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Trace element zoning in clinopyroxenes from spinel peridotite xenoliths
NUMB
Nuclear Instruments and Methods in Physics Research I375 (1993) 411-414 North-Holland
Trace element zoning in clinopyroxenes peridotite xenoliths
Beam Interactions with Materials 8 Atoms
from spine1
A. Greig a, S.H. Sie b and LA. Nicholls a a Department of Earth Sciences, Monash Unicersity, Clayton,
Victoria, 3168, Australia ’ Heavy Ion Analytical Facility, CSIRO Division of Exploration Geoscience, PO Box 136, North Ryde, NSW 2133, Australia
The excellent spatial resolution of the proton microprobe allows trace element zoning profiles to be determined within single grains of mantle minerals. Analyses of clinopyroxenes from two spine1 peridotite xenoliths from the young basaltic volcanic field of the Western Districts of Victoria show they have pronounced zoning in Ti, Zr and Sr. These zoning profiles demonstrate that regions of the lithospheric mantle have undergone recent metasomatism by infiltrating basaltic melts resulting in enrichment in Ti, Zr and Sr.
1. Introduction
Volcanics province which were erupted
Geochemical evidence suggests that, in terms of major elements, the Earth’s mantle is fairly homogeneous and broadly peridotitic in composition. The compositions of present day mantle derived volcanic rocks, however, indicate that it is heterogeneous in the distribution of incompatible trace elements (such as Ti, Zr and Sr) which are preferentially partitioned into a melt phase relative to the solid residue during partial melting. Some parts of the mantle are depleted in these eIements, while others are relatively undepleted or enriched. Chemical depletion of mantle sources can be readily explained by previous extraction of a melt fraction, but the evolution and location of enriched source regions are less clear. Fragments (xenoliths) of the lithospheric mantle are sometimes brought to the surface in mantle derived volcanic rocks and these can be used to study more directly the composition of the mantle and processes which cause chemical change. The proton microprobe has several advantages over conventional analytical techniques. Minerals in mantle peridotite xenoliths are typically OS-4 mm in diameter and as the proton microprobe can analyse a spot only 5 pm in diameter, it is possible to detect changes in trace element compositions from the core to the rim of a single grain, Details of zoning patterns can provide important information on 1) the identity of elements being introduced or removed from the grain; 2) the timing of the event; and 3) the process responsible for the zonation. We have investigated trace element zoning profiles in clinopyroxenes from mantle derived spine1 peridotite xenoliths found in alkalic basalts of the Newer 0168-583X/93/$06.00
in Western approximately
Victoria, Australia 20000 years ago.
2. Sample description A32 is a 20 cm diameter xenolith from the Anakies volcano. Clinopyroxenes are typically 1 mm in diameter. No H,O bearing minerals are present in this xenolith and CO, fluid inclusions are scarce. PS3 is a 15 cm diameter xenolith from the Mt. Porndon voicano which contains scarce ( < OS%), small (0.1 mm) grains of phlogopite, a potassic, H,O bearing mineral. Clinopyroxenes typically form 0.8 mm grains and some contain CO, fluid inclusions.
3. Analytical techniques Pyroxene separates from P53 were prepared as grain mounts, while pyroxenes from A32 were analysed in a thin section. Major and minor elements were analysed using an electron microprobe. Trace elements were determined by proton microprobe using a 3 MeV, 20 pm diameter beam. Each spot analysis was carried out for a 3 p,C integrated charge, which results in minimum detection limits at 99% confidence of 2-3 ppm. Count rates in the Si(Li) detector, set at a 135” angle with respect to the beam direction, were maintained below 4000 cps to minimize pile up effects. Measurements were carried out using a 200 pm Al filter to attenuate the intensities of major lines. To eliminate uncertainties in beam charge integration, the proton microprobe data are normalized to the Fe contents
0 1993 - Elsevier Science Publishers B.V. All rights reserved
VI. GEOLOGICAL SAMPLES
A. Greig et al. / True element zoning in clinopyroxenes
412
70 60
q
50 0.6-
-60
'
40
- 50 0.6-
160
.
