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Studies of noble gases in meteorites and in the earth

Citation

Smith, Stephen Pritchard (1979) Studies of noble gases in meteorites and in the earth. Dissertation (Ph.D.), California Institute of Technology. https://resolver.caltech.edu/CaltechTHESIS:01212016-132933875

Abstract

The isotopic and elemental abundances of noble gases in the solar system are investigated, using simple mixing models and mass-spectrometric measurements of the noble gases in meteorites and terrestrial rocks and minerals.

Primordial neon is modeled by two isotopically distinct components from the interstellar gas and dust. Neon from the gas dominates solar neon, which contains about ten times more 20Ne than 22Ne. Neon from the dust is represented in meteorites by neon-E, with 20Ne/22Ne less than 0.6. Isotopic variations in meteorites require neon from both dust and gas to be present. Mixing dust and gas without neon loss generates linear correlation lines on three-isotope and composition-concentration diagrams. A model for solar wind implantation predicts small deviations from linear mixing, due to preferential sputtering of the lighter neon isotopes.

Neon in meteorites consists of galactic cosmic ray spallation neon and at least two primordial components, neon-E and neon-S. Neon was measured in several meteorites to investigate these end- members. Cosmogenic neon produced from sodium is found to be strongly enriched in 22Ne. Neon measurements on sodium-rich samples must be interpreted with care so not to confuse this source of 22Ne with neon-E, which is also rich in 22Ne.

Neon data for the carbonaceous chondrite Mokoia show that the end member composition of neon-Si in meteorites is 20Ne/22Ne = 13.7, the same as the present solar wind. The solar wind composition evidently has remained constant since before the compaction of Mokoia.

Ca, Al-rich inclusions from the Allende meteorite were examined for correlation between neon-E and oxygen or magnesium isotopic anomalies. 22Ne and 36Ar enrichments found in some inclusions are attributed to cosmic- ray-induced reactions on Na and Cl, not to a primordial component. Neon-E is not detectably enriched in Allende.

Measurements were made to determine the noble gas contents of various terrestrial rocks and minerals, and to investigate the cycling of noble gases between different terrestrial reservoirs. Beryl crystals contain a characteristic suite of magmatic gases including nucleogenic 21Ne and 22Ne from (α,n) reactions, radiogenic 40Ar, and fissiogenic 131-136Xe from the decay of K and U in the continental crust. Significant concentrations of atmospheric noble gases are also present in beryl.

Both juvenile and atmospheric noble gases are found in rocks from the Skaergaard intrusion. The ratio 40Ar/36Ar (corrected for in situ decay of 40K) correlates with δ18O in plagioclase. Atmospheric argon has been introduced into samples that have experienced oxygen-isotope exchange with circulating meteoric hydrothermal fluids. Unexchanged samples contain juvenile argon with 40Ar/36Ar greater than 6000 that was trapped from the Skaergaard magma.

Juvenile and atmospheric gases have been measured in the glassy rims of mid-ocean ridge (MOR) pillow basalts. Evidence is presented that three samples contain excess radiogenic 129Xe and fission xenon, in addition to the excess radiogenic 40Ar found in all samples. These juvenile gases are being outgassed from the upper-mantle source region of the MOR magma. No isotopic evidence has been found here for juvenile primordial noble gases accompanying the juvenile radiogenic gases in the MOR glasses. Large argon isotopic variations in a single specimen provide a clear indication of the late-stage addition of atmospheric argon, probably from seawater.

The Skaergaard data demonstrate that atmospheric noble gases dissolved in ground water can be transferred into crustal rocks. Subduction of oceanic crust altered by seawater can transport atmospheric noble gases into the upper mantle. A substantial portion of the noble gases in mantle derived rocks may represent subducted gases, not a primordial component as is often assumed.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Geology
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Geology
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Wasserburg, Gerald J.
Thesis Committee:
  • Unknown, Unknown
Defense Date:16 May 1979
Funders:
Funding AgencyGrant Number
NSFUNSPECIFIED
Department of Energy (DOE)UNSPECIFIED
Ford Motor CompanyUNSPECIFIED
Record Number:CaltechTHESIS:01212016-132933875
Persistent URL:https://resolver.caltech.edu/CaltechTHESIS:01212016-132933875
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:9537
Collection:CaltechTHESIS
Deposited By: Benjamin Perez
Deposited On:28 Jan 2016 16:38
Last Modified:02 Dec 2020 01:09

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