Download Report

46th Lunar and Planetary Science Conference (2015)
1356.pdf
BASALTIC EUCRITES SIMILAR TO TERRESTRIAL MORBs: A GEOCHEMICAL APPROACH. A.D.
Shukla ([email protected]), D. Ray ([email protected]) and, N. Bhanadri. Physical Research Laboratory, Ahmedabad 380 009, India.
(1.04 against 1.01). No Eu anomaly (Eu/Eu*~ 1.07) is
noted in either of the two.
1000
Chondrite Normalised
100
10
N-MORB
1
E-MORB
OIB
Juvinas
0.1
Piplia
0.01
Rb Ba Th U Nb Ta La Ce Pb Pr Nd Sr Zr Hf Sm Eu Gd Tb Dy Ho Y Er Yb Lu
Fig. 1. Chondrite normalized trace element spidergram
of Piplia Kalan and Juvinas eucrite. N- MORB, EMORB and Ocean Island Basalt (OIB) (after [9]) data
are also incorporated for comparison (chondrite data
after [10]).
In bivariant plot of Ta/La (incompatible element ratio)
against Ta (least compatible element amongst the pair,
[11]), there is a steep increase of Ta/La as Ta increases
(r2=0.87) (Fig. 2). Fractional crystallization and partial
melting produces different slopes in this process identification plot, e.g. slope is steeper for partial melting
vis-à-vis fractional crystallization.
0.09
R2 = 0.87
0.08
Ta/La
Introduction: Eucrites are the most differentiated
member of HED clan or, basaltic achondrite and describe a magmatic history of the early solar system [1].
In a recent study, visible and near- IR spectroscopy
data of Asteroid 4-Vesta show mineralogical similarities with those of HED meteorites and link the latter
with 4-Vesta crust [2].
Mid-Ocean Ridge Basalts (MORBs) are the most
voluminous magmatic province of Earth (~60% of
total magma flux). Based on trace element abundance
pattern, MORBs are classified into N- MORB (normal), E-MORB (enriched) and T-MORB (transitional)
types. Here we have made an attempt to assign the
status of basaltic eucrite in terms of MORBs referred
to as terrestrial analogue. Earlier workers [3,4] have
reported a close resemblence of eucrite geochemistry
with mare basalt of the Moon. Recent geochemical
overview has also indicated a close link between lowTi mare basalts and E-MORB in terms of trace elements chemistry [5].
As eucrite petrogenesis is enigmatic, our focus in
this study involves identification of igneous processes
through the behaviour of trace element abundances of
a basaltic (noncumulate) eucrite, Piplia Kalan (Fall,
India, 1996) and its comparison with that of MORBs
since eucrites are similar to terrestrial basalts in terms
of mineralogy and texture (rapidly cooled in the form
of lava or shallow intrusion).
Analytical Techniques: Trace element compositions of whole-rock sample were carried out using
Inductively Coupled Plasma Mass Spectrometer (ICPMS, Thermo XSeries II) at Physical Research Laboratory, Ahmedabad, India. BHVO 2 was used for calibration standard during ICP-MS. Accuracy and precision is better than ±5% for most of the elements except
for Pb, it is 6%.
Results: Piplia Kalan has been classified as ‘Genomict Breccia’ based on the nature of individual
clasts which are of similar chemistry but of different
petrographic textures [6,7]. Chondrite normalized bulk
trace element data (Fig. 1) suggests Juvinas [8] is the
closest neighbor of Pipila Kalan, except the latter is
enriched in Rb, Ba, Th, U (2.5 to 16 times) and marginally depleted in Zr anf Hf (6 to7 times).
Interestingly, Piplia Kalan trace element data fall close
to E-MORB composition and these are reflected in
several ratios like Ba/LuN (Pipilia Kalan:1.80, EMORB:1.74), La/SmN (1.12 against 1.51) and Gd/YbN
0.07
0.06
Piplia Kalan (lit)
Juvinas
Present data
0.05
0.04
120
140
160
180
200
220
240
Ta (ppb)
Fig. 2. Plot of Piplia Kalan in Ta/La versus Ta (process
identification plot, after [11])
We have tested several other trace element discrimination plots to confirm the E-MORB linkages. Piplia
Kalan falls on the MORB-OIB array, towards the
enriched mantle source in Th/Yb versus Nb/Yb plot
(after [12]) while in Nb/Sm versus La/Sm plot Piplia
Kalan is more akin to T-MORB (Fig. 3). La/SmN ratio
of Piplia Kalan (1.12) falls in the range of Mid-
46th Lunar and Planetary Science Conference (2015)
Atlantic Ridge T- MORB (0.98, [13]) or Equatorial
Western Pacific T-MORB (0.95, [14]). Low Zr/Nb
(4.68) and moderate Nb/Y (0.20) ratios of Piplia Kalan
are also similar to E-MORB (8.8, 0.38) .
