Silicate inclusions (< 10 μm size) in Troodos ophiolite chromites were studied for the first time in terms of major, minor and Rare Earth Elements to investigate magmatic processes that lead to chromitite crystallization. The study was focused on silicate inclusions detected in chromite grains of chromitite pods in the enveloping dunite, and their mineral chemistry was compared with that of host peridotite phases. Samples were collected at Mount Olympus where lherzolite hosts dunite lenses that envelope chromitite pods, lenses and irregular layers. On the basis of textural and geochemical evidences two types of inclusions within chromite grains have been outlined: i) Primary inclusions within chromite in chromitites and dunites. They are mainly monophasic (diopside, olivine, or Na-Ca amphibole), rarely biphasic and comprise diopside and Na-Ca amphibole or diopside and olivine. They mainly occur at the core of host grain, and are characterized by regular rims and negative crystal shapes. ii) Secondary inclusions, characterized by irregular shapes, containing amphibole. They are richer in Mg, Ca and Cr than the primary inclusions or serpentine, and mainly occur close to chromite rims, connected to interstitial silicate matrix by fractures. Na-Ca amphibole was found only in chromitites. Diopside is Cr-rich and shows 23-25 wt% CaO; 0.5-1.8 wt% Cr2O3; 16.9-17.8 wt% MgO, and 0.7-1.8 wt% FeO. Olivine has 3.5-4.2 wt% FeO; 51.7-52.4 wt% MgO. Na-Ca amphibole has Al2O3 ranging from 5 to 9.30 wt%, Cr2O3 from 1.50 to 3.30, MgO from 21 to 23 wt%, CaO from 6 to12 wt% and NaO from 3 to 5.7 wt%. In partially serpentinized dunite enveloping chromitites, primary silicate inclusions show diopside with lower CaO content (21.6-23.5 wt%) and slightly higher FeO (1.5-2.0 wt%); olivine with lower MgO (around 50 wt%) and higher FeO (around 6.1 wt%). Compared with that in inclusions, diopside in lherzolite is lower in Cr2O3 (between 0.3 and 0.6 wt%) and shows a much wider FeO range (between 0.7 and 3.9 wt%). Olivine in lherzolite occurs as fresh crystals with 12.2-14.3 wt% FeO. Plagioclase, occuring as late impregnation, has anortitic composition showing CaO contents between 18 and 18.5 wt%. REE data obtained with the Cameca IMS 4f ion microprobe (CNR-IGG, Pavia) following an optimized analytical procedure (see Spengler et al. 2006, for details) were collected on diopside in chromite from chromitite, on enveloping dunite and in lherzolite. Total REE content (in ppm) is similar in lherzolite (REEtot=2.79-3.21) and dunite (REEtot=2.63-2.71), higher in chromitite (REEtot=6.55-13.03) and much higher in dunite close to chromitite (REEtot=16.15-23.41). Lherzolite shows a strong LREE-depletion, with (La/Yb)N=0.040-0.052. This depletion decreases moving towards dunite and chromitite, with (La/Yb)N=0.19-0.20 in dunites, (La/Yb)N=0.29-0.40 in dunites close to chromitites where (La/Yb)N=0.11-0.78. LREE depletion in diopside from lherzolites suggests that lherzolites underwent a melting event. On the other hand the presence of primary amphibole only in inclusions suggests that a fluid-rich melt played an important role in chromitite crystallization. Finally, the contrasting REE behaviour of dunite suggest that primary restitic dunites coexist with metasomatic dunites. We make the hypothesis that a melting event that depleted lherzolite diopside in LREE and formed restitic dunites was followed by the income of a fluid-rich melt responsible for the formation of REE-rich chromitite and secondary dunite. Spengler D., Van Roermund H.L.M., Drury M.R, Ottolini L., Mason P.R.D., Davies G.R.: Deep origin and hot melting of an Archaean orogenic peridotite massif in Norway. Nature, 440, (13 April Issue), (2006), 913-917. Silicate inclusions (< 10 μm size) in Troodos ophiolite chromites were studied for the first time in terms of major, minor and Rare