Trace-Element Geochemistry Spring 2009

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1 MIT OpenCurseWare Trace-Element Gechemistry Spring 2009 Fr infrmatin abut citing these materials r ur Terms f Use, visit:

2 Lecture 14 In Exchange Chrmatgraphy (pertinent reference is Navn and Stlper, Gechemical Cnsequences f Melt Perclatin: The Upper Mantle as a Chrmatgraphic Clumn, Jur. Gel., 95, , 1987) In the simple case basaltic melt ascends thrugh an verlying peridtite via prus flw and equilibrium is attained; fr trace elements this prcess is analgus t batch melting. Hwever, prir t attaining equilibrium there is a transient apprach t equilibrium. Tday we are interested in understanding this transient case. Cnsider a prus slid that is in equilibrium with respect t majr elements but nt fr trace elements. Nte this is an unrealistic special case but it leads t a first rder understanding f the effects f a chrmatgraphic prcess n abundances f trace elements. We can subsequently cnsider the mineralgical changes within the peridtite matrix caused by majr element disequilibrium. We begin by evaluating what happens as an extic melt ascends thrugh a prus slid matrix with trace element cncentratin C s in the matrix and Cm in the fluid ccupying the pres. We assume that the cncentratin f the trace element C m in the extic intrduced fluid is nt equal t C m (see Figure 49). Cnsequently this melt interacts with matrix and cnverts the slid t be in equilibrium with the intrduced melt. That is C s = DC m. Therefre as the melt passes thrugh the clumn and equilibrates the trace element cmpsitin f the slid and melt change.

3 Slid frm C s t C s. Melt frm C m t C m Cnsider Infiltratin f an Extic Melt int Overlying Peridtite Initial State Initiatin f Prcess Prus slid with Cncentratins: C s in slid C m in melt D = C s /C m Intrduce extic melt with C m C m Figure by MIT OpenCurseWare. Figure 49. Infiltratin f an extic melt int an verlying prus peridtite; the infiltrating melt and initial slid matrix are nt in equilibrium. Initially as melt with C m enters at the bttm f the clumn, melt with C m leaves at the tp, but at sme critical time (t c ) the main mass f intrduced melt reaches the tp f the clumn. This melt will be made up f all elements that did nt interact with the matrix, i.e. majr elements (assumed) and D = 0 (i.e., perfectly incmpatible) elements. What abut trace elements with D > 0? Cncentratin frnts, the change frm C m t C m will travel thrugh the slid at rates that are prprtinal t D. The equatins describing this prcess are in Navn and Stlper (1987) wh describe the prcess as fllws (see Figure 50):

4 at t = 0 extic melt is intrduced C m C melt at t = t c melt exits at the tp f this clumn C m = C s D bttm Distance in Chrmatgraphic Clumn tp Figure by MIT OpenCurseWare. Figure 50. Develpment f cncentratin frnts in a chrmatgraphic clumn. At the beginning f the prcess, i.e. at time (t) =, melt with trace element cncentratin, Cm is intrduced at the bttm f the clumn (left end). At t = tcritical all elements that d nt interact with the slid matrix, i.e. perfectly incmpatible elements with D=0, exit the clumn (right end). Hwever trace elements with D>0 react with the matrix and develp cncentratin frnts as shwn fr elements with D = 0.4 (red line) and D = 0.2 (green line). Ahead f the frnt, which is mving frm left t right, the matrix has its initial cncentratin, C s and the fluid has a cncentratin given by C = m C s /D but behind the frnt the intrduced fluid has its cncentratin f C m and it has cnverted the matrix t have a cncentratin Cs = D C m. Figure is adapted frm Figure 1a f Navn and Stlper (1987). Als Navn and Stlper state: The essence f the kind f prcess that we envisin is: Melt is cntinuusly intrduced at the base f a clumn f rck and mves upward thrugh it by permeable flw. The melt is initially ut f equilibrium with the rck matrix and the tw interact chemically. A snapsht f the clumn at any time wuld shw fr each element a cncentratin frnt, abve which the melt is in equilibrium with the initial matrix f the clumn and belw which the melt is unchanged frm when it was intrduced int the base f the clumn. Abve the cncentratin frnt, the matrix retains the initial clumn cmpsitin; belw it, the matrix has changed t be in equilibrium with the melt flwing int the clumn. The width f the frnt depends n the effectiveness f dispersive prcesses, such as diffusin, in the clumn. If we lked at a mvie f the clumn, we wuld see the cncentratin frnt fr each element sweeping upwards, with the frnts f the mre incmpatible elements mving faster than thse f the mre cmpatible nes. If we sat n the tp f the clumn and sampled the liquids that emerged, we wuld first cllect melt identical t that f an infinitesimal degree f partial melting f the initial matrix material. As the cncentratin frnt f each element reached the tp f the clumn (the frnts wuld arrive in rder f increasing cmpatibility), we wuld bserve the melts changing frm the cncentratin f that element in the incipient melt f the matrix t that f the melt flwing int the clumn s base. Eventually, the cncentratin frnts f even the mst cmpatible elements will have reached ur psitin, and thereafter, the melts that emerge will be identical t thse injected int the base f the clumn.

