NASA Technical Reports Server (NTRS) 20090006826: Early Petrogenesis and Late Impact(?) Metamorphism on the GRA 06128/9 Parent Body

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EARLY PETROGENESIS AND LATE IMPACT(?) METAMORPHISM ON THE GRA 06128/9 PARENT
BODY. L. E. Nyquist 1, C.-Y. Shih2 and Y. D. Reese3. 1KR/NASA Johnson Space Center, Houston, TX 77058. E-
mail: laurence.e.nyquist @nasa.gov. 2ESCG Jacobs-Sverdrup, Houstson, TX 77058. 3Mail Code JE-23,
ESCG/Muniz Engineering, Houston, TX, 77058.

Introduction: Initial studies of GRA06128 and
Achondrite GRA 06128/9
GRA06129 (hereafter GRA 8 and GRA 9) suggested
GRA 8 GRA 9
that these alkalic meteorites represent partial melts of a 2
parent body of approximately chondritic composition 3 WRs Px2(r)
[1-4]. A 147Sm-143Nd isochron age of 4.545±0.087 Ga arth 1 Plag41 (Lr)eaches Earth Px1(r) Px1(r)
was found for GRA 8, but plagioclase (oligoclase) plus E
o 0
whole rock and leachate samples gave an apparent el t CHUR
si~ne3tc.eo5rn pGdreaatr eyads a tagone b uoepf tph~ee3r .c5lir myGsiatt a tl[ol5i z]a.a ttTiiomhnee a~og4fe .m 5o4ef tGGamaR oAargp e8h ;iw stmhaes. 142ε Nd r --21 T14(εr6eS (lm 1t4o/21 NY44d9S)8Em)a=r=t4h0.=5.0 4-0096.±2006±.±0003.0.600 2G14a
Here we extend Sm-Nd and Rb-Sr analyses to GRA 9.
147Sm-143Nd isochron age: 147Sm-143Nd analyses of -3
0.15 0.20 0.25 0.30
whole rock and mineral separates of GRA 9 agree well
147Sm/144Nd
with those of GRA 8 (Fig. 1). Combined data for Figure 2. 146Sm-142Nd isochron for GRA 06128/9.
whole rock and pyroxene residues after leaching de- 0.0060±0.0014 and ε(142Nd) = -0.26±0.02 at
Earth
fine an age of 4.559±0.096 Ga. Initial εNd,CHUR [6] and (147Sm/144Nd)CHUR = 0.1967 (Fig. 2). The 146Sm-142Nd
εNd,HEDR [7,8] are +0.9±0.9 and +0.0±0.9, resp. A pla- age relative to 146Sm/144Sm = 0.0057±0.0005 at 4.542
gioclase sample from GRA 9 after leaching extends Ga ago [8] is 4.549±0.036 Ga. The weighted average
the secondary isochron previously observed for GRA of the 147Sm-143Nd and 146Sm-142Nd ages is
8. Data for plagioclase separates from both samples 4.550±0.034 Ga, and is interpreted as the crystalliza-
combined with data for whole rocks and whole rock tion age of both samples.
