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The Effect of Assimilation, Fractional Crystallization, and Ageing on U

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The Effect of Assimilation, Fractional Crystallization, and Ageing on U Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 72 (2008) 4136–4145 www.elsevier.com/locate/gca The effect of assimilation, fractional crystallization, and ageing on U-series disequilibria in subduction zone lavas Fang Huang *, Lili Gao, Craig C. Lundstrom Department of Geology, University of Illinois at Urbana-Champaign, 1301 W. Green Street, IL 61801, USA Received 22 October 2007; accepted in revised form 27 May 2008; available online 14 June 2008 Abstract Although most arc lavas have experienced significant magma differentiation, the effect of the differentiation process on U-series disequilibria is still poorly understood. Here we present a numerical model for simulating the effect of time-dependent magma differentiation processes on U-series disequilibria in lavas from convergent margins. Our model shows that, in a closed system with fractional crystallization, the ageing effect can decrease U-series disequilibria via radioactive decay while in an open system, both ageing and bulk assimilation of old crustal material serve to reduce the primary U-series disequilibria. In contrast, with recharge of refresh magma, significant 226Ra excess in erupted lavas can be maintained even if the average residence time is longer than 8000 years. The positive correlations of (226Ra/230Th) between Sr/Th or Ba/Th in young lavas from convergent margins have been widely used as evidence of fluid addition generating the observed 226Ra excess in subduction zones. We assess to what extent the positive correlations of (226Ra/230Th) with Sr/Th and Ba/Th observed in the Tonga arc could reflect AFC process. Results of our model show that these positive correlations can be produced during time-dependent magma differentiation at shallow crustal levels. Specifically, fractional crystallization of plagioclase and amphibole coupled with contemporaneous decay of 226Ra can produce positive correlations between (226Ra/230Th) and Sr/Th or Ba/Th (to a lesser extent). Therefore, the corre- lations of (226Ra/230Th) with Sr/Th and Ba/Th cannot be used to unambiguously support the fluid addition model, and the strength of previous conclusions regarding recent fluid addition and ultra-fast ascent rates of arc magmas is significantly lessened. Ó 2008 Published by Elsevier Ltd. 1. INTRODUCTION powerful tools for constraining the time-scale of subduction zone magmatism. Hundreds of U-series analyses for conver- Convergent margins play a critical role in Earth’s geo- gent margin lavas have now been reported (see Turner et al. chemical cycling, crust-mantle interactions and generation (2003) for a recent review) leading to four important general of new continental crust. During subduction, hydrous fluids observations: (1) most young lavas have 238U excess over are released from the subducted oceanic slab and added to 230Th but a substantial number also have 238U depletion or the overlying mantle wedge. In response to the addition of 238U–230Th equilibrium (e.g., Turner et al., 1996, 2003; water, partial melting of mantle peridotite produces H2O Sigmarsson et al., 2002; Dosseto et al., 2003); (2) almost all rich subduction zone magmas (e.g., Gill, 1981; Grove and young lavas have 231Pa excess over 235U(Pickett and Kinzler, 1986; Grove et al., 2006). Because parent-daughter Murrell, 1997; Bourdon et al., 1999; Thomas et al., 2002; disequilibrium will decay back to secular equilibrium in 5 Dosseto et al., 2003; Turner et al., 2006; Huang and half-lives of a daughter nuclide, U-series disequilibria are Lundstrom, 2007); (3) most lavas have 226Ra excesses which negatively correlate with SiO2 content and positively * Corresponding author. correlated with Sr/Th or Ba/Th (Turner et al., 2000, 2001; E-mail addresses: [email protected], huangfang426@hotmail. Turner and Foden, 2001); and (4) convergent margin lavas on com (F. Huang). average have higher U/Th than MORB (Turner et al., 2003). 0016-7037/$ - see front matter Ó 2008 Published by Elsevier Ltd. doi:10.1016/j.gca.2008.05.060 The effect of time-dependent AFC on U-series disequilibria in young lavas 4137 Most lavas at convergent margins do not represent magma chamber (Fig. 1), its mass changes according to the 0 0 0 primitive magmas but instead have experienced significant equation: dM m=dt ¼ M a þ M r À M c, where Mm is the total 0 crustal level differentiation (e.g., Gill, 1981). A number of mass of magma in the chamber, M c is the mass change rate 0 studies have emphasized the importance of fractional crys- by fractional crystallization, M a is the rate of change of 0 tallization and assimilation of crustal materials in generat- mass due to assimilation, and M r is the rate of change of ing the compositional variations in