Hibiya, Yuki et al. (2019) The origin of the unique achondrite Northwest Africa 6704: Constraints from petrology, chemistry and Re–Os, O and Ti isotope systematics, Geochimica et Cosmochimica Acta, 245, 597-627, doi: 10.1016/j.gca.2018.04.031. < 20191213103257-634355 > pub

Map on Earth or stone. Places and analyses are shown.

  • abstract:

    Northwest Africa (NWA) 6704 is a unique achondrite characterized by a near-chondritic major element composition with a remarkably intact igneous texture. To investigate the origin of this unique achondrite, we have conducted a combined petrologic, chemical, and 187Re–187Os, O, and Ti isotopic study. The meteorite consists of orthopyroxene megacrysts (En55–57Wo3–4Fs40–42; Fe/Mn = 1.4) up to 1.7 cm in length with finer interstices of olivine (Fa50–53; Fe/Mn = 1.1–2.1), chromite (Cr# ∼ 0.94), awaruite, sulfides, plagioclase (Ab92An5Or3) and merrillite. The results of morphology, lattice orientation analysis, and mineral chemistry indicate that orthopyroxene megacrysts were originally hollow dendrites that most likely crystallized under high super-saturation and super-cooling conditions (1–102 °C/h), whereas the other phases crystallized between branches of the dendrites in the order of awaruite, chromite → olivine → merrillite → plagioclase. In spite of the inferred high super-saturation, the remarkably large size of orthopyroxene can be explained as a result of crystallization from a melt containing a limited number of nuclei that are preserved as orthopyroxene megacryst cores having high Mg# or including vermicular olivine. The Re–Os isotope data for bulk and metal fractions yield an isochron age of 4576 ± 250 Ma, consistent with only limited open system behavior of highly siderophile elements (HSE) since formation. The bulk chemical composition is characterized by broadly chondritic absolute abundances and only weakly fractionated chondrite-normalized patterns for HSE and rare earth elements (REE), together with substantial depletion of highly volatile elements relative to chondrites. The HSE and REE characteristics indicate that the parental melt and its protolith had not undergone significant segregation of metals, sulfides, or silicate minerals. These combined results suggest that a chondritic precursor to NWA 6704 was heated well above its liquidus temperature so that highly volatile elements were lost and the generated melt initially contained few nuclei of relict orthopyroxene, but the melting and subsequent crystallization took place on a timescale too short to allow magmatic differentiation. Such rapid melting and crystallization might occur as a result of impact on an undifferentiated asteroid. The O–Ti isotope systematics (Δ17O = −1.052 ± 0.004, 2 SD; ε50Ti = 2.28 ± 0.23, 2 SD) indicate that the NWA 6704 parent body sampled the same isotopic reservoirs in the solar nebula as the carbonaceous chondrite parent bodies. This is consistent with carbonaceous chondrite-like refractory element abundances and oxygen fugacity (FMQ = −2.6) in NWA 6704. Yet, the Si/Mg ratio of NWA 6704 is remarkably higher than those of carbonaceous chondrites, suggesting significant nebular fractionation of forsterite in its provenance.

  • doi: 10.1016/j.gca.2018.04.031
  • modified at 8 h ago


(a) O isotopes_Tanaka and Nakamura (2013) on SIRA Thermo-Fisher MAT253


(a) XRF on ThermoARL XRF (at Washington State University)


