Evidence for the return of subducted continental crust
in Samoan lavas
Matthew G. Jackson
1
, Stanley R. Hart
2
, Anthony A.P. Koppers
3,4
, Hubert Staudigel
3
,
Jasper Konter
3
, Jerzy Blusztajn
2
, Mark Kurz
2
& Jamie A. Russell
3
1
Massachusetts Institute of Technology – Woods Hole Oceanographic Institution Joint
Program, Woods Hole, MA 02543-1525, USA
2
Woods Hole Oceanographic Institution, Woods Hole, MA 02543-1525, USA
3
Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA
92093-0225, USA
4
College of Oceanic & Atmospheric Sciences, Oregon State University, Corvallis, OR
97331-5503, USA
Substantial quantities of terrigenous sediments are known to enter the
mantle at subduction zones, but little is known about their fate in the mantle
1
.
Subducted sediment may be entrained in buoyantly upwelling plumes and returned
to the earth’s surface at hotspots
2-5
, but the proportion of recycled sediment in the
mantle is small and clear examples of recycled sediment in hotspot lavas are rare
6,7
.
We report here remarkably enriched
87
Sr/ Sr
86
and
143
Nd/ Nd
144
isotope signatures
(up to 0.720830 and 0.512285, respectively) in Samoan lavas from three dredge
locations on the underwater flanks of Savai’i island, Western Samoa. The
submarine Savai’i lavas represent the most extreme
87
Sr/ Sr
86
isotope compositions
reported for ocean island basalts (OIBs) to date. The data are consistent with the
presence of a recycled sediment component (with a composition similar to upper
continental crust, or UCC) in the Samoan mantle. Trace element data show similar
affinities with UCC—including exceptionally low Ce/Pb and Nb/U ratios
8
—that
complement the enriched
87
Sr/ Sr
86
and
143
Nd/ Nd
144
isotope signatures. The
geochemical evidence from the new Samoan lavas radically redefines the
2007-01-00868 Jackson page 1
composition of the EM2 (enriched mantle 2)
9
mantle endmember, and points to the
presence of an ancient recycled UCC component in the Samoan plume.
The earth’s mantle, as sampled by OIBs erupted at hotspots, is chemically and
isotopically heterogeneous. However, the origin of the geochemical heterogenteity of the
mantle is not well understood. One model for the geochemical evolution of the mantle
assumes that much of the chemical diversity is a result of subduction, a tectonic process
that introduces enriched oceanic crust and compositionally heterogeneous sediment into a
largely primitive (or slightly depleted) mantle
5,10,11
. Following subduction, these surface
materials mix with a peridotitic mantle, thus imprinting their enriched chemical and
isotopic signatures on its various domains. A number of isotopically-distinct
geochemical reservoirs, as sampled by OIBs, have resulted from this process. The
isotopic end-members are often referred to as HIMU (high ‘µ’ or
238
U/
204
Pb), EM1
(enriched mantle 1) and EM2 (enriched mantle 2) and DMM (depleted MORB [mid-
ocean ridge basalt] mantle)
9
. While the most radiogenic Pb isotope ratios observed in the
HIMU component have been proposed to result from a contribution of recycled oceanic
crust
9,12
, most models for the creation of the EM1 and EM2 mantle reservoirs invoke a
small portion of lithologically distinct sediments that have been recycled into the
mantle
9,13
.
The volcanically active Samoan Islands and seamounts define a hotspot track with
a classical EM2 pedigree
7,14,15
. The first high precision
87
Sr/
86
Sr and
143
Nd/
144
Nd
measurements from Samoan lavas were interpreted as evidence of sediment recycling
5
.
Recently, however, the proposed recycled sediment origin of the enriched Samoan basalts
2007-01-00868 Jackson page 2
has been called into question (see Supplementary Discussion), and an alternative model
favoring source enrichment by metasomatic processes was proposed
7
. The extreme
isotopic and chemical enrichment in the new Samoan EM2 lavas exhibit distinctly
continental fingerprints, and argue for a role for a component similar to ancient recycled
UCC in the Samoan plume (see Supplementary Discussion for ALIA dredge locations
and geochemical data).