30
- 40
20.4
16.0
170
17.0
160
17.6
150
0 0 0
17.4
140
170 100
0
200 Microns
Fig. 1 Proton
300
400
20.0
0
19.6
;
19.4
. 100
0
500
200
300
400
Micronsfrom
from rim
500
600
700
rim
microprobe analyses (in ppm) for Sr and Zr and electron microprobe analyses (in wt.%) for TiO, and CaO along core to rim traverses in clinopyroxenes from samples A32 and P53.
determined by the electron probe. The penetration depth of the proton beam used is - 60 km, although the effective depth of analysis is less due to self-absorption effects and the rapid drop in excitation efficiency with decreasing energy. For the X-ray lines of interest in this study, 95% of the total yield is attained from the first - 40 km. The thin section had a thickness of < 30 p,m, and thus minor corrections (< 5%) were required to account for contributions from the glass slide which contained some elements of interest. The epoxy used did not contribute to any of the analytical lines. Olivine grains in the section were used to monitor the thickness. The correction procedure is facilitated by the use of the program LAYER [I] and Geo_TRACE [2]. Tests of the CSIRO system and procedure against elemental and geological standards have established an accuracy of better than 5% for trace elements.
4. Results
l), concentrations
from sample A32 (fig. 1 and table of most elements are approximately
Table 1 Electron and proton
microprobe
In clinopyroxene
Sample Mineral
0
19.6
17.2
130
20.2
a
TiO, [wt.%] CaO [wt.%] Sr [ppml Y hvml Zr hvml a cpx = clinopyroxene;
analyses
constant from the centre of the grain to within about 250-300 Frn of the rim, whereupon CaO and Sr begin to decrease while TiO, and Zr increase. At around 200 p,m from the rim, the rate of change in the concentrations of TiO,, Zr and Sr decreases dramatically, giving the profiles distinct “S”- and “Z’‘-shapes. Orthopyroxene from A32 shows increases in TiO,, Zr and CaO from core to rim (Table 1). Clinopyroxene from sample P53 shows steadily decreasing CaO and increasing Sr contents from the core of the grain to its rim (fig. 1 and table 1). The profiles for TiO, and Zr are parallel, with almost constant concentrations from the core to about 200 km from the rim where they start to increase sharply, giving their profiles distinct “U”-shapes.
5. Interpretation There are two likely causes of the compositional zoning in these pyroxenes. They may have been produced by changes in pressure or temperature which affect intermineral equilibria and result in exchange and transfer reactions, or they may have been caused
of pyroxenes
A32 rim
A32 core
A32 rim
A32 core
P53 rim
P53 core
cpx
cpx
opx
opx
cpx
cpx
1.23 17.51 134*5 22+2 89+5
0.64 18.36 176f6 21+2 42+3
0.34 1.11 <3 <3 8+1
0.1 0.63 <3 <3 <3
0.48 19.43 215+5 14+1 62+3
0.30 20.52 124+4 11*1 23+_2
opx = orthopyroxene.
A. Greig et al. / True element zoning in clinopyroxenes
by metasomatism, whereby minerals react with an externally derived melt or fluid which has introduced elements. Griffin and co-workers [3] modelled the shape of zoning profiles that could be produced by two contrasted scenarios. The first involves simple diffusion of an element from the grain boundary (held at constant composition) into the grain, which produces a U-shaped profile (ref. [3], fig. 3a). The second scenario involves an initially homogeneous grain being overgrown by new material of different composition. Subsequent interdiffusion between the overgrowth and the core then results in distinctive Z-shaped or S-shaped profiles (ref. [3], fig. 3b) depending on whether the concentration of an element is higher or lower in the overgrowth compared to the core. 5.1. A32 The distinctive Z- and S-shaped trace element zoning profiles shown by clinopyroxene in sample A32 suggest they formed by crystallization of an overgrowth about 200 km wide of clinopyroxene which was rich in Ti and Zr and poor in Sr relative to the original grain. This was followed by diffusion of Ti and Zr from the overgrowth into the core and Sr from the core into the overgrowth as diffusive equilibration commenced. Orthopyroxene rims in this xenolith are also enriched in Ti and Zr relative to their cores (table 1). As the other mineral phases in this xenolith contain negligible amounts of Ti, Zr and Sr, the zoning is unlikely to be due to reactions between coexisting minerals. These elements are more likely to have been introduced externally by an infiltrating melt or fluid: i.e., metasomatism has taken place, and although the clinopyroxene overgrowth has a low Sr content relative to the core, the bulk xenolith will also have been enriched in Sr due to crystallization of new clinopyroxene. In