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Comment on “Boron content and isotopic composition of oceanic basalts: Geochemical and cosmochemical implications” by M. Chaussidon and A. Jambon
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Earth and Planetary Science Letters 128 (1994) 727-730
Comment on "Boron content and isotopic composition of oceanic basalts: Geochemical and cosmochemical implications" by M. Chaussidon and A. Jambon Chen-Feng You Scripps Institution of Oceanography, La Jolla, CA 92093-0208, USA Received 22 April 1994; accepted 22 August 1994
Recently, Chaussidon and Jambon presented results of their ion microprobe study of boron concentrations and isotopic compositions in oceanic basaltic glasses [1]. Although this study has extended the information on the geochemistry of B and ~ n B in mid-ocean ridge basalts ( E - M O R B and N-MORB), back-arc basin basalts (BABB) and ocean island basalts (OIB), their calculation of the maximum amount of B recycled in subduction zones warrants special caution. In the discussion, they assumed end-member compositions for the primary upper mantle (B = 0.4 p p m and 3liB = --7%o) and the altered oceanic crust (70 ppm, + 7%0) to place an upper limit on the amount of B recycled into the upper mantle. Based on these assumptions, it was concluded that less than 2% of the B in subducted oceanic crust was added to the mantle source in order to explain the low ~ l l B values in the oceanic basalts ( - 7.4-0.6%o). Two comments regarding the calculation of Chaussidon and Jambon will be presented in this paper. First, sediments constitute one of the most important B reservoirs in subduction zones and should be considered in any budget calculation. Secondly, there is an important fractionation effect of both B and ~11B in the slab during subduction and this effect should be assessed. In
order to clarify the second comment, I present results of an hydrothermal experiment aimed at obtaining a better constraint on the e n d - m e m b e r composition of B and 61~B in the subducted oceanic crust. The experimental results demonstrate that significant modification of B and ~l~B can occur at elevated temperatures. The exchangeable B with heavy isotopic composition ( g l i B = ~ +15%o) in sediments is released at low temperatures (T < 150°C) and the remaining B is enriched in l°B (611B = ~ - 1 0 % o ) . Any involvement of this sedimentary B will complicate the calculation of Chaussidon and Jambon. In addition, I re-evaluate the importance of slab-derived fluids in subduction zones, based on the available data on B and 61LB in OIB [1] and in H a l m a h e r a arc volcanics [2,3]. Boron is an excess volatile element. Its unique chemical characteristics (e.g., the high incompatibility in magmatic processes, the strong affinity to aqueous solutions at elevated temperatures and the large isotopic variations in natural systems) have made B and ~ n B powerful tracers for subduction recycling, especially when combined with l°Be [4,5]. Marine sediments and altered oceanic basalts, with B concentrations up to or more than 100 p p m [2,4], are the two main reservoirs for boron
0012-821X/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSD1 0 0 1 2 - 8 2 1 X ( 9 4 ) 0 0 1 8 3 - 9
728
C.-F. You/Earth and Planetary Science Letters 128 (1994) 727-730
in the u p p e r crust. The high B contents in arc volcanics have been attributed to contributions from subducted sediments a n d / o r altered basalt or slab-derived fluids [3,5]. At present, the argument for the involvement of subducted sediment in arc magmatism is favored due to the observed B - l ° B e systematics in arc lavas [5]. The experiment, using an autoclave hydrothermal rocking apparatus under controlled P - T conditions, was performed to monitor the degree of interaction between the d~collement sediments of O D P Site 808, Nankai Trough, and synthetic NaC1-CaCI z solutions with compositions similar to in situ pore waters (for details of procedures see [6]). The d6collement sample is a hemi-pelagic sediment and is mainly composed of clay minerals (e.g., illite, smectite, kaolinite and chlorite). The w a t e r / r o c k ratio is ~ 3 w / w at initial conditions and - 1 . 