Óceáni kéregmaradványok stabilizotópos vizsgálata = Stable isotope studies on ocean crust remnants

A vizsgált ofiolitos összletekből a fontosabb kőzettipusok feldolgozása, számos elektron mikroszondás és stabilizotóp-geokémiai elemzés történt meg. A fluidumzárvány-elemzések céljából laboratóriumi módszerfejlesztést végeztünk. Egyes gabbrók és szerpentinitek megőrizték a köpenyeredetre utaló d18O értékeket, míg mások erős 18O-dúsulást mutatnak, amit kis hőmérsékletű szerpentinesedéshez, vagy 18O-dús fluidumokkal történt izotópcseréhez köthetünk. A csapadékvíz-beáramlás mértéke meglehetősen alárendelt lehetett. Az összetételi eloszlások alapján feltételezhetjük a magmás eredetű ofiolitos kőzetegyüttes és az üledékes sorozatból felszabaduló fluidumok kölcsönhatását. A tengervíz/kőzet kölcsönhatással járó óceáni metamorfózisra jellemző erős D-dúsulás nem jelent meg a vizsgált kőzetekben. A plagiogránitok gránátja az oxigénizotóp-összetételi elemzések alapján egyértelműen metaszediment eredetűnek bizonyult. A Kőszeg-Rohonci Penninikum és a Melléte-Darnó-Szarvaskő sorozat kőzeteinek H és O izotópösszetételi eloszlása hasonló képet mutat a lassú kinyílást mutató óceáni kéregösszletekben megfigyelt összetételi tartományokhoz. A vizsgált összletek szubdukciója az eredeti köpenyösszetételi tartománytól erősen eltérő fluidumot szabadít fel, ami a köpenybe kerülve annak stabilizotóp-geokémiai módszerekkel kimutatható metaszomatózisát okozhatta, összhangban a magmás kőzeteken kapott eredményekkel. | The main rock types of the studied ophiolitic complexes were sampled as well as numerous electron microprobe and stable isotope analyses were conducted. Methodological laboratory development was also made for fluid inclusion analyses. Some of the gabbros and serpentinites have preserved their original mantle-like O isotope compositions, while others show strong 18O-enrichment due to low-temperature serpentinization or isotope exchange with 18O-enriched fluids. The infiltration of meteoric water was subordinate. On the base of stable isotope data distributions an interaction between the ophiolitic complex and the fluids released from the metasedimentary rocks can be assumed. The strong D-enrichment characteristic for seawater/rock interaction during oceanic metamorphism has not been observed in the studied rocks. The oxygen isotope compositions of garnet of plagiogranites have proven a sedimentary origin. The H and O isotope compositions of the Kőszeg-Rechnitz and the Meliata-Darnó-Szarvaskő series shows similar distributions to the isotope ranges observed for oceanic crustal complexes formed by slow spreading. The subduction of the studied complexes would have released fluids having significantly different compositions from the primary mantle ranges. The mantle metasomatism caused by such fluids can be detected by means of stable isotope geochemistry, in accordance with earlier results obtained on magmatic rocks of the Pannonian region

Stable hydrogen isotope compositions of tourmalines from the Sopron metamorphic complex: Metamorphic closure temperature and fluid composition

Stable hydrogen isotope compositions of metamorphic rocks and minerals can provide information on the origin of metamorphic fluids, which is especially important in systems that had experienced multiple metamorphic events. The Sopron orthogneiss-micaschist complex is a good target as it records signs of Variscan and Alpine metamorphic events as well as Variscan granitic magmatism. In this study tourmaline-bearing rocks (pegmatitic orthogneisses and kyanite-chlorite-muscovite schists) of the Sopron metamorphic complex were sampled and their tourmaline grains were analyzed for stable hydrogen isotope compositions (δ 2 H). The δ 2 H values (−23 ± 1‰, relative to V-SMOW) are in accordance with a fluid flux from devolatilization of subducted, seawater-containing rocks. Tourmaline-chlorite hydrogen isotope fractionations correspond to about 550 °C, indicating that δ 2 H values formed close to peak metamorphic temperatures are preserved without retrograde isotope exchange during cooling

