—Experimental studies and analysis of silicate inclusions testify to the magmatic genesis of part... more —Experimental studies and analysis of silicate inclusions testify to the magmatic genesis of part of chromian spinel from the Triassic deposits of the northeastern Siberian Platform. The compositions of melt inclusions in the chromian spinel show the participation of alkaline (potassic) magmas in their crystallization. Most data indicate the presence of magmatic systems similar to melts in the Guli ultrabasic massif in the northern Siberian Platform. Studies of the distribution of trace and rare-earth elements in the melt inclusions show the existence of several magmatic systems. These are, first of all, magmas that formed Guli-type ultrabasic massifs and gave rise to meimechites and picrites in the Maimecha–Kotui province. There are also plume-related magmatic systems with kimberlites, lamprophyres, and/or other continental “hot spots”. The composition of the melt inclusions suggests the existence of several types of the primary sources of chromian spinel in the northeastern Siberian Platform, which confirms the earlier data on the heterogeneous composition of the deposits of the diamondiferous Carnian (Upper Triassic) Stage. Applying computer modeling with the well-known COMAGMAT, PETROLOG, and WinPLtb programs as well as the Ol–Sp geothermometers based on the melt inclusions in chromian spinel from the Triassic deposits of the northeastern Siberian Platform, we have determined the P–T conditions of crystallization of minerals in the igneous rocks being the sources of the examined chromites. The temperature of liquidus crystallization of chromian spinel is 1324–1275 ºC. The P–T conditions of formation of olivine and clinopyroxene inclusions in it are estimated at ca. 4.5–4.1 kbar, 1510–1150 ºC and 3.2–1.0 kbar, 1285–1200 ºC, respectively.
<p>Directed garnet growth in a peraluminous pegmatoid from the Moldanubian zone in ... more <p>Directed garnet growth in a peraluminous pegmatoid from the Moldanubian zone in the Bohemian Massif (AT) formed almandine-spessartine garnet that crystallized at the transition from melt to subsolidus stage. One side of the garnet crystal was in contact with a solidified matrix, the other with melt, resulting in asymmetric microstructure, crystal morphology and major element compositional zoning.</p> <p>Based on major element composition, three garnet growth stages are inferred: (i) magmatic stage during which garnet grew as part of the coarse-grained assemblage Pl + Grt + Ky + Bt. This growth zone has the highest Mn content and P concentrations of 0.4 – 0.5 wt%, (ii) intermediate stage with decreasing Mn, and increasing Fe, Mg and Ca contents and Mg# (=Mg/(Mg+Fe)), (iii) subsolidus stage forming garnet reaction rims with highest Mg and Ca contents and Mg#.</p> <p>Growth stage (i) involves the development of sector zoning, which is defined by the colour of garnet and the presence of < 1 micrometer sized inclusions observed in optical light microscope (OM). These are dominated by phosphates in Grt{110} growth sectors and by rutile in Grt{112} sectors. Additionally, nanoinclusions of 20 – 50 nm size were identified by scanning transmission electron microscopy of garnet even for zones that appear inclusion-free in OM.</p> <p>Chemical compositions obtained from electron microprobe analyses integrate over garnet and the inclusions and thus also reflect the different nanoinclusion-contents of the two garnet growth sectors. Compared to the Grt{112} sector, the Grt{110} sector is c. 0.1 wt% higher in P, 100 – 150 ppm higher in Na, and 100 – 200 ppm lower in Ti, which is in line with the prevalence of Na-containing phosphates and the comparatively lower abundance of rutile in this sector.</p> <p>The following scenarios are considered for the genesis of the nanoinclusions: As sector zoning has developed during magmatic conditions of growth stage (i), overgrowth of pre-existing accessory phases is implausible, as no mechanism for selective incorporation of different phases at different facets is known. Instead, facet specific minor and trace element partitioning during garnet growth and subsequent exsolution, or alternatively, sector specific nucleation and co-growth of accessory rutile and phosphates are both reasonable explanations for the observed distribution of inclusions in specific garnet sectors. These considerations indicate facet-selective processes likely related to the different crystal structure features exposed at the interfaces in contact with the melt.