Publication Date:
2017-04-04
Description:
Shallow slab devolatilization is not only witnessed through fluid expulsion at
accretionary prisms, but is also evidenced by active serpentinite seamounts in the shallow
fore-arc region of the Mariana convergent margin. Ocean Drilling Program (ODP) Leg
195 recovered serpentinized peridotites that present a unique opportunity to study the
products of shallow level exchanges between the upper mantle and slab-derived fluids.
Similar to samples recovered during ODP Leg 125, the protoliths of these fore-arc
serpentinized peridotites are mantle harzburgites that have suffered large volume melt
extraction (up to 25%) prior to interactions with fluids released from the downgoing
Pacific Plate. Samples recovered from both ODP legs 125 and 195 show U-shaped rare
earth element (REE) patterns and very low REE abundances (0.001–0.1 chondrites).
Relative to global depleted mantle values these rocks typically have 1–2 orders of
magnitude lower high field strength elements, REE, Th, and U contents. Interestingly, all
fore-arc rocks thus far examined show extreme enrichments of fluid mobile elements
(FME: B, As, Cs, Sb, Li). Because the elemental and B, Li, and Sr isotope systematics in
these fore-arc serpentinites point to nonseawater-related processes, studies of elemental
excesses and anomalous isotopic signatures allow assessment of how much of the
subducted inventory is lost during the initial subduction process between 10 and 40 km.
On the basis of similar but substantial enrichments of FME in the Mariana fore-arc
samples recovered at ODP legs 125 and 195, we report large slab inventory depletions of
B ( 75%), Cs ( 25%), As ( 15%), Li ( 15%), and Sb ( 8%); surprisingly low
(generally less than 2%) depletions of Rb, Ba, Pb, U, Sr; and no depletions in REE and the
high field strength elements (HFSE). Such slab-metasomatized mantle wedge materials
may be dragged to depths of arc magma generation, as proposed by Tatsumi (1986) and
Straub and Layne (2002) and thus represent an unexplored class of mantle material,
different in its origins, physical properties and geochemical fingerprint from mantle rocks
traditionally used in modeling a wide range of subduction zone processes.
Description:
Published
Description:
B09205
Description:
2.3. TTC - Laboratori di chimica e fisica delle rocce
Description:
JCR Journal
Description:
reserved
Keywords:
Shallow slab fluid
;
Mariana arc-basin
;
04. Solid Earth::04.04. Geology::04.04.05. Mineralogy and petrology
;
04. Solid Earth::04.04. Geology::04.04.07. Rock geochemistry
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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