Search

The petrogenesis of basaltic arc lavas with heavy Mo isotopic compositions is controversial. Given that oceanic crust preferentially loses isotopically heavy Mo during subduction at forearc depth and the remaining Mo is isotopically light and locked in rutile, fluids derived from such crust at subarc depth cannot account for the heavy Mo isotopic compositions of basaltic arc lavas. Serpentinites can contain substantial Mo and volatiles and are a potential source of isotopically heavy Mo in basaltic arc lavas. However, the Mo isotopic composition of serpentinites and the Mo isotope behavior during serpentinite dehydration are poorly constrained. Here, we present Mo contents and isotope data for serpentinites from the South Sandwich Trench-Fracture Zone intersection, and deserpentinized peridotites and high-pressure meta-serpentinite veins from the western-central Alps and Betic Cordillera, Spain. Most of these samples have heavy Mo isotopic compositions (δ98/95Mo NIST 3134 , ranging from −0.26 ‰ to 1.84 ‰) with low Ce/Mo (0.10–23.98) and Ce/Pb (0.17–3.59) ratios, which are distinct from the compositions of eclogite-facies metabasalts and metapelites. In addition, no systematic variations in Mo contents and Mo isotopic compositions occur for the studied samples, indicating limited Mo isotope fractionation during serpentinite dehydration. The above results suggest that serpentinites and their derived fluids at subarc depths can serve as the main isotopically heavy Mo sources for basaltic arc lavas. The Mo isotope data with Ce/Mo and Ce/Pb systematics suggest that serpentinite-derived fluids or melting of serpentinite diapirs at subarc depths can explain the heavy Mo isotopic compositions of basaltic arc lavas. Furthermore, recycled oceanic slab (oceanic crust + serpentinized mantle) would has limited influence on the Mo systematics of the mantle, which explains why the mantle has maintained a nearly constant Mo isotopic composition since 3.5 Ga. [ABSTRACT FROM AUTHOR]

Fluids play a crucial role in element mobility and mass transfer at the slab-mantle interface in subduction zones. However, tracing the source and chemical composition of subduction zone fluids still remains challenging. High-pressure (HP) metamorphic leucophyllites, mainly composed of quartz, muscovite/phengite and Mg-chlorite, occur in several localities in the Eastern Alps and experienced significant Mg-metasomatism at forearc depths during the Eoalpine orogeny. They thus provide a good opportunity to explore the origin of Mg-rich fluids in the continental subduction zone. The leucophyllites are rich in Mg and poor in Fe, Na, and Ca and occur as lenses and layers in the country rocks of metagranites. Both leucophyllites and metagranites show similar whole-rock REE distribution patterns, and magmatic zircons from them exhibit similar U-Pb ages of ∼271 Ma, suggesting that both types of rock have the same protolith of early Permian granite. One metagranite that preserves magmatic composition shows a low δ26Mg value of −0.64‰, suggesting a contribution of carbonate-rich sediment when granitic melts were produced. A profile of samples from metagranite through transitional gneiss to leucophyllite in Hungary shows two trends in δ26Mg values. In the Vashegy sections, δ26Mg values firstly decrease from −0.64‰ to −1.30‰ during gneiss formation, and then increase to 0.29‰ during leucophyllite formation. In a second locality, the TV tower section, the δ26Mg values firstly decrease from −0.29‰ to −0.89‰ during gneiss formation, and then remain constant during leucophyllite formation. Two leucophyllites in Austria exhibit higher δ26Mg values of 0.05 to 0.09‰ compared to their country rocks of about −0.20‰. Based on the Mg isotope systematics and their relationships to whole-rock geochemical compositions, we propose two types of fluid in the subduction zone: (1) a low δ26Mg (<−1.3‰) fluid produced by dissolution of mainly Mg-calcite at HP conditions; (2) a high δ26Mg (>0.3‰) fluid derived from dehydration of talc-rich serpentinite. This is for the first time to find subduction zone fluids with distinct δ26Mg values at forearc depths. Both fluids possibly originated from dehydration of the sediment-serpentinite mélange at the slab-mantle interface. The fluids were heterogeneous in δ26Mg values and they metasomatized slices of continental crust along shear zones. The presence of fluids with δ26Mg values as low as −1.3‰ suggests that the dissolution of Mg-rich carbonates can be significant at forearc depths. Our results provide an excellent example of tracing different sources of subduction zone fluids by coupling Mg isotopes with other petrological and geochemical variables. They may also have great implications for the generation of magmas with variable Mg isotope compositions at convergent plate boundaries. [ABSTRACT FROM AUTHOR]

