Fractionation of Oxygen Isotopes at the Faces of Smoky Quartz

Abstract

The 180 / 160 ratio under the m-and r-faces of smoky quartz has been measured; the ratio of the growth velocities was vm/vr = \ß. The obtained separation factor 1.00020 is compared with that obtained previously for amethyst (1.00043), where the ratio vm/vr was unity.Recently we reported on a new isotope effect [1]: During the growth of natural amethyst, Si02 with different lsO content is incorporated at crystallo graphically different faces of the crystal, the 180 / 160 ratio increasing in the order z-face, r-face, m-face. Meanwhile a similar effect in natural calcite was re ported by J. A. D. Dickson [2].Care must be taken if one wants to single out such an effect because during the hydrothermal growth of the crystal the isotope composition of the mother liquid may have varied, this leading to a corresponding iso topic layering ("mantle zoning") inside the crystal, which is superimposed on the sought for isotopic "sec tor zoning".In a polished plane-parallel slice of quartz, which is cut out from the crystal parallel to the c-axis and perpendicular to an m-face, one sees the boundary between the r-and m-sectors if during the growth impurities, which cause colouring, have been incorpo rated preferentially at one of the two faces. This was the reason why for our first study we had chosen amethyst and in the present one we deal with smoky quartz. The colours of both smoky quartz and amethyst are due to aluminium and iron impurities. If the aluminium impurity Al3 + substituting silicon exReprint requests to Prof. Dr. A. Klemm, Max-Planck-Institut für Chemie, Saarstraße 23, W-6500 Mainz, FRG. ceeds the interstitial Fe3+ impurity by a large amount, radioactivity will turn the colourless quartz to a smoky quartz. On the other hand, if the amount of interstitial Fe3+ exceeds the Al3+ impurity, the amethystine colour will appear after irradiation [3].If one chooses two points on such a visible boundary between r-and m-sectors and cuts through these points parallel to the corresponding r-and m-faces, the material between these cuts in the r-and m-sectors has grown within the same time span. In order to secure that the proportion of the amounts of material m 1 m 3 Fig. 1. The position of the samples in a slice of smoky quartz, cut parallel to the c-axis and perpendicular to the m-face. Height 6 cm, thickness 3 mm. In this case, where the average ratio of the growth velocities vr and vm was 3, sampling by scraping material from the surfaces, as was done in [2], would have given results where the isotopic mantle zoning is fully involved. Near the surface the sectorial boundary changes its direction. It would therefore have been better to exclude the corresponding layers from the samples.0932-0784 / 91 / 1200-0001 $ 01.30/0. -Please order a reprint rather than making your own copy.

Oxygen isotope sector zoning in natural hydrothermal quartz

“…This is in accordance with the studies by Klemm et al (1990Klemm et al ( , 1991 who found differences of 0.4% between m and r and 0.2% between r and z for an amethyst crystal and of 0.2% between m and r in a smoky quartz. Their conclusion put forward the hypothesis of possible growth rate effects, but also a role of attachment energy.…”

“…Previous work on quartz samples by Klemm et al (1990Klemm et al ( , 1991, Onasch and Vennemann (1995), but also the work on natural calcites by Dickson (1991Dickson ( , 1996Dickson ( , and 1997 has indicated that there may be a face control on O-isotope fractionations. Such an effect may be enhanced at lower temperatures of crystallization, and/or amplified by a large trace element content, and/or related to a difference in the rate of crystallization, which in itself may also affect the trace element incorporation.…”

“…6 that, based on volume of growth, face r 2 from sample VA03 has grown faster than r 1 (Poty, 1969;Klemm et al, 1990Klemm et al, , 1991. If growth kinetics affect O-isotope fractionations in the sense described by Watson (2004), smaller d 18 O values would then be expected in the faster growing crystallographic orientation (e.g.…”

“…Structural arrangements of the crystal lattice may affect the incorporation of trace elements into crystals Reeder, 1990, 1995;Jourdan et al, 2009), and hence isotopic fractionation (Klemm et al, 1990(Klemm et al, , 1991Dickson, 1991;Onasch and Vennemann, 1995;Méheut et al, 2009). The three different types of crystallographic orientations À r, z and m in quartz have different surface structures and hence somewhat different lattice arrangements, with m having a tighter arrangement of atoms at the surface (Dowty, 1976).…”

“…Measurements of O-and C-isotope compositions in carbonates have, however, produced contradictory results and it is still not clear if isotopic sector zoning actually exists. Previous O-isotope measurements in quartz have already indicated the possibility of sector zoning, with d 18 O values that can differ by up to 2% between crystallographically different growth sectors of the same growth zone (Klemm et al, 1990(Klemm et al, , 1991Onasch and Vennemann, 1995).…”