Site U1372 | Site U1373 | Site U1374 | Site U1376 | Site U1377
IODP Expedition 330:
Louisville Seamount Trail
Site U1375 Summary
PDF file is available for download.
Background and Objectives
Site U1375 (Primary Site
LOUI-2B) on Achernar Guyot was the fourth drill site targeted during Integrated
Ocean Drilling Program (IODP) Expedition 330. This seamount has an estimated
age of ~59-63 Ma and knowledge about this feature will fill in an important gap
in the age vs. distance
relationship of the Louisville Seamount Trail and it will provide pivotal
information in the reconstruction of past plate motions and the motion of the
Louisville hotspot. Compared to Rigil Guyot and Canopus Guyot to the northwest,
this seamount is two times smaller, 29 km long and 27 km wide, and it is part
of a trail of seven small guyots and seamounts that starts with Burton Guyot at
the northern end. Site U1375 was targeted in the middle of this small edifice,
away from its shelf edges and away from any thick packages of dipping
volcaniclastics on its flanks, the latter which preferentially were targeted at
Sites U1372, U1373 and U1374. Achernar Guyot shows no evidence of tilting and
Site U1375 was placed on its summit plain at ~1259 m water depth. Sidescan
sonar reflectivity survey and 3.5 kHz sub-bottom profiling data indicate that
Site U1375 is covered with less than 15 m of pelagic sediment and seismic
reflection profiles show that this central part of Achernar Guyot is typified
by a less than 44 m of volcaniclastics overlaying igneous basement.
The original drilling plan was
to recover the soft sediment using a gravity push approach with little or no
rotation using a Rotary Core Barrel (RCB), followed by standard coring into the
volcaniclastic materials and 350 m into igneous basement. A short downhole
logging series was planned including the standard Triple Combo and FMS-Sonic
tool strings, and the third-party Göttingen Borehole Magnetometer (GBM) tool.
Drilling during ODP Leg 197 provided the first compelling
evidence for the motion of mantle plumes by documenting a large ~15° shift in
paleolatitude for the Hawaiian hotspot (Tarduno et al., 2003; Duncan et al.,
2006). This lead to two geodynamical end-member models that are being tested
during Expedition 330, namely that the Louisville and Hawaiian hotspots moved
coherently over geological time (Wessel and Kroenke 1997; Courtillot et al.
2003) or, quite the opposite, that these hotspots show considerable inter-hotspot
motions, as predicted by mantle flow models (Steinberger, 2002; Steinberger et
al., 2004; Koppers et al., 2004; Steinberger and Antretter, 2006; Steinberger
and Calderwood, 2006). The most important objective of Expedition 330 therefore
was to core deep into the igneous basement of four Louisville seamounts in
order to sample a large number of in situ lava flows ranging in age between 80
and 50 Ma. With a sufficiently large number of these independent cooling units
high-quality estimates of their paleolatitude can be determined, and any
recorded paleolatitude shift (or lack thereof) can be compared with seamounts
in the Hawaiian-Emperor seamount trail. For this reason Expedition 330 mimicked
the drilling strategy of ODP Leg 197 by drilling Louisville guyots equivalent
in age to Detroit (76-81 Ma), Suiko (61 Ma), Nintoku (56 Ma) and Koko (49 Ma)
in the Emperor seamounts, with Achernar Guyot being equivalent to Suiko
Seamount. Accurate paleomagnetic inclination data are required for the drilled
seamounts in order to establish a record of the past motion of the Louisville
hotspot, and together with high-resolution 40Ar/39Ar age
dating of the cored lava flows, these data will help us to constrain the
paleolatitudes of the Louisville hotspot between 80 and 50 Ma.
