Abstract
Optically homogeneous augite xenocrysts, closely associated with spinel–peridotite nodules, occur in alkali basalts from Hannuoba (Hebei province, China). They were studied by electron and X-ray diffraction to define the occurrence and significance of pigeonite exsolution microtextures. Sub-calcic augite (Wo34) exsolved into En62–62Fs25–21Wo13–17 pigeonite and En46–45Fs14–14Wo40–42 augite, as revealed by TEM through diffuse coarser (001) lamellae (100–300 Å) and only incipient (100) thinner ones (<70 Å). C2/c augite and P21/c pigeonite lattices, measured by CCD-XRD, relate through a(Aug)≅a(Pgt), b(Aug)≅b(Pgt), c(Aug)≠c(Pgt) [5.278(1) vs 5.189(1)Å] and β(Aug)≠β(Pgt) [106.55(1) vs 108.55(2)°]. Cell and site volumes strongly support the hypothesis that the augite xenocrysts crystallised at mantle depth from alkaline melts. After the augite xenocrysts entered the magma, (001) lamellae first formed by spinodal decomposition at a T min of about 1,100 °C, and coarsened during very rapid transport to the surface; in a later phase, possibly on cooling, incipient (100) lamellae then formed.
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Acknowledgements
The authors are indebted to E.M. Piccirillo for a critical review of the text, and to G.C. Capitani for help during the TEM study. M. Mellini and an anonymous referee improved the manuscript. F. Princivalle would like to thank the Chinese Academy of Science for funding his visit to China in 1998, and J. Feng and Y. Wanming for their precious support and help given in field sampling. The authors also acknowledge the financial support of MURST grants (COFIN '01, project 'Transformations, reactions, ordering in minerals') to F. Princivalle and P.F. Zanazzi. The English text was revised by Gabriel Walton.
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Brizi, E., Nazzareni, S., Princivalle, F. et al. Clinopyroxenes from mantle-related xenocrysts in alkaline basalts from Hannuoba (China): augite–pigeonite exsolutions and their thermal significance. Contrib Mineral Petrol 145, 578–584 (2003). https://doi.org/10.1007/s00410-003-0467-9
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DOI: https://doi.org/10.1007/s00410-003-0467-9