical and isotope signatures of carbonatitic zircons.
The borehole at the southern par t of subglacial Lake Vostok has been drilled into an ice lay er ... more The borehole at the southern par t of subglacial Lake Vostok has been drilled into an ice lay er that has been refrozen from the lake water. This ice layer contains random sediment inclusions, eight of which have been studied using state-of the-art analytical techniques. Six inclusions comprise soft aggregates consisting mainly of clay-mica minerals and micron-sized quartz grains while
Zircon grains from two Precambrian carbonatites from Fennoscandia (Siilinjärvi and Tiksheozero) w... more Zircon grains from two Precambrian carbonatites from Fennoscandia (Siilinjärvi and Tiksheozero) were studied by in-situ geochemical and isotope methods. Zircon domains which preserved primary mantle signatures were identified by a combination of microscopic investigations of thin sections of the carbonatite rocks and separated zircon grains (optical microscopy, optical microscopy combined with cathodoluminescence (OM–CL), and scanning electron microscopy). All studied samples show evidence for alteration processes caused by infiltration of late-stage carbonatite melt/s or fluid/s. This led to different changes in the geochemistry and isotope composition of altered zircon domains. For the 2.6 Ga old Siilinjärvi carbonatite complex, zircon grains underwent solid state recrystallization mainly at their rims. Zircon cores often preserved primary mantle signatures and register high HREE/LREE ratios, undisturbed U–Pb ages, εHf values close to CHUR (chondritic uniform reservoir), and δ18O values typical for the mantle. The solid state recrystallization of zircon occurred mainly in contact with a late-stage carbonatite melt or fluid and led to (i) diffusion driven loss of HREE, Th, and U, (ii) partial disturbance of theU–Pb system, (iii) a small shift of the δ18O toward lower values, and (iv) higher CL intensities in such zircon domains. Contrarily, zircons from the 2.0 Ga old Tiksheozero complex underwent a coupled dissolution– reprecipitation process. Dissolved–reprecipitated zircon domains typically have a distinctive patchy texture in BSE images and sometimes contained abundant micro-inclusions of calcite, phlogopite, apatite, and baddeleyite. The EDS spectra documented that these non-luminescent zircon regions have much higher concentrations of Ca (and sometimes Fe) indicating an “impure” zircon composition. In addition, many trace elements are highly enriched in such “impure” zircon. Enrichment of Th (and to a lower degree of U) resulted in high Th/U ratios (>1) and disturbance of the U/Pb ages. Hf and O isotope values varied widely even within a single zircon grain. Isotope signatures attest a complex crystallization history of carbonatites and can best be interpreted as melt mixing processes of different carbonatite melt pulses. The non-luminescent reprecipitated zircon domains in Tiksheozero are often rimmed by zircon with high CL-intensity recrystallized in a solid state aswell as by new zircon overgrowths. These zircon regions consist of pure zircon without elevated concentrations of Ca and Fe. Solid state recrystallization was accompanied by annealing that caused volume loss resulting in fractures that are now frequently filled with calcite (often containing a minor proportion of apatite) and phlogopite which sampled the withdrawn elements from the recrystallized zircon. Co-crystallizing calcite may thus lead to HREE depletion in newly grown zircon. It is expected that interaction with carbonatite melts can lead to similar changes in zircon grains from other sub-continental mantle rocks. Then, the primary geochemical and isotope systems of zircon may be changed despite its chemical and physical robustness. Infiltration of late-stage carbonatite melts seems to be a common process within many carbonatitic rocks that may hamper to determine the primary geochemical and isotope signatures of carbonatitic zircons.
