Mantle-Derived Volcanic Rocks

Recently, precise noble gas isotopic analyses of diamonds have also become available. Diamonds are derived from the mantle. However, diamonds are unique in that most known diamonds are of Precambrian age hence, we can hope for noble gas records of the very ancient mantle, complementary to the constraints on the present mantle provided by recent volcanics. 

Sections 62-6.5 describe in more detail the major kinds of materials from which information on the state of the mantle may be derived, with emphasis on the character of the materials and their mantle sources. Noble gas data from these materials are summarized in Sections 6.6 and 6.7. 

Most submarine volcanic rocks contain C02-filled vesicles (bubbles) in glassy margins. Because noble gases in silicate melts partition very effectively into a gas phase (i.e., their solubilities are low), it would be expected that noble gases in submarine volcanics would be found in the bubbles as will be discussed later, this seems indeed to be the case (e.g., Kurz Jenkins, 1981 Marty Ozima, 1986 Sarda Graham, 1990 Graham Sarda, 1991). The popping rocks, so-called because of 

To show this quantitatively, ordinate-intercept analysis (e.g., Takaoka Nagao, 1980) of mixing between air and nonatmospheric (mantle) Ar gives 

In an analogous way, Equation (6.1) can also be rearranged interchanging the roles of 36Arm and 36Ara, with the same approximation yielding 

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The isotope eomposition of oxygen in the Cenozoic lavas of Antarctica, measured by Wenner (1974), are mostly in the normal range for mantle-derived volcanic rocks (i.e., = -h5.0 to -h6.5%o relative to... 

To obtain information on the noble gas state in the mantle, it is necessary to analyze mantle-derived materials that have trapped mantle noble gases. Accessible samples include volcanic rocks, volcanic gases, mantle xenoliths, and diamonds. Among various mantle-derived materials, submarine volcanic rocks are particularly useful because of their wide occurrence and their relatively large (for mantle samples) amounts of trapped noble gases. So far, information has been obtained mainly from... 

The whole-rock Rb-Sr date of 1789 141 Ma (Paleoproterozoic) is the time when these rocks were recrystallized during regional metamorphism to amphibolite grade and the strontium they contained was isotopically homogenized so that all three specimens had the same initial Sr/ Sr ratio of 0.7059 0.0041. This value indicates that their protoliths contained strontium derived from the lithospheric mantle in the form of volcanic rocks or their weathering products. 

The low initial Sr/ Sr ratio of the Hope Granite in the Gabbro Hills indicates that the granitic magma contained strontium derived from mafic volcanic rocks in the underlying lithospheric mantle. 

Precambrian age of the continental crust and young volcanic rocks derived from the subcrustal mantle of the Earth by volcanic eruptions on the continents and in the ocean basins. The Sr/ Sr ratios of all carbonate rocks that formed in the oceans during the Phanerozoic fit this curve (Adapted from Faure and Mensing 2005)... 

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