26Al/27Al record in UOC chondrules: Possible effect of parent body thermal metamorphism
N.G. Rudraswami
National Institute of Oceanography , Dona Paula -403 004 Goa , India.
Introduction

2000-610x5=7)
6 Al/2 Al 02(0.1402
0.1393

Al-Mg isotope data of UOC chondrules from 3.0
to 3.3 analyzed by us [LEW86134 (L3.0),
Semarkona(LL3.0),
QUE97008(L3.05),
LEW86018(L3.1),Y791324 (LL3.1), Bishunpur(LL3.1), Adrar-003 (L/LL3.1), ALHA77176(L3.2), ALHA764 (LL3.3)].

The main objective of this work are:
050

100

1502002503005x10-52
310-6

Absence of resolved 26Mg excess in one chondrules from LEW86134, belonging tolow petrologic grade L3.0.

Rudraswami and Goswami,
2007 and Present work

Disturbed Al-Mg systematics

45

ALHA76004 (LL3.3)

ALHA77176 (L3.2)

Y791324 (LL3.1)

Adrar003 (L/LL3.1)

Bishunpur (LL3.1)

LEW86018 (L3.1)

QUE97008 (L3.05)

LEW86134 (L3.0)

semarkona (LL3.0)

Chainpur (LL3.4)

Inman (LL3.3)

10-7

The initial 26Al/27Al values in UOC (3.0-3.3) chondrules vary from :~2x10-5 to ≤10-6.
Onset of the chondrule formation: ~1Ma after CAI.
[Most of the chondrules have 26Al/27Al ratio between (5-12) x 10-6]

Data from petrologic grade 3.0 to 3.4

Number of cases

8

Literature data
Our data

The absence of radiogenic 26Mg in a chondrule from LEW86134 (3.0) and also in Krymka (L3.1)[Huss et al., 2001] is not expected because of the low temperature thermal history of the parent bodies
26Al in UOC chondrule: Role of parent body thermal processesof Absence
these UOCs.ofWe
model thermal evolution by solving the heat conduction equation numerically using
finite difference method and using standard values for the various parameters such as density, decay
constant, initial temperature, thermal conductivity, thermal diffusivity and heat capacity [Yomogida andMatsui, 1983] to further investigate this issue.

1.2x10-5-65x10

Laboratory studies suggest that Mg isotope records in plagioclase over scale length of tens ofmicron, typical size of the analyzed phases in UOCs, can be thermally reset at a temperature of ~500oC
within a time period of about 1Ma [LaTourette and Wasserburg, 1998].

6Calculations were carried out for meteorite parent bodies ≥20km in size and of chondritic
composition, with initial 26Al/27Al of 1.2510-5 (formation time 1.44 Ma after CAI) and of 6.2510-6(formation time 2.16 Ma after CAI), respectively, and considering instantaneous accretion.

4

Our calculations show that the time temperature regime (>1Ma, 500oC) will be experienced by anobject residing at ≥2km below the surface in such bodies. Note that the temperature for resetting AlMg isotope records in glassy phases (mesostasis) will be lower than plagioclase and hence theirisotopic records may be reset even at shallower depths.

20-101

Initial2627

Al/ Al x102-536820km radius
Only 26Al takenTonset =2.16Ma

10

1000

8km

10km

6km

3km

4km

2km

6001k m0246810Time (Ma)An intense chondrule formation epoch ~1.5 Ma after CAI formation and lasted ≤1Ma.

104

Initial 26Al/27Al=6.25x10-66Quinyambie (LL3.4)

A majority of them (~ 85%) lie in the range of 510-6 to 1.210-5.

Literature data(Russell et al.,1996;
Kita et al.,2000;
Huss et al.,2001;
Mostefaoui et al.,2002;
McKeegan et al.,2000

Krymka (LL3.1)

The range of initial 26Al/27Al ratios in the analyzed chondrules varies from1.9510-5 to 5x10-6 with a sole exception that has an initial value of <10-6.

20

10-5

semarkona (LL3.0)

Results

350

1

Identified areas, typically tens of microns across, were analyzed to determine theirmineralogical composition using electron microprobe (EPMA; Cameca SX-100).
Such phases, were analyzed for Al-Mg isotope systematics using a Cameca ims-4f
secondary ion mass spectrometer (SIMS) [Goswami et al., 1994].