contrast to the trace elements, the zoning in CaO shown by both pyroxenes is probably not solely due to metasomatism as changes in temperature will cause coexisting clinopyroxene and orthopyroxene to exchange Ca [4]. The CaO zoning in pyroxenes from this xenolith reflects re-equilibration in response to heating from a temperature of around 1000°C to llOO”C, based on two pyroxene thermometery [5]. The timing of the metasomatism can be constrained as initial zoning profiles will be largely eliminated in a time of r2/4D, where r is the radius of the grain and D is the diffusion coefficient (see ref. 161). Using the experimentally determined rate of diffusion of Sr in clinopyroxene of about lo-l4 cm2/s at 1100°C [7], this corresponds to a time of 2000 years. Therefore the preservation of zoning in Sr suggests that the metasomatism occurred only shortly (geologically speaking) before the xenolith was eruuted, but it is unlikely to have oc-
413
curred while the xenolith was entrained in the host magma as the time taken to transport xenoliths from the mantle to the surface is only on the order of 2-3 days. Candidates for metasomatic agents in the upper mantle are melts and H,O-CO, fluids. A melt is more likely in this case as experimental studies on basaltic magmas [S] show that they crystallize Ti-Al rich clinopyroxene in a pressure interval which encompasses the likely depth of origin of the xenolith (15-20 kbar). If the clinopyroxene overgrowths were crystallized from an infiltrating basaltic melt, experimentally determined clinopyroxene/ basalt partition coefficients [9] predict that the melt contained about 4% TiO,, 400 ppm Zr, 1450 ppm Sr and 20 ppm Y. These values are similar to those of primitive alkalic basalts from the Newer Volcanics province [lo]. These, or similar basalts are therefore capable of crystallizing clinopyroxcne overgrowths with the appropriate composition. The source of the melt was probably a basaltic intrusion in the mantle at some (unknown) distance from the volume of mantle from which the xenolith was derived, and this intrusion would also have been responsible for the heating undergone by the xenolith material. The metasomatic melt is closely related in time to, and was possibly a precursor of, the host magma that brought the xenolith to the surface. 5.2. P53 The U-shaped element zoning profiles shown by clinopyroxene in P53 are consistent with simple diffusion of Ti, Zr and Sr into the pre-existing grain from the grain boundary. These elements are also likely to have been introduced by an externally derived infiltrating melt or fluid. Although the phlogopite in this xenolith will also contain significant Ti, Zr and Sr, it is very rare and probably formed from the same melt or fluid. The presence of this hydrous mineral and CO, fluid inclusions in other minerals indicates some H ,OCO, fluid was involved in the metasomatism, but the proportion appears to have been minor. Experimental evidence also suggests that the solubilities of Ti and Zr in H,O-CO, fluids in the mantle will be very low relative to those in melts [ll]. A melt containing a small proportion of volatile components may therefore be a more probable agent for the metasomatism simply on the basis of relative efficiency. The crystallization of phlogopite and lack of new clinopyroxene growth in P53 suggests the melt had a more evolved composition than a typical basalt. At lOOO”C, the estimated equilibration temperature of the xenolith, the diffusion rate of Sr is fast enough (- 1015 cm2/sec [7]) to eliminate zoning in clinopyroxene in about 30000 years. Thus this metasomatic event also occurred only shortly prior to entrainment of the xenolith in the host magma. VI. GEOLOGICAL SAMPLES
414
A. Greig et al. / True element zoning in clinopyroxenes
6. Conclusions
Trace element zoning profiles in clinopyroxenes in peridotite xenoliths determined by proton microprobe demonstrate recent, localised metasomatism of the lithospheric upper mantle beneath Western Victoria by infiltrating basaltic melts. The clinopyroxene overgrowth seen in sample A32 probably crystallized from an infiltrating primitive alkalic basalt whereas the crystallization of phlogopite and lack of new clinopyroxene growth in P53 suggests the melt had a more evolved composition than a typical basalt. This metasomatism is a consequence of the extensive basaltic volcanism which has occurred in Western Victoria during the last 4.5 Myr.
References [l] D.R. Cousens, C.G. Ryan, S.H. Sie and W.L. Griffin, Proc. 5th Australian Conf. on Nuclear Techniques of Analysis, Lucas Heights, NSW, ISSN 0811-9422, Australian Inst. Nucl. Sci. Eng. (1987) 58.
