 |
| Titel |
A Dynamic study of Mantle processes applying In-situ Methods to Compound Xenoliths: implications for small to intermediate scale heterogeneity |
| VerfasserIn |
Ioannis Baziotis, Paul Asimow, Antonios Koroneos, Theodoros Ntaflos, Giampero Poli |
| Konferenz |
EGU General Assembly 2013
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250076804
|
|
|
|
|
|
| Zusammenfassung |
| The mantle is the major geochemical reservoir of most rock-forming elements in the Earth.
Convection and plate-tectonic driven processes act to generate local and regional
heterogeneity within the mantle, which in turn through thermal and chemical interactions
modulates ongoing geophysical processes; this feedback shapes the dynamics of the deep
interior. Consequently, these processes contribute to the evolution of the earth throughout its
geological history.
Up to now, the heterogeneity of the mantle has been extensively studied in terms of
conventional methods using basalt chemistry, bulk rock and mineral major and trace element
analysis of isolated xenolith specimens of varying lithology, and massif exposures. The
milestone of the present study, part of an ongoing research project, is the application of
in-situ analytical methods such as microprobe, LA-ICP-MS and high resolution
SEM in order to provide high quality major and trace element analyses as well
as elemental distribution of the coexisting phases in the preserved intra-mantle
lithologies,
Particularly, in the context of the current study we used selected compound xenoliths
from San Carlos (Arizona, USA), Kilbourne Hole (New Mexico, USA), Cima Dome and
Dish Hill suites (California, USA), San Quintin (Baja California, Mexico) and Chino Valley
(Arizona, USA), from the Howard Wilshire collection archived at the Smithsonian Institution.
The selection of these compound xenoliths was based upon freshness and integrity of
specimens, maximum distance on both sides of lithologic contacts, and rock types thought
most likely to represent subsolidus juxtaposition of different lithologies that later partially
melted in contact. The San Carlos samples comprise composite xenoliths with websterite,
lherzolite and clinopyroxenite layers or clinopyroxenite veins surrounded by lherzolite or
orthopyroxenite-rich rims. The Kilbourne Hole suite comprises spinel-(olivine)
clinopyroxenite and orthopyroxenite dikes cutting spinel lherzolite (Irving 1980). The Dish
Hill volcanic field contains lherzolite xenoliths with amphibole-rich veins previously
interpreted by Wilshire et al. (1980) in terms of reaction of a H2O- and Fe-rich fluid
with the lherzolite host producing notable losses of Mg and Al. The Cima volcanic
field, located in the southern Basin and Range province, provides samples with
contacts among such diverse lithologies as Cr-diopside spinel peridotite, websterite,
gabbro, clinopyroxenite and wehrlite (Wilshire et al. 1991). The San Quintin example
contains large websterite veins crosscutting lherzolite matrix. Chino Valley xenoliths
show cumulate textures with alternating orthopyroxene- and clinopyroxene-rich
layers.
We discuss and assign the observed reaction textures and mechanisms between the
different minerals (e.g. olivine, clinopyroxene, orthopyroxene), layers (e.g. clinopyroxenite
and lherzolite) and the consequent compositional changes across/among the minerals and the
lithologic contacts. Thus, we characterize the possible genetic origin scenarios
for the observed contacts between adjacent lithologies of the composite xenoliths.
Those examples, most likely to represent original subsolidus contacts that underwent
partial melting together, will have the most direct relevance as benchmarks for
application of future kinetic models of melt extraction from heterogeneous mantle
lithologies.
References
Irving, A.J. (1980). Petrology and Geochemistry of Composite Ultramafic Xenoliths in
Alkalic Basalts and Implications for Magmatic Processes within the Mantle. American
Journal of Science, 280, 389-426.
Wilshire, H.G., et al. (1980). Amphibole-Rich Veins in Lherzolite Xenoliths, Dish Hill
and Deadman Lake, California. American Journal of Science, 280, 576-593.
Wilshire, H.G., et al. (1991). Petrology of Lower Crustal and Upper Mantle Xenoliths
from the Cima Volcanic Field, California. Journal of Petrology, 32(1), 169-200. |
|
|
|
|
|
|