Boron isotopes as indicators of possible precursors to boron-rich granulite-facies Neoproterozoic paragneisses from the Larsemann Hills, Prydz Bay, East Antarctica  


Authors / CoAuthors

Macgregor, J. | Grew, E.S. | De Hoog, C. | Harley, S.L. | Hinton, R.W. | Carson, C.

Abstract

The ca. 1000 Ma Brattstrand Paragneiss in the Larsemann Hills contains a unique assemblage of borosilicate-rich rocks: tourmaline (Tur) quartzite, prismatine (Prs) leucogneiss and grandidierite (Gdd) borosilicate gneiss. In situ analyses with a Cameca ims 4f ion microprobe gave '11B (= {[sample11B/10B / SRM 95111B/10B] - 1} × 1000) to be '3.0 to '14.3? in Tur, '9.6 to '18.1' in Prs and '1.9 to '8.7' in Gdd (1s mostly 1-2' per sample). In anatectic pegmatites, Tur '11B = 4.8 to '12.1'; comparison with host rock Tur implies melting and crystallization from melt together did not fractionate B isotopes. With two exceptions, average '11B increases in a given sample Prs < Tur < Gdd with Prs B 4.8±1.6' lighter and Gdd B 2.8±1.9' heavier than Tur B. This regularity is consistent with the preference of 10B for tetrahedral sites (Prs) and 11B for trigonal sites (Tur, Gdd) and crystallization in near isotopic equilibrium. The precursor of the B-rich rock least changed by metamorphism, Tur quartzite, is interpreted to be a product of pre-metamorphic, hydrothermal B-metasomatism. If there had been no '11B decrease from devolatization during metamorphism, quartzite Tur '11B ('8.7 to '5.7') constrains '11B of premetamorphic fluid to be '3 to 0' (2008 Tur-fluid '11B for 200 C), consistent with a continental source. However, more likely devolatization decreased Tur '11B, and '11B > 0' in the premetamorphic fluid, so an alternative precursor, such as mud volcanoes, should be considered.

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nonGeographicDataset

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73314

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Keywords

  • External Publication
( Theme )
  • isotopes
( Theme )
  • Antarctic data
  • AQ
Australian and New Zealand Standard Research Classification (ANZSRC)
  • Earth Sciences
  • Published_Internal

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2012-01-31T00:00:00

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