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Creation and evolution of the oceanic lithosphere: contributions from ocean drilling
Kathryn M. Gillis (The University of Victoria)
SEOS, P.O. Box 3055, Victoria, BC V8W 3P6, [email protected]
Between the time that oceanic lithospheric plates are created at spreading centers and subducted back
into the mantle, tectonic plates interact with the hydrosphere, biosphere, and each other. Ocean drilling
has played an essential role in our understanding of the products, fluxes, and processes resulting from
plate tectonic and magmatic cycles, and will be required in the future to resolve fundamental questions
that remain unanswered. In the following sections, I highlight some of the successes of the Ocean
Drilling Program, and describe outstanding questions and drilling strategies for resolving these questions.
This abstract summarizes the views of a group of international scientists who participated in a planning
meeting for the Integrated Ocean Drilling Program (COMPLEX meeting, May, 1999). The COMPLEX
report will be available from the JOI office in March 2000 (see the web site: http://www.joi-odp.org/).
The Legacy of the Ocean Drilling Program
Recent achievements in ocean drilling of the lithosphere have changed fundamentally our view of the
crust and upper mantle. Using the strategy of offset drilling in tectonic windows, ocean drilling has
shown that the lower ocean crust formed at fast- and slow- spreading ridges differs dramatically in
composition, structure, and degree and style of alteration (Fig. 1). Deep drilling in intact upper crust at
Hole 504B near the Costa Rica Rift complements offset drilling of lower crust and mantle in tectonic
windows. Hole 504B has penetrated most of the sheeted dike complex, validating ophiolite models for
the shallow ocean crust, but it has demonstrated that the seismic layer 2/3 boundary in this setting lies
above the dike-gabbro transition and probably is an alteration front rather than a lithologic boundary.
Complementary studies at two sites in the Atlantic and at two sites on Jurassic crust in the Pacific and
Indian Oceans have provided remarkable views of the physical properties of older altered crust, helping
to determine rates of crustal alteration.
A series of offset holes drilled into the active TAG hydrothermal deposit on the Mid-Atlantic Ridge
demonstrated that the evolution of this sulfide mound is intimately linked to entrainment of ambient
seawater and the precipitation of massive anhydrite, a mineral that is not preserved in ophiolite-hosted
massive sulfide deposits (Humphris et al., 1995). This observation has changed our current models for
the formation of these types of ore deposits, and has provided new explanations for the lithologies of the
massive sulfides. A complementary sediment-hosted massive sulfide deposit of extraordinary dimensions
and character was also sampled in Middle Valley on the Juan de Fuca Ridge, providing new insights into
ore accretion and modification processes (Zierenberg et al., 1998). This deposit is underlain by a
stockwork feeder zone and a silicified, diagenitic alteration front that acts as a hydrologic, thermal, and
chemical barrier, and, possibly, as an "aquifer" for the mineralizing solution.
Ocean drilling has led to the discovery of microbial life deep in the ocean crust, in both sediments and
volcanic basement. The range of temperatures and chemical conditions in which these microbes can
survive is startling. Drilling provides the only sampling opportunities that allow documentation of the insitu extent and character of the global subsurface biosphere.
Future Directions of Integrated Ocean Drilling Program
These recent achievements pose important new questions and lay a solid foundation for future ocean
drilling. Newly developed drilling, sampling, analytical, and in-situ technologies now make possible
numerous experimental approaches that Earth Scientists could only wish for in the past. Understanding
oceanic lithospheric architecture and evolution requires that we address three primary questions:
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Creation and evolution of the oceanic lithosphere
(A) How are the oceanic crust and upper mantle created and modified during their life cycle, from the
ridge to the trench?
(B) What are the time-integrated fluxes of fluids, heat, and chemistry associated with lithospheric
creation and modification, and what are their impacts on global cycles?
(C) What is the specific hydrologic, magmatic, tectonic, and biologic processes involved in lithospheric
creation and modification?