5 w / w at the end of the experiment. This experiment basically mimics the behavior of B and 8 ~ B during early subduction zone processes. The experiment was conducted over a t e m p e r a t u r e range of 25-350°C, at 800 bar, and the entire experiment lasted for about 45 days. Fifteen fluid samples ( ~ 7 cm 3 for each) were extracted after ~ 3 days interaction at every 2 5 50°C increment. In addition, the initial and final sediments were analyzed. The fluid samples were analyzed for major and trace elements, including both B and 61~B. The fluid B contents were determined by a modified B-curcumin method and the ~IlB were analyzed by negative ion thermal ionization mass spectrometer (TIMS) with a precision of 1.2%o (95% C.L.) [7]. The sediment B and 6 ~ B values were analyzed via isotope dilution T I M S ( ~ 1%) and Cs2BO ~- positive ion TIMS (0.4%o, 95% C.L.), respectively [4]. The ddcollement sediment has a concentration of ~ 117 ppm, with a 6 ~ B of ~ - 4 . 7 % 0 . The high mobilization of B at elevated P - T conditions is demonstrated well in the hydrothermal interaction experiment (Fig. 1). The B concentration in the solutions increases rapidly with t e m p e r a t u r e during the heating episode and remains rather constant while cooling (Fig. la). The 6liB value decreases drastically during the early heating stage, indicating release of the exchangeable B (~11B = ~ + 15%o [2]), and remains some-
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t
t
t
II
l[
t
t
1500 B (~M) 1000 |
500 (a) i
i
i
100
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Temperature
400
(°C)
30
20
811B
10
(
t
t
t
t t
t t
t t
t (b)
-1C
i
i
i
100
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Temperature
400
(°C)
Fig. 1. V a r i a t i o n in b o r o n c o n c e n t r a t i o n a n d i s o t o p i c c o m p o s i tion in t h e e x t r a c t e d fluids of t h e h y d r o t h e r m a l w a t e r sedim e n t i n t e r a c t i o n e x p e r i m e n t to m i m i c t h e B b e h a v i o r d u r i n g the e a r l y s u b d u c t i o n z o n e p r o c e s s e s . (a) B v e r s u s T in the e x t r a c t e d fluids. (b) 611B v e r s u s T in the e x t r a c t e d fluids. O = the h e a t i n g e p i s o d e ; • = t h e c o o l i n g e p i s o d e .
what scattered through the entire cooling process (Fig. lb). The fluid B contents and 611B suggest a mixing trend between an exchangeable fraction ( ~ 20 p p m , ~ + 15%o) and a fixed fraction ( ~ 100 ppm, ~ - 1 0 % o ) . The slight curvature and some scatter in the data (Fig. 2) can be attributed to the effects of chemisorption at low temperatures a n d / o r redissolution of the residual pore water salt in the sediment with a 611B of ~ 25%o [7]. In this experiment about 35% of total B in the
C,-F. You/Earth and Planetary Science Letters 128 (1994) 727-730
sediment was removed (from 117 to 70 ppm) and the ~11B in the residual changed significantly (from ~ - 4 . 7 to - 1 1 . 8 % o ) after 45 days. These data emphasize the importance of chemical and isotopic modification as a result of the temperature increase during normal oceanic crust subduction and this effect has to be taken into account in any subduction zone budget consideration. Another important point shown here is that a large change in sediment ~ l B occurs at relatively low temperatures. Furthermore, the large amount of B mobilization observed both in the sediments and the fluids supports the argument of significant return of B to the ocean during slab subduction [2,7]. This isotopic modification was not considered in the calculation of Chaussidon and Jambon, who assumed a fixed composition (611B = + 7 % c ) for altered oceanic crust through the entire subduction process. The calculation of Chaussidon and Jambon is improper because of the preferential mobilization of B with high 6~'B during subduction, as is shown by the experimen-
28
18 811B
-2 B
Initial Sed.
-12
Final Sed. . . . . . . . .
1 0 -4
i
10 -3
........ 10 2
1/13(pM -:)
Fig. 2. Plot of 61:B versus 1 / B (/xM l) in fluid samples of the hydrothermal experiment (the 611B of the initial sediments and the final sediments are also indicated). These data suggest a mixing trend between the exchangeable B and the lattice-bound B in sediments. The slight curvature and some scatter in the data can be attributed to the effects of chemisorption at low temperatures or redissolution of residual pore water salt in sediments with 6HB of ~ 25%0 [7]. Significant mobilization of sediment B (35%) with large isotope fractionation (from ~ - 4 . 7 to - 11.8%o) occurred after 45 days at temperatures lower than 350°C. Symbols as in Fig. 1.