Expedition 330 also aimed to provide important insights into
the magmatic evolution and melting processes that produced and constructed
Louisville volcanoes while progressing from their shield to post-shield, and
maybe post-erosional, volcanic stages. Existing data from dredged lavas suggest
that the mantle source of the Louisville hotspot has been remarkably
homogeneous for as much as 80 m.y. (Cheng et al., 1987; Hawkins et al., 1987;
Vanderkluysen et al., 2011). In addition, all dredged basalts are predominantly
alkalic and possibly represent a mostly alkalic shield-building stage, which is
in contrast to the tholeiitic shield-building stage of volcanoes in the
Hawaiian-Emperor seamount trail (Hawkins et al., 1987; Vanderkluysen et al.,
2011). Therefore, the successions of lava flows cored during Expedition 330
will help us to characterize the Louisville seamount trail as the product of a primary hotspot and to test the long-lived homogeneous
geochemical character of its mantle source. Analyses of melt inclusions,
volcanic glass samples, primitive basalts, high-Mg olivines and clinopyroxene
phenocrysts will provide further constraints on the asserted homogeneity of the
Louisville plume source, its compositional evolution between 80 and 50 Ma,
potential mantle plume temperatures, and its magma genesis, volatile outgassing
and differentiation. Finally, basalts and sediments cored at Site U1375 were planned
to be used for a range of secondary objectives such as searching for active
microbial life in the old seamount basements and to find fossil traces of these
microbes left behind in volcanic glasses and biofilms on the rocks.
The 322 nmi voyage to Site U1375
(Prospectus Site LOUI-2B) on Achernar Guyot was accomplished at an average
speed of 10.2 knots and the vessel arrived on site at 0330 hr on 25 January.
The bit tagged seafloor at a depth
of 1269.0 mbrf (= 1258 mbsf). Hole U1375A was spudded with the rotary core
barrel (RCB) assembly at 1345 hr on 25 January. Almost immediately the driller
experienced erratic high torque indicating that we were attempting to core
though loose rocks and boulders. Frequent overpulls of up to 90 kip were required
to keep the drill string rotating freely. At 2130 hr coring of Hole U1375A was
terminated at a depth of 11.5 mbsf because of unstable hole conditions. The
average recovery for the two cores was 13%.
The vessel was offset 300 m at 315
degrees and a second attempt at coring this site was initiated when Hole U1375B
was spudded at 2345 hr. After penetrating 8.5 m with increasing difficulty and
constantly fighting unstable hole conditions, operations at this site were
terminated as well. The lone core retrieved had an average recovery of 7%. It
was concluded that the formation on the top of this seamount consisted of a
sedimentary breccia loosely held together in a "soft" carbonate
matrix that quickly disintegrated during drilling, leaving mainly loose pebbles
behind. The vessel departed for an approved alternate site located 91 nmi NNW
from Site U1375 at 1945 hr on 26 January. The total time spent on Site U1375 was 34.3 hours or 1.4 days.
Sediment at Site U1375 was mostly
restricted to Hole U1375A, and represents a pelagic cap and an older sediment
cover of Achernar Guyot. Two stratigraphic units were defined on the basis of
compositional and textural characteristics of the sediment at macroscopic and
microscopic scales. The uppermost part of Hole U1375A (Unit I) was retrieved
with only poor (~2%) recovery with few cuttings retrieved in the core catcher
of Core 330-U1375A-1R. The cuttings indicate that Unit I probably contains a
young (late Miocene to Holocene) sedimentary cover composed of sandy
foraminiferal ooze. The sediment resembles that recovered in the uppermost
parts of Sites U1372 on Canopus Guyot and U1374 on Rigil Guyot, and is
interpreted to represent a pelagic cap on top of the drowned seamount. An older
sedimentary cover (Unit II) in Hole U1375A occurs between 8.50 and 10.11 mbsf,
which includes from top to bottom: (1) a ferromanganese-phosphate encrustation
at ~8.50 mbsf; (2) an early to middle Paleocene grain-supported, poorly-sorted,
multicolor basalt conglomerate between ~8.50 and 9.34 mbsf; and (3) an altered,
monolithic, matrix-supported, poorly-sorted, multicolor basalt breccia between
9.34 and the bottom of the hole at 10.11 mbsf. The composition and texture of
the sediment suggest that Unit II includes a hemipelagic interval probably
deposited after drowning of Achernar Guyot at Site U1375, and on top of an
older debris flow deposit.