These questions cover product, fluxes and processes, respectively, and will be addressed by exploring
lithosphere in different locations and of different ages. One common thread that runs through many of the
outstanding questions is the role of fluids as agents of change through the lithospheric life cycle, from
initial creation through aging to subduction and entry into the upper mantle. In the remainder of this
abstract, I highlight a series of more specific questions that are related to the three primary questions
listed above.
What is the nature of the lower crust and upper mantle, and how should the ophiolite analogy be
applied?
New oceanic lithosphere is the primary product of seafloor spreading. The nature of the uppermost crust
is readily assessed by submersible observations, dredging of exposed outcrops, and drilling, but the
nature of the lower crust remains unconstrained. Marine seismic and land-based ophiolite studies led to
the formulation of a layered model for the structure of the oceanic crust, in which pillow lavas and
sheeted dikes are underlain by gabbro and mantle peridotite. But gabbros and mantle peridotites are
exposed on the seafloor at slow spreading ridges, even in wide regions where the seismic structure
appears to be “normally” layered. In addition, many ophiolite volcanic rocks have chemical affinities
with modern arc rocks. The validity of the ophiolite model for fast-spreading ocean crust hinges on the
composition of the lower crust. Typical ophiolite lower crustal gabbros show abundant modal and
compositional layering, and a lack of sub-solidus plastic deformation. The lack of continuous sampling
has made it impossible to determine whether and where these features are present in both fast-spread and
slow-spread lower crust. It is crucial to obtain such samples via ocean drilling, both in tectonic
exposures and via drilling of intact crustal sections.
Can we correlate between lithostratigraphy, seismic stratigraphy, and hydrostratigraphy?
The seismic velocity structure of oceanic crust and upper mantle allow us to make large-scale inferences
about their composition and evolution, but with the acquisition of new data and samples, even the most
fundamental correlation upon which many of our ideas are based must now be questioned. Seismic
properties result from a combination of bulk mineralogy, pore distribution, cracking, alteration,
temperature, and pressure, none of which is well understood except in a few locations and over short
depth intervals. Non-uniqueness in our interpretations of remote seismic data can be addressed only by
direct correlation with rock properties, requiring good recovery, in-situ measurements, and coupled
experiments in several settings.
For example, seismic layer 2A in young crust hosts vigorous hydrothermal circulation, but layer 2A
disappears when the crust is quite young. In contrast, global heat flow compilations indicate that
significant advective heat loss continues until the crust is at least 65 Ma, and regional surveys indicate
that local convection may continue within some of the oldest remaining seafloor. What is the nature of
layer 2A and why do the seismic properties of the shallowest ocean crust change over time? Does the
base of layer 2A correspond to the top of the dike section, a porosity transition within the extrusive pile,
or are there other causes in different locations?
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Creation and evolution of the oceanic lithosphere
What is the interplay between magmatic, tectonic, biological, and hydrothermal processes?
We can describe qualitative relations between a range of processes involving evolution of the
lithosphere, but the quantitative mechanisms are poorly understood. Heat and magmatic input at the ridge
crest provide the driving force for high-temperature hydrothermal circulation, and off-axis magmatism
may contribute to rejuvenated circulation after the crust has aged. We now have evidence that both
magmatic and hydrothermal fluids interact with the lower crust during hydrothermal circulation. How
much exchange of elements and volatiles takes place between hydrothermal and magmatic fluids?
Magmatic injection likely provides conduits for fluid flow, by physically splitting the rock and allowing
the penetration of cold seawater, creating additional fracture porosity by thermal contraction, but how
long does this dynamic porosity remain open, and how much of the surrounding lithosphere becomes
geochemically altered by the channeled flow? What fraction of the fluid, solute, and heat flux occurs
through channeled flow, and how much occurs through more ubiquitous flow? How do “blooms” of
bacterial colonies influence the permeability of the rock, and do changes in bacterial populations help to
control temperatures within the rock, optimizing conditions for growth? What role do bacteria play in
mediating the deposition of clays and other alteration minerals over a range of thermal and chemical
conditions?
What are the fluxes of heat and mass between the lithosphere and the ocean?
