Snow
Jonathan E.
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The isotope geochemistry of abyssal peridotites and related rocks
Snow
Jonathan E.
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Thesis
This dissertation studies several aspects of the formation of the Earth's
oceanic mantle and crust, using a variety of geologic techniques, principally
major elements, radiogenic isotopes and trace elements, but including petrography,
mineral chemistry, x-ray diffraction, seafloor geomorphology, and analysis
of the tectonics of fracture zones. The first chapter is an introduction to the
problems to be addressed in this work. The second chapter examines the composition
of basalts erupted near the Atlantis II Fracture Zone on the Southwest
Indian Ridge. Trends in major element compositions of those basalts can be related
directly to the nearby presence of the fracture zone. The effects of mantle
composition and crustal level lateral transport of magma in the rift system can be
ruled out by the analysis of isotopes and the geomorphology of the fracture zone
floor. This is the best demonstration to date of a transform fault effect on basalt
compositions. In trying to quantify putative transform fault effects documented
at other fracture zones, no systematic correlation of transform offset age with
mantle temperature change can be found, suggesting that mantle composition
and lateral transport phenomena play a larger than expected role in the evolution
of those areas. The third chapter relates to oceanic mantle rocks as they are altered at or
near the Earth's surface. The major elements which make up abyssal peridotites
are extensively redistributed by the alteration they have undergone. Mg is
shown to be extracted from the peridotites, and a variety of trace elements
added. This elemental redistribution is taken as evidence for extensive Mg
transport by circulating waters. Since the solubility of Mg-bearing minerals in
hydrothermal solutions is quite limited, much lower temperatures and much
higher water /rock ratios are required to explain the major element compositions
of the peridotites than had previously been assumed. The behavior of the Nd, Sr and Os isotopic systems during seafloor alteration
was also studied. The isotope systematics of these rocks strongly support
the hypothesis of high water /rock ratios in the formation of serpentinized
abyssal peridotites. Nonetheless, Nd and Sr reside in a phase which is resistant
to alteration (clinopyroxene) and the concentration of Os is high relative to that
of seawater, so that it too appears resistant to alteration. Primary mantle isotopic
signatures may be obtained from abyssal peridotites by careful analysis, even of
extremely weathered rocks.
Radiogenic strontium in excess of what could be introduced by seawater
contamination or in situ radiogenic growth in a reasonable period of time was
also found. These observations confirm earlier work which had been discredited
for many years. The only plausible mechanism for the formation of this
"orphan" S7Sr is that it is introduced as part of a sedimentary component which
infiltrates the rock during metamorphism and/ or weathering. The 87Sr may be
contained by or sorbed onto extremely fine clay particulates, or colloidal suspensions,
as opposed to the dissolved ionic Sr which is normally thought of as
characterizing the Sr isotopic composition of seawater. The high water/rock
ratios required by the bulk isotopic analysis, as well as the pervasive elemental
redistribution arguing for extensive near-surface weathering at high water /rock
ra.tios strongly support this hypothesis. Given pervasive percolation of water
throughout the samples, sufficient radiogenic, sediment-derived strontium may
be drawn deep into the crust in the course of its weathering to cause such high
B7SrfB6Sr ratios.
The fourth chapter deals exclusively with primary mantle isotopic information
from abyssal peridotites. This is the first study which has attempted to
relate the Os isotopic system in the oceanic mantle to other isotopic systems and
to trace elements. It is possible, with some extreme assumptions, to model the
range of Os isotopes in the oceanic mantle alone in a standard model of formation
of the depleted mantle by extraction of the crust. The additional constraints
provided by the study of Nd isotopes in depleted mantle rocks from the oceans
show that partial melt extraction and the formation of a depleted reservoir alone
are not sufficient to account for the range of both N d and Os isotopes in the
Earth's mantle. Possible mechanisms for the decoupling of the Os and Nd isotopic
systems include elemental fractionation via the porous flow of basalt
through the mantle, mantle metasomatism, recycling of a subducted component
in the mantle and core formation. The core extraction model is pursued in some
detail. Such core extraction models can account for the distributions and isotopic
compositions of compatible and incompatible trace elements in the Earth's mantle,
but they are highly non-unique, and thus difficult to test.