Chapter 5 VOLATILES IN THE MANTLE LITHOSPHERE : MODES OF OCCURRENCE AND CHEMICAL COMPOSITIONS

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 * E-mail address: mjzhang@lzu.edu.cn. Phone: +86-931-891-2403; Fax: +86-931-891-2449, Corresponding author: Dr. Mingjie Zhang, Institute of Geological Science, Lanzhou University, P. R. China. No part of this digital document may be reproduced, stored in a retrieval system or transmitted in any form or by any means. The publisher has taken reasonable care in the preparation of this digital document, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained herein. This digital document is sold with the clear understanding that the publisher is not engaged in rendering legal, medical or any other professional services. Mingjie Zhang, Yaoling Niu and Peiqing Hu 172 etc., whereas at shallow levels, the mantle lithosphere as reflected in mantle xenoliths displays 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.