Earth Elements to investigate magmatic processes that lead to chromitite crystallization. The study was focused on silicate inclusions detected in chromite grains of chromitite pods in the enveloping dunite, and their mineral chemistry was compared with that of host peridotite phases. Samples were collected at Mount Olympus where lherzolite hosts dunite lenses that envelope chromitite pods, lenses and irregular layers. On the basis of textural and geochemical evidences two types of inclusions within chromite grains have been outlined: i) Primary inclusions within chromite in chromitites and dunites. They are mainly monophasic (diopside, olivine, or Na-Ca amphibole), rarely biphasic and comprise diopside and Na-Ca amphibole or diopside and olivine. They mainly occur at the core of host grain, and are characterized by regular rims and negative crystal shapes. ii) Secondary inclusions, characterized by irregular shapes, containing amphibole. They are richer in Mg, Ca and Cr than the primary inclusions or serpentine, and mainly occur close to chromite rims, connected to interstitial silicate matrix by fractures. Na-Ca amphibole was found only in chromitites. Diopside is Cr-rich and shows 23-25 wt% CaO; 0.5-1.8 wt% Cr2O3; 16.9-17.8 wt% MgO, and 0.7-1.8 wt% FeO. Olivine has 3.5-4.2 wt% FeO; 51.7-52.4 wt% MgO. Na-Ca amphibole has Al2O3 ranging from 5 to 9.30 wt%, Cr2O3 from 1.50 to 3.30, MgO from 21 to 23 wt%, CaO from 6 to12 wt% and NaO from 3 to 5.7 wt%. In partially serpentinized dunite enveloping chromitites, primary silicate inclusions show diopside with lower CaO content (21.6-23.5 wt%) and slightly higher FeO (1.5-2.0 wt%); olivine with lower MgO (around 50 wt%) and higher FeO (around 6.1 wt%). Compared with that in inclusions, diopside in lherzolite is lower in Cr2O3 (between 0.3 and 0.6 wt%) and shows a much wider FeO range (between 0.7 and 3.9 wt%). Olivine in lherzolite occurs as fresh crystals with 12.2-14.3 wt% FeO. Plagioclase, occuring as late impregnation, has anortitic composition showing CaO contents between 18 and 18.5 wt%. REE data obtained with the Cameca IMS 4f ion microprobe (CNR-IGG, Pavia) following an optimized analytical procedure (see Spengler et al. 2006, for details) were collected on diopside in chromite from chromitite, on enveloping dunite and in lherzolite. Total REE content (in ppm) is similar in lherzolite (REEtot=2.79-3.21) and dunite (REEtot=2.63-2.71), higher in chromitite (REEtot=6.55-13.03) and much higher in dunite close to chromitite (REEtot=16.15-23.41). Lherzolite shows a strong LREE-depletion, with (La/Yb)N=0.040-0.052. This depletion decreases moving towards dunite and chromitite, with (La/Yb)N=0.19-0.20 in dunites, (La/Yb)N=0.29-0.40 in dunites close to chromitites where (La/Yb)N=0.11-0.78. LREE depletion in diopside from lherzolites suggests that lherzolites underwent a melting event. On the other hand the presence of primary amphibole only in inclusions suggests that a fluid-rich melt played an important role in chromitite crystallization. Finally, the contrasting REE behaviour of dunite suggest that primary restitic dunites coexist with metasomatic dunites. We make the hypothesis that a melting event that depleted lherzolite diopside in LREE and formed restitic dunites was followed by the income of a fluid-rich melt responsible for the formation of REE-rich chromitite and secondary dunite. Spengler D., Van Roermund H.L.M., Drury M.R, Ottolini L., Mason P.R.D., Davies G.R.: Deep origin and hot melting of an Archaean orogenic peridotite massif in Norway. Nature, 440, (13 April Issue), (2006), 913-917.

An (EMPA+SIMS) investigation of silicate inclusions in Troodos chromitites / A. Merlini, G. Grieco, V. Diella, L. Ottolini. - In: EPITOME. - ISSN 1972-1552. - 2007:(2007 Sep), pp. 444-444. ((Intervento presentato al 6. convegno Geoitalia tenutosi a Rimini nel 2007.

