The Chemical Composition of Subducting Sediments Journal Article uri icon

DCO ID 11121/1218-3632-8348-1147-CC

is Contribution to the DCO

  • YES

year of publication

  • 2014

abstract

  • More than a decade since the estimation of global subducting sediment (GLOSS), an expanding inductively coupled plasma mass spectrometry dataset for trace elements motivates a new assessment of the bulk sediment flux into deep-sea trenches and an update, GLOSS-II. The specific focus here is on key elements in solid earth recycling that are now better constrained by new data: Li, Be, B, Nb, Ta, Pb, Th, and U. The abundances of these elements reflect, to first order, dilution of continental detritus by biogenic opal and carbonate and, second, various continental and marine processes, such as the extent of continental weathering (Li/K2O), hydrogenous versus hydrothermal oxides (Pb/Fe), biophosphate accumulation and exposure age on the seafloor (Th and U), and organic carbon burial (U). The Be/K2O ratio (0.86 ± 0.04) and Nb/Ta ratio (14 ± 1) of terrigenous marine sediments reflect average upper continental crust and indeed refine those estimates. Given these new systematics, along with newly published data, revised bulk sediment compositions are calculated for 16 of the 25 trenches included in GLOSS. Two major additional margins are newly assessed (New Zealand-Hikurangi and Chile). Many element budgets are substantially revised for individual trenches, particularly Rb, Cs, Nb, and Pb. A new global weighted average, GLOSS-II, is similar to GLOSS (within 10% relative abundance) for many elements, with more significant changes (up to 50%) in Rb, Cs, Rb/Sr, Nd/Hf, and U/Th. The updated trench estimates provide new insights into sediment recycling at subduction zones, particularly for Li, Be, and Nb. The mass flux of Li subducted into different trenches correlates strongly with the Li/Y ratio in adjacent volcanic arcs, pointing to a direct control on Li enrichment in arcs by subducted sediment. Such a control may be consistent with Li-isotope variations in some arcs, with little fractionation required in the slab or mantle. On the other hand, arcs do not reflect the common Be/K2O ratio of most sediments and show fractionation to higher and lower ratios. These fractionations may relate to the different thermal structure of different slabs and/or subducted sediment versus basalt contributions. Sediments vary significantly in their Nb anomaly (i.e., Nb depletion with respect to La and Th), and some arcs inherit the sedimentary Nb anomaly (e.g., South Sandwich) with no new fractionation in the subduction zone, while others (e.g., Marianas and Aleutians) likely require residual rutile in the slab. The Nb anomaly is thus a nearly ubiquitous feature in arcs that has a complex origin, dependent on at least three variables: the sedimentary Nb anomaly, the presence of rutile in the slab (likely driven by Fe2 +, Fe3 +, and/or Ti abundances in the sediments), and Nb/La variation in the mantle. Trench sediments and GLOSS-II should thus continue to be useful in elucidating processes occurring in subduction zones, the continents, and the mantle.

volume

  • 4