PI C.G. Ryan, D.R. Cousens,
S.H. Sie, W.L. Griffin, G.F. Suter and E. Clayton, Nucl. Instr. and Meth. B47 (1990) 55. [31 W.L. Griffin et al., Geochim. Cosmochim. Acta 53 (1989) 561. [41 P. Bertrand and J.C.C. Mercier, Earth Planet. Sci. Lett. 79 (1986) 109. [51 A. Greig and I.A. Nicholls, in Abstracts IAVCEI General Assembly, Santa Fe, New Mexico, Miner. Resour. Bull. 131 (1989) 114. Contrib. Mineral. Petrol. 161 D. Smith and S.N. Ehrenberg, 86 (1984) 274. S.R. Hart and N. Shimuzu, Geochim. 171 M. Sneeringer, Cosmochim. Acta 48 (1984) 1589. b31 D.H. Green, Earth Planet. Sci. Lett. 17 (1973) 456. 191 T.H. Green, S.H. Sie and J. Adam, Proc. 7th Australian Conf. on Nuclear Techniques of Analysis University of Melbourne, ISSN 0811-9422, Australian Inst. Nucl. Sci. Eng. (1991) 135. M.T. McCulloch and S.S. Sun, [lOI W.F. McDonough, Geochim. Acta Cosmochim. 49 (1985) 2051. eds. M.A. Men[Ill D.H. Eggler, in: Mantle Metasomatism, zies and C.J. Hawkesworth (Academic Press, London, 1987) p. 21.
Nuclear Instruments and Methods in Physics Research I375 (1993) 411-414 North-Holland
Trace element zoning in clinopyroxenes peridotite xenoliths
Beam Interactions with Materials 8 Atoms
from spine1
A. Greig a, S.H. Sie b and LA. Nicholls a a Department of Earth Sciences, Monash Unicersity, Clayton,
Victoria, 3168, Australia ’ Heavy Ion Analytical Facility, CSIRO Division of Exploration Geoscience, PO Box 136, North Ryde, NSW 2133, Australia
The excellent spatial resolution of the proton microprobe allows trace element zoning profiles to be determined within single grains of mantle minerals. Analyses of clinopyroxenes from two spine1 peridotite xenoliths from the young basaltic volcanic field of the Western Districts of Victoria show they have pronounced zoning in Ti, Zr and Sr. These zoning profiles demonstrate that regions of the lithospheric mantle have undergone recent metasomatism by infiltrating basaltic melts resulting in enrichment in Ti, Zr and Sr.
1. Introduction
Volcanics province which were erupted
Geochemical evidence suggests that, in terms of major elements, the Earth’s mantle is fairly homogeneous and broadly peridotitic in composition. The compositions of present day mantle derived volcanic rocks, however, indicate that it is heterogeneous in the distribution of incompatible trace elements (such as Ti, Zr and Sr) which are preferentially partitioned into a melt phase relative to the solid residue during partial melting. Some parts of the mantle are depleted in these eIements, while others are relatively undepleted or enriched. Chemical depletion of mantle sources can be readily explained by previous extraction of a melt fraction, but the evolution and location of enriched source regions are less clear. Fragments (xenoliths) of the lithospheric mantle are sometimes brought to the surface in mantle derived volcanic rocks and these can be used to study more directly the composition of the mantle and processes which cause chemical change. The proton microprobe has several advantages over conventional analytical techniques. Minerals in mantle peridotite xenoliths are typically OS-4 mm in diameter and as the proton microprobe can analyse a spot only 5 pm in diameter, it is possible to detect changes in trace element compositions from the core to the rim of a single grain, Details of zoning patterns can provide important information on 1) the identity of elements being introduced or removed from the grain; 2) the timing of the event; and 3) the process responsible for the zonation. We have investigated trace element zoning profiles in clinopyroxenes from mantle derived spine1 peridotite xenoliths found in alkalic basalts of the Newer 0168-583X/93/$06.00
in Western approximately
Victoria, Australia 20000 years ago.
2. Sample description A32 is a 20 cm diameter xenolith from the Anakies volcano. Clinopyroxenes are typically 1 mm in diameter. No H,O bearing minerals are present in this xenolith and CO, fluid inclusions are scarce. PS3 is a 15 cm diameter xenolith from the Mt. Porndon voicano which contains scarce ( < OS%), small (0.1 mm) grains of phlogopite, a potassic, H,O bearing mineral. Clinopyroxenes typically form 0.8 mm grains and some contain CO, fluid inclusions.
