caa a22 4500 465791298 CHVBK 20180323112025.0 cr unu---uuuuu 170327e19900201xx s 000 0 eng 10.1007/BF00306438 doi (NATIONALLICENCE)springer-10.1007/BF00306438 Isotopic studies of processes in mafic magma chambers: II. The Skaergaard Intrusion, East Greenland [Elektronische Daten] [Brian Stewart, Donald DePaolo] Isotopic ratios of Nd and Sr have been measured in a suite of samples spanning most of the exposed stratigraphy of the Skaergaard intrusion in order to detect and quantify input (such as assimilated wallrock and fresh magma) into the magma chamber during crystallization. Unlike δ18O and δD, Nd and Sr isotope ratios do not appear to have been significantly affected by circulation of meteoric waters in the upper part of the intrusion. Variations in initial 87Sr/86Sr and εNd suggest that the Skaergaard magma chamber was affected during its crystallization by a small amount (2%-4%) of assimilation of Precambrian gneiss wallrock (high 87Sr/86Sr, low εNd) and possibly recharge of uncontaminated magma. Decreases in εNd and increases in 87Sr/86Sr during the early stages (0%-30%) of crystallization give way to approximately unchanging isotopic ratios through crystallization of the latest-deposited cumulates. Modelling of assimilation-fractional crystallization-recharge processes using these data as constraints shows that the assimilation rate must have been decreasing throughout crystallization. In addition, the isotope data allow replenishment by an amount of uncontaminated magma equal to 20%-30% of the total intrusion mass, occurring either continuously or in pulses over the first 75% of crystallization. Comparison of the recharge models with published Mg/(Mg+Fe2+) data from Skaergaard cumulates shows that the modelled replenishment rates are not inconsistent with available major element data, although significant recharge during the final ∼25% of crystallization can be ruled out. The isotope data show that the Skaergaard magma could have incorporated only a small amount of the gneiss that it displaced from the floor of the chamber; assimilation appears to have taken place primarily across a partially molten zone that formed at the roof from the wallrock that was dislodged during emplacement. In the latest stages of crystallization (>75% crystallized), the Skaergaard magma may have become stratified into two separately-convecting layers, effectively insulating Layered Series cumulates from further contamination. Springer-Verlag, 1990 Stewart Brian Department of Earth and Space Sciences, U.C.L.A., 90024, Los Angeles, CA, USA aut DePaolo Donald Berkeley Center for Isotope Geochemistry, Department of Geology and Geophysics, University of California, 94720, Berkeley, CA, USA aut Contributions to Mineralogy and Petrology Springer-Verlag 104/2(1990-02-01), 125-141 0010-7999 104:2<125 1990 104 410 https://doi.org/10.1007/BF00306438 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1007/BF00306438 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Stewart Brian Department of Earth and Space Sciences, U.C.L.A., 90024, Los Angeles, CA, USA aut NATIONALLICENCE 700 1- DePaolo Donald Berkeley Center for Isotope Geochemistry, Department of Geology and Geophysics, University of California, 94720, Berkeley, CA, USA aut NATIONALLICENCE 773 0- Contributions to Mineralogy and Petrology Springer-Verlag 104/2(1990-02-01), 125-141 0010-7999 104:2<125 1990 104 410 Metadata rights reserved Springer special CC-BY-NC licence nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-springer