Radiogenic strontium

Here, Sr denotes radiogenic Sr from in situ decay. However, most rocks do contain common strontium, which must be subtracted from the measured 87Sr to determine 87Sr ... 

The rubidium-strontium geochronometer used in the Rb-Sr geochronological method is based on the radioactive 3 -decay of Rb to Sr. The growth of radiogenic Sr in a Rb rich mineral can be described by the following Equation (9.6). In the rubidium-strontium age dating method, the radioactive Rb isotope with a natural isotope abundance of 27.85 % and a half-hfe of 4.88 X 10 ° years is fundamental to the 3 decay to the isobar Sr. The equation for the Rb-Sr method can be derived from the general equation of radioactive decay (Equation 8.8 in Section 8.8) ... 

Radiogenic isotope ratios of OIE are shown on Figure 15. These diagrams display remarkably similar topologies as the respective MORE data shown in Figures 4-6. Strontium isotope ratios are negatively correlated with neodymium and hafnium isotopes, and correlations between strontium, neodymium, and hafnium isotopes and lead... 

When ultramafic rocks were initially analyzed for strontium isotopes (Hurley et al, 1964 Roe, 1964 Stueber and Murthy, 1966 Lanphere, 1968) only whole rocks were analyzed. Such data led to numerous erroneous conclusions about the relationship between mantle samples and basalts due to the pervasive alteration suffered by many ultramafic rocks, enhancing the amount of radiogenic strontium present. Subsequent studies have shown that acid-washed mineral separates, particularly diopside and garnet, are the most reliable means of obtaining the primary, unaltered... 

A number of eclogites have both radiogenic neodymium isotopes and radiogenic strontium relative to bulk Earth. For these samples, some of the Sr/ Sr values exceed modem and Archean seawater compositions (see Chapter 6.17). Hence, in a subduction-type model, the radiogenic strontium isotopic composition of the eclogite protohth is not solely inherited from oceanic cmst but is likely to be a time-integrated response to Rb/Sr enrichment during hydrothermal alteration... 

Th/La, and the Aleutians show strong positive correlation between Ba/La and Th/La. Figure 13 also shows that most oceanic arc lavas show positively correlated barium, thorium, lanthanum, and Sr/ Sr, while oceanic Aleutian arc lavas, and continental arc lavas, show wide variation in barium, thorium, and lanthanum that is not correlated with Sr/ Sr or, e.g., ° Pb/ ° Pb. Thus, for example, enrichments in thorium can occur in primitive arc lavas without incorporation of radiogenic strontium and lead, (and unradio-genic neodymium) from subducted sediments (Kelemen et at, 2003b). 

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