</p> <p>We conclude that facet specific formation of nanoinclusions is an important factor controlling the trace element composition of pegmatoid garnet apart from bulk melt composition and pT-conditions. When interpreting P, Na and Ti contents in garnet, the potential presence of nanoinclusions that are invisible in the optical light microscope needs to be accounted for, as they may be more widespread in pegmatoid garnet than expected.</p> <p>The study was funded by the Austrian Science Fund (FWF): I4285-N37 and the Slovenian Research Agency (ARRS): N1-0115.</p>
Typically, 40Ar/39Ar dating of phlogopites from deep-seated xenoliths of kimberlite pipes produce... more Typically, 40Ar/39Ar dating of phlogopites from deep-seated xenoliths of kimberlite pipes produces estimates that suggest much older ages than those when these pipes were intruded. High-pressure (3 GPa) laboratory experiments enabled the authors to explore the behaviour of argon in the phlogopite structure under the conditions that correspond to the mantle, at the temperatures (from 700 to 1000 °С), far exceeding closure temperature of the K/Ar isotopic system. “Volume diffusion” remains foremost for describing the mobility of argon in phlogopite at high pressures. The mantle material age can be estimated through the dating of the phlogopites from deep-seated xenoliths of kimberlites, employing the 40Ar/39Ar method, subject to correction for a partial loss of radiogenic 40Ar when xenolith moves upwards to the Earth’s surface. The obtained data served as the basis for proposing the behaviour model of the K/Ar isotopic system of minerals in conditions of great depths (lower crust, man...
Today, it is known that the majority of diamonds are crystallized mostly from a metasomatic agent... more Today, it is known that the majority of diamonds are crystallized mostly from a metasomatic agent close in the main characteristics to carbonatite melts acting upon mantle rocks, and therefore, diamonds are located in the interstitial space of these rocks. So far, diamond has never been found included in other kimberlitic or xenolithic minerals. We have found a diamond inclusion inside the kimberlitic olivine grain, which is the first find of its kind. The diamond crystal is to have been captured by the growing olivine at quite high temperatures (more than 1400 °C) early in the history of the cratonic lithospheric mantle formation. The event had taken place long before the depleted peridotite cooled down to the temperature of the Middle Archean cratonic geotherm corresponding to the diamond stability field at depths where carbonatite melts can react with depleted peridotite, making it a diamond-bearing rock. On the one hand, this find provides evidence that diamonds can crystallize ...
34 LILE-enriched chromium titanates of the magnetoplumbite (AM12O19) and crichtonite 35 (ABC18T2O... more 34 LILE-enriched chromium titanates of the magnetoplumbite (AM12O19) and crichtonite 35 (ABC18T2O38) groups were recognized as abundant inclusions in orthopyroxene grains in a 36 mantle-derived xenolith from the Udachnaya-East kimberlite pipe, Daldyn field, Siberian craton. 37 The studied xenolith consists of three parts: an orthopyroxenite, a garnet clinopyroxenite, and a 38 garnet-orthopyroxene intermediate domain between the two. Within the host enstatite (Mg# 39 92.6) in the orthopyroxenitic part of the sample titanate inclusions are associated with Cr-spinel, 40 diopside, rutile, Mg-Cr-ilmenite, and pentlandite. Crichtonite-group minerals also occur as 41 lamellae inclusions in pyrope grains of the intermediate domain adjacent to the orthopyroxenite, 42 as well as in interstitial to enstatite oxide intergrowths together with Cr-spinel, rutile, and 43 ilmenite. 44 Yimengite-hawthorneite inclusions in enstatite contain (wt%) 3.72-8.04 BaO, 2.05-3.43 45 K2O, and 0.06-0.48 CaO. The...
