Shear zones in the upper mantle - relation between geochemical enrichment and deformation in mantle peridotites
- Univ. of London (England)
Textural variations in mantle-derived spinel peridotites have previously been interpreted as evidence of the existence of asthenospheric mantle diapirs, indicating deformational heterogeneity on a large lateral scale (kilometers to tens of kilometers). However, many volcanic vents entrain both deformed and undeformed xenoliths, and field relations in peridotite massifs show the scale of alternation between deformed and undeformed peridotite to be small (centimeters to meters) because of the presence of numerous lithospheric shear zones. Some rare xenoliths contain both deformed and undeformed peridotite. These is also an apparent relation between deformation and the growth of metasomatic minerals; amphibole is often concentrated in strongly deformed zones in peridotite massifs and deformed xenoliths, although it is also found in crosscutting veins and aureoles around such veins. A relation can also be seen between deformation and indicators of geochemical enrichment. Clinopyroxenes from many deformed spinel peridotites show light rate earth element (REE) enrichment, whereas clinopyroxenes from undeformed spinel peridotites commonly have mid-ocean ridge basalt (MORB)-type light REE-depleted patterns. Sr and Nd radiogenic isotopic compositions of undeformed peridotites are generally MORB-like, and have low {epsilon}Sr and high {epsilon}Nd. In contrast, deformed peridotites, with or without amphibole, often have higher {epsilon}Sr and low {epsilon}Nd values, indicating geochemical enrichment by large ion lithophile (LIL)- and light REE-enriched fluids or melts. These observations can be used to infer that the shallow mantle contains lithospheric ductile shear zones in which metasomatic fluids precipitated amphibole and clinopyroxene.
- OSTI ID:
- 6213105
- Journal Information:
- Geology; (USA), Journal Name: Geology; (USA) Vol. 18:4; ISSN GLGYB; ISSN 0091-7613
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
580000* -- Geosciences
ALKALINE EARTH ISOTOPES
CHEMISTRY
CORRELATIONS
DEFORMATION
EARTH MANTLE
ELEMENTS
ENRICHMENT
FLUIDS
GEOCHEMISTRY
IGNEOUS ROCKS
ISOTOPES
METALS
METAMORPHIC ROCKS
NEODYMIUM ISOTOPES
PERIDOTITES
PLUTONIC ROCKS
RARE EARTHS
RESERVOIR FLUIDS
ROCKS
SHEAR
STRONTIUM ISOTOPES
TEXTURE