Northwest Africa 6693 Electronic Annex

Electronic Annex for
Northwest Africa 6693: A new type of FeO-rich achondrite with low Δ17O
Paul H. Warren1, Alan E. Rubin1, Junko Isa1, Steve Brittenham2,
Insu Ahn3,4 and Byeon-Gak Choi4
1
Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, USA
2
3
4
4860 Glenbrook Drive, Boise, ID 83704, USA
Korean Polar Research Institute, Incheon 406-840, Korea
Earth Science Education, Seoul National University, Seoul 151-748, Korea
Corresponding author is Warren: pwarren@ucla.edu, tel. 310-825-3202, fax 310-206-3051
Submitted to Geochimica et Cosmochimica Acta, May 2012
The purpose of this Electronic Annex is simply to add a variety of figures that, due to
considerations of length and/or color-reproduction costs, could not be included in the regular paper.
EA-1. Low-magnification (width: 14 mm) crossed-nicols view of a fairly representative area of NWA
6693. The rock is dominated by pigeonite oikocrysts, a few of which are shown here almost in their
entirety: a bright blue crystal (right), a pale olivine-grey crystal (lower left), and a crystal at
extinction (black, upper right). The irregular outlines of these oikocrysts, most conspicuously the two
that dominate the right half of this view (i.e., the blue one and the one at extinction), suggest a
corroded/embayed texture. Olivine chadacrysts are a variety of (mostly bright) colors. The section is
abnormally thick, and consequently, conveniently, the extremely anhedral feldspar appears mostly as
various shades of orange (instead of the normal grey), although one relatively small grain (lowercentral area, amoeboid black zone extending up into the olive-grey oikocryst) is at extinction. The
round-edged black areas in the two upper corners are off-section (background glass).
1
EA-2. Low-magnification (width: 13 mm) crossed-nicols view of a fairly representative area of NWA
6926. The rock is dominated by pigeonite oikocrysts, several of which have large portions of the
crystal visible here as, clockwise from left: a pale bluish green crystal; a crystal along top of the view
that grades from orange (its lower left) to magenta (upper right); a crystal in lower right quadrant that
grades from blue (its upper right) to magenta; and a teal-colored crystal on the bottom-central part of
the image. Olivine chadacrysts are a variety of (mostly bright) colors. The black areas in the two left
corners are off-section (background glass).
2
EA-3. Contrast-enhanced version of the central portion of Fig. 8, with red ovals showing distribution of
tiny merrillites, which are clustered in the same region as the fine-grained olivines (white). High
contrast also reveals fine exsolution lamellae (dark grey) associated (typically, for NWA 6693) with
the margins of pyroxenes. The near-black phase is Na-feldspar.
3
Cr-spinel
EA-4. BSE image (width: 1.75 mm) of the End 1 area of NWA 6693. The area of Fig. 3 is shown by
box. The two largest olivines (light grey) have irregular outlines with apparently embayed margins.
Other major phases are pyroxene (medium grey) and feldspar (dark grey). Blacks are voids
(including cracks and larger, more equant voids that probably are “pluck” artifacts). Visible minor
phases are awaruite metal (white) and Cr-spinel, as three typically equant grains (arrows).
4
EA-5. BSE image of End 1 portion of NWA 6693 showing (much like Fig. 3) pyroxene (medium grey)
mantling olivine (light grey), with Cr-spinel (near-white) locally in between, and feldspar (dark grey)
farther beyond the pyroxene mantle.
5
EA-6. Crossed-nicols views of region around the cluster of fine, rounded olivines shown in Fig. 8.
Widths (shorter dimensions) are 6 mm (left) and 1.1 mm (right). The fine olivine cluster is associated
with a single outcrop of feldspar (light grey), but the context of that host feldspar is not obviously
unusual. It is a portion of a larger optically continuous feldspar crystal (see additional outcrops below
and to the left in the low-magnification image; a different feldspar, to the right of the fine olivine
cluster, appears as a very pale yellow). The outcrop is between two different pigeonite oikocrysts
(medium grey, orange) and close to a third oikocryst (brown). Most of the smaller, bright-colored
grains are olivine, although not the yellow grain in the upper right of the low-magnification view;
that is pigeonite.
6
EA-7. Crossed-nicols view (width: 9.3 mm) of the atypical End 1 region of NWA 6693. Of the many
pigeonite crystals in this area, only two (pale beige at left, truncated mix of orange-red grading to
blue near lower right corner) are oikocrysts more than 2 mm across.
7
EA-8. BSE images: In the upper image, from the End 1 area of NWA 6693, the main, light grey phase is
pigeonite. Near-white grain in center is Cr-spinel. Dark grey anhedral phase is feldspar, which
(below and to the right of the Cr-spinel) shows “pincer”-like partial envelopment of lobes of
pigeonite. The lower image, from NWA 6926, shows a melt inclusion within Cr-spinel (near-white).
This particular melt inclusion is unusual in having a second major phase (merrillite, medium grey
toward top) in addition to felsic glass (darker grey). Lower left end of the inclusion plucked out
during sectioning, leaving Cr-spinel that slopes below the plane of the section, and consequently
appears slightly dark.