729
I0 6 2 511B
° -2 -6 -10 0.0
I
0.4
0.8
1.2
I/B (ppm-1) Fig. 3. Plot of 6lIB versus 1 / B (ppm t) in Halmahera arc volcanics [2,3] and in Hawaii OIB [1]. The good linear correlation suggests mixtures between a MORB end-member and an enriched end-member with high 6HB. C)= Halmahera arc volcanics; • = Hawaii OIB.
tal results. Any involvement of the residual sedimentary B in the slab will change the calculation drastically. Results of B and 611B in Hawaii OIB [1] and in H a l m a h e r a arc volcanics [2,3], as well as the experimental data, have important implications for the effect of slab-derived fluids on the chemical composition of arc lavas. The H a l m a h e r a arc volcanics and Hawaii OIB show a linear correlation in the plot of 1 / B versus 6JIB, suggesting mixtures between M O R B (B < 1 ppm, ~ - 10%e) and an enriched end-member (61~B = - +6%o, Fig. 3). Results of the hydrothermal experiment, as well as of several field studies (e.g., D S D P Site 477 [2]), indicate that B in sediments that have been subjected to progressive metamorphism will have a low 6 ' : B (an analogy to the residual B in the oceanic crust after deep subduction). If this is true, then the residual B in the subducted crust cannot be the observed enriched end-member in Fig. 3. This argument is further strengthened by the fact that B is highly mobile during progressive dehydration upon subduction [7,8]. On the other hand, the B-enriched e n d - m e m b e r can be reasonably interpreted in terms of involvement of fluids generated at shallow depth and enriched in 6~'B. A subduction model involving enrichment of 13, 1°Be, or other incompatible elements at rather shallow depth and, then, through subsequent
730
C.-F. You/Earth and Planetary Science Letters 128 (1994) 727-730
multi-stage hydration/dehydration reaction of hydrous m i n e r a l s in the slab at d e p t h , can g e n e r ate t h e c h e m i c a l characteristics of arc volcanics (e.g., B and 6HB, i n c o m p a t i b l e e l e m e n t s , U / T h ratio and l ° B e) [6,9].
Acknowledgements This w o r k is s u p p o r t e d by N S F grant O C E - 9 2 16786 to J.M. G i e s k e s a n d A.J. Spivack. T h e a u t h o r thanks R. R o s e n b a u e r a n d J.L. Bischoff at U S G S , M e n l o P a r k for t h e ir h e l p in h y d r o t h e r real e x p e r i m e n t s . C o m m e n t s f r o m P. Castillo, J.M. G i e s k e s and L.-H. C h a n on the early draft are greatly a p p r e c i a t e d . Special t h a n k to H. S t a u d i g e l for his critical review. [UC]
References [1] M. Chaussidon and A. Jambon, Boron content and isotopic composition of oceanic basalts: Geochemical and cosmochemical implications, Earth Planet. Sci. Lett. 121, 277-291, 1994.
[2] A.J. Spivack, Boron Isotope Geochemistry, 183 pp., MIT/WHOI, 1986. [3] M.P. Palmer, Boron isotope systematics of Halmahera arc (Indonesia) lava: Evidence for involvement of the subducted slab, Geology 19, 215-217, 1991. [4] T. Ishikawa and E. Nakamura, Boron isotope systematics of marine sediments, Earth Planet. Sci. Lett. 117, 567-580, 1993. [5] J.D. Morris, W.P. Leeman and F. Tera, The subducted component in island arc lava: constraints from the Be isotopes and B-Be systematics, Nature 344, 31-36, 1990. [6] C.F. You, J.D. Morris, J.M. Gieskes, R. Rosenbauer, S. Zheng, X. Xu, T.L. Ku and J.L Bischoff, Mobilization of Be in sedimentary column at convergent margins, Geochim. Cosmochim. Acta, in press. [7] C.F. You, A.J. Spivack, J.H. Smith and J.M. Gieskes, Mobilization of boron in convergent margins: implications for the boron geochemical cycle, Geology 21, 201-210, 1993. [8] A.E. Moran, V.B. Sisson and W.P. Leeman, Boron depletion during progressive metamorphism: implications for subduction processes, Earth Planet. Sci. Lett. 111, 331 349, 1992. [9] J.H. Davies and D.J. Stevenson, Physical model of source region of subduction zone volcanics, J. Geophys. Res. 97, 2037-2070, 1992.