3km

1000

Data on Initial 26Al/27Al at the time of Chondrule formation

Bishunpur (LL3.1)

Phases with high Al/Mg ratio present within the chondrules were identified frombackscattered electron images of the sections.

5kmTime (Ma)

Initial 26Al/27Al Ratio

Thin sections mapped using a scanning electron microscope equipped with anenergy dispersive X-ray analyzer.

1500

1400 0

Undisturbed Al-Mg systematicsExperimental approach

7km

500

27Al/24Mg

Samples selection
Thermal metamorphism is an important parameter in our choice of samples. Nine
Unequilibrated Ordinary Chondrites (UOCs) were selected for the present work areamong the least metamorphosed chondritic meteorites; we have excluded petrologic
grade 3.4 in our study as earlier data suggest possible evidence of thermal
metamorphism in chondrules from these UOCs [Huss et al., 2001].

9km

1km

0.1375Investigate the possibility of parent body thermal processes that may haveaffected the Al-Mg isotopic systematics in UOC chondrules of low
metamorphic grades.

Initial 26Al/27Al=1.25x10-50.14110.1384To infer the exact duration of chondrules formation in the solar nebula fromstudies of chondrule of low metamorphic grade (≤3.3)

20km radius
Only 26Al taken
Tonset =1.44Ma

20km

Temperature (K)These UOCs are expected to experience temperatures of ~250 C (3.0) too~450 C (3.4) in their parent bodies. Such temperatures are not sufficient to disturb
Al-Mg isotope systematics in Al-rich phases (plagioclase) present in UOC chondruleseven if they sustain for about 10 Ma [LaTourette and Wasserburg, 1998]. Thus, the2627
above variation in initial Al/ Al would suggest a very long duration of close to three
million years for chondrule formation.

2500

Time in million years

o-5
0
1x
1.2=7 Al) 02/26 Al(0.1420

26Mg/24Mg

Chondrules from Unequilibrated Ordinary Chondrites (UOCs) having petrologic
26
27-6-5
grades 3.0 to 3.4 display a range of initial Al/ Al varying from <10 to ~ 2 10[Russell et al., 1996, Kita et al., 2000, Huss et al., 2001, Mostefaoui et al., 2002,
Rudraswami and Goswami., 2007]

0.1429Temperature (K)Refractory Ca-Al-rich Inclusions (CAIs) and Chondrules present in meteorites are
considered as products of high temperature processes taking place in the solar
nebula, the gas and dust cloud surrounding the early sun.

Further, if we include contribution from the other heat source, 60Fe as well (initial 60Fe/56Fe ~10-6),
and also consider an extended regolith surface (with poor thermal conductivity), resetting of Al-Mg
isotope records in the above cases can take place for chondrules residing even closer to the surface,nominally between 1 and 2 km.

We, therefore, propose that the occasional chondrules with low initial 26Al/27Al ratio (having< 5 10-6) present in UOCs of petrologic grades 3.0 to 3.2, and whose number increases
significantly in UOCs of petrologic grades >3.3, experienced such a thermal episode as anindependent entity, perhaps due to impact related dynamic mixing on the parent body, prior
to their final incorporation into the host meteorite.

Conclusion: Parent body thermal processes can explain presence of chondruleswith low initial 26Al/27Al in UOC of various petrologic grades.

Reference
Kita N.T. et al. (2000) Geochim. Cosmochim. Acta, 64, 3912-3922.
Mostefaoui S. et al. (2002) Meteorit. Planet. Sci., 37, 421-438.
Huss G.R. et al. (2001) Meteorit. Planet. Sci., 36, 975-997.
Russell S. et al. (1996) Science, 273, 757-762.
Latourette T. & Wasserburg G. J. (1998) Earth Planet. Sci. Lett., 158, 91-108.
Rudraswami.N.G. and Goswami.J.N. (2007) Earth Planet. Sci. Lett., 257, 231-244.
Goswami J.N. et al. (1994) Proc. of Indian acad. Sci, 103, 57-82.
Yomogida K. and Matsui T. (1983) J. Geophys. Res. 88, 9513-9533.

FOR FURTHER INFORMATION PLEASE CONTACTRudraswami N G
Geological Oceanography Division
National Institute of Oceanography
Dona Paula- 403 004, Goa,India
E-mail: rudra@nio.org
Phone No : +91-832-2630358