A granoblastic basalt viewed under the microscope (picture is 2.3 mm across). Magnification shows a rock formed of small rounded mineral grains annealed together (plagioclase: white, pyroxene: light green and light brown, and magnetite or ilmenite: black). They may look inoffensive, but these rocks are the hardest material ever drilled in more than four decades of scientific ocean drilling. The rocks are very abrasive and aggressive to the drilling and coring tools, and difficult to penetrate. However, the samples recovered provide a treasure trove of information, recording the rocks’ initial crystallization as a basaltic dike then their reheating at the top of the mid-ocean ridge magma chamber. These rocks represent the heat exchanger where thermal energy from the cooling and solidifying melt in the magma chamber below is exchanged with seawater infiltrating from the oceans, leading to the “black smoker-type” hot (>350°C) water vents on the seafloor. Credit: IODP/USIO |
Integrated
Ocean Drilling Program (IODP) Expedition 335 Superfast Spreading Rate
Crust 4 recently completed operations in Ocean Drilling Program (ODP)
Hole 1256D, a deep scientific borehole that extends more than 1500
meters below the seafloor into the Pacific Ocean’s igneous crust – rocks
that formed through the cooling and crystallization of magma, and form
the basement of the ocean floor.
An
international team of scientists led by co-chief scientists Damon
Teagle (National Oceanographic Center Southampton, University of
Southampton in the UK) and Benoît Ildefonse (CNRS, Université
Montpellier 2 in France) returned to ODP Hole 1256D aboard the
scientific research vessel, JOIDES Resolution, to sample a complete
section of intact oceanic crust down into gabbros.
This expedition was the fourth in a series and builds on the efforts of three expeditions in 2002 and 2005.
Gabbros
are coarse-grained intrusive rocks formed by the slow cooling of
basaltic magmas. They make up the lower two-thirds of the ocean crust.
The intrusion of gabbros at the mid-ocean ridges is the largest igneous
process active on our planet with more than 12 cubic kilometers of new
magma from the mantle intruded into the crust each year. The minerals,
chemistry, and textures of gabbroic rocks preserve records of the
processes that occur deep within the Earth’s mid-ocean ridges, where new
ocean crust is formed.
“The
formation of new crust is the first step in Earth’s plate tectonic
cycle,” explained Teagle. “This is the principal mechanism by which heat
and material rise from within the Earth to the surface of the planet.
And it’s the motion and interactions of Earth’s tectonic plates that
drive the formation of mountains and volcanoes, the initiation of
earthquakes, and the exchange of elements (such as carbon) between the
Earth’s interior, oceans, and atmosphere.”
Granoblastic dike samples were recovered in abundance by fishing tools during successive hole remediation operations. Science party members Sumiyo Miyashita and Yoshiko Adashi from Niigata University, Japan, examine large rock samples from Hole 1256D. |
“Understanding
the mechanisms that construct new tectonic plates has been a major,
long-standing goal of scientific ocean drilling,” added Ildefonse, “but
progress has been inhibited by a dearth of appropriate samples because
deep drilling (at depths greater than 1000 meters into the crust) in the
rugged lavas and intrusive rocks of the ocean crust continues to pose
significant technical challenges.”
ODP
Hole 1256D lies in the eastern equatorial Pacific Ocean about 900
kilometers to the west of Costa Rica and 1150 kilometers east of the
present day East Pacific Rise. This hole is in 15 million year old crust
that formed during an episode of “superfast” spreading at the ancient
East Pacific Rise, when the newly formed plates were moving apart by
more than 200 millimeters per year (mm/yr).
“Although
a spreading rate of 200 mm/yr is significantly faster than the fastest
spreading rates on our planet today, superfast-spread crust was an
attractive target,” stated Teagle, “because seismic experiments at
active mid-ocean ridges indicated that gabbroic rocks should occur at
much shallower depths than in crust formed at slower spreading rates. In
2005, we recovered gabbroic rocks at their predicted depth of
approximately 1400 meters below the seafloor, vindicating the overall
‘Superfast’ strategy.”
Previous
expeditions to Hole 1256D successfully drilled through the erupted
lavas and thin (approximately one-meter-wide) intrusive “dikes” of the
upper crust, reaching into the gabbroic rocks of the lower crust. The
drilling efforts of Expedition 335 were focused just below the
1500-meter mark in the critical transition zone from dikes to gabbros,
where magma at 1200°C exchanges heat with super-heated seawater
circulating within cracks in the upper crust. This heat exchange occurs
across a narrow thermal boundary that is perhaps only a few tens of
meters thick.
In
this zone, the intrusion of magma causes profound textural changes to
the surrounding rocks, a process known as contact metamorphism. In the
mid-ocean ridge environment this results in the formation of very
fine-grained granular rocks, called granoblastic basalts, whose
constituent minerals recrystallize at a microscopic scale and become
welded together by magmatic heat. The resulting metamorphic rock is as
hard as any formation encountered by ocean drilling and sometimes even
tougher than the most resilient of hard formation drilling and coring
bits.
Expedition
335 reentered Hole 1256D more than five years after the last expedition
to this site. The expedition encountered and overcame a series of
significant engineering challenges, each of which was unique, although
difficulties were not unexpected when drilling in a deep, uncased,
marine borehole into igneous rocks.
Laying out a fishing tool on the rig floor. An illustration of the hard work by the drill ship crew. Credit: IODP/USIO, Photo by Benoît Ildefonse |
The
patient, persistent efforts of the drilling crew successfully cleared a
major obstruction at a depth of 920 that had initially prevented
reentry into the hole to its full depth of 1507 meters. Then at the
bottom of the hole the very hard granular rocks that had proved
challenging during the previous Superfast expedition were once more
encountered. Although there may only be a few tens of meters of these
particularly tenacious granoblastic basalts, their extreme toughness
once more proved challenging to sample– resulting in the grinding down
of one of the hardest formation coring bits into a smooth stump.
A
progressive, logical course of action was then undertaken to clear the
bottom of the hole of metal debris from the failed coring bit and
drilling cuttings. This effort required the innovative use of
hole-clearing equipment such as large magnets, and involved over 240
kilometers of drilling pipe deployments (trips) down into the hole and
back onto the ship. (The total amount of pipe “tripped” was roughly
equivalent to the distance from Paris to the English coast, or from New
York City to Philadelphia, or Tokyo to Niigata). These efforts returned
hundreds of kilograms of rocks and drill cuttings, including large
blocks (up to 5 kilograms) of the culprit granoblastic basalts that
hitherto had only been very poorly recovered through coring. A limited
number of gabbro boulders were also recovered, indicating that
scientists are tantalizingly close to breaking through into the gabbroic
layer.
Expedition
335 operations also succeeded in clearing Hole 1256D of drill cuttings,
much of which appear to have been circulating in the hole since earlier
expeditions.
“We
recovered a remarkable sample suite of granoblastic basalts along with
minor gabbros, providing a detailed picture of a rarely sampled, yet
critical interval of the oceanic crust,” Ildefonse observed. “Most
importantly,” he added, “the hole has been stabilized and cleared to its
full depth, and is ready for deepening in the near future.”
IODP Expedition 335, Superfast Spreading Rate Crust 4