Earth and Planetary Science Letters 128 (1994) 727-730
Comment on "Boron content and isotopic composition of oceanic basalts: Geochemical and cosmochemical implications" by M. Chaussidon and A. Jambon Chen-Feng You Scripps Institution of Oceanography, La Jolla, CA 92093-0208, USA Received 22 April 1994; accepted 22 August 1994
Recently, Chaussidon and Jambon presented results of their ion microprobe study of boron concentrations and isotopic compositions in oceanic basaltic glasses [1]. Although this study has extended the information on the geochemistry of B and ~ n B in mid-ocean ridge basalts ( E - M O R B and N-MORB), back-arc basin basalts (BABB) and ocean island basalts (OIB), their calculation of the maximum amount of B recycled in subduction zones warrants special caution. In the discussion, they assumed end-member compositions for the primary upper mantle (B = 0.4 p p m and 3liB = --7%o) and the altered oceanic crust (70 ppm, + 7%0) to place an upper limit on the amount of B recycled into the upper mantle. Based on these assumptions, it was concluded that less than 2% of the B in subducted oceanic crust was added to the mantle source in order to explain the low ~ l l B values in the oceanic basalts ( - 7.4-0.6%o). Two comments regarding the calculation of Chaussidon and Jambon will be presented in this paper. First, sediments constitute one of the most important B reservoirs in subduction zones and should be considered in any budget calculation. Secondly, there is an important fractionation effect of both B and ~11B in the slab during subduction and this effect should be assessed. In
order to clarify the second comment, I present results of an hydrothermal experiment aimed at obtaining a better constraint on the e n d - m e m b e r composition of B and 61~B in the subducted oceanic crust. The experimental results demonstrate that significant modification of B and ~l~B can occur at elevated temperatures. The exchangeable B with heavy isotopic composition ( g l i B = ~ +15%o) in sediments is released at low temperatures (T < 150°C) and the remaining B is enriched in l°B (611B = ~ - 1 0 % o ) . Any involvement of this sedimentary B will complicate the calculation of Chaussidon and Jambon. In addition, I re-evaluate the importance of slab-derived fluids in subduction zones, based on the available data on B and 61LB in OIB [1] and in H a l m a h e r a arc volcanics [2,3]. Boron is an excess volatile element. Its unique chemical characteristics (e.g., the high incompatibility in magmatic processes, the strong affinity to aqueous solutions at elevated temperatures and the large isotopic variations in natural systems) have made B and ~ n B powerful tracers for subduction recycling, especially when combined with l°Be [4,5]. Marine sediments and altered oceanic basalts, with B concentrations up to or more than 100 p p m [2,4], are the two main reservoirs for boron
0012-821X/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSD1 0 0 1 2 - 8 2 1 X ( 9 4 ) 0 0 1 8 3 - 9
728
C.-F. You/Earth and Planetary Science Letters 128 (1994) 727-730
in the u p p e r crust. The high B contents in arc volcanics have been attributed to contributions from subducted sediments a n d / o r altered basalt or slab-derived fluids [3,5]. At present, the argument for the involvement of subducted sediment in arc magmatism is favored due to the observed B - l ° B e systematics in arc lavas [5]. The experiment, using an autoclave hydrothermal rocking apparatus under controlled P - T conditions, was performed to monitor the degree of interaction between the d~collement sediments of O D P Site 808, Nankai Trough, and synthetic NaC1-CaCI z solutions with compositions similar to in situ pore waters (for details of procedures see [6]). The d6collement sample is a hemi-pelagic sediment and is mainly composed of clay minerals (e.g., illite, smectite, kaolinite and chlorite). The w a t e r / r o c k ratio is ~ 3 w / w at initial conditions and - 1 . 