PETROLOGY OF THE NWA 7325 ACHONDRITE − METEORITE FROM MERCURY, THE UREILITE PARENT BODY, OR A PREVIOUSLY UNSAMPLED ASTEROID?

1Cyrena A. Goodrich, 2Noriko T. Kita, and 2Daisuke Nakashima

References: [1] Irving A.J. et al. (2013) LPSC 44, #2164. [2] Weider S. et al. (2012) JGR 117, 15 pp. [3] Kita N.T. et al. (2014) this meeting. [4] Sutton S.R. et al. (2014) this meeting. [5] Bischoff A. et al. (2013) EPSC 8, 427. [6] Goodrich C.A. & Delaney J. (2000) GCA 64, 149. [7] Keil K. (2007) Chemie der Erde 67, 37. [8] Ikeda Y. et al. (2000) Ant. Met. Res. 13, 177. [9] Kita N.T. et al. (2004) GCA 68, 4213. [10] Cohen B.A. et al. (2004) GCA 68, 4249. [11] Goodrich C.A. & Wilson L. (2014) this meeting. [12] Sanborn M.E. et al. (2013) MSM 76, #5220.

CONCLUSIONS

(a)

Fig. 1. Collage of back-scattered electron images (BEI) of part of a thin section of NWA 7325. plag = plagioclase; px = pyroxene; ol = olivine.

1Planetary Science Institute, 1700 E. Ft. Lowell Drive, Tucson, AZ 85719 USA (cgoodrich@psi.edu). 2WiscSIMS, Dept. of Geoscience, U. Wisconsin, 1215 W. Dayton St., Madison, WI 53706 USA.

INTRODUCTION NWA 7325 is an ungrouped achondrite that was suggested to be from Mercury [1], based on its very low Fe content and Al/Si and Mg/Si ratios compared with Messenger data [2]. We further investigate the petrology of NWA 7325 and examine possible connections to a variety of meteorite types. In [3,4] we report in-situ oxgyen isotope and trace element analyses, Cr isotope composition, and in-situ Cr, V and Ti valence measurements (see talks friday afternoon).

Petrography and Mineral Compositions

NWA 7325 Related to Ureilites?

(b)

Petrography: NWA 7325 has a protogranular to poikilitic texture of ~0.25-1 mm-sized high-Ca pyroxene and ~0.1-0.7 mm-sized olivine grains surrounded or poikilitically enclosed by plagioclase (Fig. 1). Olivine grains are rounded with concave segments (described by [1] as “lobate”) and commonly occur as partial or complete mantles around pyroxenes (Fig. 2a). Modal abundances (~25-30% pyx, 10-15% oliv, 55-60% plag) are similar to those of [1,5]. The sections also contain several % voids with shapes similar to the olivine.

ol

plag

px

Mineral Compositions: Olivine is homogeneous Fo 97.5±0.1 Its Fe/Mg-Fe/Mn (Fig. 3) and CaO-Cr2O3 (Fig. 4) compositions are unique compared with olivine in other primitive achondrites. Large pyroxene grains are homogeneous Wo 45.3±0.2, mg# 98.2±0.2 with 2.8±0.1% Al2O3, 0.92±0.07% Cr2O3, 0.17±0.2% Na2O, and TiO2 ≤0.05%. Many grains show thin lamellae that appear to be shock lamellae (Fig. 2c).

ol

plag

Plagioclase is mottled (Fig. 2b, 5) with variations from ~An 85−94 (avg. An 89.6). It shows reaction with pyroxene: 1) precipitation of tiny wisps of Ca-pyroxene adjacent to grains of pyroxene and olivine (Fig. 2b); 2) invasion into pyroxene grains as veins [5]; and 3) resorption of pyroxene inclusions, with re-precipitation of more calcic pyroxene and more sodic plagioclase (Fig. 5). Troilite with ≤ 6% Cr (and lamellae of daubreelite) is a common minor phase (Fig. 2d). It contains metal with ≤15% Ni and 3.8% Co.

px

plag

ol

1 mm

NWA 7325

px (a)

(d)

px

(c)

Fig. 2. BEI of NWA 7325. (a) Olivine (ol) mantling pyroxene (px) grains. Voids showing similar morphology may once have been olivine. (b) Wisps of pyroxene in plagioclase adjacent to larger grains of pyroxene and olivine; pyroxenes show reacted edges but olivines do not. (c) lamellae in pyroxene, showing no compositional difference. (d) bleb of troilite with thin, Cr-rich lamellae, probably daubrèelite (dbr).