10
MORB
T-MORB (MAR)
T-MORB (EPR)
N-MORB
E-MORB
OIB
Th/Yb
1
Pipilia Kalan
Juvinas
0.1
0.01
0.1
12
8
MORB
T-MORB (MAR)
T-MORB (EPR)
N-MORB
E-MORB
OIB
6
Pipilia Kalan
Juvinas
10
Nb/Sm
1
Nb/Yb
10
100
4
2
0
0
1
2
3
La/Sm
4
5
6
Fig. 3 Trace element bivariant plots (Th/Yb versus
Nb/Yb and Nb/Sm versus La/Sm) for global MORB
and non cumulate Eucrite (Piplia Kalan and Juvinas)
In Nb/U versus U/Th plot, Piplia Kalan falls within the
MORB-OIB array while Juvinas and Mid-Atlantic
Ridge T-MORB are more akin to OIB (Fig. 4).
Nb/U
100
MORB
T-MORB (MAR)
T-MORB (EPR)
N-MORB
E-MORB
OIB
10
PK
Juvinas
1
0.1
U/Th
1
Fig. 4. Plot of Global MORB and Piplia Kalan eucrite
in Nb/U versus U/Th binary diagram after [15].
1356.pdf
Discussion: Our present geochemical comparison
of Piplia Kalan eucrite with MORB suggests the following:
(i) Piplia Kalan magma was evolved through partial
melting-dominated process rather than fractional crystallization.
(ii) Based on trace element abundances, Piplia Kalan is
more akin to E-MORB, more specifically between TMORB and E-MORB.
(iii) Incompatible trace element ratios of Piplia Kalan
closely resemble with T-MORB of Mid-Atlantic Ridge
and Equatorial Pacific and fall well within the terrestrial MORB-OIB array.
(iv) High La/SmN (>1) and low Zr/Nb (<10) ratios
along with overall resemblance to E-MORB further
suggest that the source mantle composition of Piplia
Kalan was relatively fertile as compared to terrestrial
depleted N-MORB. Barring minor depletion of high
field strength elements and higher Nb/Yb ratio (~2.5)
trace element ratios of Piplia Kalan in general is consistent with a probable mixing of E-MORB and NMORB-like mantle source.
It is inferred from the limited trace element
data of Piplia Kalan that E-MORB could be considered
a potential terrestrial analogue for Main Group Eucrite.
MORB-like chemical composition may be comparable
with planery crust of other basaltic terrestrial planets
and therefore call for future investigation.
Acknowledgement: We are thankful to Dr. S. Ghosh
for helpful comments.
References: [1] Mittlefehldt D.W. et al. (1998) Rev. Mineral. 36, 4.1-4.195. [2] De Sanctis M.C. et al. (2012) Science
336, 697-700. [3] Duke M.B. and Silver, L.T. (1967) Geochim. Cosmochim. Acta 31, 1637-1665. [4] Ruzicka A. et
al. (2001) Geochim. Cosmochim. Acta 65, 979-997. [5] Ray
D. and Misra S. (2012) EPSC abstract 7, 2012-73-1. [6] Shukla A.D. et al. (1997) Met. Planet. Sci. 32, 611-615. [7] Buchanan P.C. et al. (2000) Met. Planet. Sci. 35, 609-615. [8]
Barrat J.A. et al. (2007) Geochim. Cosmochim. Acta 71,
4108-4124. [9] Sun S.-S. and McDonough W.F. (1989) Geol.
Soc. Spec. London, 42, 313-345. [10] McDonough W.F. and
Sun S.S. (1995) Chem. Geol. 120, 223-253. [11] White
W.M. (2008) Geochemistry In: Encyclopedia of Earth
Sciences, Macmillan, New York (online version). [12]
Pearce J.A. and Peate D.W. (1995) Annu. Rev. Earth Planet.
Sci. 23, 251-285. [13] White W.M. and Klein E.M. (2014)
Treatise on Geochemistry, 2nd ed, 457-496. [14] Park S.-H.
et al. (2006) Earth Planet. Sci. Lett. 248, 700-714. [15]
Hofmann A.W. (2003). Treatise on Geochemistry 2, 61-101.