Stable isotope compositions of the Penninic ophiolites of the Kõszeg-Rechnitz series

Abstract The ophiolitic rocks of the easternmost Penninic unit, the Kőszeg-Rechnitz series, were analyzed for their H, C and O stable isotope compositions. Serpentinite, gabbro, blueschist, talc deposits, ophicarbonates, as well as calcite and inclusion fluids from quartz segregation veins were analyzed in order to determine the effects of different metamorphic events on the stable isotope compositions. The oxygen isotope compositions have a wide range depending on rock type and locality. Gabbro and serpentinite of Bienenhütte (Bernstein Window) have preserved mantle-like δ18O values (5.9 to 6.3‰; all values are in ‰ relative to V-SMOW), whereas the serpentinite of Glashütten and Rumpersdorf (Kőszeg-Rechnitz Window) and the silicate minerals of the ophicarbonate rocks show a strong 18O-enrichment (up to 16.2‰). The 18O-enrichment may have been induced by low-temperature serpentinization or interaction with 18O-rich fluids that had been in equilibrium with sedimentary rocks. Contrary to the O isotope compositions, the H isotope compositions seem to be homogeneous in the entire series, with D values of −63 ± 7‰. Only some serpentinite rocks were depleted in D (down to −106‰), usually regarded as a result of interaction with meteoric water infiltrating during late-stage metamorphism. The meteoric water infiltration was rather limited, as even samples taken directly from slickensides within serpentinite bodies preserved isotopic compositions close to those of the bulk series. H and O isotope compositions of fluids mobilized in the metasedimentary rocks of the Penninic unit during the main metamorphic stage were determined by analyzing inclusion fluids and calcites in quartz-carbonate veins. The isotope compositions indicate interaction between these fluids and the ophiolite series, although relative deuterium enrichment has been preserved in the ophiolitic rocks. The strong D-enrichment characteristic for oceanic crust that has experienced high-temperature interaction with seawater was not detected. However, the H isotope compositions obtained for the Kőszeg-Rechnitz series indicate that subduction of the Penninic oceanic crust and the associated devolatilization may have been potentially responsible for mantle metasomatism, resulting in H isotope compositions of about −40‰, similar to the range determined from mantle-derived amphibole megacrysts (Demény et al. 2005). To conclude, the present dataset is discussed in the light of earlier studies on the formation of the Sopron leucophyllite

Partial dolomitization of foreslope and toe-of-slope facies of a Carnian carbonate platform, Transdanubian Range, Hungary

Partial and pervasive dolomitization of foreslope and toe-of-slope deposits of an early Carnian carbonate platform was investigated to understand the process and mechanism of dolomitization. Based on petrographic observations and C and O isotope data, the dolomitization took place in a near-surface to shallow burial setting; seawater of slightly elevated salinity was likely the dolomitizing fluid. The circulation system was maintained by reflux of evaporated sea water and geothermal heating of cold seawater derived from the surrounding deeper basin. The dolomitization was mostly controlled by the permeability of the platform-derived calcareous sediments

A stable isotope study on Cretaceous magmatic influences in the Transdanubian Mid-Mountains

Results of earlier stable isotope studies on lamprophyre-carbonatite bodies and red calcite dykes in the Transdanubian Mid-Mountains are summarized in this paper. Their genetics have similarities in that they were formed during the Cretaceous and that magmatic fluids presumably played a significant role in the formation. A tentative model is proposed in which magmatism and magmatic fluid circulation would appear in the Transdanubian Mid-Mountains in different forms depending on spatial and/or temporal positions; i.e. emplacement of mantle-derived lamprophyre-carbonatite bodies during the Middle-Upper Cretaceous that induced carbonate vein formation and formation of red calcite dykes of Aptian to Campanian age that show magmatic delta-13C and deltaD signatures and can be related to movements of deep-seated igneous fluids