Serpentinite-derived fluids play a crucial role in mass exchange between the Earth's crust and mantle. However, there is currently a limited understanding of how serpentinite-derived fluids influence the Si isotope composition of mantle and subducted crustal materials. Whiteschists in the Dora-Maira Massif of the Western Alps and leucophyllites in Austria of the Eastern Alps were formed through serpentinite-derived fluid metasomatism of metagranite protoliths. Thus, they can provide valuable insights into the geochemical diversity of such rocks resulting from fluid–rock interactions. A special focus is on Si isotope compositions, which were analyzed in whiteschists, leucophyllites, and their corresponding country rocks. The aim of this paper is to investigate the impact of serpentinite-derived metasomatic fluids on the Si isotope compositions of subducted continental crust. With the exception of one whiteschist sample, which contains high amounts of pyrope, and one pyrope megablast, all other whiteschists and leucophyllites exhibit Si isotope compositions similar to those of their country rocks. This observation suggests that the Si isotopes of whiteschists and leucophyllites were inherited from their respective country rocks; fluid metasomatism did not significantly modify the Si isotope compositions of whiteschists and leucophyllites. Because pyrope has much lower δ30Si values than quartz and phengite as further major mineral constituents of the studied whiteschists, the samples with high pyrope contents reveal lighter Si isotopes than those containing lower amounts of pyrope. Interestingly, the metagranites and the majority of whiteschists and leucophyllites exhibit Si isotope compositions below the igneous array, implying that their protoliths are S-type granites. These granites commonly contain sedimentary components derived from their sources. Our results document that Si behaves immobile during serpentinite-derived fluid metasomatism of the subducted continental crust. Combined with previous results, we were able to show that slab derived fluids, including serpentinite-derived fluids, produce only a minimal influence on the Si isotope composition of the overlying mantle wedge and its derived magmas. Thus, the Si isotope compositions of rocks, even if highly affected by metasomatizing fluids, can be used as significant fingerprints that refer to their unaltered precursors. • Whiteschist and leucophyllite are formed by serpentinite-derived fluid metasomatism. • The δ30Si values of most whiteschists and leucophyllites are same to country rocks. • Serpentinite-derived fluids did not affect Si isotopes of metagranites and mantle. • Pyrope has much lower δ30Si values than quartz and phengite. • The samples with high pyrope contents have lighter Si isotopes. [ABSTRACT FROM AUTHOR]

Deep-Earth cycling of nitrogen (N) along the subduction pathway, remains relatively poorly understood, particularly that related to deep subduction of continental crust. The ultrahigh-pressure (UHP) Dora-Maira Massif whiteschists in the Western Alps and their lower- P-T equivalent leucophyllites in the Eastern Alps show a decrease in N concentration and an increase in δ15N relative to their presumed country rock protoliths. On average, the whiteschists contain 20.7 ± 10.5 ppm N (mean ± 1σ) with δ15N air = +2.7 ± 2.5‰ (mean ± 1σ) and the leucophyllites contain 59.8 ± 12.3 ppm N with δ15N air = +4.3 ± 2.0‰, whereas the metagranitic country rocks from Dora-Maira contain 41.3 ± 12.5 ppm N with δ15N air = −1.9 ± 3.2‰ and the country rocks from the Eastern Alps contain 91.1 ± 42.5 ppm N with δ15N air = +1.7 ± 2.1‰. The loss of N and isotope shift in the whiteschists and leucophyllites from these two localities was accompanied by loss of LILE (Rb, Ba), Sr, CaO, Na 2 O, and FeO total. The other stable isotope systems applied to study both suites (Mg, Fe, O, Li and Ba) show no obvious correlation with variations in δ15N. Infiltration by and interaction with a serpentinite-derived fluid depleted in N and LILE could have led to leaching of N from the country rock protoliths and production of the +4.6‰ (Dora-Maira Massif) and +2.6‰ (Eastern Alps) isotope shifts adequately explained using a Rayleigh distillation model. The N exchange and leaching process is best explained as involving N 2 -NH 4 or NH 3 -NH 4 at about 550–600 °C, the latter the approximate temperatures at which the fluid infiltration is thought to have occurred. These results provide new insights regarding N behavior and isotopic fractionation during fluid-rock interactions occurring in UHP rocks and point to the importance of H 2 O-rich fluids from serpentinite dehydration as agents of metasomatism along deep subduction interfaces. Furthermore, the data for the metagranites indicate the potential of this lithology to retain significant amounts of N to at least 100 km and beyond sub-arc depths. • Alps whiteschists/leucophyllites contain less N with higher δ15N than protoliths. • Lowered N and elevated δ15N resulted from leaching during fluid-rock interaction. • The metasomatic fluid was most likely derived from serpentinite dehydration. • Metasomatism of this type could mobilize NH 3 /N 2 fluids into the upper mantle. • Metagranites can retain significant amounts of N during continental subduction. [ABSTRACT FROM AUTHOR]

Whole rock lithium (Li) and Li isotopic compositions of coesite-bearing whiteschists and their country rock metagranites from the Dora-Maira Massif in the Western Alps were investigated to understand their behavior during fluid-rock interaction in mantle depths of continental subduction zones. The whiteschists reveal relatively low δ7Li values (−6.4‰ to +2.4‰, with an average value of −2.1‰) in comparison to the country rock metagranites (δ7Li = −1.2‰ to +3.6‰, with an average value of +1.4‰). The δ7Li values of whiteschists do not show any correlations with major elements (e.g. Mg, Fe), Li, or isotopes (i.e., δ26Mg and δ18O values) and cannot be explained by kinetic fractionation. Hence, the low δ7Li values of whiteschists result from interactions with metasomatic fluids containing isotopically light Li that occurred in deep portions of a continental subduction zone. Fluid-rock interaction modeling suggests that the metasomatic fluids contain low δ7Li values (down to −4‰) and moderate Li concentrations (50 to 150 ppm), which were probably derived from the dehydration of serpentinites in the subduction channel. The presence of such low δ7Li fluids in deep subduction zones could cause Li isotopic heterogeneity in the earth's interior. • The whiteschists have relatively low δ7Li values (−6.4‰ to +2.4‰). • Isotopically light Li compositions resulted from metasomatism with fluids. • The metasomatic fluids were probably derived from the dehydration of serpentinites. • Such low δ7Li fluids could cause Li isotopic heterogeneity in the Earth's mantle. [ABSTRACT FROM AUTHOR]