Isotope Geochemistry: Understanding the Earth's Evolution through Radiogenic Isotopes, Study notes of Geochemistry

Download Isotope Geochemistry: Understanding the Earth's Evolution through Radiogenic Isotopes and more Study notes Geochemistry in PDF only on Docsity! Isotope Geochemistry Chapter 6 145 4/10/11 Radiogenic Isotope Geochemistry I: The Mantle INTRODUCTION The initial use of radioactive and radiogenic isotopes in geology was directed exclusively toward geochronology. The potential geochemical applications became apparent only later. One of the first to recognize the potential of radiogenic isotope studies was Paul Gast, who was a student of Al Nier, whom we have already met. In what was one of the first papers to apply radiogenic isotopes to a geo- chemical problem (the composition of the mantle), Gast (1960) described the principles of Sr isotope geochemistry as follows: In a given chemical system the isotopic abundance of 87Sr is determined by four parameters: the isotopic abundance at a given initial time, the Rb/Sr ratio of the system, the decay constant of 87Rb, and the time elapsed since the initial time. The isotopic composition of a particular sample of strontium, whose history may or may not be known, may be the result of time spent in a number of such systems or environments. In any case the isotopic composition is the time-integrated result of the Rb/Sr ratios in all the past envi- ronments. Local differences in the Rb/Sr will, in time, result in local differences in the abundance of 87Sr. Mixing of material during processes will tend to homogenize these local variations. Once homogenization occurs, the isotopic composition is not further affected by these processes. Because of this property and be- cause of the time-integrating effect, isotopic compositions lead to useful inferences concerning the Rb/Sr ra- tio of the crust and of the upper mantle. It should be noted that similar arguments can be made for the ra- diogenic isotopes of lead, which are related to the U/Pb ratio and time. Gast's first sentence is simply a statement of the radiogenic growth equation for the Rb-Sr system: 87Sr 86Sr = 87Sr 86Sr ! "# $ %& 0 + 87Rb 86Sr e't (1( ) 6.01 Gast's statement remains a valid and succinct summary of the principles of isotope geochemistry. The only change we need to make is to modify the last sentence to add mention of the radiogenic isotopes of Nd, Hf, Os, etc. A principal objective of geology is to understand how the Earth evolved from its initial state to its present one. Radiogenic isotope geochemistry is uniquely suited for this sort of study because an iso- tope ratio such as 87Sr/86Sr is a function not only of the differentiation processes which fractionate Rb from Sr, it is also a function of the time at which the fractionation occurred. On a continuously evolv- ing Earth, ancient features tend to be destroyed by subsequent processes. For example, erosion of rock to form a new sedimentary rock destroys information about the large-scale structure of the pre-existing rock. If the sediment is sufficiently coarse-grained, some information about the fine-scale structure of parent rock will be preserved. If the sediment produced is fine-grained, such as shale, no structural in- formation is preserved. Some chemical information might be preserved in this case, although often very little. Isotopic ratios of the shale, however, preserve information not only about the chemistry of the parent, but also about its age of formation. Similarly, melting of the sediment to form granite de- stroys all structural information, but often considerable chemical information is preserved. Isotope ra- tios, however, preserve, albeit incompletely, information about both chemistry and time. As Gast said, the 87Sr/86Sr ratio is a function of the time-integrated Rb/Sr. Ultimately, we can draw much broader inferences than merely the time-integrated Rb/Sr ratio. Rb and Sr are both trace ele- Docsity.com Isotope Geochemistry Chapter 6 146 4/10/11 ments, and together account for only a few ppm of the mass of the Earth. However, Rb and Sr