5 w / w at the end of the experiment. This experiment basically mimics the behavior of B and 8 ~ B during early subduction zone processes. The experiment was conducted over a t e m p e r a t u r e range of 25-350°C, at 800 bar, and the entire experiment lasted for about 45 days. Fifteen fluid samples ( ~ 7 cm 3 for each) were extracted after ~ 3 days interaction at every 2 5 50°C increment. In addition, the initial and final sediments were analyzed. The fluid samples were analyzed for major and trace elements, including both B and 61~B. The fluid B contents were determined by a modified B-curcumin method and the ~IlB were analyzed by negative ion thermal ionization mass spectrometer (TIMS) with a precision of 1.2%o (95% C.L.) [7]. The sediment B and 6 ~ B values were analyzed via isotope dilution T I M S ( ~ 1%) and Cs2BO ~- positive ion TIMS (0.4%o, 95% C.L.), respectively [4]. The ddcollement sediment has a concentration of ~ 117 ppm, with a 6 ~ B of ~ - 4 . 7 % 0 . The high mobilization of B at elevated P - T conditions is demonstrated well in the hydrothermal interaction experiment (Fig. 1). The B concentration in the solutions increases rapidly with t e m p e r a t u r e during the heating episode and remains rather constant while cooling (Fig. la). The 6liB value decreases drastically during the early heating stage, indicating release of the exchangeable B (~11B = ~ + 15%o [2]), and remains some-
2500
2000
t t
t
t
t
II
l[
t
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i
i
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(°C)
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811B
10
(
t
t
t
t t
t t
t t
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-1C
i
i
i
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Temperature
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(°C)
Fig. 1. V a r i a t i o n in b o r o n c o n c e n t r a t i o n a n d i s o t o p i c c o m p o s i tion in t h e e x t r a c t e d fluids of t h e h y d r o t h e r m a l w a t e r sedim e n t i n t e r a c t i o n e x p e r i m e n t to m i m i c t h e B b e h a v i o r d u r i n g the e a r l y s u b d u c t i o n z o n e p r o c e s s e s . (a) B v e r s u s T in the e x t r a c t e d fluids. (b) 611B v e r s u s T in the e x t r a c t e d fluids. O = the h e a t i n g e p i s o d e ; • = t h e c o o l i n g e p i s o d e .
what scattered through the entire cooling process (Fig. lb). The fluid B contents and 611B suggest a mixing trend between an exchangeable fraction ( ~ 20 p p m , ~ + 15%o) and a fixed fraction ( ~ 100 ppm, ~ - 1 0 % o ) . The slight curvature and some scatter in the data (Fig. 2) can be attributed to the effects of chemisorption at low temperatures a n d / o r redissolution of the residual pore water salt in the sediment with a 611B of ~ 25%o [7]. In this experiment about 35% of total B in the
C,-F. You/Earth and Planetary Science Letters 128 (1994) 727-730
sediment was removed (from 117 to 70 ppm) and the ~11B in the residual changed significantly (from ~ - 4 . 7 to - 1 1 . 8 % o ) after 45 days. These data emphasize the importance of chemical and isotopic modification as a result of the temperature increase during normal oceanic crust subduction and this effect has to be taken into account in any subduction zone budget consideration. Another important point shown here is that a large change in sediment ~ l B occurs at relatively low temperatures. Furthermore, the large amount of B mobilization observed both in the sediments and the fluids supports the argument of significant return of B to the ocean during slab subduction [2,7]. This isotopic modification was not considered in the calculation of Chaussidon and Jambon, who assumed a fixed composition (611B = + 7 % c ) for altered oceanic crust through the entire subduction process. The calculation of Chaussidon and Jambon is improper because of the preferential mobilization of B with high 6~'B during subduction, as is shown by the experimen-
28
18 811B
-2 B
Initial Sed.
-12
Final Sed. . . . . . . . .
1 0 -4
i
10 -3
........ 10 2
1/13(pM -:)
Fig. 2. Plot of 61:B versus 1 / B (/xM l) in fluid samples of the hydrothermal experiment (the 611B of the initial sediments and the final sediments are also indicated). These data suggest a mixing trend between the exchangeable B and the lattice-bound B in sediments. The slight curvature and some scatter in the data can be attributed to the effects of chemisorption at low temperatures or redissolution of residual pore water salt in sediments with 6HB of ~ 25%0 [7]. Significant mobilization of sediment B (35%) with large isotope fractionation (from ~ - 4 . 7 to - 11.8%o) occurred after 45 days at temperatures lower than 350°C. Symbols as in Fig. 1.
