Publication Date:
2022-05-26
Description:
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1992
Description:
Variations in the abundances of elements and radiogenic isotopes in mantle derived
peridotites and volcanic rocks are chemical integrals over time, space, and process, which
ultimately contain information about the role of convection in the earth's mantle in creating,
maintaining, and destroying geochemical heterogeneities. Successful inversion of these
integrals requires extensive knowledge of the geochemical behavior of elements, the length
scales of chemical variability, the evolution with time of geologic systems, the physical
properties of mantle rocks, and the driving forces of phenomena which govern heat and
mass transport in a dynamic earth. This dissertation attempts to add to this knowledge by
examining the trace element and isotope geochemistry of mantle peridotites and oceanic
island basalts, and by studying aspects of the flow of viscous fluids driven by thermal
buoyancy.
The trace element and isotopic systematics of peridotites and associated mafic layers
from the Ronda Ultramafic Complex, southern Spain (Chapter 2), provides information
bearing on the geochemical behavior of the highly incompatible elements U, Th, and Pb in
the mantle, and on the length scales of geochemical variability in a well exposed peridotite
massif. Garnet is demonstrated to be a significant host for U in the mantle, and together
with clinopyroxene, these two minerals control the abundances and partitioning
relationships of U and Th during the melting of anhydrous peridotite. Clinopyroxene,
plagioclase, and to a lesser extent garnet are hosts for Pb in mantle peridotite; however, the
role of trace sulfide may exert some control over the abundance and partitioning of Pb in
some samples. Due to the possibility that Pb is partitioned into sulfide, the U/Pb, Th/Pb,
and Ce/Pb ratios measured in clinopyroxene are likely to be higher than the bulk rock.
U-Pb age systematics of garnet-clinopyroxene pairs from Ronda peridotites and mafic
layers indicate Pb isotopic equilibrium in these samples up to 20-50 Ma ago. The Pb-Pb
systematics of garnet- and spinel-facies peridotites and mafic layers indicate a heterogeneity
on the order of 3 Ga old. This Pb isotope signature may have been created within the
massif 3 Ga ago, or may have been metasomatically imprinted on the massif 1.3 Ga ago by
basaltic melts with island arc affinities. The isotopic evolution of Ronda is consistent with at 1.3 Ga ago, and was subsequently incorporated into the subcontinental lithosphere. The
very low U, Th, and Pb concentrations in depleted peridotite indicate that recycled crustal
materials, with U-Th-Pb concentrations 102-104 times higher than peridotite, will have a
larger influence on the isotopic composition of Pb in the mantle than on the Sr and Nd
isotopic composition.
An investigation of the trace element and isotopic compositions of clinopyroxenes
in peridotite xenoliths from Savaii, Western Samoa and Tubuai, Austral Islands (Chapter 3)
reveals geochemical signatures which are not present in basalts from these islands, due to
the inherent averaging of melting processes. The data indicate similarities in the melting
and melt segregation processes beneath these isotopically extreme islands. Samples with
LREE depleted clinopyroxenes, with positive Zr and negative Ti anomalies, are the result
of poly baric fractional melting of peridotite in the garnet- and spinel lherzolite stability
fields, with the Savaii samples having experienced a larger mean degree of melting than the
Tubuai samples. The extreme fractionation of HREE in the Savaii samples requires that
they have melted to the clinopyroxene-out point (about 20%) while retaining residual
garnet; the low concentrations ofHREE in these same samples requires a further 10-20%
melting in the spinel lherzolite stability field. The extremely high total degrees of melting
experienced by the Savaii samples (33-42%), as well as the high degree of melting in the
garnet lherzolite stability field, suggests a mantle plume origin for these xenoliths.