share some of their properties with other elements of their group: Rb with the alkalis, Sr with the alkaline earths. So Rb/Sr fractionations tell us something about alkali/alkaline earth fractionations. In addi- tion, since Rb is highly incompatible and Sr only moderately so, Rb/Sr fractionations tell us something about the fractionation of incompatible elements from compatible or less incompatible elements. Simi- larly Sm/Nd variations are related to variations between light and heavy rare earths, as well as incom- patible–less incompatible element variations. We have some knowledge of the processes that fraction- ate the alkalis and alkaline earths and the light and heavy rare earths. Thus knowledge of variations in these element ratios al- lows us to limit the range of possible processes oc- curring within the Earth; the time parameter in equation 6.01 allows us the limit the range of possible times at which this fractionation oc- curred. DEFINITIONS: TIME- INTEGRATED AND TIME AVERAGED Gast stated that the 87Sr/86Sr is a function of the time-integrated Rb/Sr ratio. What did he mean by “time- integrated”? Suppose the 87Rb/86Sr ratio evolves in some reservoir in some com- plex way. Let’s allow the 87Rb/86Sr to be an arbi- trary function of time such as 87Rb/86Sr = t + sin(5 t/π) +1. This is shown plotted in Fig. 6.01a. If we integrate 87Rb/86Sr with respect to time, we get the area un- der the curve, of course. From that, we can find the average 87Rb/86Sr simply by dividing the area under the curve by (t – t0), which is 3.3. The 87Sr/86Sr would evolve as shown by the solid line in Fig. 6.01b. Just as we can calculate the average 87Rb/86Sr ratio from the area under the curve in figure Figure 6.01. (a) 87Rb/86Sr is shown as changing in a hypothetical reser- voir as some arbitrary function of time. The average 87Rb/86Sr may be calculated by integrating 87Rb/86Sr with respect to time, and then divid- ing by (t – t0). (b) Evolution of 87Sr/86Sr for the case where 87Rb/86Sr is a function of time as shown in Fig. 6.01a. The dashed line shows the growth of 87Sr/86Sr if 87Rb/86Sr is constant and equal to 3.3. Docsity.com Isotope Geochemistry Chapter 6 Spring 2011 149 4/10/11 The Sr-Nd-Hf Picture Figure 6.02 shows the Sr and Nd isotopic characteristics of the Earth’s major silicate reservoirs. We begin by focusing on the picture of the mantle provided by isotopic compositions of basalts‡ derived from the suboceanic mantle. Initially, we’ll focus on oceanic basalts and avoid continental ones for two reasons: 1.) many are contaminated by the continental crust through which they ascend and 2.) the sub- continental lithosphere, from which some of the continental basalts are derived, does not convect and hence many not be as well mixed and as representative of the mantle as oceanic basalts derived from the convecting mantle. Figure 6.03 shows the isotopic compositions of oceanic basalts – mid-ocean ridge basalts and oceanic island basalts in detail. Mid-ocean ridge basalts (MORB)are those erupted at plate boundaries along the Earth’s 50,000 km mid-ocean ridge system. Oceanic island basalts (OIB) are those that erupt on oce- anic island volcanoes, such as the Hawaiian ones, and include oceanic islands, such as Iceland, that lie on mid-ocean ridges. The first observation is that Nd and Sr isotope ratios are inversely correlated and ‡ By definition, basalt is an extrusive igneous rock (i.e., lava) with less than 52% SiO2. Basalt is the primary product of melting of the mantle. Mid-ocean ridge basalts are simply those lavas erupted at mid-ocean ridge spreading cen- ters, i.e., divergent plate boundaries. Figure 6.03. Sr and Nd isotope ratios of oceanic island basalts. The gradient in the MORB field indi- cates data density: the vast majority of MORB plot at the high εNd end of the spectrum. Adapted from White (2010). Docsity.com Isotope Geochemistry Chapter 6 Spring 2011 150 4/10/11 the region occupied by the oceanic basalt data is often referred to as the “mantle array”. The second observation is that, although there is overlap, MORB have the lower 87Sr/86Sr