Argon isotopic studies of minerals in kimberlites, mantle xenoliths and diamonds, from selected southern African localities

Miscellaneous · Sun Jan 01 00:00:00 EST 1989

OSTI ID:7011224

Phillips, D

The occurrence, composition, behavior and origin of excess argon components, in mantle phases hosted by southern African kimberlites, is evaluated using furnace step-heating and laser-probe analytical techniques. Laser-probe {sup 40}Ar/{sup 39}Ar analyses of phlogopite from the swartruggens kimberlite dyke (145 Ma), and Premier diatreme ({approximately}1200 Ma) lherzolite xenoliths, yielded apparent ages decreasing from high ages at grain centres to values approaching the age of kimberlite intrusion, along grain margins. The old apparent ages are attributed excess radiogenic argon, with high {sup 40}Ar/{sup 39}Ar ratios (> 15,000), incorporated prior to kimberlite intrusion under conditions of locally high argon partial pressure. The preservation of the excess argon components is dependent on the timing of melt devolatilization, temperature, cooling rate and the characteristic radius for argon diffusion. Swartruggens phlogopite grains also display chlorine zonations, measured by a neutron activation technique and the laser probe. Fluorine contents, determined by electron microprobe were uniform. Halogen analyses of Premier xenolith phlogopite revealed minor variations. {sup 40}Ar/{sup 39}Ar laser-probe analyses of eclogitic suite clinopyroxene inclusions in diamonds from the Premier kimberlite yielded an age of 1198 {plus minus} 6 Ma, indistinguishable from the inferred time of intrusion of the host kimberlite ({approximately}1200 Ma). This implies diamond formation synchronous with, or no more than {approximately}20 Ma before kimberlite generation. The associated initial {sup 40}Ar/{sup 39}Ar ratio of 334 {plus minus} 50 is similar to the present day atmospheric composition. It is suggested that late-stage equilibration with {sup 36}Ar-rich fluids, derived either from primordial mantle, or from subducted atmospheric argon, is the most likely explanation for this low {sup 40}Ar/{sup 39}Ar value.

• Research Organization: Princeton Univ., NJ (USA)
• OSTI ID: 7011224
• Resource Relation: Other Information: Thesis (Ph. D.)
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
AFRICA
GEOLOGIC DEPOSITS
AGE ESTIMATION
GEOCHEMISTRY
ACTIVATION ANALYSIS
ARGON 36
ARGON 39
ARGON 40
CHLORINE
DEVOLATILIZATION
DIAMONDS
DIFFUSION
DIKES
ELECTRON MICROPROBE ANALYSIS
ENVIRONMENTAL TRANSPORT
EQUILIBRIUM
FLUORINE
GEOLOGIC HISTORY
INTRUSION
ISOTOPE DATING
ISOTOPE RATIO
KIMBERLITES
METEORIC WATER
MINERALOGY
MINERALS
NEUTRON REACTIONS
ARGON ISOTOPES
BARYON REACTIONS
BETA DECAY RADIOISOTOPES
BETA-MINUS DECAY RADIOISOTOPES
CARBON
CHEMICAL ANALYSIS
CHEMISTRY
ELEMENTAL MINERALS
ELEMENTS
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
GEOLOGIC STRUCTURES
GROUND WATER
HADRON REACTIONS
HALOGENS
HYDROGEN COMPOUNDS
IGNEOUS ROCKS
ISOTOPES
LAMPROPHYRES
LIGHT NUCLEI
MASS TRANSFER
MICROANALYSIS
NONMETALS
NUCLEAR REACTIONS
NUCLEI
NUCLEON REACTIONS
OXYGEN COMPOUNDS
RADIOISOTOPES
ROCKS
STABLE ISOTOPES
VOLCANIC ROCKS
WATER
YEARS LIVING RADIOISOTOPES
580000* - Geosciences