The Llandoverian black shales of the Prades Mountains, SW Catalonian Coastal Ranges, contain seve... more The Llandoverian black shales of the Prades Mountains, SW Catalonian Coastal Ranges, contain several metamorphosed stratiform sulfide deposits. The mineralized interval, up to 30 m in thickness, consists of interbedded sulfide-rich (mostly pyrrhotite) shales, feldspar-rich layers and apatite beds. The ore contains Zn, Cu, Pb, Au, Ag and PGE. Whole-rock trace-element analyses were performed by ICP-MS, and the results were normalized to NASC reference standard. The REE patterns show enrichment in Eu (La) and a strong depletion in Ce. This distribution is compatible with REE mostly inherited from seawater, but a significant hydrothermal component is inferred for Eu. Profiles of redox-sensitive trace elements show great V, Cr, Ni, Co, Mo and U enrichments with respect to NASC standard. Part of this enrichment could derive from a direct precipitation from seawater, favoured by the euxinic conditions of the Silurian basin. Nevertheless, V (up to 5444 ppm) and Cr (up to 640 ppm) contents would require additional sources. These elements could be scavenged from seawater by exhalative particles in a hydrothermal derived plume that finally accumulated on the seafloor. In contrast, high Ni, Co and Mo values could be of hydrothermal origin. Sm–Nd isotopic analyses of feldspar-rich layers yielded an isochron age of 437±57 Ma (Llandoverian). These results, as well as the fine-grained textures, the lack of evidences of replacement and the pre-deformational and pre-metamorphic character support the syngenetic origin of the mineralization. Trace-element geochemistry and Sm–Nd isotopes are consistent with a submarine-exhalative origin of the mineralization processes, and suggest that the feldspar-rich levels are metaexhalites.
This study is an attempt to unravel the tectono-metamorphic history of high-grade metamorphic roc... more This study is an attempt to unravel the tectono-metamorphic history of high-grade metamorphic rocks in the Eastern Erzgebirge region. Metamorphism has strongly disturbed the primary petrological genetic characteristics of the rocks. We compare geological, geochemical, and petrological data, and zircon populations as well as isotope and geochronological data Ž. Ž. for the major gneiss units of the Eastern Erzgebirge; 1 coarse-to medium-grained AInner Grey GneissB, 2 fine-grained Ž. Ž. AOuter Grey GneissB, and 3 ARed GneissB. The Inner and Outer Grey Gneiss units MP–MT overprinted have very similar geochemical and mineralogical compositions, but they contain different zircon populations. The Inner Grey Gneiss is Ž. found to be of primary igneous origin as documented by the presence of long-prismatic, oscillatory zoned zircons 540 Ma and relics of granitic textures. Geochemical and isotope data classify the igneous precursor as a S-type granite. In contrast, Outer Grey Gneiss samples are free of long-prismatic zircons and contain zircons with signs of mechanical rounding through sedimentary transport. Geochemical data indicate greywackes as main previous precursor. The most euhedral zircons are Ž. zoned and document Neoproterozoic ca. 575 Ma source rocks eroded to form these greywackes. U–Pb-SHRIMP measurements revealed three further ancient sources, which zircons survived in both the Inner and Outer Grey Gneiss: Ž. Ž. Ž. These results point to absence of Grenvillian type sources and derivation of the crust from the West African Craton. The granite magma of the Inner Grey Gneiss was probably derived through in situ melting of the Outer Grey Gneiss sedimentary protolith as indicated q Electronic supplements available on the journal's homepage: http:rrwww.elsevier.comrlocaterlithos 0024-4937r01r$-see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž. PII: S 0 0 2 4-4 9 3 7 0 0 0 0 0 6 6-9
We investigated the isotope composition (O, C, Sr, Nd, Pb) in mineral separates of the two Precam... more We investigated the isotope composition (O, C, Sr, Nd, Pb) in mineral separates of the two Precambrian carbonatite complexes Tiksheozero (1.98 Ga) and Siilinjärvi (2.61 Ga) from the Karelian–Kola region in order to obtain information on Precambrian mantle heterogeneity. All isotope systems yield a large range of variations. The combination of cathodoluminescence imaging with stable and radiogenic isotopes on the same samples and mineral separates indicates various processes that caused shifts in isotope systems. Primary isotope signatures are preserved in most calcites (O, C, Sr, Pb), apatites (O, Sr, Nd), amphiboles (O), magnetites (O), and whole rocks (Sr, Nd). The primary igneous C and O isotope composition is different for both complexes (Tiksheozero: δ 13 C = − 5.0‰, δ 18 O = 6.9‰; Siilinjärvi: δ 13 C = −3.7‰, δ 18 O = 7.4‰) but very uniform and requires homogenization of both carbon and oxygen in the carbonatite melt. The lowest Sr isotope ratios of our carbonates and apatites from the Archaean Siilinjärvi (0.70137) and the Palaeoproterozoic Tiksheozero (0.70228) complexes are in the range of bulk silicate earth (BSE). Positive ε Nd values of the two carbonatites point to very early Archaean enrichment of Sm/Nd in the Fennoscandian mantle. No HIMU components could be detected in the two complexes, whereas Tiksheozero carbonatites give the first indication of Palaeoproterozoic U depletion for Fennoscandia. Sub-solidus exchange processes with water during emplacement and cooling of carbonatites caused an increase in the oxygen isotope composition of some carbonates and probably also an increase of their 87 Sr/ 86 Sr ratio. A larger increase of initial Sr isotope ratios was found in carbonatized silicic rocks compared to carbonatite bodies. The Svecofennian metamorphic overprint (1.9– 1.7 Ga) caused reset of Rb/Sr (mainly mica) and Pb/Pb (mainly apatite) isochron systems.
