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Durham University

Department of Earth Sciences

Profile

Publication details for Professor Yaoling Niu

Zhang, M.J., Niu, Yaoling & Hu, P.Q. (2009). Volatiles in the mantle lithosphere: Occurrence modes and chemical compositions. In The lithosphere: Geochemistry, geology and geophysics. Anderson, J.E. & Coates, R.W. New York: Nova Science Publishers, Inc. 171-212.

Author(s) from Durham

Abstract

Volatiles play important roles in chemical differentiation of the Earth, in concentrating
economic metals, and in regulating earth’s surface environments by means of magmatism,
metasomatism, degassing and recycling. Mantle rocks and rocks derived from the mantle such
as basalts, mantle xenoliths and ophiolitic peridotites are materials available to investigate the
ways in which volatiles may store in the mantle, their compositions, and probable histories.
The laser Raman spectroscopy, Infrared spectrometry and ion microprobe in combination with
micro-thermometry are non-destructive methods to analyze volatile compositions trapped in
fluid inclusions. On the other hand, vacuum crushing and stepwise heating are methods
employed to extract the volatiles and measure their chemical and isotopic compositions using
mass spectrometry. An improved vacuum stepwise heating technique can effectively separate
volatiles in different occurrence modes in mantle materials, which in combination with mass
spectrometry can yield excellent and highly reproducible analytical data.
Volatiles in the mantle occur in various forms such as free element or molecular species
along grain boundaries, carbonate, sulfide or hydrous minerals, fluid inclusions or charged
species dispersed in mineral structures (e.g., OH-), structural defects or vacancies. Volatiles
trapped in structural defects and vacancies are volumetrically significant. Large amount of
hydrogen occurs as free H2 species, not OH- as previously thought.
Volatiles in the mantle are mixtures of primordial volatiles and recycled volatiles with
characteristic chemical compositions. Volatiles in the sub-continental lithospheric mantle
(SCLM) vary with depth and mantle reservoirs. Deep portions of mantle lithosphere in the
diamond stability field have higher contents of reduced volatile species such as H2 and CO varying volatile compositions; initial volatiles trapped during primary crystallization stage are
dominated by reduced species like CO, H2. In contrast, metasomatic volatiles are more
oxidized such as CO2 and SO2 etc.
Volatiles in mantle source regions of oceanic basalts are all dominated by H2O and CO2
with minor CO, CH4, N2, and H2; the abundances vary with tectonic settings. MORB are
depleted in volatiles as a result of source depletion in its history, whereas abundant volatiles in
IAB and BABB are probably originated from subduction devolatilization. OIB may have
abundant volatiles inherited form the undegassed mantle with a recycled component. Volatiles
in ancient oceanic lithosphere as recorded in ophiolites are all dominated by CO2 with minor
amounts of other volatile species.狑?