(a) SEM-EDS on SEM-EDS
Nwa6704 thumb NWA 6704 < 20111129133337-438-094 >pub
  • classification: achondrite
  • physical-form: aliquot
  • status: (unknown)
  • description: Achondrite; Northwest Africa
  • modified at 2020-03-24
name box-type global-id
binder of The origin of the unique achondrite Northwest Africa 6704: Constraints from petrology, chemistry and Re–Os, O and Ti isotope systematics pub container 20191213103323-776143
name device size global-id
NWA 6704 fraction-4 pub SIRA Thermo-Fisher MAT253 3 20191217104046-838124
NWA 6704 fraction-3 pub SIRA Thermo-Fisher MAT253 3 20191217104046-810524
NWA 6704 fraction-2 pub SIRA Thermo-Fisher MAT253 3 20191217104045-369593
NWA 6704 fraction-1 pub SIRA Thermo-Fisher MAT253 3 20191217104045-330910
NWA 6704 fraction-5 pub SIRA Thermo-Fisher MAT253 3 20191217104046-459662
NWA 6704-#3 -ave pub ICPMS Thermo-Fisher Element XR 30 20191213153919-836204
NWA 6704-#3-b pub ICPMS Thermo-Fisher Element XR 30 20191213153918-969699
NWA 6704-#3-a pub ICPMS Thermo-Fisher Element XR 30 20191213153917-942883
NWA 6704-#6-Px pub ICPMS Thermo-Fisher Element XR 38 20191213153922-356003
NWA 6704-#3-c pub ICPMS Thermo-Fisher Element XR 30 20191213153919-335971
NWA 6704-#6-Pl pub ICPMS Thermo-Fisher Element XR 38 20191213153921-583147
NWA 6704-#6 pub ICPMS Thermo-Fisher Element XR 46 20191213153920-567129
NWA 6704_major pub ThermoARL XRF (at Washington State University) 11 20191213150621-402387
dO pub SIRA Thermo-Fisher MAT253 2 20190527114716-532700
XMg pub EPMA JEOL JXA-8530F 1 20190527114744-204098
NWA 6704 (b) Opx_coarse-parts pub EPMA JEOL JCMA-733MKII/JXA-8900L (Tokyo) 15 20191213145651-704928
NWA 6704 (b) Vermicular olivine-coarse-parts pub EPMA JEOL JCMA-733MKII/JXA-8900L (Tokyo) 9 20191213145652-590802
NWA 6704 (b) olivine-fine-parts pub EPMA JEOL JCMA-733MKII/JXA-8900L (Tokyo) 9 20191213145652-084744
NWA6704_TS-6 pub SEM-EDS 8 20191213114855-861018
NWA 6704 (b) Vermicular olivine-fine-parts pub EPMA JEOL JCMA-733MKII/JXA-8900L (Tokyo) 10 20191213145652-465804
NWA 6704 (b) Opx_fine-parts pub EPMA JEOL JCMA-733MKII/JXA-8900L (Tokyo) 15 20191213145651-100116
NWA 6704 (b) Opx_relict opx in fine-parts pub EPMA JEOL JCMA-733MKII/JXA-8900L (Tokyo) 15 20191213145652-642298
NWA 6704 (b) olivine-coarse-parts pub EPMA JEOL JCMA-733MKII/JXA-8900L (Tokyo) 9 20191213145652-185143
NWA 6704 (b) Chr-coarse-parts pub EPMA JEOL JCMA-733MKII/JXA-8900L (Tokyo) 9 20191213145653-300968
NWA 6704 (b) Chr-fine-parts pub EPMA JEOL JCMA-733MKII/JXA-8900L (Tokyo) 9 20191213145653-984580
NWA 6704 (b) Pl-coarse-parts pub EPMA JEOL JCMA-733MKII/JXA-8900L (Tokyo) 10 20191213145653-637780
NWA 6704 (b) Pl-fine-parts pub EPMA JEOL JCMA-733MKII/JXA-8900L (Tokyo) 10 20191213145653-367220
NWA 6704 (b) Merrillite pub EPMA JEOL JCMA-733MKII/JXA-8900L (Tokyo) 10 20191213145654-766407
NWA 6704 (b) glass incl in Ol pub EPMA JEOL JCMA-733MKII/JXA-8900L (Tokyo) 13 20191213145654-746167
NWA 6704 (b) glass incl in Ol-K-rich pub EPMA JEOL JCMA-733MKII/JXA-8900L (Tokyo) 13 20191213145654-465490
NWA 6704 (b) glass incl in Chr pub EPMA JEOL JCMA-733MKII/JXA-8900L (Tokyo) 14 20191213145654-420607