3. Analytical techniques Pyroxene separates from P53 were prepared as grain mounts, while pyroxenes from A32 were analysed in a thin section. Major and minor elements were analysed using an electron microprobe. Trace elements were determined by proton microprobe using a 3 MeV, 20 pm diameter beam. Each spot analysis was carried out for a 3 p,C integrated charge, which results in minimum detection limits at 99% confidence of 2-3 ppm. Count rates in the Si(Li) detector, set at a 135” angle with respect to the beam direction, were maintained below 4000 cps to minimize pile up effects. Measurements were carried out using a 200 pm Al filter to attenuate the intensities of major lines. To eliminate uncertainties in beam charge integration, the proton microprobe data are normalized to the Fe contents
0 1993 - Elsevier Science Publishers B.V. All rights reserved
VI. GEOLOGICAL SAMPLES
A. Greig et al. / True element zoning in clinopyroxenes
412
70 60
q
50 0.6-
-60
'
40
- 50 0.6-
160
.
30
- 40
20.4
16.0
170
17.0
160
17.6
150
0 0 0
17.4
140
170 100
0
200 Microns
Fig. 1 Proton
300
400
20.0
0
19.6
;
19.4
. 100
0
500
200
300
400
Micronsfrom
from rim
500
600
700
rim
microprobe analyses (in ppm) for Sr and Zr and electron microprobe analyses (in wt.%) for TiO, and CaO along core to rim traverses in clinopyroxenes from samples A32 and P53.
determined by the electron probe. The penetration depth of the proton beam used is - 60 km, although the effective depth of analysis is less due to self-absorption effects and the rapid drop in excitation efficiency with decreasing energy. For the X-ray lines of interest in this study, 95% of the total yield is attained from the first - 40 km. The thin section had a thickness of < 30 p,m, and thus minor corrections (< 5%) were required to account for contributions from the glass slide which contained some elements of interest. The epoxy used did not contribute to any of the analytical lines. Olivine grains in the section were used to monitor the thickness. The correction procedure is facilitated by the use of the program LAYER [I] and Geo_TRACE [2]. Tests of the CSIRO system and procedure against elemental and geological standards have established an accuracy of better than 5% for trace elements.
4. Results
l), concentrations
from sample A32 (fig. 1 and table of most elements are approximately
Table 1 Electron and proton
microprobe
In clinopyroxene
Sample Mineral
0
19.6
17.2
130
20.2
a
TiO, [wt.%] CaO [wt.%] Sr [ppml Y hvml Zr hvml a cpx = clinopyroxene;
analyses
constant from the centre of the grain to within about 250-300 Frn of the rim, whereupon CaO and Sr begin to decrease while TiO, and Zr increase. At around 200 p,m from the rim, the rate of change in the concentrations of TiO,, Zr and Sr decreases dramatically, giving the profiles distinct “S”- and “Z’‘-shapes. Orthopyroxene from A32 shows increases in TiO,, Zr and CaO from core to rim (Table 1). Clinopyroxene from sample P53 shows steadily decreasing CaO and increasing Sr contents from the core of the grain to its rim (fig. 1 and table 1). The profiles for TiO, and Zr are parallel, with almost constant concentrations from the core to about 200 km from the rim where they start to increase sharply, giving their profiles distinct “U”-shapes.
5. Interpretation There are two likely causes of the compositional zoning in these pyroxenes. They may have been produced by changes in pressure or temperature which affect intermineral equilibria and result in exchange and transfer reactions, or they may have been caused
of pyroxenes
A32 rim
A32 core
A32 rim
A32 core
P53 rim
P53 core
cpx
cpx
opx
opx
cpx
cpx
1.23 17.51 134*5 22+2 89+5
0.64 18.36 176f6 21+2 42+3
0.34 1.11 <3 <3 8+1
0.1 0.63 <3 <3 <3
0.48 19.43 215+5 14+1 62+3
0.30 20.52 124+4 11*1 23+_2
opx = orthopyroxene.