Abstract Garnet–(olivine) websterite xenoliths from the lithospheric mantle of the central and no... more Abstract Garnet–(olivine) websterite xenoliths from the lithospheric mantle of the central and northeastern parts of the Siberian Craton contain exsolution microstructures after Si- and Ti-rich precursor garnets. We petrographically, geochemically, and thermobarometrically investigated 13 such xenoliths from the Mir, Obnazhennaya, and Udachnaya kimberlite pipes. All samples contain garnet grains with needle- to lamellae-shaped precipitates (up to 3.0 vol%), including Ti-oxide and/or pyroxene. Orthopyroxene and clinopyroxene grains host oriented lamellae of complementary Ca-rich and Ca-poor pyroxene, respectively, in addition to lamellae of garnet and Ti- and/or Cr-oxides. The common exsolution lamellae assemblages in garnet and pyroxene imply that exsolution occurred during cooling from high-temperature precursors. Exsolution is unlikely to have resulted from variations in pressure, given experimental and thermodynamic constraints. Host mineral partitioning of transition metal and lanthanide elements with different diffusivities record temperatures that range between those of local geotherms and a dry pyroxenite solidus. Inferred magmatic minimum temperatures of 1500–1700 °C satisfy the physical conditions predicted from experimental studies of the solubility of excess Si and Ti in garnet. Granular inclusions of all major minerals within each other imply an overlapping crystallisation history. The reconstructed compositions of the websterite whole-rocks have high MgO contents (15.7–35.7 wt%). A plot of MgO/SiO2 versus SiO2 forms an array, apart from the compositions of natural websterites that formed by interaction of peridotite with basaltic or siliceous melts. The array overlaps the compositional range of komatiite flows from Commondale and Barberton, South Africa, including spinifex, massive, and cumulate subtypes of komatiites. Other major and minor element abundances and ratios of the Siberian websterite suite resemble those of South African Al-enriched komatiites and are distinct from melt–rock reaction websterites. Therefore, the mineral microstructures and geochemistry of the Siberian websterites are suggestive of the former presence of a thermal anomaly. We propose that mantle plume activity or a similar form of lower-mantle ascent played a major role in stabilising cratonic nuclei before amalgamation of the present-day Siberian Craton.
Abstract40Ar/39Ar dating of phlogopite from kelyphitic rims around garnet grains from the Udachna... more Abstract40Ar/39Ar dating of phlogopite from kelyphitic rims around garnet grains from the Udachnaya–Vostochnaya kimberlite pipe in the Sakha (Yakutia) Republic (Russia) revealed that when this mineral has contact with a kimberlite melt its age corresponds (within error limits) to that of the formation of the kimberlite pipe, thus indicating that the method may be used for dating kimberlites and related rocks. In mantle xenoliths, kelyphitic phlogopites rimming garnet grains partially lose radiogenic Ar, which results in a complex age spectrum. Rejuvenation of the K/Ar system in them is determined by the thermal impact of the kimberlite melt on captured rocks.
—Experimental studies and analysis of silicate inclusions testify to the magmatic genesis of part... more —Experimental studies and analysis of silicate inclusions testify to the magmatic genesis of part of chromian spinel from the Triassic deposits of the northeastern Siberian Platform. The compositions of melt inclusions in the chromian spinel show the participation of alkaline (potassic) magmas in their crystallization. Most data indicate the presence of magmatic systems similar to melts in the Guli ultrabasic massif in the northern Siberian Platform. Studies of the distribution of trace and rare-earth elements in the melt inclusions show the existence of several magmatic systems. These are, first of all, magmas that formed Guli-type ultrabasic massifs and gave rise to meimechites and picrites in the Maimecha–Kotui province. There are also plume-related magmatic systems with kimberlites, lamprophyres, and/or other continental “hot spots”. The composition of the melt inclusions suggests the existence of several types of the primary sources of chromian spinel in the northeastern Siberian Platform, which confirms the earlier data on the heterogeneous composition of the deposits of the diamondiferous Carnian (Upper Triassic) Stage. Applying computer modeling with the well-known COMAGMAT, PETROLOG, and WinPLtb programs as well as the Ol–Sp geothermometers based on the melt inclusions in chromian spinel from the Triassic deposits of the northeastern Siberian Platform, we have determined the P–T conditions of crystallization of minerals in the igneous rocks being the sources of the examined chromites. The temperature of liquidus crystallization of chromian spinel is 1324–1275 ºC. The P–T conditions of formation of olivine and clinopyroxene inclusions in it are estimated at ca. 4.5–4.1 kbar, 1510–1150 ºC and 3.2–1.0 kbar, 1285–1200 ºC, respectively.