subduction zone mass due to recharge. If there is no recharge of fresh mag- magmas (e.g., DePaolo, 1981; Hildreth and Moorbath, ma, the AFCR model reduces to AFC, which reduces fur- 1988; Beard et al., 2005). DePaolo (1981) provided equa- ther to pure fractional crystallization (FC) model if the tions for quantifying the geochemical effects of assimilation rate of assimilation is zero. A description of nomenclatures and fractional crystallization (AFC) and applied them to and parameters used is given in Table 1. open-system magma evolution. Because U-series nuclides usually have small partition coefficients (<0.01) between 2.1. Trace element concentrations most silicate minerals and melt (Blundy and Wood, 2003), fractional crystallization by itself should not significantly Similar to equation 1a of DePaolo (1981), we assume affect U-series disequilibria generated by earlier magmatic that the concentration of a trace element in the recharging 0 processes (such as melting or fluid addition). However, be- magma is constant ðCmÞ such that the instantaneous rate of cause magma residence times could vary from tens of years change of mass of a trace element is given by: to 105 years (e.g., Halliday et al., 1989; Heath et al., 1998; dðÞM C dM dC Hawkesworth et al., 2000; Zellmer et al., 2000; Cooper et m m ¼ C m þ M m dt m dt m dt al., 2001; Condomines et al., 2003; Reagan et al., 2003; 0 0 0 0 Asmerom et al., 2005; Jicha et al., 2005), contemporaneous ¼ M aCa þ M rCm À M cDCm ð1Þ decay of short-lived nuclides (such as 226Ra) during AFC where Cm is the concentration of the trace element and D is processes could significantly affect the extent of U-series the bulk partition coefficient of the element between the disequilibria in young lavas (Condomines et al., 1995). In- crystallized minerals and melt. deed, previous works have addressed the variation of If dMm ¼ 0, magma evolution is similar to a zone refining 226 230 dt ( Ra/ Th) due to fractional crystallization and magma process, where the mass of the magma chamber is always replenishment for the case of the Ardoukoba Volcano (Asal 0 M m. Eq. (1) can be integrated as: rift, Djibouti) (Vigier et al., 1999) and variation of 226 230 r C r C0 r C r C0 ( Ra/ Th) with time in a closed system to constrain 0 a a þ r m ÀrcDt a a þ r m Cm ¼ Cm À e þ ð2Þ the cooling rate of magma chambers (Blake and Rogers, rcD rcD 0 0 2005). Besides the effect of fractional crystallization, ageing, M a M r where ra ¼ 0 , defined as the assimilation rate, rr ¼ 0 , the Mm 0 Mm and replenishment (Hughes and Hawkesworth, 1999; Mc recharge rate, and rc ¼ 0 , the crystallization rate. M m Hawkesworth et al., 2000), U-series disequilibria can also dMm In general, dt 6¼ 0. For a slowly cooling magma cham- be changed by addition of assimilation of crustal materials. dMm ber, dt < 0. Eq. (1) can be re-written as: Recently, Touboul et al. (2007) reported a model simulating U-series disequilibrium variations in closed and open sys- 0 dCm rcD raCa þ rrCm tems assuming that crystallization rate is a linear function þ 1 þ Cm ¼ ð3Þ dlnF ra þ rr À rc ra þ rr À rc of the mass of magma in the chamber which exponentially decreases with time. where F is the fraction of melt remaining in the 0 0 0 M m MaþM rÀMc In this paper, we present a more general time-dependent magma chamber F ¼ 0 ; and dF ¼ 0 dt M m Mm AFC model assuming a constant rate of change for magma ¼ ðÞra þ rr À rc dt. An analytical solution for Eq. (3) is: mass with time. This model can be used to simulate the var- 0 0 ÀraþrrÀrcþrcD raCa þ rrCm ÀraþrrÀrcþrcD iation in U-series disequilibria due to assimilation of crustal raþrrÀrc raþrrÀrc Cm ¼ CmF þ 1 À F materials, fractional crystallization, and magma recharge ra þ rr À rc þ rcD coupled with ageing. It allows for simulation of both closed ð4Þ and open system (assimilation and recharge) behavior and If rr = 0, Eq. (5) is same to equation (6a) in DePaolo (1981). accounts for decay of short-lived U-series nuclides in the calculation. The aim of this model is to provide a tool for 2.2. Short-lived U-series nuclides better assessing the effect of magma differentiation pro- cesses on U-series disequilibria and to provide an alterna- If the residence time of magma is comparable to or long- tive explanation for the positive correlations between 226 er than the half-lives of the short-lived isotopes, radioactive Ra excess and Sr/Th or Ba/Th. decay must be taken into account for assessing the change of mass in U-series nuclides in a magma chamber. The var- 2. EQUATIONS FOR TIME-DEPENDENT AFC iation of mass of a daughter nuclide with time can be writ- MODEL ten as: Our model follows the basic equations of DePaolo dðÞM mCm;D 0 0 0 0 ¼ M aCa;D þ M rCm;D À M cDDCm;D (1981), starting with a general scenario including assimila- dt tion, fractional crystallization, and continuous recharge kPM mmDCm;P þ À kDM mCm;D ð5Þ (AFCR) processes.
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