Calcareous nannofossils and
planktonic foraminifers observed in the unconsolidated sandy foraminiferal ooze
of Unit I display an age range of latest Miocene-Holocene. The first core from
Hole U1375A has penetrated 8.5 m of supposedly soft pelagic sediment, however,
almost all sediment was flushed from the core liner during recovery. Only a
small amount of disturbed sediment was retained in the core catcher in Core
U1375-1R and was defined as Unit I. In contrast, calcareous nannofossils and
planktonic foraminifers present in Section U1375A-2R-1 allowed for a
preliminary age assignment of early Paleocene for the Unit IIA conglomerate,
indicating a more than 55 million year interval represented by the unconformity
between Units I and II.
penetrated to a total depth of 11.5 mbsf and recovered 1.61 m of sedimentary
rocks containing five types of volcanic clasts. The clast types found in Units
IIA and IIB include aphyric basalt, moderately olivine-augite-phyric basalt,
moderately augite-olivine-plagioclase-phyric basalt, and highly
olivine-augite-phyric basalt. Hole U1375B penetrated to a total depth of 8.5
mbsf and recovered 57 cm of igneous rock. Unit I, the only unit to be defined
for Hole U1375B, is composed of moderately olivine(-augite)-phyric microgabbro
(dolerite) with olivine and augite phenocrysts that reach over 10 mm in size.
The entire succession recovered from Holes U1375A and U1375B has
undergone secondary alteration by low temperature water-rock interactions
and/or weathering. The overall alteration of the volcanic clasts in sedimentary
units from Hole U1375A ranges from slight to high (between 10% and 60%),
whereas the moderately olivine (-augite) -phyric
microgabbro (dolerite) from Hole U1375B varies from moderately to highly
altered (55%). Plagioclase and augite are generally well-preserved, both as
phenocrysts and in the groundmass throughout the entire igneous portion of the
core. Olivine is typically completely altered to iddingsite, hematite, Fe-oxyhydroxydes
and carbonates. Alteration phases are mostly carbonates (Mg-calcite), brown
clay minerals and other secondary phases (iddingsite, Fe oxyhydroxides,
goethite). Additionally, the microgabbro from Hole U1375B is characterized by
millimeter thick veins of goethite.
Structural features in Hole U1375A are veins and vein networks within
sedimentary clasts, and geopetals in the surrounding sediments. The reliable
geopetals are horizontal, indicating this part of Archenar Seamount has not
been tilted since deposition of the geopetal infilling material. Veins and vein
networks are common within the clasts, and are up to 8 mm wide, although
typically much thinner. In Hole U1375B several veins are present within the
microgabbro (dolerite). Most of these veins are steeply dipping, with thinner
conjugate veins at shallow dips.
One sample of the Unit I microgabbro (dolerite) from Hole U1375B was
analyzed chemically. It is moderately altered and highly evolved, and
represents one of the most alkalic rocks recovered during Expedition 330. Data
for the sample lie in the field of basanite and tephrite in a total alkalis (Na2O+K2O)
vs. SiO2 diagram. It appears to be the product of crystal
fractionation dominated by olivine, and to a lesser extent, augite. It has
slightly lower Zr and Y for its TiO2 content than igneous rocks from
Sites U1372 through U1374 and Site U1376, suggesting that it may represent a
different magma type.