<p>Directed garnet growth in a peraluminous pegmatoid from the Moldanubian zone in ... more <p>Directed garnet growth in a peraluminous pegmatoid from the Moldanubian zone in the Bohemian Massif (AT) formed almandine-spessartine garnet that crystallized at the transition from melt to subsolidus stage. One side of the garnet crystal was in contact with a solidified matrix, the other with melt, resulting in asymmetric microstructure, crystal morphology and major element compositional zoning.</p> <p>Based on major element composition, three garnet growth stages are inferred: (i) magmatic stage during which garnet grew as part of the coarse-grained assemblage Pl + Grt + Ky + Bt. This growth zone has the highest Mn content and P concentrations of 0.4 – 0.5 wt%, (ii) intermediate stage with decreasing Mn, and increasing Fe, Mg and Ca contents and Mg# (=Mg/(Mg+Fe)), (iii) subsolidus stage forming garnet reaction rims with highest Mg and Ca contents and Mg#.</p> <p>Growth stage (i) involves the development of sector zoning, which is defined by the colour of garnet and the presence of < 1 micrometer sized inclusions observed in optical light microscope (OM). These are dominated by phosphates in Grt{110} growth sectors and by rutile in Grt{112} sectors. Additionally, nanoinclusions of 20 – 50 nm size were identified by scanning transmission electron microscopy of garnet even for zones that appear inclusion-free in OM.</p> <p>Chemical compositions obtained from electron microprobe analyses integrate over garnet and the inclusions and thus also reflect the different nanoinclusion-contents of the two garnet growth sectors. Compared to the Grt{112} sector, the Grt{110} sector is c. 0.1 wt% higher in P, 100 – 150 ppm higher in Na, and 100 – 200 ppm lower in Ti, which is in line with the prevalence of Na-containing phosphates and the comparatively lower abundance of rutile in this sector.</p> <p>The following scenarios are considered for the genesis of the nanoinclusions: As sector zoning has developed during magmatic conditions of growth stage (i), overgrowth of pre-existing accessory phases is implausible, as no mechanism for selective incorporation of different phases at different facets is known. Instead, facet specific minor and trace element partitioning during garnet growth and subsequent exsolution, or alternatively, sector specific nucleation and co-growth of accessory rutile and phosphates are both reasonable explanations for the observed distribution of inclusions in specific garnet sectors. These considerations indicate facet-selective processes likely related to the different crystal structure features exposed at the interfaces in contact with the melt.</p> <p>We conclude that facet specific formation of nanoinclusions is an important factor controlling the trace element composition of pegmatoid garnet apart from bulk melt composition and pT-conditions. When interpreting P, Na and Ti contents in garnet, the potential presence of nanoinclusions that are invisible in the optical light microscope needs to be accounted for, as they may be more widespread in pegmatoid garnet than expected.</p> <p>The study was funded by the Austrian Science Fund (FWF): I4285-N37 and the Slovenian Research Agency (ARRS): N1-0115.</p>
Typically, 40Ar/39Ar dating of phlogopites from deep-seated xenoliths of kimberlite pipes produce... more Typically, 40Ar/39Ar dating of phlogopites from deep-seated xenoliths of kimberlite pipes produces estimates that suggest much older ages than those when these pipes were intruded. High-pressure (3 GPa) laboratory experiments enabled the authors to explore the behaviour of argon in the phlogopite structure under the conditions that correspond to the mantle, at the temperatures (from 700 to 1000 °С), far exceeding closure temperature of the K/Ar isotopic system. “Volume diffusion” remains foremost for describing the mobility of argon in phlogopite at high pressures. The mantle material age can be estimated through the dating of the phlogopites from deep-seated xenoliths of kimberlites, employing the 40Ar/39Ar method, subject to correction for a partial loss of radiogenic 40Ar when xenolith moves upwards to the Earth’s surface. The obtained data served as the basis for proposing the behaviour model of the K/Ar isotopic system of minerals in conditions of great depths (lower crust, man...
Today, it is known that the majority of diamonds are crystallized mostly from a metasomatic agent... more Today, it is known that the majority of diamonds are crystallized mostly from a metasomatic agent close in the main characteristics to carbonatite melts acting upon mantle rocks, and therefore, diamonds are located in the interstitial space of these rocks. So far, diamond has never been found included in other kimberlitic or xenolithic minerals. We have found a diamond inclusion inside the kimberlitic olivine grain, which is the first find of its kind. The diamond crystal is to have been captured by the growing olivine at quite high temperatures (more than 1400 °C) early in the history of the cratonic lithospheric mantle formation. The event had taken place long before the depleted peridotite cooled down to the temperature of the Middle Archean cratonic geotherm corresponding to the diamond stability field at depths where carbonatite melts can react with depleted peridotite, making it a diamond-bearing rock. On the one hand, this find provides evidence that diamonds can crystallize ...