Petrogenesis: The subchondritic Mn/Mg of olivine in NWA 7325 (Fig. 3) and its plagioclase- and high-Ca pyroxene-rich mineralogy suggest that it is a cumulate. It appears to have crystallized from an incompatible element-depleted melt, under conditions of extremely low oxygen fugacity [4]. Pyroxene and olivine were the earliest phases, with plagioclase growing around them. Plagioclase was remelted and reacted with pyroxene, e.g.,

diopside + anorthite = wollastonite + albite Based on the presence of shock lamellae in the pyroxenes, and analogy to enstatite chondrite impact melt rocks with similar textures [7], this was probably caused by shock, rather than internal heating [5].

Minor elements in olivine and pyroxene (Fig. 3,4) indicate that NWA 7325 does not belong to a known primitive achondrite group. Further, the combination of An-rich plagioclase and very magnesian mafics is not observed in any group (Fig. 6). However, a similar assemblage (clast γ-8, Fig. 3,4,6) occurs as one of the indigenous feldspathic lithologies in polymict ureilites [8-11]. The oxygen isotope composition of NWA 7325 [1,3] is consistent with this ureilitic lithology [9]. Trace elements in plagioclase [3] and minor elements in olivine (Fig. 3,4) further support a connection. Nevertheless, Cr isotope compositions [3,12] indicate that NWA 7325 cannot be derived from the ureilite parent body (UPB). We suggest that the UPB and the parent body of NWA 7325 share chemical and oxygen isotope properties, and experienced similar melting processes. Oxygen isotope compositions suggest formation beyond the ice line, in the outer asteroid belt, rather than the inner Solar System (see [3] and talk friday afternoon).

Petrogenesis as a Cumulate

Sample and Procedures: Two sections were prepared from a new, ~2.2 g sample of NWA 7325 (shown above). BEI/SEI/EDS observations utilized the Zeiss EVO 50 XVP scanning electron microprobe at U. Mass. Mineral compositions and X-ray maps were obtained using the Cameca SX-50 electron microprobe at U. Mass.

Fig. 5. Inclusions of pyroxene reacted with surrounding plagioclase in NWA 7325. (a) Reaction results in Ca-enriched mantle on pyroxene and Na-enrichment in adjacent plagioclase. (b) Combined Na (red), Ca (green) and Al (blue) X-ray map of [a]. (c) Highly-resorbed inclusion. (d) Similar to [a]. In addition, bright needles around the outside appear to be wollastonite (Wo).

plag

px

(a) (b)

plag px

Na-rich

Na-rich

(c) plag

px

Wo?

(d)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.70

20

40

60

80

brachinites & brachinite-like Divnoe low-Ca pyroxene ureilites augite-bearing ureilites lodranites winonaites

mol

ar F

e/Mn

in o

livin

emolar Fe/Mg in olivine

γ-8

NWA 7325igneous fractionation

redox

angritesto

GRA 06128

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90.0

0.1

0.2

0.3

0.4 NWA 7325

brachinites & brachinite-like GRA 06128/06129 Divnoe main group ureilites acapulcoites & lodranites winonaites NWA 6693/6704

wt. %

CaO

in o

livin

e

wt. % Cr2O3 in olivine

γ-8

to angrites

Fig. 6. An of plagioclase vs. mg# of olivine or pyroxene for various meteorite types (many sources) compared with NWA 7325. Red = feldspathic clasts in polymict ureilites [8-11].

(see [3] and talk friday afternoon).

20 30 40 50 60 70 80 90 1000

20

40

60

80

100 ADOR-like

ureilite - labradoritic

An o

f pla

gioc

lase

mg# of pyroxene or olivine

NWA 7325

GRA 06128

Divnoe

brachinites

angrites

γ-8

NWA 6693

acapulcoites/lodranites

winonaitesureilite - albitic

NWA 7325 is a cumulate rock that does not belong to a known achondrite group. In many characteristics it resembles a rare, Mg-An-rich indigenous lithology found as clasts in polymict ureilites. However, based on Cr isotopes, it cannot be derived from the ureilite parent body. Like ureilites, it probably formed beyond the ice line, in the outer regions of the disk.

px

px

void

troilite

dbr

(see talks friday afternoon).

Fig. 3. Subchondritic Mn/Mg of NWA 7325 olivine is unusual compared with other primitive achondrites and suggests origin as an extreme cumulate [6]. Mg-An-rich lithology (γ-8) in polymict ureilites [8], has similar composition.

Fig. 4. CaO and Cr2O3 contents of olivine in NWA 7325 are more like those of olivine in main group ureilites or Mg-An-rich lithology (γ-8) in polymict ureilites [8] than those of olivine in any other primitive achondrites.