Zircon grains from two Precambrian carbonatites from Fennoscandia (Siilinjärvi and Tiksheozero) w... more Zircon grains from two Precambrian carbonatites from Fennoscandia (Siilinjärvi and Tiksheozero) were studied by in-situ geochemical and isotope methods. Zircon domains which preserved primary mantle signatures were identified by a combination of microscopic investigations of thin sections of the carbonatite rocks and separated zircon grains (optical microscopy, optical microscopy combined with cathodoluminescence (OM–CL), and scanning electron microscopy). All studied samples show evidence for alteration processes caused by infiltration of late-stage carbonatite melt/s or fluid/s. This led to different changes in the geochemistry and isotope composition of altered zircon domains. For the 2.6 Ga old Siilinjärvi carbonatite complex, zircon grains underwent solid state recrystallization mainly at their rims. Zircon cores often preserved primary mantle signatures and register high HREE/LREE ratios, undisturbed U–Pb ages, ε Hf values close to CHUR (chondritic uniform reservoir), and δ 18 O values typical for the mantle. The solid state recrystallization of zircon occurred mainly in contact with a late-stage carbonatite melt or fluid and led to (i) diffusion driven loss of HREE, Th, and U, (ii) partial disturbance of the U–Pb system, (iii) a small shift of the δ 18 O toward lower values, and (iv) higher CL intensities in such zircon domains. Contrarily, zircons from the 2.0 Ga old Tiksheozero complex underwent a coupled dissolution– reprecipitation process. Dissolved–reprecipitated zircon domains typically have a distinctive patchy texture in BSE images and sometimes contained abundant micro-inclusions of calcite, phlogopite, apatite, and baddeleyite. The EDS spectra documented that these non-luminescent zircon regions have much higher concentrations of Ca (and sometimes Fe) indicating an " impure " zircon composition. In addition, many trace elements are highly enriched in such " impure " zircon. Enrichment of Th (and to a lower degree of U) resulted in high Th/U ratios (>1) and disturbance of the U/Pb ages. Hf and O isotope values varied widely even within a single zircon grain. Isotope signatures attest a complex crystallization history of carbonatites and can best be interpreted as melt mixing processes of different carbonatite melt pulses. The non-luminescent reprecipitated zircon domains in Tiksheozero are often rimmed by zircon with high CL-intensity recrystallized in a solid state as well as by new zircon overgrowths. These zircon regions consist of pure zircon without elevated concentrations of Ca and Fe. Solid state recrystallization was accompanied by annealing that caused volume loss resulting in fractures that are now frequently filled with calcite (often containing a minor proportion of apatite) and phlogopite which sampled the withdrawn elements from the recrystallized zircon. Co-crystallizing calcite may thus lead to HREE depletion in newly grown zircon. It is expected that interaction with carbonatite melts can lead to similar changes in zircon grains from other sub-continental mantle rocks. Then, the primary geochemical and isotope systems of zircon may be changed despite its chemical and physical robustness. Infiltration of late-stage carbonatite melts seems to be a common process within many carbonatitic rocks that may hamper to determine the primary geochemical and isotope signatures of carbonatitic zircons.
