Minor Element Zoning of Olivine in Type Iia Chondrules in Semarkona

Introduction: The detailed interpretations of chondrule thermal histories are difficult and controversial. The olivine in Type IA chondrules may be relicts of granular olivine aggregates or formed by fractional crystallization [1,2]. Type IIA chondrules may have been heated and cooled once or multiple times [3,4]. Criteria in addition to texture would be useful for determining the extent of melting, number of events and cooling history. A new tool that can clarify how olivine crystals formed is X-ray intensity mapping of minor elements such as P [5,6,7] and it reveals unexpected features in many chondrules investigated. Experimental Techniques: We used a section of Semarkona MNHN ns2 and made X-ray scanning images of chondrule olivine using the Cameca SX100 at the Université Paris VI. We used 15keV, high beam current (500nA) and high dwell times (100 ms) with 1μm spacing. BSE intensities and P, Cr, Ca, and Al or Mn counts were measured simultaneously using LTAP, LLIF, LPET and TAP crystals to form images. Quantitative analyses of olivine were done on the Cameca SX100 microprobe at the Université Blaise Pascal, along traverses with 1μm spacing. We used 15 keV, and 150nA for less abundant elements. The lowest detection limits were 71 ppm (P), 76 ppm (Na), 93 ppm (Al) 140 ppm (Ca) and 210 ppm (Cr). Results: Mean P2O5 concentrations in olivine grains in Type IIA chondrules range from 0.013 to 0.115 wt.%, always above the detection limit. The maximum values, occurring both in interior zones and at edges for P2O5 are 0.11 to 0.53 wt.%. The data are consistent with those of [3], <0.06 to 0.17 wt.% for chondrule mean olivine and <0.06 to 0.5 wt.% for chondrule maxima. By contrast, Type IA olivine has less than the detection limit of 71 ppm for P. Maps of PKα intensity in Type IIA chondrules in Semarkona MNHN ns2 reveal several kinds of zonation not evident in maps for other elements. The zonation varies from obvious (Fig. 1,2) to very faint, and from fairly continuous to sharply discontinuous. P maps also reveal the presence of relict grains. Concentric-hourglass zoning. Many olivine crystals show discontinuous concentric zoning in hourglass form for P, whereas other elements e.g. Cr show a much simpler pattern (Fig. 1). Oscillatory concentric zonation. P zonation is very common, especially as oscillatory concentric zonation, Fig. 1 Concentric-hourglass zoning in P (left); reverse/normal zoning in Cr (right). which documents stages in olivine growth. In many cases P zoning shows that the central region has the form of a hopper crystal (Fig. 2), which has subsequently filled in. Melt inclusions (white in Fig. 2) are common in such grains.