leachates give an apparent age of 3.4±0.4 Ga. Phos- 87Rb-87Sr data: Whole rock, whole rock leachate,
phates contain the majority of the REE in GRA, and and whole rock residue data for GRA 8 showed that
are sampled by the leachates. Thus, both whole rocks the leachates are contaminated with Sr having 87Sr/86Sr
and whole rock leachates behave as approximately >~0.715 [5]. Sr isotopic data for GRA 9 are signifi-
closed Sm-Nd isotopic systems, and lie at the intersec- cantly less contaminated. Fig. 3 shows the Rb-Sr data
tion of the primary and secondary isochrons. Plagio- for both samples minus the leachate data. Data for
clase is susceptible to Nd isotopic reequilibration in acid-washed mineral separates shown with small inte-
part because of the low REE abundances in plagio- rior yellow dots in the figure define apparent ages of
clase. We interpret the secondary isochron age as an 4.31±0.45 and 4.36±0.61 Ga for GRA 8 and GRA 9,
upper limit to a time of metamorphism during which resp. Although the two ages are similar, the GRA 8
the Nd isotopic composition in plagioclase was partly isochron has a higher initial (87Sr/86Sr) = 0.69975±65
I
reequilibrated. compared to 0.69935±77 for GRA 9. Both ages are
146Sm-142Nd isochron age: Data for px from both within analytical uncertainty of the Sm-Nd age, but
GRA 8 and GRA 9 combined with data for whole appear to reflect some Sr isotopic reequilibration as
rocks and whole rock leachates define 146Sm/144Sm = observed for the plagioclase Sm-Nd data. A 4.55 Ga
(Sm-Nd age) reference isochron passed through the
Achondrite - GRA 06128/9
GRA 8 GRA 9 Achondrite GRA 06128/9
0.5150
T=4.559±0.096 Ga Px1(r) WR Plag(r)
Px2(r) WR1 Plag1(r)
0.5140 εNd, CHUR=+0.9±0.9 Px1(r) 0.706 Px2(r)
44Nd 0.5130 εNdW, HRE,D WR=R 10.0±0.9 WR1(r) OpqM(rM)(r)Px2(r) Sr GRIT(AS=8r4) =.M301i.n6± 90R9.e47s55i ±dG6ua5es Px1G(r)RA 8 WWRR(1r()r)
1431Nd/ 00..55111200 Plag(LrP)elaacWgh1R,( Lr1)e(la)ch1 LWeaRc1h 460.15 104.270Sm/1404.2N5d 0.30 8786Sr/ 0.705 WWRR1 GRA 9 Px2(r)Plag(Mr)M(r) I(STr(,S GmR-NAd9))==40..5659 9G1a14
0.5100 Plag(r) T =3.4±0.4 GεaNd 02 -96Ma +96Ma 0.704 Px1(r) GRTA=94 .M36in± 0R.e6s1i dGuaes
Plag -2 I(Sr)=0.69935±77
0.10 0.15 0.20 0.25 0.30
0.07 0.08 0.09 0.10
147Sm/144Nd
87Rb/86Sr
Figure 1. 147Sm-143Nd isochron for GRA 06128/9.
Figure 3. Rb-Sr data for GRA 06128/9.
GRA 9 data gives (87Sr/86Sr) = 0.699114±0.000029 Δ17O ~ -0.21 for the GRA meteorites may link
I
which we interpret as the best determination of them to the parent body of the brachinite achondrites,
(87Sr/86Sr) for GRA 9. If most of the GRA 8 data are possibly as flotation cumulates [4]. A bulk sample of
I
compromised by contamination, it is possible that it Brachina has Sr, Nd, and Sm abundances ~4X and Rb
shares the same (87Sr/86Sr)I because two data points of abundances ~3X lower than GRA, consistent with a
GRA 8 (WR(r) and WR1(r)) lie nearly within their common parent body [5]. The O-isotopic compositions
respective error limits on the extension of the 4.55 Ga of silicate inclusions in IAB iron meteorites also are
isochron through the GRA 9 data. Nevertheless, our similar to those of the GRA samples. An “andesitic”
preferred interpration is that the GRA 8 lithology crys-
plagioclase/diopside silicate inclusion from the Caddo
tallized with a higher (87Sr/86Sr) as would be sug-
I County IAB iron [9] has feldspar of composition Ab
80-
gested by its higher whole rock Rb/Sr ratio.