NWA 6704 (b) Awaruite-coarse-part pub EPMA JEOL JCMA-733MKII/JXA-8900L (Tokyo) 4 20191213145655-605906
NWA 6704 (b) Awaruite-fine-part pub EPMA JEOL JCMA-733MKII/JXA-8900L (Tokyo) 4 20191213145655-100309
NWA 6704 (b) Pentlandite pub EPMA JEOL JCMA-733MKII/JXA-8900L (Tokyo) 7 20191213145655-669001
NWA 6704 (b) pub TIMS Thermo-Fisher Triton 3 20191217094355-607170
NWA 6704 (b) Heazlewoodite pub EPMA JEOL JCMA-733MKII/JXA-8900L (Tokyo) 6 20191213145655-335653
NWA 6704 (b) Chr fraction 1-1 pub ICPMS Thermo-Fisher Neptune 3 20191217113801-059045
NWA 6704 (b) Chr fraction 1-2 pub ICPMS Thermo-Fisher Neptune 3 20191217113801-057740
NWA 6704 (b) pub ICPMS 5 20191217094341-245323
NWA 6704 (b) Chr fraction 1-3 pub ICPMS Thermo-Fisher Neptune 3 20191217113801-285370
NWA 6704 (b) Chr fraction 1-4 pub ICPMS Thermo-Fisher Neptune 3 20191217113801-080859
NWA 6704 (b) Chr fraction 2-1 pub ICPMS Thermo-Fisher Neptune 3 20191217113801-938372
NWA 6704 (b) Chr fraction 2-2 pub ICPMS Thermo-Fisher Neptune 3 20191217113802-897394
NWA 6704 (b) Chr fraction 2-3 pub ICPMS Thermo-Fisher Neptune 3 20191217113802-204609
NWA 6704 (b) Chr fraction 2-4 pub ICPMS Thermo-Fisher Neptune 3 20191217113802-522749
NWA 6704_silicate pub TIMS Thermo-Fisher Triton 3 20191217094356-160855
NWA 6704_metal pub TIMS Thermo-Fisher Triton 3 20191217094356-445724
NWA 6704 (d) pub TIMS Thermo-Fisher Triton 3 20191217094356-361570
NWA 6704 (c) pub TIMS Thermo-Fisher Triton 3 20191217094356-517930
NWA 6704 (a) pub TIMS Thermo-Fisher Triton 3 20191217094355-211541
NWA 6704 (d) pub ICPMS 5 20191217094341-556223
NWA 6704 (a) pub ICPMS 5 20191217094341-284020
NWA 6704 (c) pub ICPMS 5 20191217094341-561992
NWA 6704_metal pub ICPMS 5 20191217094341-923033
NWA 6704_silicate pub ICPMS 5 20191217094341-807324
NWA6704_TS-7 pub SEM-EDS 8 20191213114856-983682
name spots global-id
caption ID assembly stone
Table 6. The oxygen isotope composition of NWA 6704 bulk meteorite, compared with those of paired meteorites. pub 20191217104125-305436 oxygen 5
Table 4b. Major and trace element abundances determined in NWA 6704 by ICPMS. pub 20191213155114-725819 trace (Hibiya et al., 2019) 7
Table 4a. Major element abundances determined in NWA 6704 by X-ray fluorescence. pub 20191213151314-579090 major (Hibiya et al., 2019) 1
Table 5. Highly siderophile element abundances (ppb) and 187Re–187Os systematics of NWA 6704 metal, silicate, and bulk fractions. pub 20191217094509-298441 HSE (Hibiya et al., 2019) 6
Table 2. Modal mineralogy of NWA 6704 thin sections (in vol.%). pub 20191213114608-735967 mode (Hibiya et al., 2019) 2
Table 7. Ti isotope data for chromite fractions of NWA 6704 and terrestrial materials. pub 20191217113823-960024 Ti is. (Hibiya et al., 2019) 8
Table 3. The representative EPMA data (in wt.%) for individual phases. pub 20191213145747-313844 major_min (Hibiya et al., 2019) 19
manual
  • name
  • Hibiya, Yuki
  • Archer, Gregory J.
  • Tanaka, Ryoji
  • Sanborn, Matthew E.
  • Sato, Yuya
  • Iizuka, Tsuyoshi
  • Ozawa,Kazuhito
  • Walker, Richard J.
  • Yamaguchi, Akira
  • Yin, Qing-Zhu
  • Nakamura, Tomoki
  • Irving, Anthony J.