Major and trace element composition of Skaergaard plagioclase; geochemical evidence for changes in magma dynamics during the final stage of crystallization of the Skaergaard intrusion |
| |
| Authors: | Yun D Jang H R Naslund |
| |
| Institution: | (1) Department of Geological Sciences and Environmental Studies, State University of New York at Binghamton, Binghamton, NY 13902, USA e-mail: Naslund@binghamton.edu, US |
| |
| Abstract: | Plagioclase separates from the Layered Series (LS), Upper Border Series (UBS), and Marginal Border Series (MBS) of the Skaergaard
intrusion were analyzed to examine major and trace element variations. In general, plagioclase from the LS, UBS, and MBS show
similar trends in major elements vs. crystallization: SiO2, Na2O, and K2O progressively increase, and CaO and MgO progressively decrease with fractionation. No abrupt changes in the trends of major
components of Skaergaard plagioclase during the differentiation of the intrusion are observed. Trace elements in plagioclase
reflect changes in the Skaergaard magma and changes in plagioclase distribution coefficients with differentiation. Sr, Ga,
and probably Ba are included elements in Skaergaard plagioclase, but were excluded from the other cumulus phases, and as a
result systematically increased in the magma and plagioclase during differentiation. Be, Cs, Hf, Rb, Ta, U, and Zr, and the
transition metals Co, Cr, Cu, Ni, Sc, V, and Zn were excluded elements in Skaergaard plagioclase, and remained low in plagioclase
during differentiation. Changes in the abundances of these elements in plagioclase during differentiation reflect changes
in their abundance in the magma. With the exception of the lower zone, which is enriched in the light rare earth elements,
rare earth elements in LS plagioclase, in general, increase with differentiation of the Skaergaard intrusion, but decrease
dramatically at the UZa/UZb boundary where abundant apatite first appears. Rare earth elements in UBS plagioclase followed
a similar trend to LS plagioclase, except during the initial and final stages of differentiation. UBS plagioclase is much
more enriched in rare earth elements during the final 20% of crystallization, except for Eu, which is similar in plagioclase
from the two series. The observed trends suggest that the floor and roof sequences became isolated from each other and that
the floor sequence may have been more reducing and the roof sequence more oxidizing during the final 20% of crystallization.
As the Skaergaard magma ceased convection, or convected as isolated cells, during the final stages of differentiation, volatile
elements may have accumulated in the UBS magma, resulting in an increase in ƒO2, and a decrease in Eu/Sm in UBS plagioclase. The observed trends of rare earth elements in plagioclase from the LS and UBS
fit well with theoretical calculations that assume closed-system crystallization, and would be difficult to reconcile with
any model requiring significant discharge of magma from the chamber during the final 20% of crystallization. The enrichment
of light rare earth elements in plagioclase, suggests that the lower part of the intrusion re-equilibrated with a late, light
rare earth element-rich fluid or melt. The recharge model proposed by earlier workers to explain anomalous Sr and Nd isotopes
appears unlikely in light of the two to fourfold enrichment of light rare earth elements in these samples.
Received: 1 October 1999 / Accepted: 14 May 2000 |
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