An Or [14] similar to that for GRA. It has similar
(87Sr/86Sr): Fig. 4 compares (87Sr/86Sr) for the 84 12-16 3
I I Rb and Sr abundances as GRA, and Sm and Nd abun-
GRA 8/9 samples to values for (a) Caddo County sili-
dances ~2X higher. A model calcuation [5]showed that
cate inclusion 3Ba (CC-3Ba) [9,10], (b) a silicate in-
REE abundances in GRA 9 [3] could be derived from
clusion from the Campo del Cielo IAB iron meteorite
those in the Caddo County 3Ba inclusion by plagio-
[10], and the Efremovka CAI E38. For GRA 8,
clase accumulation. Fig. 5 shows this model applied to
(87Sr/86Sr) is in complete agreement with that for CC-
I Sm and Sr abundances in GRA 8 and 9. The required
3Ba. Evolution from (87Sr/86Sr) ~0.698934 for the
I enrichment in Sr and REE for CC-3Ba can be modeled
CAI at typical chondritic 87Rb/86Sr (μ) ~0.82 would
by ~15-20% equilibrium partial melting of a chondri-
require ~15 Ma for GRA 9 and ~38 Ma for GRA 8,
tric precursor [10]. This model is more compatible
resp. These values probably are maxima. Similar cal-
with a sizable parent body than ones relying solely on
culations relative to (87Sr/86Sr) ~0.698972 for the
I disequilibrium melting of chondritic precursors, for
LEW86010 angrite [11] gives time intervals of ~ 12
Ma and ~35 Ma, resp. Time intervals this long are Petrogenesis Model of GRA 06128/9
consistent with the Sm-Nd age, but exceed the ~3 Ma 3
35% Melt (from 15%PM) Experimental glasses by
Al-Mg model formation interval reported by [12]. + 65% Plag accumulation Disequilibrium partial melting
(Nyquist et al. 2008) @1200°C; IW-1, 13%PM
(Feldstein et al., 2001)
Achondrite GRA 06128/9 and 2 1-hour
Silicate Inclusions in Iron Meteorites IAB pm) 21-days
0.7005 m (p 7-days
0.7000 GRA 0C6V1 μ2=80.24 μC=I,0 L.8, 1L-L0,. 8H4 S 1 ParCtial Cme-lti3ngBGaRaccAum uP9lalatigo n 3-days
86Sr/Sr)i 0.6995 GRA 06129 BrPaclahgina 00 CHUR 50GRA 8 100 150
87( Campo del Cielo Sr (ppm)
Figure 5. Model for Sm and Sr variation during petrogene-
0.6990 Caddo County
I(Sr)=0.698934 sis of GRA 8/9. Solid symbols are measured values for CC-
μ=(87Rb/86Sr)SOURCE (4.5C68A IGa) 3Ba [10] and GRA 8,9 (this investigation.)
0.6985 example (cf. [12, 15]).
3.8 4.0 4.2 4.4 4.6
Time before Present (Ga) References: [1] Treiman A. H. et al. (2008) LPS
Figure 4. Initial 87Sr/86Sr for GRA 8/9 and inclusions in the XXXIX, Abstract #2214. [2] Arai T. et al. (2008) ibid. Ab-
Caddo County and Campo del Cielo iron meteorites [10]. stract #2465. [3] Shearer C. K. et al. (2008) ibid. Abstract
#1825. [4] Zeigler R. A. et al. (2008) ibid. Abstract #2456.
Discussion: The “young” secondary Sm-Nd [5] Nyquist L. E. et al. (2008) Meteoritics & Planet. Sci., 43,
isochron age of ~3.4 Ga is similar to Ar-Ar degassing A119. [6] Jacobsen S. B. and Wasserburg G. J. (1984) EPSL,
ages found for some eucrites, and suggests impact- 67, 137-150. [7] Nyquist L. et al. (2006) GCA, 70, 5990-
resetting by the same “cataclysmic” meteoroid bo- 6015. [8] Nyquist L. E. et al. (2008) LPS XXXIX, Abstract #
mardment within the solar system ~3.4-4.1 Ga ago that 1437. [9] Takeda H. et al. (2000) GCA, 64, 1311-1327. [10]
affected the HEDPB [13]. Textural evidence that GRA Liu Y. Z. et al (2003) LPS XXXIV, Abstract #1983. [11]
8/9 was severely shocked was cited by several authors Nyquist L. E. et al. (1994) Meteoritcs, 29, 872-885. [12]
(e.g., [1], [2]); [2] suggested extensive post-shock an- Shearer C. K. et al. (2008) Am. Mineral., 93 1937-1940. [13]
nealing. Different (87Sr/86Sr)I values for GRA 8 and 9 Bogard D. D. (1995) Meteoritics, 30, 244-268. [14] Takeda
indicate they are products of different magmatic H. et al. (2002) Meteoritics & Planet. Science, 37, A139.
events. Both observations suggest a sizable parent [15] Feldstein S. N. et al. (2001) Meteoritics & Planet.
body. Science, 36, 1421-1441.