A large majority of the xenolith clinopyroxenes from both Savaii and Tubuai are
LREE enriched to varying degrees, and many samples display significant intergrain trace
element heterogeneity. This highly variable yet systematic heterogeneity was the result of
metasomatism by percolating melts undergoing chromatographic trace element
fractionation. The trace element compositions of some LREE enriched clinopyroxenes are
consistent with the percolating melt being typical oceanic island basalt. The clinopyroxenes
with the highest LREE concentrations from both islands, which also have very low Ti and
Zr concentrations and large amounts of grain-boundary hosted Ba, require that the
percolating melt in these cases had the trace element signature of carbonatite melt. The
isotopic composition of one of these "carbonatitic" samples from Tubuai is similar to
basalts from this island. The isotopic composition of clinopyroxene in a "carbonatitic"
sample from Savaii records 87Sr/86Sr and l43Nd/l44Nd values of .71284 and .512516
respectively, far in excess of the most extreme Samoa basalt values (87Sr/86Sr=.70742,
143Nd/l44Nd=.51264). These "carbonatitic" signatures indicate the presence of volatilerich,
isotopically extreme components in the mantle beneath Tubuai and Savaii, which
likely have their origins in recycled crustal materials. The Re-Os isotope systematics of oceanic island basalts from Rarotonga, Savaii,
Tahaa, Rurutu, Tubuai, and Mangaia are examined (Chapter 4). Os concentration
variations suggest that olivine, or a low Re/Os phase associated with olivine, controls the
Os concentration in basaltic magmas. The Savaii and Tahaa samples, with high 87Sr/86Sr
and 207Pb/204Pb ratios (EMII), as well as basalts from Rarotonga, have 1870s/1860s ratios
of 1.026-1.086, within the range of estimates of bulk silicate earth and depleted upper
mantle. The basalts from Rurutu, Tubuai, and Mangaia (Macdonald hotspot), characterized
by high Pb isotope ratios (HIMU), have 1870sfl860s ratios of 1.117-1.248, higher than
any estimates for bulk silicate earth, and higher than Os isotope ratios of metasomatized
peridotites. The high 1870s/1860s ratios indicate the presence of recycled oceanic crust in
the mantle sources of Rurutu, Tubuai, and Mangaia. Inversion of the isotopic data for
Mangaia (endmember HIMU) indicate that the recycled crustal component has Rb/Sr,
Sm/Nd, Lu/Hf, and Th/U ratios which are very similar to fresh MORB glasses, and U/Pb
and Th/Pb ratios which are within the range of MORB values, but slightly higher than
average N-MORB. These results indicate that the low-temperature alteration signature of altered oceanic crust may be largely removed during subduction, and that oceanic crust was
recycled into to the lower mantle source of the Macdonald hotspot plume. Furthennore, the
high 187Os/l86Os ratios of the Tubuai and Mangaia basalts indicates that percolation
through depleted mantle peridotite (187Os/186Qs=1.00-1.08), observed to occur in the
Tubuai xenoliths, had little influence on the composition of the erupted basalts.
A fluid dynamic model for mantle plumes is developed (Chapter 5) by examining a
vertical, axisymmetric boundary layer originating from a point source of heat, and
incorporating experimentally constrained rheological and physical properties of the mantle.
Comparison of linear (n=l) and non-Newtonian (olivine, n=3) rheologies reveals that non-Newtonian
plumes have narrower radii and higher vertical velocities than corresponding
Newtonian plumes. The non-Newtonian plumes also exhibit "plug flow" at the conduit
axis, providing a mechanism for the transport of deep mantle material, through the full
depth of the mantle, in an unmixed state. Plumes are demonstrated to entrain ambient
mantle via the horizontal conduction of heat, which increases the buoyancy and lowers the
viscosity of mantle at the plume boundary. Streamlines calculated from the fluid dynamic
model demonstrate that most of the entrained mantle originates from below 1500 km depth.
Parameterization of the entrainment mechanism indicates that the factional amount of
entrained mantle is lower in stronger, hotter plumes due to their higher vertical velocities.
Examination of the global isotopic database for oceanic island basalts reveals the
presence of a mantle component (FOZO), common to many hotspots worldwide,
characterized by depleted 87Sr/86Sr and 143Nd/l44Nd, radiogenic 206,207,208Pb/204Pb, and
high 3He/4He. This component is isotopically distinct from the source of MORB; thus,
with the exception of ridge centered hotspots such as Iceland and the Galapagos, upper
mantle does not appear to be a component in most hotspots, in agreement with entrainment
theory. The combined fluid dynamic and isotopic results indicate that both FOZO and the
enriched mantle components (EMI, EMil, and HIMU) are located in the lower mantle.
Furthermore, high 3He/4He in FOZO precludes an origin for FOZO-bearing plumes in a
thermal boundary layer at 670 km depth in the mantle. Since a 670 km thermal boundary
layer would be replenished by the downward motion of the upper mantle, an origin for
FOZO at 670 km would require either 1) a high 3He/4He signature in the MORB source, or
2) entrainment of MORB mantle into intraplate plumes, neither of which is observed in the
OIB isotope data. This indicates that the 670 km discontinuity is not a barrier to mantle
convection. The preservation of isotopically different upper and lower mantles does not
require layered convection, but is probably the result of an increasing residence time with
depth in the mantle, possibly caused by an increase in the mean viscosity of the mantle with
depth.
Keywords:
Geochemistry
;
Earth sciences
;
Fluid dynamics
;
Peridotite
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Format:
application/pdf
Permalink