Characterization of physical
properties was conducted for material recovered at Site U1375. The data sets
are mutually consistent and fall within the ranges expected based on the
identified lithologies. In Hole U1375A, magnetic susceptibility, bulk density,
and natural gamma ray radiation all show a moderate decrease downhole likely
due to a reduction in basaltic clasts, but may also be affected by the
fragmented nature of the recovered material. The 60 cm of microgabbro recovered
in Hole U1375B generally has values of magnetic susceptibility and bulk density
larger than observed in Hole U1375A and similar values of natural ray
radiation. In Hole U1375A, values of GRA-derived bulk density, MS, and NGR each
show a moderate decrease downhole, which may be related to a reduction in
basaltic clasts, but may also be affected by the fragmented nature of the
recovered material. Material from both holes show a color reflectance that is
more yellow than blue. In terms of redness versus greenness, the sedimentary
rocks of Hole U1375A are consistently more red than green, whereas the igneous
rocks from Hole U1375B are more neutral.
The single, 57 cm long microgabbro unit that may not be in situ had an
average inclination of 36.3 ± 1.6°
(determined from archive half-core data using Fisher statistics). A single
discrete sample taken from the same unit has been interpreted as having a
broadly consistent direction.
No microbiology samples were taken from Site U1375 samples.
Cheng, Q., Park,
K.-H., MacDougall, J.D., Zindler, A., Lugmair, G.W., Hawkins, J., Lonsdale, P.,
Staudigel, H. (1987). Isotopic evidence for a hotspot origin of the Louisville
seamount chain. In: B.H. Keating, P. Fryer, R. Batiza, G.W. Boehlert
(Editors), Seamounts, islands and atolls. American Geophysical Union Monograph,
Washington, 43: 283-296.
Courtillot, V., Davaille, A., Besse, J., Stock, J. (2003).
Three distinct types of hotspots in the Earth's mantle. Earth
and Planetary Science Letters, 205: 295-308.
Duncan, R.A., Tarduno, J.A. and Scholl, D.W. (2006). Leg 197 Synthesis:
Southward motion and geochemical variability of the Hawaiian Hotspot. In:
Proceedings of the Ocean Drilling Program, Scientific Results. R.A. Duncan,
J.A. Tarduno, T.A. Davies and D.W. Scholl.
Hawkins, J.W., Lonsdale, P.F., Batiza, R.
(1987). Petrologic evolution of the Louisville seamount chain. In: B.H. Keating, P. Fryer, R. Batiza (Editors), Seamounts,
islands and atolls. American Geophysical Union Monograph, Washington, 43:
Koppers, A.A.P., Duncan,
R.A., Steinberger, B. (2004). Implications of a non-linear 40Ar/39Ar
age progression along the Louisville seamount trail for models of fixed and
moving hotspots. Geochemistry Geophysics Geosystems 5(1). Paper Number
2003GC000671. 22 pp.
Steinberger, B. (2002). Motion of the Easter Island hotspot relative to hotspots on the Pacific plate. Geochem. Geophys.
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Steinberger, B., Sutherland,
R., and O'Connell, R. J. (2004). Mantle flow models constrained by
revised global plate motions successfully predict the Emperor-Hawaii and other
hotspot-related seamount chains. Nature, 430, 167-173,
Steinberger, B. and Antretter, M. (2006). Conduit
diameter and buoyant rising speed of mantle plumes: Implications for the motion
of hotspots and shape of plume conduits. Geochemistry Geophysics Geosystems
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Steinberger, B. and Calderwood, A. (2006). Models
of large-scale viscous flow in the Earth's mantle with constraints from mineral
physics and surface observations. Geophysical Journal International, 167, 1461-1481, doi:10.1111/j.1365-246X.2006.03131.x.
Tarduno, J.A., Duncan, R.A., Scholl, D.W., Cottrell, R.D., Steinberger,
B., Thordarson, T., Kerr, B.C., Neal, C.R., Frey. F.A., Torii, M., Carvallo, C.
(2003). The Emperor Seamounts: Southward motion of the Hawaiian hotspot plume
in Earth's mantle, Science, 301, 1,064-1,069.
Vanderkluysen, L., Mahoney, J.J., Koppers, A.A.P. and Lonsdale, P. (2011).
Geochemical Evolution of the Louisville Seamount Chain. In Preparation.
Kroenke, L.W. (1997). A geometric technique for relocating hotspots and refining
absolute plate motions. Nature, 387: 365-369.