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<oai_dc:dc xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:oai_dc="http://www.openarchives.org/OAI/2.0/oai_dc/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.openarchives.org/OAI/2.0/oai_dc/ http://www.openarchives.org/OAI/2.0/oai_dc.xsd">
  <dc:creator>Rosalind Tuthill Helz</dc:creator>
  <dc:date>1973</dc:date>
  <dc:description>&lt;p class="chapter-para"&gt;The phase relations of three basalts, the Picture Gorge tholeiite, the 1921 Kilauea olivine tholeiite, and the 1801 Hualalai alkali basalt, were studied at 5 kb water pressure, 680–1000°C, at the oxygen fugacities of the quartz-fayalite-magnetite (QFM) and hematite-magnetite (HM) buffers.&lt;/p&gt;&lt;p class="chapter-para"&gt;In the range 680–850 °C, the crystalline assemblage on the QFM buffer is dominantly hornblende+ plagioclase, ± ilmenite, magnetite, sphene, fayalitic olivine, and phlogopitic mica. From 875 to 1000 °C the crystalline assemblage is hornblende+ olivine± augite+ ilmenite± magnetite. A melt phase is present from 700 to 1000 °C; a vapor phase was present in all charges.&lt;/p&gt;&lt;p class="chapter-para"&gt;The hornblendes formed on the QFM buffer range in composition from common green hornblendes at low temperatures to kaersutitic hornblendes at 1000 °C. A1(IV) and Ti increase temperature. AI(VI) passes through a maximum near 825 °C, decreasing both above and below this temperature. AI(IV) is proportional to the sum A1(VI)+2Ti. There is a positive linear correlation of approximately 3 : 1 between AI(IV) and the number of cations in the A-site. The most likely explanation for this correlation at present is that the substitution of AI(VI) or Ti&lt;sup&gt;+4&lt;/sup&gt;for a divalent cation creates local charge imbalances in the amphibole structure which can be compensated only by further A-site substitution. There also appears to be a correlation between the a-cell dimension of hornblende and the A-site occupancy. Above a thresh hold value of approxmately 0.5 cations in A,&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;i&gt;a&lt;/i&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;increases as A-site occupancy increases.&lt;/p&gt;&lt;p class="chapter-para"&gt;Phase relations on the hematite-magnetite buffer are considerably simpler. The hornblendes show relatively little change in composition as temperature increases, and in the tholelitic compositions break down at or below 970 °C 35–60 °C above the first appearance of augite±olivine. The melting of hornblende is incongruent in all cases. The Fe-Ti oxides are pseudo-brookite and titanohematite; at 1000 °C these oxides make up 10 per cent by weight of the assemblage and contain most of the Tio&lt;sub&gt;2&lt;/sub&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;and FeO in the charge.&lt;/p&gt;&lt;p class="chapter-para"&gt;The patterns of hornblende variation observed in this study compare closely with those reported in a wide range of experimental and field data. The appearance of high-TiO&lt;sub&gt;2&lt;/sub&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;kaersutitic hornblendes in the tholeities at 1000° C, P&lt;sub&gt;H2O&lt;/sub&gt;= 5 kb on the QFM buffer implies that the restricted occurence of kaersutite in nature (where it is associated only with mafic to intermediate alkalic rocks) is controlled by volatile content (H&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;,&lt;/sub&gt;F&lt;sub&gt;2&lt;/sub&gt;)rather than by differences in condensed bulk composition.&lt;/p&gt;</dc:description>
  <dc:format>application/pdf</dc:format>
  <dc:identifier>10.1093/petrology/14.2.249</dc:identifier>
  <dc:language>en</dc:language>
  <dc:publisher>Oxford Academic Press</dc:publisher>
  <dc:title>Phase relations of basalts in their melting range at PH2O = 5 kb as a function of oxygen fugacity: Part I. Mafic phases</dc:title>
  <dc:type>article</dc:type>
</oai_dc:dc>