The borehole at the southern par t of subglacial Lake Vostok has been drilled into an ice lay er ... more The borehole at the southern par t of subglacial Lake Vostok has been drilled into an ice lay er that has been refrozen from the lake water. This ice layer contains random sediment inclusions, eight of which have been studied using state-of the-art analytical techniques. Six inclusions comprise soft aggregates consisting mainly of clay-mica minerals and micron-sized quartz grains while
Zircon grains from two Precambrian carbonatites from Fennoscandia (Siilinjärvi and Tiksheozero) w... more Zircon grains from two Precambrian carbonatites from Fennoscandia (Siilinjärvi and Tiksheozero) were studied by in-situ geochemical and isotope methods. Zircon domains which preserved primary mantle signatures were identified by a combination of microscopic investigations of thin sections of the carbonatite rocks and separated zircon grains (optical microscopy, optical microscopy combined with cathodoluminescence (OM–CL), and scanning electron microscopy). All studied samples show evidence for alteration processes caused by infiltration of late-stage carbonatite melt/s or fluid/s. This led to different changes in the geochemistry and isotope composition of altered zircon domains. For the 2.6 Ga old Siilinjärvi carbonatite complex, zircon grains underwent solid state recrystallization mainly at their rims. Zircon cores often preserved primary mantle signatures and register high HREE/LREE ratios, undisturbed U–Pb ages, εHf values close to CHUR (chondritic uniform reservoir), and δ18O values typical for the mantle. The solid state recrystallization of zircon occurred mainly in contact with a late-stage carbonatite melt or fluid and led to (i) diffusion driven loss of HREE, Th, and U, (ii) partial disturbance of theU–Pb system, (iii) a small shift of the δ18O toward lower values, and (iv) higher CL intensities in such zircon domains. Contrarily, zircons from the 2.0 Ga old Tiksheozero complex underwent a coupled dissolution– reprecipitation process. Dissolved–reprecipitated zircon domains typically have a distinctive patchy texture in BSE images and sometimes contained abundant micro-inclusions of calcite, phlogopite, apatite, and baddeleyite. The EDS spectra documented that these non-luminescent zircon regions have much higher concentrations of Ca (and sometimes Fe) indicating an “impure” zircon composition. In addition, many trace elements are highly enriched in such “impure” zircon. Enrichment of Th (and to a lower degree of U) resulted in high Th/U ratios (>1) and disturbance of the U/Pb ages. Hf and O isotope values varied widely even within a single zircon grain. Isotope signatures attest a complex crystallization history of carbonatites and can best be interpreted as melt mixing processes of different carbonatite melt pulses. The non-luminescent reprecipitated zircon domains in Tiksheozero are often rimmed by zircon with high CL-intensity recrystallized in a solid state aswell as by new zircon overgrowths. These zircon regions consist of pure zircon without elevated concentrations of Ca and Fe. Solid state recrystallization was accompanied by annealing that caused volume loss resulting in fractures that are now frequently filled with calcite (often containing a minor proportion of apatite) and phlogopite which sampled the withdrawn elements from the recrystallized zircon. Co-crystallizing calcite may thus lead to HREE depletion in newly grown zircon. It is expected that interaction with carbonatite melts can lead to similar changes in zircon grains from other sub-continental mantle rocks. Then, the primary geochemical and isotope systems of zircon may be changed despite its chemical and physical robustness. Infiltration of late-stage carbonatite melts seems to be a common process within many carbonatitic rocks that may hamper to determine the primary geochemical and isotope signatures of carbonatitic zircons.
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Papers by Boris V Belyatsky
by in-situ geochemical and isotope methods. Zircon domains which preserved primary mantle signatures were
identified by a combination of microscopic investigations of thin sections of the carbonatite rocks and separated
zircon grains (optical microscopy, optical microscopy combined with cathodoluminescence (OM–CL), and scanning
electron microscopy). All studied samples show evidence for alteration processes caused by infiltration of
late-stage carbonatite melt/s or fluid/s. This led to different changes in the geochemistry and isotope composition
of altered zircon domains. For the 2.6 Ga old Siilinjärvi carbonatite complex, zircon grains underwent solid state
recrystallization mainly at their rims. Zircon cores often preserved primary mantle signatures and register high
HREE/LREE ratios, undisturbed U–Pb ages, εHf values close to CHUR (chondritic uniform reservoir), and δ18O
values typical for the mantle. The solid state recrystallization of zircon occurred mainly in contact with a
late-stage carbonatite melt or fluid and led to (i) diffusion driven loss of HREE, Th, and U, (ii) partial disturbance
of theU–Pb system, (iii) a small shift of the δ18O toward lower values, and (iv) higher CL intensities in such zircon
domains. Contrarily, zircons from the 2.0 Ga old Tiksheozero complex underwent a coupled dissolution–
reprecipitation process. Dissolved–reprecipitated zircon domains typically have a distinctive patchy texture in
BSE images and sometimes contained abundant micro-inclusions of calcite, phlogopite, apatite, and baddeleyite.
