Hydrous Minerals and Carbonates in Peridotite Inclusions from the Green Knobs and Bull Park Kimberlitic Diatremes on the Colorado Plateau

  1. F.R. Boyd and
  2. Henry O.A. Meyer
  1. Douglas Smith

Published Online: 19 MAR 2013

DOI: 10.1029/SP016p0345

The Mantle Sample: Inclusion in Kimberlites and Other Volcanics

The Mantle Sample: Inclusion in Kimberlites and Other Volcanics

How to Cite

Smith, D. (1979) Hydrous Minerals and Carbonates in Peridotite Inclusions from the Green Knobs and Bull Park Kimberlitic Diatremes on the Colorado Plateau, in The Mantle Sample: Inclusion in Kimberlites and Other Volcanics (eds F.R. Boyd and H. O.A. Meyer), American Geophysical Union, Washington, D.C.. doi: 10.1029/SP016p0345

Author Information

  1. Department of Geological Sciences, University of Texas At Austin, Austin, Texas 78712

Publication History

  1. Published Online: 19 MAR 2013
  2. Published Print: 1 JAN 1979

ISBN Information

Print ISBN: 9780875902135

Online ISBN: 9781118664858

SEARCH

Keywords:

  • Amphibole composition;
  • Chlorite crystals;
  • Green knobs peridotite inclusions;
  • Hydration;
  • Hydrous minerals;
  • Mineral compositions

Summary

Over 90% of the peridotite inclusions at the Green Knobs and Buell Park diatremes contain hydrous phases which appear to have formed before the inclusions were incorporated in the kimberlitic eruptions, and a few percent contain magnesite of similar origin. Evidence that amphibole, chlorite, titanoclinohumite, and magnesite were formed before eruption includes observations that these phases were deformed together with anhydrous silicates, that they occur in apparent equilibrium metamorphic textures, and that they show systematic compositional variations together with anhydrous silicates. Evidence for the time of antigorite formation is ambiguous.

Amphibole (16–2 wt% Al2O3) , chlorite (20–12%), and pyroxene (6–0.3%) become less aluminous with increased rock hydration, and orthopyroxene becomes less calcic. Titanoclinohumite contains the same TiO2 content (5.5%) in different assemblages. Antigorite contains up to 5 weight % Al2O3 plus Cr2O3. Though compositional zoning and persistence of relict phases are common, local assemblages may have formed in equilibrium: magnesite-diopside and antigorite-enstatite are possible equilibrium pairs.

Hydration may have occurred over a range of water fugacities at temperatures below 700°C and depths from 45 to 60 plus km. Analysis of an Fe- Mg gradient in olivine indicates that some hydration preceded kimberlite emplacement by less than 25 million years. The fluid responsible for hydration was water-rich: constituents other than water and CO2 were not added to the rocks. The hydrated peridotites likely represent continental mantle, not fragments of an oceanic slab. Intrusion of minette magma into hydrated peridotite may have caused dehydration reactions which triggered nonmagmatic kimberlitic eruptions.