An (EMPA+SIMS) investigation of silicate inclusions in Troodos chromitites

A. Merlini
Primo
;
G. Grieco
Secondo
;
2007

Abstract

Silicate inclusions (< 10 μm size) in Troodos ophiolite chromites were studied for the first time in terms of major, minor and Rare Earth Elements to investigate magmatic processes that lead to chromitite crystallization. The study was focused on silicate inclusions detected in chromite grains of chromitite pods in the enveloping dunite, and their mineral chemistry was compared with that of host peridotite phases. Samples were collected at Mount Olympus where lherzolite hosts dunite lenses that envelope chromitite pods, lenses and irregular layers. On the basis of textural and geochemical evidences two types of inclusions within chromite grains have been outlined: i) Primary inclusions within chromite in chromitites and dunites. They are mainly monophasic (diopside, olivine, or Na-Ca amphibole), rarely biphasic and comprise diopside and Na-Ca amphibole or diopside and olivine. They mainly occur at the core of host grain, and are characterized by regular rims and negative crystal shapes. ii) Secondary inclusions, characterized by irregular shapes, containing amphibole. They are richer in Mg, Ca and Cr than the primary inclusions or serpentine, and mainly occur close to chromite rims, connected to interstitial silicate matrix by fractures. Na-Ca amphibole was found only in chromitites. Diopside is Cr-rich and shows 23-25 wt% CaO; 0.5-1.8 wt% Cr2O3; 16.9-17.8 wt% MgO, and 0.7-1.8 wt% FeO. Olivine has 3.5-4.2 wt% FeO; 51.7-52.4 wt% MgO. Na-Ca amphibole has Al2O3 ranging from 5 to 9.30 wt%, Cr2O3 from 1.50 to 3.30, MgO from 21 to 23 wt%, CaO from 6 to12 wt% and NaO from 3 to 5.7 wt%. In partially serpentinized dunite enveloping chromitites, primary silicate inclusions show diopside with lower CaO content (21.6-23.5 wt%) and slightly higher FeO (1.5-2.0 wt%); olivine with lower MgO (around 50 wt%) and higher FeO (around 6.1 wt%). Compared with that in inclusions, diopside in lherzolite is lower in Cr2O3 (between 0.3 and 0.6 wt%) and shows a much wider FeO range (between 0.7 and 3.9 wt%). Olivine in lherzolite occurs as fresh crystals with 12.2-14.3 wt% FeO. Plagioclase, occuring as late impregnation, has anortitic composition showing CaO contents between 18 and 18.5 wt%. REE data obtained with the Cameca IMS 4f ion microprobe (CNR-IGG, Pavia) following an optimized analytical procedure (see Spengler et al. 2006, for details) were collected on diopside in chromite from chromitite, on enveloping dunite and in lherzolite. Total REE content (in ppm) is similar in lherzolite (REEtot=2.79-3.21) and dunite (REEtot=2.63-2.71), higher in chromitite (REEtot=6.55-13.03) and much higher in dunite close to chromitite (REEtot=16.15-23.41). Lherzolite shows a strong LREE-depletion, with (La/Yb)N=0.040-0.052. This depletion decreases moving towards dunite and chromitite, with (La/Yb)N=0.19-0.20 in dunites, (La/Yb)N=0.29-0.40 in dunites close to chromitites where (La/Yb)N=0.11-0.78. LREE depletion in diopside from lherzolites suggests that lherzolites underwent a melting event. On the other hand the presence of primary amphibole only in inclusions suggests that a fluid-rich melt played an important role in chromitite crystallization. Finally, the contrasting REE behaviour of dunite suggest that primary restitic dunites coexist with metasomatic dunites. We make the hypothesis that a melting event that depleted lherzolite diopside in LREE and formed restitic dunites was followed by the income of a fluid-rich melt responsible for the formation of REE-rich chromitite and secondary dunite. Spengler D., Van Roermund H.L.M., Drury M.R, Ottolini L., Mason P.R.D., Davies G.R.: Deep origin and hot melting of an Archaean orogenic peridotite massif in Norway. Nature, 440, (13 April Issue), (2006), 913-917. Silicate inclusions (< 10 