The patterns of heat flow from the earth’s mantle through the ocean crust are fundamentally affected by
two phenomena. One is the localization of magmatic processes primarily along constructional plate
boundaries, resulting in the familiar theoretical conductive heat flow curve that decays with the inverse
square root of lithospheric age. The second phenomenon is the infiltration of seawater into fractured and
porous crust and mantle, which redistributes heat by advection.
This process of advective fluid flow, coupled with chemical reactivity between seawater and oceanic
rocks, leads ultimately to major fluxes of heat and mass within the lithosphere and between the
lithosphere and the overlying ocean. The formation of giant economic ore deposits is one important result
of mass transfer through oceanic lithosphere.
Alteration budgets for the crust and upper mantle need to be quantified at all stages of the life cycle of an
oceanic plate, from its birth at a spreading center to beyond the time at which the plate is subducted, as
well as for plates created at the full range of spreading rates and in various sedimentation regimes.
Recent results indicate, for instance, that the hydrologic isolation of the crust occurs at various rates and
efficiencies depending on the sedimentation rates and sediment types, as well as basement topography
(Fisher and Becker, 2000). Thus, ventilation of the upper crust may continue for variable periods of time.
The cycling of seawater through the ocean crust and mantle must be understood in terms of global cycles
of water, carbon, oxygen, metals, and other elements, but needs to be resolved in terms of specific
lithospheric properties. The rates of cycling depend fundamentally on plate tectonic rates, so that firstorder differences in seafloor spreading rates have profound consequences for seawater chemistry, global
ocean-atmosphere cycles, and, indeed, life on earth.
What are the roles of bacteria in alteration and mineralization?
Recognition that microbial populations exist deep within the oceanic crust has spurred interest in
evaluating the impact of that activity on the alteration of the crust. Along fractures in otherwise fresh
basaltic glass, channels that appear to be the result of microbial activity radically increase the surface
area of glass that is exposed to seawater. Hence, it has been postulated that microbial activity may
influence the kinetics and extent of hydrothermal alteration. However, we have no constraints on the
diversity or pervasiveness of organisms with depth, temperature, or age of the crust, nor on how the rates
of microbially mediated alteration compare to those uninfluenced by microbes. Drilling provides the only
way to identify and quantify the subsurface biosphere, and to assess their role in crustal alteration.
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Creation and evolution of the oceanic lithosphere
References
Carlson, R.L., 1998, Seismic velocities in the uppermost oceanic crust: age dependence and the fate of
layer 2A, J. Geophys. Res., 103, 7069-7077.
Humphris, S.E., Herzig, P., Miller, J., et al., 1995, The internal structure of an active sea-floor massive
sulphide deposit Nature, 377, 713-716.
Zierenberg, R.A., Fouquet, Y., Miller, D.J., et al., The deep structure of a seafloor hydrothermal deposit,
Nature, 392, 485-488.
Fisher, A. and Becker, K., 2000, Channelized fluid flow in oceanic crust reconciles heat-flow and
permeability data, Nature, 403, 71-74.
Biography
K. Gillis was a co-chief scientist for ODP Leg 147, has served on several advisory ODP committees,
including the Lithosphere Panel and the PPG “Architecture of the Oceanic Lithosphere”, and was cochair for the Crustal Architecture session for the ODP COMPLEX meeting. Her research is aimed at
understanding fluid-rock interactions in oceanic hydrothermal systems.
Figure 1. Schematic record of penetration into oceanic lithosphere based on crustal age and a possible scenario for
proposed new drilling targets. Solid lines indicate holes completed to date. All other lines indicate proposed new
drilling targets. While some progress has been made drilling upper crustal sections in a few settings, relatively little
of the lower crustal and upper mantle has been drilled. There is also a major gap in drilling into all levels of crust of
intermediate age (10-100 Ma). The average age of oceanic crust is 65 Ma, the average age of subducting crust is 75
Ma, and the average age at which advective heat loss from the crust ends is 65 Ma. Dashed lines indicate proposed
complete intact crustal holes into shallow crust and dots indicate proposed holes through intact crust and upper
mantle rocks.