The most isotopically-enriched Samoan whole rock
87
Sr/
86
Sr signature (0.720469,
Mg# = 57.2) is recorded in a trachyandesite, dredge sample D115-21, which was taken
from the southwestern flank of Savai’i. Clinopyroxene (cpx) mineral separates from the
same sample yielded an even higher
87
Sr/
86
Sr ratio (.721630). A trachybasalt (D115-18)
hosts the second most enriched
87
Sr/
86
Sr (0.718592, Mg# = 58.7), and cpx mineral
separates from the sample also gave more enriched ratios (0.720232-0.720830). Six
other lavas recovered in the same dredge also exhibit enriched
87
Sr/
86
Sr ratios (0.708175-
0.716394, Mg#’s from 52.0-65.1). Dredge D118, located on the far western end of the
Savai’i lineament, contained an alkali basalt with enriched
87
Sr/
86
Sr (0.710337, measured
on fresh cpx). Dredge D128, taken on the northeastern flanks of Savai’i, yielded a
transitional basalt with a high
87
Sr/
86
Sr ratio (0.712500, Mg# = 70.5) and several other
basalts with less enriched
87
Sr/
86
Sr (0.706397-0.708170, Mg#’s from 61.2-63.9). Dredge
D114, taken on the southwestern flanks of Savai’i, provides younger shield basalts of
transitional chemistry and normal
87
Sr/
86
Sr (0.705422-0.705435, Mg#’s are 67.2 and
76.3).
The
87
Sr/
86
Sr isotopes in the basalts from all three ultra-enriched sampling
localitites are complemented by enriched (low)
143
Nd/
144
Nd and the lowest
3
He/
4
He ratios
2007-01-00868 Jackson page 3
(4.31-4.93 times atmospheric, Ra) observed in Samoan basalts. Together the new data
extend the Samoan isotope array to a region outside the global OIB field (Fig. 1). Highly
enriched EM2 signatures have previously been observed only in metasomatized xenoliths
from Savai'i (
87
Sr/
86
Sr up to 0.712838
16
), and the Samoan EM2 basalts provide the first
evidence that the enriched component hosted in these xenoliths also occurs as erupted
basalts. The enriched
87
Sr/
86
Sr and
143
Nd/
144
Nd isotope ratios, coupled with the low
3
He/
4
He, are consistent with a recycled UCC component in the mantle source of the
Samoan EM2 basalts.
The UCC reservoir exhibits several diagnostic trace element characteristics that
can be useful for detecting its presence in Samoan EM2 lavas. Compared to OIB and
MORB lavas, UCC displays exceptional depletion in Nb (and Ta), Ti and Eu, and
enrichment in Pb (Fig. 2). Samoan basalts have trace element characteristics that are
increasingly similar to UCC with more enriched
87
Sr/
86
Sr and
143
Nd/
144
Nd values (Fig. 3).
While the most depleted basalts from Samoa show slight positive anomalies in Nb and
Ti, the magnitude of these anomalies decreases monotonically toward the most enriched
Samoan EM2 basalts. Similarly, a correlation exists between greater Pb enrichment and
increasing isotopic enrichment in Samoan basalts. Importantly, the Eu anomaly is
increasingly negative in the most isotopically-enriched Samoan EM2 lavas (excluding
basalts with MgO < 6.5 wt.%), and the Rb/Sr and U/Pb are too low in the lower (or
middle) continental crust
17
to be consistent with the new Samoan Sr and Pb isotope data;
these observations rule out the involvement of lower (or middle) continental crust.
Furthermore, rare xenoliths with enriched
87
Sr/
86
Sr and
143
Nd/
144
Nd from the
subcontinental lithospheric mantle (SCLM)
9
suggest that this mantle domain can be
2007-01-00868 Jackson page 4
isotopically-enriched. However, the SCLM does not appear to exhibit the trace element
anomalies observed in the most isotopically-enriched Samoan lavas
18
. Instead, isotope
ratios and trace element anomalies (Nb, Ti, Eu and Pb) in Samoan basalts generate arrays
that trend toward a composition similar to UCC.
We can exclude a shallow origin for the anomalous enrichment observed in the
Samoan EM2 lavas. Due to the close proximity of the Tonga Trench, located only 120
km south of Savai’i, rapid cycling of sediment from the subduction zone into the Samoan
plume was proposed as a mechanism for generating the extreme isotopic enrichment in
Samoan lavas
14
. However, at the time the submarine Savai’i lavas were erupted 5 Ma
ago (A.K. et al., manuscript in preparation), plate reconstructions indicate that the
northern terminus of the Tonga Trench was located 1,300 km to the west of Savai’i
19
, and
sediment input from the Tonga Trench can be ruled out as a source of enrichment in these
lavas.
Evidence from Pb isotopes suggests that it is unlikely that shallow level
contamination by modern marine sediments is responsible for the isotopic enrichment in
the Samoan EM2 basalts. In Δ
207
Pb/
204
Pb – Δ
208
Pb/
204
Pb isotope space, Samoan basalts
and global marine sediments
1
exhibit non-overlapping fields with diverging trends (Fig.
4). Moreover, three composite cores taken from the Samoan region, and a single
ferromanganese crust from the flanks of Savai’i, plot in the global marine sediment field
and exhibit no geochemical relationship with the extremely enriched Samoan lavas. It is
also unlikely that the Samoan plume has been contaminated by stranded continental crust,
such as found beneath the Kerguelen plateau
20
and the southern Mid-Atlantic ridge
21
,
2007-01-00868 Jackson page 5