Samarium, abundance

A clast from Mount Padbury has a mg of 36, a molar FeO/MnO of 36, and a flat REE pattern at 9-10 X Cl chondrite abundances (Mittlefehldt, 1979)—all within the ranges for basaltic eucrites. However, many basaltic clasts are distinct in major element composition, with higher mg s and lower molar FeO/MnO ratios than those of basaltic eucrites, and have LREE-depleted patterns and (Eu/Sm)ci > 1— patterns unknown among unaltered basaltic eucrites. Some gabbro clasts are similar to cumulate eucrites in major-and trace-element contents, but many are distinct in having extreme depletions in the most incompatible elements (Mittlefehldt, 1979 Rubin and Mittlefehldt, 1992). In extreme cases, samarium abundances are only 0.02-0.03 X Cl chondrites (Rubin and Jerde, 1987 Rubin and Mittlefehldt, 1992), much less than the 1-2 X Cl typical of cumulate eucrites. These clasts have (Eu/Sm)ci of 220-260, the most extreme ratios known among solar system igneous rocks (Mittlefehldt et al., 1992). 

Samarium is the 39th most abundant element in the Earths crust and the fifth in abundance (6.5 ppm) of all the rare-earths. In 1879 samarium was first identified in the mineral samarskite [(Y, Ce U, Fe) (Nb, Ta, Ti )Ojg]. Today, it is mostly produced by the ion-exchange process from monazite sand. Monazite sand contains almost all the rare-earths, 2.8% of which is samarium. It is also found in the minerals gadolmite, cerite, and samarskite in South Africa, South America, Australia, and the southeastern United States. It can be recovered as a byproduct of the fission process in nuclear reactors. 

Samarium occurs in nature widely distributed but in trace quantities, always associated with other rare earth metals. The two most important minerals are (i) monazite, which is an orthophosphate of thorium and the rare earths and (ii) bastanasite, which is a rare earth fluocarbonate. The samarium content of these ores is about 2%, as oxide. It also is found in precambri-an granite rocks, shales, and certain minerals, such as xenotime and basalt. Its abundance in the earth s crust is estimated to be 7.05 mg/kg. 

In 1901 Eugene-Anatole Demargay in Paris showed that the samples of samarium and gadolinium produced until that time harboured yet another rare-earth element, which he named generously after all of Europe europium. This element is in fact one of the most naturally abundant of the group the Earth s crust contains twice as much europium as tin. It is harvested today largely for a very special and useful property its emission of very pure red and blue light. 

In some cases, thermal neutrons can also be used to measure the absolute abundances of other elements. Transforming the neutron spectrum into elemental abundances can be quite involved. For example, to determine the titanium abundances in lunar spectra, Elphic et at. (2002) first had to obtain FeO estimates from Clementine spectral reflectances and Th abundances from gamma-ray data, and then estimate the abundances of the rare earth elements gadolinium and samarium from their correlations with thorium. They then estimated the absorption of neutrons by major elements using the FeO data and further absorption effects by gadolinium and samarium, which have particularly large neutron cross-sections. After making these corrections, the residual neutron absorptions were inferred to be due to titanium alone. 

Isotope abundances which are free from all sources of bias are defined as absolute isotope abundances. The absolute isotope composition of elements can be measured by MC-TIMS and MC-ICP-MS via gravimetric synthetic mixtures or standard solutions from highly enriched isotopes, as demonstrated for neodymium,11 erbium13 and samarium,11 13 99 respectively. 

As reported by Olmez and Gordon (University of Maryland), the concentration pattern of rare earth elements on fine airborne particles (less than 2.5 micrometers in diameter) is distorted from the crustal abundance pattern in areas influenced by emissions from oil-fired plants and refineries. The ratio of lanthanum (La) to samarium (Sm) is often greater than 20 (crustal ratio is less than 6). The unusual pattern apparently results from tlie distribution of rare earths in zeolite catalysts used in refining oil. Oil industry emissions have been found to perturb the rare earth pattern even in very remote locations, such as the Mauna Loa Observatory in Hawaii. 

Samarium is regarded as a relatively abundant lanthanoid. It occurs to the extent of about 4.5 to 7 parts per million in Earth s crust. That makes it about as common as boron and two other lanthanoids, thulium and gadolinium. 

The perception that lanthanide metals were rare and therefore inaccessible or expensive was a contributing factor to their long-lasting neglect and slow develo nent as useful synthetic tools, hi fact, rare earths in general are relatively plentiful in terms of their abundance in the earth s crust. Samarium and ytterbium occur in proportions nearly equal to those of boron and tin, for example. Modem separation methods have made virtually all of the lanthanides readily available in pure form at reasonable cost. 

Beyond the broad major-element constraints afforded by seismic imaging, the abundance of many trace elements in the mantle clearly records the extraction of core (Chapters 2.01 and 2.15) and continental crust (Chapter 2.03). Estimates of the bulk composition of continental cmst (Volume 3) show it to be tremendously enriched compared to any estimate of the bulk Earth in certain elements that are incompatible in the minerals that make up the mantle. Because the crust contains more than its share of these elements, there must be complementary regions in the mantle depleted of these elements—and there are. The most voluminous magmatic system on Earth, the mid-ocean ridges, almost invariably erupt basalts that are depleted in the elements that are enriched in the continental crust (Chapter 2.03). Many attempts have been made to calculate the amount of mantle depleted by continent formation, but the result depends on which group of elements is used and the assumed composition of both the crust and the depleted mantle. If one uses the more enriched estimates of bulk-continent composition, the less depleted estimates for average depleted mantle, and the most incompatible elements, then the mass-balance calculations allow the whole mantle to have been depleted by continent formation. If one uses elements that are not so severely enriched in the continental cmst, for example, samarium and neodymium, then smaller volumes of depleted mantle are required in order to satisfy simultaneously the abundance of these elements in the continental cmst and the quite significant fractionation of these elements in the depleted mantle as indicated by neodymium isotope systematics. 

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