A large number of mantle (peridotite and pyroxenite) xenoliths occurs in basaltic diatreme pipes and lava flows from Hyblean Plateau (Sicily, Southern Italy). Peridotites consist mainly of spinel-facies protogranular-textured harzburgites and lherzolites (Fo90, En89, Cr-diopside, Cr-rich spinel). Major elements distribution and abundance are consistent with their depleted character (high MgO, low Al2O3) related to one or more melt extraction events, but evidences of modal and cryptic metasomatism do exist. For instance, phlogopite of metasomatic origin rarely occurs in these peridotites. Geochemical evidences of metasomatism consist of LREE and some LILE enrichment (e.g. LaN/YbN = 9-19), Sr-Nd and He isotopic signatures. We performed in situ Os isotope analyses on 11 sulfide grains from 4 mantle peridotites. Sulfides in these peridotites are Ni-rich, often rimmed by, or enclosing, a Cu-rich phase. Sulfides occur mainly related to the widespread serpentine network. A few sulfides are olivine-enclosed but their size is too small (< 50 m) to be analysed. The studied sulfides has sub-chondritic 187Os/188Os, ranging from 0.110408 to 0.124398. 187Re/188Os ranges from 0.0296463 to 1.41346, being sub-chondritic in all samples but one (sample GE12). The occurrence of supra-chondritic 187Re/188Os indicates the secondary, probably recent, addition of Re to the system, but the Os isotope composition attests its provenance from a low-187Os/188Os (< PUM) reservoir. The negative correlation between 187Os/188Os and 187Re/188Os in the other samples also supports the Os metasomatic origin, rather suggesting the mixing between two metasomatic reservoirs with different Re-Os signatures. Calculated Re-depletion age (TRD) gives Neo and Mesoproterozoic ages that is the minimum melt-depletion age. Hyblean peridotites all experienced melt extraction event(s) – determining removal of Re from the system - and underwent metasomatic Re-enriching event(s). The occurrence of sulfides with different Os isotope signatures in sample GE12 confirms the importance of using in situ technique in such petrological study
METASOMATISM IN THE HYBLEAN PERIDOTITES: THE OS ISOTOPE CONTRIBUTION / Sapienza G.T.; Griffin W.L.; O'Reilly S.Y.; Morten L.; Scribano V.. - In: EPITOME. - ISSN 1972-1552. - STAMPA. - 1:(2005), pp. 304-305. (Intervento presentato al convegno 5° FIST Geoitalia 2005 tenutosi a Spoleto nel 21-23 Settembre 2005).
METASOMATISM IN THE HYBLEAN PERIDOTITES: THE OS ISOTOPE CONTRIBUTION
SAPIENZA, GIOVANNA TIZIANA;MORTEN, LAURO;
2005
Abstract
A large number of mantle (peridotite and pyroxenite) xenoliths occurs in basaltic diatreme pipes and lava flows from Hyblean Plateau (Sicily, Southern Italy). Peridotites consist mainly of spinel-facies protogranular-textured harzburgites and lherzolites (Fo90, En89, Cr-diopside, Cr-rich spinel). Major elements distribution and abundance are consistent with their depleted character (high MgO, low Al2O3) related to one or more melt extraction events, but evidences of modal and cryptic metasomatism do exist. For instance, phlogopite of metasomatic origin rarely occurs in these peridotites. Geochemical evidences of metasomatism consist of LREE and some LILE enrichment (e.g. LaN/YbN = 9-19), Sr-Nd and He isotopic signatures. We performed in situ Os isotope analyses on 11 sulfide grains from 4 mantle peridotites. Sulfides in these peridotites are Ni-rich, often rimmed by, or enclosing, a Cu-rich phase. Sulfides occur mainly related to the widespread serpentine network. A few sulfides are olivine-enclosed but their size is too small (< 50 m) to be analysed. The studied sulfides has sub-chondritic 187Os/188Os, ranging from 0.110408 to 0.124398. 187Re/188Os ranges from 0.0296463 to 1.41346, being sub-chondritic in all samples but one (sample GE12). The occurrence of supra-chondritic 187Re/188Os indicates the secondary, probably recent, addition of Re to the system, but the Os isotope composition attests its provenance from a low-187Os/188Os (< PUM) reservoir. The negative correlation between 187Os/188Os and 187Re/188Os in the other samples also supports the Os metasomatic origin, rather suggesting the mixing between two metasomatic reservoirs with different Re-Os signatures. Calculated Re-depletion age (TRD) gives Neo and Mesoproterozoic ages that is the minimum melt-depletion age. Hyblean peridotites all experienced melt extraction event(s) – determining removal of Re from the system - and underwent metasomatic Re-enriching event(s). The occurrence of sulfides with different Os isotope signatures in sample GE12 confirms the importance of using in situ technique in such petrological studyI documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
