TY - JOUR
T1 - Experimental study of boron geochemistry
T2 - implications for fluid processes in subduction zones
AU - You, C. F.
AU - Spivack, A. J.
AU - Gieskes, J. M.
AU - Rosenbauer, R.
AU - Bischoff, J. L.
N1 - Funding Information:
Acknowledgnaen&--Waep preciateth e two helpful reviewsb y J. Ryan and G. Hemminga nd also the associateed itorK . Mezgefro r handlingt his manuscriptT. his studyw as supportedb y NSF grant GCE 92-16786to J. M. Gieskesa ndA . J. Spivack.
PY - 1995/6
Y1 - 1995/6
N2 - A comprehensive experimental study, utilizing an autoclave hydrothermal apparatus with a 10B isotopic tracer, has been conducted to monitor the geochemical behavior of sediment B during early subduction zone processes. The partition coefficient of exchangeable B (KD) was determined over a temperature range of 25-350°C, at 800 bars and a water/rock ratio of 3-1.5 w/w. These KD are shown to be a complex function of temperature, pH, and possibly mineralogy. At low temperatures, KD is significantly high at ∼4 in contrast to the value of essentially zero at temperatures higher than ∼100°C. A KD of zero represents no B adsorption, implying efficient mobilization of exchangeable B at shallow depths during sediment subduction. Our experimental results demonstrate high mobilization of bulk B in sediments (both exchangeable and lattice bound) at elevated temperatures (200-350°C), in good agreement with previous observations of B in metasediments indicating progressive depletion during metamorphism. In addition, this study emphasizes the importance of a possible water/rock ratio dependence of B mobilization. In other words, the degree of sedimentary B mobilization in subduction zones strongly depends on the local thermal structure and porosity distribution. In low geothermal gradient areas, large amounts of porewater are expelled before significant B mobilization has occurred, so that some sedimentary B will survive and get into the deeper parts of the subduction zone. Our results imply that efficient mobilization of B from the subducted slab must occur and that arc magmatism recycles most of the remaining subducted B back to surface reservoirs. A reconsideration of the B budget in subduction zones provides critical information with respect to B sources and sinks in the ocean.
AB - A comprehensive experimental study, utilizing an autoclave hydrothermal apparatus with a 10B isotopic tracer, has been conducted to monitor the geochemical behavior of sediment B during early subduction zone processes. The partition coefficient of exchangeable B (KD) was determined over a temperature range of 25-350°C, at 800 bars and a water/rock ratio of 3-1.5 w/w. These KD are shown to be a complex function of temperature, pH, and possibly mineralogy. At low temperatures, KD is significantly high at ∼4 in contrast to the value of essentially zero at temperatures higher than ∼100°C. A KD of zero represents no B adsorption, implying efficient mobilization of exchangeable B at shallow depths during sediment subduction. Our experimental results demonstrate high mobilization of bulk B in sediments (both exchangeable and lattice bound) at elevated temperatures (200-350°C), in good agreement with previous observations of B in metasediments indicating progressive depletion during metamorphism. In addition, this study emphasizes the importance of a possible water/rock ratio dependence of B mobilization. In other words, the degree of sedimentary B mobilization in subduction zones strongly depends on the local thermal structure and porosity distribution. In low geothermal gradient areas, large amounts of porewater are expelled before significant B mobilization has occurred, so that some sedimentary B will survive and get into the deeper parts of the subduction zone. Our results imply that efficient mobilization of B from the subducted slab must occur and that arc magmatism recycles most of the remaining subducted B back to surface reservoirs. A reconsideration of the B budget in subduction zones provides critical information with respect to B sources and sinks in the ocean.
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U2 - 10.1016/0016-7037(95)00137-9
DO - 10.1016/0016-7037(95)00137-9
M3 - Article
AN - SCOPUS:0028792771
VL - 59
SP - 2435
EP - 2442
JO - Geochmica et Cosmochimica Acta
JF - Geochmica et Cosmochimica Acta
SN - 0016-7037
IS - 12
ER -