The EDS spectra documented that these non-luminescent zircon regions have much higher concentrations of Ca
(and sometimes Fe) indicating an “impure” zircon composition. In addition, many trace elements are highly
enriched in such “impure” zircon. Enrichment of Th (and to a lower degree of U) resulted in high Th/U ratios
(>1) and disturbance of the U/Pb ages. Hf and O isotope values varied widely even within a single zircon
grain. Isotope signatures attest a complex crystallization history of carbonatites and can best be interpreted as
melt mixing processes of different carbonatite melt pulses. The non-luminescent reprecipitated zircon domains
in Tiksheozero are often rimmed by zircon with high CL-intensity recrystallized in a solid state aswell as by new
zircon overgrowths. These zircon regions consist of pure zircon without elevated concentrations of Ca and Fe.
Solid state recrystallization was accompanied by annealing that caused volume loss resulting in fractures that
are now frequently filled with calcite (often containing a minor proportion of apatite) and phlogopite which
sampled the withdrawn elements from the recrystallized zircon. Co-crystallizing calcite may thus lead to HREE
depletion in newly grown zircon. It is expected that interaction with carbonatite melts can lead to similar
changes in zircon grains from other sub-continental mantle rocks. Then, the primary geochemical and isotope
systems of zircon may be changed despite its chemical and physical robustness. Infiltration of late-stage
carbonatite melts seems to be a common process within many carbonatitic rocks that may hamper to determine
the primary geochemical and isotope signatures of carbonatitic zircons.
by in-situ geochemical and isotope methods. Zircon domains which preserved primary mantle signatures were
identified by a combination of microscopic investigations of thin sections of the carbonatite rocks and separated
zircon grains (optical microscopy, optical microscopy combined with cathodoluminescence (OM–CL), and scanning
electron microscopy). All studied samples show evidence for alteration processes caused by infiltration of
late-stage carbonatite melt/s or fluid/s. This led to different changes in the geochemistry and isotope composition
of altered zircon domains. For the 2.6 Ga old Siilinjärvi carbonatite complex, zircon grains underwent solid state
recrystallization mainly at their rims. Zircon cores often preserved primary mantle signatures and register high
HREE/LREE ratios, undisturbed U–Pb ages, εHf values close to CHUR (chondritic uniform reservoir), and δ18O
values typical for the mantle. The solid state recrystallization of zircon occurred mainly in contact with a
late-stage carbonatite melt or fluid and led to (i) diffusion driven loss of HREE, Th, and U, (ii) partial disturbance
of theU–Pb system, (iii) a small shift of the δ18O toward lower values, and (iv) higher CL intensities in such zircon
domains. Contrarily, zircons from the 2.0 Ga old Tiksheozero complex underwent a coupled dissolution–
reprecipitation process. Dissolved–reprecipitated zircon domains typically have a distinctive patchy texture in
BSE images and sometimes contained abundant micro-inclusions of calcite, phlogopite, apatite, and baddeleyite.
The EDS spectra documented that these non-luminescent zircon regions have much higher concentrations of Ca
(and sometimes Fe) indicating an “impure” zircon composition. In addition, many trace elements are highly
enriched in such “impure” zircon. Enrichment of Th (and to a lower degree of U) resulted in high Th/U ratios
(>1) and disturbance of the U/Pb ages. Hf and O isotope values varied widely even within a single zircon
grain. Isotope signatures attest a complex crystallization history of carbonatites and can best be interpreted as
melt mixing processes of different carbonatite melt pulses. The non-luminescent reprecipitated zircon domains
in Tiksheozero are often rimmed by zircon with high CL-intensity recrystallized in a solid state aswell as by new
zircon overgrowths. These zircon regions consist of pure zircon without elevated concentrations of Ca and Fe.
Solid state recrystallization was accompanied by annealing that caused volume loss resulting in fractures that
are now frequently filled with calcite (often containing a minor proportion of apatite) and phlogopite which
sampled the withdrawn elements from the recrystallized zircon. Co-crystallizing calcite may thus lead to HREE
depletion in newly grown zircon. It is expected that interaction with carbonatite melts can lead to similar
changes in zircon grains from other sub-continental mantle rocks. Then, the primary geochemical and isotope
systems of zircon may be changed despite its chemical and physical robustness. Infiltration of late-stage
carbonatite melts seems to be a common process within many carbonatitic rocks that may hamper to determine
the primary geochemical and isotope signatures of carbonatitic zircons.