8
EA-9. Macroscopic view (width: 33 mm) of the olivine-rich enclave before it was used to make one of
our thin sections. The enclave is the green, roughly pentagon-shaped area centered shortly below the
center of the image. Unfortunately, the dark green rim along the bottom and left sides of the enclave
is not definitely present in the final thin section. However, the rim is probably represented by a 0.5mm thick layer of near-pure olivine (upper left in Fig. EA-10); and perhaps also a more distal layer,
roughly 1 mm thick, dominated by blocky pigeonites (white and orange in Fig. EA-11). The two grey
grains shortly left of the olivine-rich enclave are probably exceptionally large Cr-spinels. Black in
lower right marks the edge of the slab.
9
0.5 mm
EA-10. Backscattered-electron image of the olivine-rich enclave. Olivine is light grey. The other main
phases are subequal proportions of feldspar (darkest grey) and pigeonite (medium grey). Rare white
grains are Cr-spinels (off-white) and awaruites (bright white). The feldspar manifests a “graphic”
intergrowth texture, with crystal margins aligned dominantly at angles of about 70 and 330 degrees
(clockwise) relative to vertical. The enclave’s rim (upper left) appears to consist of a 0.5-mm thick
layer of near-pure olivine, and perhaps also a more distal layer, roughly 1 mm thick, dominated by
blocky pigeonites.
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EA-11. Low-magnification (width: 10 mm) view of the olivine-rich enclave with partly crossed nicols.
The separate olivine oikocrysts can be seen as: pink (upper left), green to grey-green (center), pink
again (center-right), light blue (upper right), dark green (lower right, small in area), and lime green
(also small in area, immediately above and to the right of the “lower right” green). As discussed in
the text, the first three of these (green, pink, green) go to optical extinction together and probably
originated as one large oikocryst. White and orange grains that appear to constitute an outer rim of
the enclave (cf. Fig. EA-10) are pigeonites.
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EA-12 Reflected-light, crossed nicols view of bubbles just below the surface of a polished plug sample
of NWA 6693. This is a lower-magnification, wide-context counterpart to Fig. 9. Additional
relatively large bubbles are visible near the top end of the large feldspar, and a few more are scattered
within the adjacent mafic silicate grains. Hexagonal dark grey grain just below scale bar is Cr-spinel.
12
50 μm
25 μm
EA-13. Transmitted-light views of a cluster of mostly parallel-aligned bubbles within NWA 6693. The
main host phase, reddish-orange in the upper, crossed-nicols view, is albitic feldspar. The
“southwest” tip of the bubble cluster extends into pigeonite (light bluish-gray). Perceptible at close
inspection of the higher magnification, uncrossed-nicols image are fine planar lamellae, parallel to
the prevailing orientation of the bubble cluster, within the feldspar.
13
EA-14. BSE image showing exsolution along with complex shock-mobilized feldspar and Cr-spinel
features within a NWA 6693 pyroxene. Most of image (medium grey) is low-Ca pyroxene. Within it
are scattered blebs and lamellae (red ovals) of augite. Feldspar, or feldspar-like glass (very dark
grey), is difficult to distinguish from void space. The approximately diagonal black feature that
trends NNE-SSW near the center of the image is a large crack; and a few other small cracks, and
equant-shaped voids, are present. Linear feature “A” and the many linear dark features parallel to
“A” (blue ovals) are almost entirely feldspar, probably shock-injected into the pyroxene. Note that
many of the feldspar veins appear to have been broken and displaced by a few micrometers. Also, the
“A” feldspar vein may be related, through a discontinuous series of shorter veins that trend
clockwise, to the “B” vein. The alignment of the “B” vein appears (uniquely, in this area; however,
cf. Fig. EA-15) parallel with the augite exsolution lamellae. Orange oval indicates a train of tiny Crspinels, a very common feature in NWA 6693.
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EA-15. BSE image showing shock-mobilized feldspar (or feldspar-like glass) and long narrow voids
(bubbles?) within a NWA 6693 pyroxene. The image’s high brightness setting makes pyroxene
appear light grey, while feldspar is medium grey, and easily distinguishable from void space (black).
Large white grains are olivine. Note that the feldspar veins and elongate voids are generally
discontinuous, and these features, as well as some narrow cleavage cracks, show closely similar
(near-vertical) alignment.
15
EA-16. Color images of two NWA 6693 pyroxene overgrowth grains. Views are 1.7 mm wide, planepolarized light on the left, crossed nicols on the right, and correspond to greyscale images in Fig. 11.
In the upper right image, the overgrowth mantle appears as a slightly greenish shade of grey; it
almost surrounds a core that is mostly blue, along with (near the overgrowth contact) some red and
beige. In the lower right image, the overgrowth appears a light-medium grey; it occurs only on the
left and bottom side of the core, which is light orange. The darker orange above and to the right of
the core is a distinct pigeonite crystal; its orange color is probably mere coincidence. In both these
cases (although not for all NWA 6693 overgrowth grains), the optical extinction angle is precisely
the same for the overgrowth-mantle as for the core.
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T (ºC), based on olivine Ca content
1300
1200
ureilites
LEW 88763
Brachina
1100
brachinites
1000
NWA1500
72415
NWA 6693
MIL 090340
900
HED
intrusives
60025
800
lunar
intrusives
76535
Divnoe
700
45
50
55
60
65
70
75
80
85
90
95
100
olivine Fo (mol%)
EA-17. Temperature of equilibration based on Ca partitioning between olivine and clinopyroxene
[method of Köhler and Brey, 1990], with results for NWA 6693 and a variety of other intrusive or
restititic extraterrestrial igneous rocks.
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