ratios and highest εNd than OIB. Variations in radiogenic isotope ratios in basalts result from variations in parent-daughter ratios in the mantle over great lengths of time. For example, to create a 1 εNd variation in 143Nd/144Nd would require a variation in 147Sm/144Nd of 0.02, or about 10%, that existed for 1.4 Ga. We infer MORB come from a source with lower time-integrated 87Rb/86Sr and higher 147Sm/144Nd than OIB. Nd isotopes systematics can help put the relative differences between MORB and OIB in context be- cause 142Nd/144Nd ratios provide some constraints on the Sm/Nd ratio of the silicate Earth and primi- tive mantle. As we saw in the last Chapter, 142Nd/144Nd ratios of all modern terrestrial materials are about 18 ppm higher than those of ordinary chondrites. How different the Sm/Nd ratio of the Earth must be from chondritic depends somewhat on the very early history of the Earth, but Caro and Bour- don (2010) estimate the 147Sm/144Nd ratio of the bulk Earth as 0.2082±25 compared to a chondritic value in the range of of 0.1960–0.1966. This would mean that the bulk silicate Earth, or at least the observable part of it (if a hidden low 147Sm/144Sm reservoir exists), has εNd = 6.9±1.9. Assuming the “mantle array in Figure 6.03, Caro and Bourdon estimated the 87Sr/86Sr of the bulk (observable) silicate Earth as 0.7030±0.004. These values are illustrated on Figure 6.02 as grey bands. On average, MORB have εNd = 8.53 and 87Sr/86Sr = 0.70289; so-called “normal MORB”, which are those that are light rare earth de- pleted, have εNd = 9.25 and 87Sr/86Sr = 0.70273 (White, 2011). Thus MORB have higher time-integrated 147Sm/144Nd lower 87Rb/86Sr than bulk Earth, – one that is relatively depleted in more incompatible ele- ments, such as Rb and Nd compared to more compatible ones such as Sr and Sm. For this reason, we often speak of MORB as being derived from a “depleted”, meaning incompatible element-depleted, reservoir. Although we tend to think of MORB and OIB as being isotopically distinct, there is in fact consider- Figure 6.04. Comparison of Nd isotope ratio analyses of 1679 MORB and 2380 OIB from the PetDB and GEOROC databases. While distributions overlap, the two groups have different means and different spreads, with these differences being highly statistically significant. Docsity.com Isotope Geochemistry Chapter 6 Spring 2011 151 4/10/11 able overlap in the isotopic compositions of these groups. This is illustrated in Figure 6.04 for εNd. Nevertheless, the means of the two are quite different, 8.5 and 5.0 respectively. MORB also clearly have a more uniform isotopic compositions with less dispersion than OIB, implying the reservoir from which MORB are derived is less heterogeneous. In the above discussion, we have transformed the isotopic information into information about parent- daughter ratios. The next step is to consider those processes that can fractionate, or change, the parent- daughter ratios. Rb is more incompatible than Sr, while Nd is more incompatible than Sm. This sug- gests the mantle has been affected by a process that removes the more incompatible elements. Partial melting and extraction of the melt is such a process. If, as we supposed above, the crust has formed by extraction of the mantle, then the depleted isotopic signature of the mantle might just reflect formation of the continental crust. The source of MORB appears to have been more depleted, perhaps having lost a greater melt fraction, than the source of OIB. There are, however, other explanations for the differ- ence between MORB and OIB, which we will consider later. Figure 6.05 shows 176Hf/177Hf ratios plotted against Nd isotope ratios. As we expect from what we know of the behavior of the parents relative to the daughters, εHf is negatively correlated with 87Sr/86Sr and positively correlated with εNd. From the intersection of the eHf–εNd array with εNd = 6.9, Caro and Figure 6.05. Hf and Nd isotope ratios of the suboceanic mantle as sampled by oceanic basalts. Grey bands show the inferred isotopic composition of the bulk silicate Earth. Docsity.com