5 Figure 51 shws cncentratins in melts exiting a chrmatgraphic clumn fr elements with varius D at varius times, t =, t = t c and t = 2 c. At t= the cncentratin prfile (red line) is that f the intrduced melt; its cncentratin reaches a maximum f C m /C s = 10 fr D < 0.1 because this melt is assumed t be a 10% batch melt f the initial matrix. At t = t c the prus melt initially in the clumn (blue line) ranges frm C m /Cs = 1000 fr D = and 1 fr D = 1; i.e. the 1/D limit. Hwever, at t = t c (green line) there is an abrupt change in abundance rati f elements with D values slightly greater r lesser than This prcess is analgus t the in exchange clumns used by analytical gechemists t separate Sm frm Nd, Rb frm Sr, and Pb frm U and Th. C melt /C s 1000 t = t c = melt present in clumn initially t = 2t c t = = fluid entering clumn frmed by 10% melting D Figure by MIT OpenCurseWare. Figure 51. Enrichment f melt relative t initial slid, i.e. C melt / C s, versus D shwing the cncentratin frnts f incmpatible elements with varius D at t =, t = t c and t = 2t c. At t = the cncentratin prfiles (red line) is that f the intrduced melt. This melt is assumed t be a 10% melt f the initial matrix (C s ) s its C m / C s reaches a maximum f 10 when D < F. At t = t c the prus melt in the initial clumn (blue line) has cncentratin given by the 1/D limit. At t = 2t c the melt (green line) shws an abrupt cncentratin frnt with elements with D>0.3 retaining the initial prus melt cntents whereas elements with D<0.3 have cntents equal t the intrduced melt. Figure is adapted frm Figure 1b f Navn and Stlper (1987). 4

6 Hence a distinctive characteristic f melts that reflect a chrmatgraphic prcess are large differences in ratis f incmpatible elements with very similar slid/melt partitin cefficients. Hwever, we have never fund a tempral sequence f lavas shwing abrupt changes in abundance ratis f trace elements with similar D. Very likely the explanatin is that magmas easily mix; in fact, mixing is inherent in melting mdels such as accumulated fractinal and cntinuus melts as well as in dynamic melting (Figure 40). Hwever, there are strng indicatrs f the chrmatgraphic prcess in the trace element cntents f peridtites, the presumed slid matrix invlved in chrmatgraphy. It is very likely that evidence fr the chrmatgraphic prcess is retained in slids because diffusin is slw in slids and mixing f slids is difficult.

7 Figure 52 illustrates the prcess; the paper by Takazawa et al. (1992) is an example. In Exchange Reactin Between Perclating Z1 Melt and Peridtite Clinpyrxenes/Chndrite CPX equilibrated with Melt (Cm) Melt (C m ) Original CPX Z2 La Nd Dy Yb Z1 Z2 Extic Melt (Cm) Figure by MIT OpenCurseWare. Figure 52. Right: A peridtite matrix in the upper mantle is infiltrated by an extic melt whse rare-earth element cncentratins (La, Nd, Dy, Yb) are nt in equlibrium with the matrix. Left: Apprach f matrix t equilibrium with the extic melt fr time <tcritical with slid state diffusin explicitly included in the calculatin (see Takazawa et al., 2002). Lwer panel shws the REE pattern (chndrite-nrmalized) fr the riginal clinpyrxene (cpx) in the matrix peridtite, the REE pattern fr the intrduced melt and cpx in equilibrium with this melt. Middle panel shws the cpx at psitin Z2 (right panel) in the peridtite clumn. Here the cpx is nearly fully equilibrated with the intrduced melt as indicated by the vertical arrws shwing that La and Nd cncentratins in the matrix (slid line in middle panel) have nearly reached the La and Nd cntents f cpx in equilibrium with the intrduced melt (upper dashed line). Upper panel shws the cpx at psitin Z1 (right panel) in the peridtite clumn. Here the cncentratin frnts f Nd, Dy and Yb have nt yet reached this lcatin in the clumn; nly La cncentratins have been increased frm the level in the riginal cpx. The distinctive gechemical feature imparted by the chrmatgraphic prcess is the abrupt inflectin at Nd shwing a marked increase in La/Nd rati. Such inflectins are cmmn in peridtites (see Jhnsn et al., 1990; Takazawa et al., 1992).