• journal_title：Geology
• Contributor：William E. Stone ; Etienne Deloule ; Michelle S. Larson ; C. Michael Lesher
• Publisher：Geological Society of America
• Date：1997-
• Format：text/html
• Language：en
• Identifier：10.1130/0091-7613(1997)025<0143:EFHHMM>2.3.CO;2
• journal_abbrev：Geology
• issn：0091-7613
• volume：25
• issue：2
• firstpage：143

Prevailing petrogenetic models for Precambrian high-MgO melts such as komatiites invoke crystallization from nearly anhydrous melts (≪0.5% H₂O) generated by partial melting of mantle peridotite at temperatures of (≤ 1900 °C and pressures of (18 GPa. However, ultramafic cumulate and gabbro zones of komatiitic and other high-MgO units in Precambrian greenstone belts contain vesicles and minor to major amounts (≤ 25%) of igneous amphibole. The textures (oikocrysts, rims on intercumulate pyroxene, and mineral inclusions within orthocumulate olivine) and the water-rich compositions (1.00%–2.50% H₂O) of igneous amphiboles from the Archean Abitibi belt indicate crystallization in situ from significantly hydrous melts while the melt fraction was still as high as 40%–50%. Comparisons to experimental phase equilibria suggest that the residual melts from which the amphiboles crystallized contained 3%–4% H₂O, and adjustments for fractional crystallization suggest that the initial melts may have contained as much as 2% H₂O. H₂O contents of this magnitude would require substantial revision of the nearly anhydrous models for Precambrian high-MgO melts, possibly permitting generation at lower temperatures and pressures, lowering their densities and viscosities, increasing their eruptibility, and enhancing the formation of spinifex textures.