34 LILE-enriched chromium titanates of the magnetoplumbite (AM12O19) and crichtonite 35 (ABC18T2O... more 34 LILE-enriched chromium titanates of the magnetoplumbite (AM12O19) and crichtonite 35 (ABC18T2O38) groups were recognized as abundant inclusions in orthopyroxene grains in a 36 mantle-derived xenolith from the Udachnaya-East kimberlite pipe, Daldyn field, Siberian craton. 37 The studied xenolith consists of three parts: an orthopyroxenite, a garnet clinopyroxenite, and a 38 garnet-orthopyroxene intermediate domain between the two. Within the host enstatite (Mg# 39 92.6) in the orthopyroxenitic part of the sample titanate inclusions are associated with Cr-spinel, 40 diopside, rutile, Mg-Cr-ilmenite, and pentlandite. Crichtonite-group minerals also occur as 41 lamellae inclusions in pyrope grains of the intermediate domain adjacent to the orthopyroxenite, 42 as well as in interstitial to enstatite oxide intergrowths together with Cr-spinel, rutile, and 43 ilmenite. 44 Yimengite-hawthorneite inclusions in enstatite contain (wt%) 3.72-8.04 BaO, 2.05-3.43 45 K2O, and 0.06-0.48 CaO. The...
Abstract Garnet–(olivine) websterite xenoliths from the lithospheric mantle of the central and no... more Abstract Garnet–(olivine) websterite xenoliths from the lithospheric mantle of the central and northeastern parts of the Siberian Craton contain exsolution microstructures after Si- and Ti-rich precursor garnets. We petrographically, geochemically, and thermobarometrically investigated 13 such xenoliths from the Mir, Obnazhennaya, and Udachnaya kimberlite pipes. All samples contain garnet grains with needle- to lamellae-shaped precipitates (up to 3.0 vol%), including Ti-oxide and/or pyroxene. Orthopyroxene and clinopyroxene grains host oriented lamellae of complementary Ca-rich and Ca-poor pyroxene, respectively, in addition to lamellae of garnet and Ti- and/or Cr-oxides. The common exsolution lamellae assemblages in garnet and pyroxene imply that exsolution occurred during cooling from high-temperature precursors. Exsolution is unlikely to have resulted from variations in pressure, given experimental and thermodynamic constraints. Host mineral partitioning of transition metal and lanthanide elements with different diffusivities record temperatures that range between those of local geotherms and a dry pyroxenite solidus. Inferred magmatic minimum temperatures of 1500–1700 °C satisfy the physical conditions predicted from experimental studies of the solubility of excess Si and Ti in garnet. Granular inclusions of all major minerals within each other imply an overlapping crystallisation history. The reconstructed compositions of the websterite whole-rocks have high MgO contents (15.7–35.7 wt%). A plot of MgO/SiO2 versus SiO2 forms an array, apart from the compositions of natural websterites that formed by interaction of peridotite with basaltic or siliceous melts. The array overlaps the compositional range of komatiite flows from Commondale and Barberton, South Africa, including spinifex, massive, and cumulate subtypes of komatiites. Other major and minor element abundances and ratios of the Siberian websterite suite resemble those of South African Al-enriched komatiites and are distinct from melt–rock reaction websterites. Therefore, the mineral microstructures and geochemistry of the Siberian websterites are suggestive of the former presence of a thermal anomaly. We propose that mantle plume activity or a similar form of lower-mantle ascent played a major role in stabilising cratonic nuclei before amalgamation of the present-day Siberian Craton.
Abstract40Ar/39Ar dating of phlogopite from kelyphitic rims around garnet grains from the Udachna... more Abstract40Ar/39Ar dating of phlogopite from kelyphitic rims around garnet grains from the Udachnaya–Vostochnaya kimberlite pipe in the Sakha (Yakutia) Republic (Russia) revealed that when this mineral has contact with a kimberlite melt its age corresponds (within error limits) to that of the formation of the kimberlite pipe, thus indicating that the method may be used for dating kimberlites and related rocks. In mantle xenoliths, kelyphitic phlogopites rimming garnet grains partially lose radiogenic Ar, which results in a complex age spectrum. Rejuvenation of the K/Ar system in them is determined by the thermal impact of the kimberlite melt on captured rocks.
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