μm size) in Troodos ophiolite chromites were studied for the first time in terms of major, minor and Rare Earth Elements to investigate magmatic processes that lead to chromitite crystallization. The study was focused on silicate inclusions detected in chromite grains of chromitite pods in the enveloping dunite, and their mineral chemistry was compared with that of host peridotite phases. Samples were collected at Mount Olympus where lherzolite hosts dunite lenses that envelope chromitite pods, lenses and irregular layers. On the basis of textural and geochemical evidences two types of inclusions within chromite grains have been outlined: i) Primary inclusions within chromite in chromitites and dunites. They are mainly monophasic (diopside, olivine, or Na-Ca amphibole), rarely biphasic and comprise diopside and Na-Ca amphibole or diopside and olivine. They mainly occur at the core of host grain, and are characterized by regular rims and negative crystal shapes. ii) Secondary inclusions, characterized by irregular shapes, containing amphibole. They are richer in Mg, Ca and Cr than the primary inclusions or serpentine, and mainly occur close to chromite rims, connected to interstitial silicate matrix by fractures. Na-Ca amphibole was found only in chromitites. Diopside is Cr-rich and shows 23-25 wt% CaO; 0.5-1.8 wt% Cr2O3; 16.9-17.8 wt% MgO, and 0.7-1.8 wt% FeO. Olivine has 3.5-4.2 wt% FeO; 51.7-52.4 wt% MgO. Na-Ca amphibole has Al2O3 ranging from 5 to 9.30 wt%, Cr2O3 from 1.50 to 3.30, MgO from 21 to 23 wt%, CaO from 6 to12 wt% and NaO from 3 to 5.7 wt%. In partially serpentinized dunite enveloping chromitites, primary silicate inclusions show diopside with lower CaO content (21.6-23.5 wt%) and slightly higher FeO (1.5-2.0 wt%); olivine with lower MgO (around 50 wt%) and higher FeO (around 6.1 wt%). Compared with that in inclusions, diopside in lherzolite is lower in Cr2O3 (between 0.3 and 0.6 wt%) and shows a much wider FeO range (between 0.7 and 3.9 wt%). Olivine in lherzolite occurs as fresh crystals with 12.2-14.3 wt% FeO. Plagioclase, occuring as late impregnation, has anortitic composition showing CaO contents between 18 and 18.5 wt%. REE data obtained with the Cameca IMS 4f ion microprobe (CNR-IGG, Pavia) following an optimized analytical procedure (see Spengler et al. 2006, for details) were collected on diopside in chromite from chromitite, on enveloping dunite and in lherzolite. Total REE content (in ppm) is similar in lherzolite (REEtot=2.79-3.21) and dunite (REEtot=2.63-2.71), higher in chromitite (REEtot=6.55-13.03) and much higher in dunite close to chromitite (REEtot=16.15-23.41). Lherzolite shows a strong LREE-depletion, with (La/Yb)N=0.040-0.052. This depletion decreases moving towards dunite and chromitite, with (La/Yb)N=0.19-0.20 in dunites, (La/Yb)N=0.29-0.40 in dunites close to chromitites where (La/Yb)N=0.11-0.78. LREE depletion in diopside from lherzolites suggests that lherzolites underwent a melting event. On the other hand the presence of primary amphibole only in inclusions suggests that a fluid-rich melt played an important role in chromitite crystallization. Finally, the contrasting REE behaviour of dunite suggest that primary restitic dunites coexist with metasomatic dunites. We make the hypothesis that a melting event that depleted lherzolite diopside in LREE and formed restitic dunites was followed by the income of a fluid-rich melt responsible for the formation of REE-rich chromitite and secondary dunite. Spengler D., Van Roermund H.L.M., Drury M.R, Ottolini L., Mason P.R.D., Davies G.R.: Deep origin and hot melting of an Archaean orogenic peridotite massif in Norway. Nature, 440, (13 April Issue), (2006), 913-917.
Settore GEO/09 - Georisorse Miner.Appl.Mineral.-Petrogr.per l'amb.e i Beni Cul
set-2007
Federazione Italiana di Scienze della Terra
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