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Remote Sensing of Hydrogen in Earth’s Mantle

Department of Geology and Geophysics Yale University New Haven, Connecticut, 06520-8109, U.S.A., e-mail: shun-ichiro.karato@yale.edu

The first 20% of the full text of this article appears below.

	INTRODUCTION

 
Hydrogen¹ in Earth’s interior is known to play a key role in a number of processes. Consequently, inferring the distribution of hydrogen is a critical step in our study of dynamics and evolution of Earth. Usually, hydrogen distribution is inferred from two types of samples at the surface. First, a magma may contain hydrogen (water), and under some conditions the water content of the magma can be quenched upon cooling. In these cases, measurements of the water content of the magma provide us with some constraints on the hydrogen (water) content of the source region (if we know the partitioning of hydrogen between magmas and the source rocks, and the degree of melting). Second, hydrogen content of some xenoliths transported by magma can be measured. They provide a direct clue as to the hydrogen content in a region from which a xenolith has been carried. However, these direct, petrologic approaches have two major problems. Firstly, the sampling is limited by the distribution of volcanoes, and even if there are volcanoes that carry rocks from Earth’s interior, the depth extent that volcanoes sample rocks is limited (usually <200 km). Secondly, there is no guarantee that the hydrogen content that one measures on these samples actually represents the hydrogen content in a region where these samples came from. For example, hydrogen is known to diffuse very easily so hydrogen dissolved in minerals could diffuse out during the transport of a rock, or conversely, a piece of rock may acquire extra hydrogen during its ascent. Also hydrogen atoms dissolved in minerals may precipitate in the mineral to form fluid inclusions or micro-scale hydrous minerals. In summary, the direct method to infer the distribution of hydrogen from rock samples has major limitations, and an alternative approach, i.e., remote sensing hydrogen content from geophysical observations . . . [Full Text of this Article]