HR: 14:55h
AN: DI43A-06 [Abstracts]
TI: Chlorine and Potassium Flux Into the Mantle via Subduction of Oceanic Crust: Constraints From Melt Inclusions in HIMU Lavas
AU: * Szramek, L A
EM: szramek@mail.utexas.edu
AF: Department of Geological Sciences, Jackson School of Geosicences, The University of
Texas, 1 University Station C1100, Austin, TX 78712, United States
AU: Lassiter, J C
EM: lassiter1@mail.utexas.edu
AF: Department of Geological Sciences, Jackson School of Geosicences, The University of
Texas, 1 University Station C1100, Austin, TX 78712, United States
AB:
Previous studies have estimated the chlorine content of recycled oceanic crust as ranging from <50 ppm [1] up
to ~200 ppm [2,3]. One of the main discrepancies in these estimates is the fraction of K assumed to be
retained in the subducted slab. In order to better constrain estimates of Cl and K recycling, we have analyzed
olivine-hosted melt inclusions in HIMU lavas from the island of Raivavae to determine Cl concentrations as well
as Cl/K2O. K/Nb, and Cl/Nb ratios. Raivavae lavas span a wide range of lead isotopic values, with
206Pb/204Pb ranging from ~19.3 to 21.3. Previous isotopic and trace element studies suggest
that the lavas derive from a mantle source containing ancient dehydrated oceanic crust.
Chlorine and K2O concentrations range from 40-5300 ppm and 0.22-3.5 wt.% respectively. The majority
of Cl/K2O ratios range from 0.01-0.12, Cl/Nb ratios range from 5-25, and K/Nb ratios range from 50-350.
Cl/K2O, Cl/Nb, and Cl concentrations are positively correlated with 206Pb/204Pb. Within
individual inclusions, there is a positive correlation between Nb/Zr and Cl/Nb. No correlation exists between host
olivine forsterite content and Cl/K2O ratios or Cl concentrations. In contrast, K/Nb is negatively correlated
with 206Pb/204Pb.
The correlation between source signatures, e.g., 206Pb/204Pb and Nb/Zr, with Cl concentrations,
Cl/K2O, and Cl/Nb, and lack of correlation with host forsterite content suggest that Cl concentrations are not
controlled by shallow assimilation processes. This suggests that the elevated Cl concentrations, Cl/K2O,
Cl/Nb are a result of chlorine recycling into the deep mantle and not from assimilation of Cl-rich brine as
suggested by [1].
To estimate the amount of chlorine and potassium retained in recycled oceanic crust, we have looked at Cl/Nb
and K/Nb ratios because Nb should be retained during slab dehydration. Using an average Cl/Nb ratio of
~18 in HIMU lavas and an average N-MORB Nb content of 5.6 ppm, we estimate ~100 ppm Cl in
subducted oceanic crust after slab dehydration, intermediate between previous estimates of [1] and [3]. Using an
average K/Nb ratio of ~100 and 5.6 ppm Nb, we obtain ~560 ppm K contained in the subducted slab.
This estimate is lower than that assumed by [3]. Given an estimated Cl content in altered oceanic crust of
~157-322 ppm [2] and K content of ~4650 ppm [4], we estimate ~30-65% of the Cl and
~12% of the K in altered crust is retained in the slab and eventually returned to the deep mantle.
Using the above estimates of ~100 ppm Cl and ~560 ppm K in subducted oceanic crust we can
evaluate the net flux of Cl and K into the deep mantle. Fresh oceanic crust has ~20 ppm Cl whereas
subducted crust has ~100 ppm Cl. This suggests a net transfer of Cl into the deep mantle through the plate
tectonic cycle. Future Cl isotope measurements of HIMU lavas may allow for further constraints on the amount of
Cl in HIMU mantle that is derived from the hydrosphere. In contrast, estimated K in fresh oceanic crust
(~500 ppm) is essentially the same as K in subducted crust (~ 560 ppm). Therefore unlike Cl, K is
currently in a steady state in the mantle with no net transfer into or out of the deep mantle.
[1] Lassiter et al., EPSL 202 (2002) 525-540.
[2] Philippot et al., EPSL 161 (1998) 33-44.
[3] Stroncik and Haase, Geology 32 (2004) 945-948.
[4] Staudigel et al., EPSL 130 (1995) 169-185.
DE: 1025 Composition of the mantle
DE: 1030 Geochemical cycles (0330)
DE: 1038 Mantle processes (3621)
DE: 1043 Fluid and melt inclusion geochemistry
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting