GEOL 135 Lab REE partitioning in melts and granite weathering The Rare Earth Elements REE The lanthanide series is formed by filling of 4f orbitals and its constituents are commonly termed the rare earth elements REE The common valence state is 3 for all REE over a wide range of oxygen fugacity Ce4 can occur in highly oxidized environments at the earth s surface and Eu2 in reducing environments in the crust and mantle The rare earth elements are lithophile elements low electronegativities lead to the formation of highly ionic bonds which substitute into many silicates and phosphates Ionic radii of the lanthanides decreases with increasing atomic number from La to Lu called the lanthanide contraction Because of their high charge and large radii the rare earth elements are usually relatively incompatible in silicate minerals However due to the lanthanide contraction the heavier rare earths are smaller and thus can fit within some lattice sites although there must be charge compensation for the 3 valence so incompatibility decreases with increasing Z 1 Producing REE diagrams REE diagrams are plotted as concentration on the y axis and each REE represented on the x axis Using the chondrite REE data in ppm by weight plot the diagram for REE La though Lu corresponding to the chondrite composition on Table 1 using excel try making a LINE plot Why does the element concentration vary significantly in this fashion Since the effect plotted for chondrite REE affects all rocks we typically normalize all REE analyses based on the chondrite values so we can see effects associated with other phenomena Plot the N and E type MORBs N MORB is a normal mid ocean ridge basalt E Morb is an enriched MORB OIB Ocean Island Basalt a mantle plume material and a granite as CHONDRITE NORMALIZED compositions on an REE plot discuss the major differences Table 1 selected REE data for selected rocks to use in these exercises 2 Rayleigh Fractionation of elements as melts crystallize As melts crystallize elements enter certain minerals according to a partitioning coefficient KD for a comprehensive list of KD values see http earthref org cgi bin er cgi s kdd s0 main cgi Using the chondrite normalized data above as a starting point and the KD data in Table 2 plot how Sm Nd Eu and Yb would change as a function of melt crystallization as Plagioclase and Clinopyroxene crystallize and then discuss how an OIB mantle material could be produced La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu KD KD Use the Rayleigh Fractionation equation for this PlagioclaseClinopyroxene Cimelt Ci0rockF KD 1 0 135 0 0536 0 062 0 0858 0 088 0 15 0 028 0 1873 Where the initial melt concentration for any element is Cimelt the 0 036 0 291 concentration of any element in the rock or mineral under consideration 0 73 0 458 is Ci0rock KD is the partition coefficient and F is the fraction of melt 0 04 0 82 remaining Here the REE concentrations for the chondrite is the initial 0 026 0 53 0 055 0 442 0 048 0 44 0 041 0 387 Table 2 selected KD values for Plagioclase and Clinopyroxenes 0 036 1 09 0 031 0 43 0 025 0 433 3 REE changes as granite weathers REE are often conservative in other words REE distributions of minerals forming sediments are representative of the parent rocks they come from with some caveats We will investigate how REE changes as a function of granite weathering and the minerals REEs are coming from as granite dissolves Table 3 lists REE for Granite and the major minerals in granite Data from Aubert et al 2001 GCA in addition to soil river sediment soil porewater and river water analyses Plot the REE patterns of each sample and calculate apparent KD for each mineral Investigate this dataset and address the following REE ppm HPT Granite Plagioclase Orthoclase Apatte Muscovite Biotte top Soil river sediment soil porewater river water If river sediment is deposited and eventually forms a fluvial sedimentary rock how well is the REE preserved and what elements see the most difference Address why these specific REE distributions may be different Discuss this with supporting graphs and calculations La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 16 3 37 51 4 66 17 28 4 14 0 35 3 09 0 49 2 45 0 37 0 82 0 12 0 69 0 09 7 74 17 19 2 03 7 4 1 58 0 33 1 03 0 14 0 64 0 1 0 23 0 03 0 2 0 03 3 97 8 94 1 01 3 71 0 83 0 48 0 59 0 08 0 33 0 05 0 11 0 01 0 09 0 01 103 28 298 05 51 21 276 8 132 83 9 99 109 1 16 8 71 36 9 37 16 93 2 33 11 87 1 45 3 15 7 93 0 95 3 49 0 89 0 04 0 66 0 11 0 56 0 09 0 18 0 03 0 15 0 02 60 72 147 08 17 73 66 22 14 05 0 24 9 8 1 36 5 9 0 86 1 86 0 26 1 37 0 19 17 63 38 27 4 35 15 4 2 72 0 25 1 54 0 18 0 76 0 13 0 31 0 05 0 27 0 04 16 4 38 9 4 8 20 5 5 5 0 9 6 2 1 5 2 0 88 2 3 0 3 1 86 0 27 188 4 114 5 12 8 58 17 6 3 9 19 8 3 6 19 3 2 8 7 1 7 10 9 1 3 21 34 7 2 41 17 3 1 20 3 8 22 4 1 12 2 13 2