We present measurements of hydrogen and oxygen isotopes in MORB glasses
from Macquarie Island (SW. Pacific Ocean) coupled with determination of
bulk H2O content by two independent techniques: total dehydration and
FTIR. The incompatible trace elements in these glasses vary by a factor
of 12 to 17, with K2O varying from 0.1 to 1.7 wt.%; these ranges
reflect a variable degree of closed-system mantle melting, estimated
from 1 to 15%. Water concentrations determined by the two techniques
match well, yielding a range from 0.25 to 1.49 wt.% which correlates
positively with all of the measured incompatible trace elements,
suggesting that water is un-degassed, and behaves conservatively during
mantle melting. Also, the agreement between the FTIR-determined and
extracted water contents gives us confidence that the measured isotopic
values of hydrogen reflect that of the mantle. Comparison of the range
of water content with that of other incompatible trace elements allows
estimation of the water partition coefficient in lherzolite, 0.0208
(ranging from 0.017 to 0.023), and the water content in the source, 386
ppm (ranging from 370 to 440 ppm). We observe a fairly narrow range in
delta D and delta O-18 values of -75.5 +/- 4.5 parts per thousand and
5.50 +/- 0 .05 parts per thousand respectively, that can be explained by
partial melting of normal lherzolitic mantle. The measured delta D and
delta O-18 values of Macquarie Island glasses that range from nepheline-
to hypersthene-normative, and from MORB to EMORB in composition, are
identical to those in average global MORB. The observed lack of
variation of delta D and delta O-18 with 1 to 15% degree of mantle
melting is consistent with a bulk melting model of delta D and delta
O-18 fractionation, in which water is rapidly scavenged into the first
partial melt. The narrow ranges of delta D and delta O-18 in normal
mantle are mostly due to the buffering effect of clino- and
orthopyroxenes in the residual assemblage; additionally, fast ``wet''
diffusion of oxygen and hydrogen isotopes through the melting regions
may further smooth isotopic differences. (C) 2012 Elsevier B.V. All
rights reserved.