Anomalously high delta D and micro-scale hydrogen isotope heterogeneities in the mantle: Ion microprobe analysis of amiboles from peridotite xenoliths at Nushan, eastern China
Q. Xia et al., Anomalously high delta D and micro-scale hydrogen isotope heterogeneities in the mantle: Ion microprobe analysis of amiboles from peridotite xenoliths at Nushan, eastern China, CHIN SCI B, 46(6), 2001, pp. 505-509
Hydrogen isotopic compositions of four amphibole grains from three pieces o
f Iherzolite xenoliths in Cenozoic basanites of Nushan, eastern China have
been analyzed by ion microprobe. deltaD values of all analyzed points range
from -94 parts per thousand to +46 parts per thousand, some of which are m
uch higher than the highest deltaD (+8 parts per thousand) reported previou
sly for mantle materials. The heterogeneities of D/H ratios within single g
rains have been observed, the variation of deltaD is up to 80 parts per tho
usand on the scale of less than 400 mum. No correlation between hydrogen is
otopic ratios and hydrogen contents can be found, implying that the scatter
of deltaD values could not result from a late shallow process such as hydr
ogen loss or hydrothermal alterations and should be considered as inherited
from the source at depth. Chemical compositions of Nushan amphiboles are v
ery homogeneous, excluding the fact that the scatter of deltaD values could
arise from variable fractionation factors between a single fluid source an
d minerals. Therefore, metasomatic fluids responsible for the formation of
Nushan amphiboles should be heterogeneous and result in the observed large
variable and anomalously high deltaD values of amphiboles, We suggested tha
t such metasomatic fluids could be related to magma degassing in the mantle
source. Based on the D-H diffusion data and the scale of hydrogen isotope
heterogeneities, it was inferred that the mantle metasomatism took place so
on before the eruption of host magma.