729
I0 6 2 511B
° -2 -6 -10 0.0
I
0.4
0.8
1.2
I/B (ppm-1) Fig. 3. Plot of 6lIB versus 1 / B (ppm t) in Halmahera arc volcanics [2,3] and in Hawaii OIB [1]. The good linear correlation suggests mixtures between a MORB end-member and an enriched end-member with high 6HB. C)= Halmahera arc volcanics; • = Hawaii OIB.
tal results. Any involvement of the residual sedimentary B in the slab will change the calculation drastically. Results of B and 611B in Hawaii OIB [1] and in H a l m a h e r a arc volcanics [2,3], as well as the experimental data, have important implications for the effect of slab-derived fluids on the chemical composition of arc lavas. The H a l m a h e r a arc volcanics and Hawaii OIB show a linear correlation in the plot of 1 / B versus 6JIB, suggesting mixtures between M O R B (B < 1 ppm, ~ - 10%e) and an enriched end-member (61~B = - +6%o, Fig. 3). Results of the hydrothermal experiment, as well as of several field studies (e.g., D S D P Site 477 [2]), indicate that B in sediments that have been subjected to progressive metamorphism will have a low 6 ' : B (an analogy to the residual B in the oceanic crust after deep subduction). If this is true, then the residual B in the subducted crust cannot be the observed enriched end-member in Fig. 3. This argument is further strengthened by the fact that B is highly mobile during progressive dehydration upon subduction [7,8]. On the other hand, the B-enriched e n d - m e m b e r can be reasonably interpreted in terms of involvement of fluids generated at shallow depth and enriched in 6~'B. A subduction model involving enrichment of 13, 1°Be, or other incompatible elements at rather shallow depth and, then, through subsequent
730
C.-F. You/Earth and Planetary Science Letters 128 (1994) 727-730
multi-stage hydration/dehydration reaction of hydrous m i n e r a l s in the slab at d e p t h , can g e n e r ate t h e c h e m i c a l characteristics of arc volcanics (e.g., B and 6HB, i n c o m p a t i b l e e l e m e n t s , U / T h ratio and l ° B e) [6,9].
Acknowledgements This w o r k is s u p p o r t e d by N S F grant O C E - 9 2 16786 to J.M. G i e s k e s a n d A.J. Spivack. T h e a u t h o r thanks R. R o s e n b a u e r a n d J.L. Bischoff at U S G S , M e n l o P a r k for t h e ir h e l p in h y d r o t h e r real e x p e r i m e n t s . C o m m e n t s f r o m P. Castillo, J.M. G i e s k e s and L.-H. C h a n on the early draft are greatly a p p r e c i a t e d . Special t h a n k to H. S t a u d i g e l for his critical review. [UC]
References [1] M. Chaussidon and A. Jambon, Boron content and isotopic composition of oceanic basalts: Geochemical and cosmochemical implications, Earth Planet. Sci. Lett. 121, 277-291, 1994.
[2] A.J. Spivack, Boron Isotope Geochemistry, 183 pp., MIT/WHOI, 1986. [3] M.P. Palmer, Boron isotope systematics of Halmahera arc (Indonesia) lava: Evidence for involvement of the subducted slab, Geology 19, 215-217, 1991. [4] T. Ishikawa and E. Nakamura, Boron isotope systematics of marine sediments, Earth Planet. Sci. Lett. 117, 567-580, 1993. [5] J.D. Morris, W.P. Leeman and F. Tera, The subducted component in island arc lava: constraints from the Be isotopes and B-Be systematics, Nature 344, 31-36, 1990. [6] C.F. You, J.D. Morris, J.M. Gieskes, R. Rosenbauer, S. Zheng, X. Xu, T.L. Ku and J.L Bischoff, Mobilization of Be in sedimentary column at convergent margins, Geochim. Cosmochim. Acta, in press. [7] C.F. You, A.J. Spivack, J.H. Smith and J.M. Gieskes, Mobilization of boron in convergent margins: implications for the boron geochemical cycle, Geology 21, 201-210, 1993. [8] A.E. Moran, V.B. Sisson and W.P. Leeman, Boron depletion during progressive metamorphism: implications for subduction processes, Earth Planet. Sci. Lett. 111, 331 349, 1992. [9] J.H. Davies and D.J. Stevenson, Physical model of source region of subduction zone volcanics, J. Geophys. Res. 97, 2037-2070, 1992.