A. Greig et al. / True element zoning in clinopyroxenes
by metasomatism, whereby minerals react with an externally derived melt or fluid which has introduced elements. Griffin and co-workers [3] modelled the shape of zoning profiles that could be produced by two contrasted scenarios. The first involves simple diffusion of an element from the grain boundary (held at constant composition) into the grain, which produces a U-shaped profile (ref. [3], fig. 3a). The second scenario involves an initially homogeneous grain being overgrown by new material of different composition. Subsequent interdiffusion between the overgrowth and the core then results in distinctive Z-shaped or S-shaped profiles (ref. [3], fig. 3b) depending on whether the concentration of an element is higher or lower in the overgrowth compared to the core. 5.1. A32 The distinctive Z- and S-shaped trace element zoning profiles shown by clinopyroxene in sample A32 suggest they formed by crystallization of an overgrowth about 200 km wide of clinopyroxene which was rich in Ti and Zr and poor in Sr relative to the original grain. This was followed by diffusion of Ti and Zr from the overgrowth into the core and Sr from the core into the overgrowth as diffusive equilibration commenced. Orthopyroxene rims in this xenolith are also enriched in Ti and Zr relative to their cores (table 1). As the other mineral phases in this xenolith contain negligible amounts of Ti, Zr and Sr, the zoning is unlikely to be due to reactions between coexisting minerals. These elements are more likely to have been introduced externally by an infiltrating melt or fluid: i.e., metasomatism has taken place, and although the clinopyroxene overgrowth has a low Sr content relative to the core, the bulk xenolith will also have been enriched in Sr due to crystallization of new clinopyroxene. In contrast to the trace elements, the zoning in CaO shown by both pyroxenes is probably not solely due to metasomatism as changes in temperature will cause coexisting clinopyroxene and orthopyroxene to exchange Ca [4]. The CaO zoning in pyroxenes from this xenolith reflects re-equilibration in response to heating from a temperature of around 1000°C to llOO”C, based on two pyroxene thermometery [5]. The timing of the metasomatism can be constrained as initial zoning profiles will be largely eliminated in a time of r2/4D, where r is the radius of the grain and D is the diffusion coefficient (see ref. 161). Using the experimentally determined rate of diffusion of Sr in clinopyroxene of about lo-l4 cm2/s at 1100°C [7], this corresponds to a time of 2000 years. Therefore the preservation of zoning in Sr suggests that the metasomatism occurred only shortly (geologically speaking) before the xenolith was eruuted, but it is unlikely to have oc-
413
curred while the xenolith was entrained in the host magma as the time taken to transport xenoliths from the mantle to the surface is only on the order of 2-3 days. Candidates for metasomatic agents in the upper mantle are melts and H,O-CO, fluids. A melt is more likely in this case as experimental studies on basaltic magmas [S] show that they crystallize Ti-Al rich clinopyroxene in a pressure interval which encompasses the likely depth of origin of the xenolith (15-20 kbar). If the clinopyroxene overgrowths were crystallized from an infiltrating basaltic melt, experimentally determined clinopyroxene/ basalt partition coefficients [9] predict that the melt contained about 4% TiO,, 400 ppm Zr, 1450 ppm Sr and 20 ppm Y. These values are similar to those of primitive alkalic basalts from the Newer Volcanics province [lo]. These, or similar basalts are therefore capable of crystallizing clinopyroxcne overgrowths with the appropriate composition. The source of the melt was probably a basaltic intrusion in the mantle at some (unknown) distance from the volume of mantle from which the xenolith was derived, and this intrusion would also have been responsible for the heating undergone by the xenolith material. The metasomatic melt is closely related in time to, and was possibly a precursor of, the host magma that brought the xenolith to the surface. 5.2. P53 The U-shaped element zoning profiles shown by clinopyroxene in P53 are consistent with simple diffusion of Ti, Zr and Sr into the pre-existing grain from the grain boundary. These elements are also likely to have been introduced by an externally derived infiltrating melt or fluid. Although the phlogopite in this xenolith will also contain significant Ti, Zr and Sr, it is very rare and probably formed from the same melt or fluid. The presence of this hydrous mineral and CO, fluid inclusions in other minerals indicates some H ,OCO, fluid was involved in the metasomatism, but the proportion appears to have been minor. Experimental evidence also suggests that the solubilities of Ti and Zr in H,O-CO, fluids in the mantle will be very low relative to those in melts [ll]. A melt containing a small proportion of volatile components may therefore be a more probable agent for the metasomatism simply on the basis of relative efficiency. The crystallization of phlogopite and lack of new clinopyroxene growth in P53 suggests the melt had a more evolved composition than a typical basalt. At lOOO”C, the estimated equilibration temperature of the xenolith, the diffusion rate of Sr is fast enough (- 1015 cm2/sec [7]) to eliminate zoning in clinopyroxene in about 30000 years. Thus this metasomatic event also occurred only shortly prior to entrainment of the xenolith in the host magma. VI. GEOLOGICAL SAMPLES
414
A. Greig et al. / True element zoning in clinopyroxenes
6. Conclusions
Trace element zoning profiles in clinopyroxenes in peridotite xenoliths determined by proton microprobe demonstrate recent, localised metasomatism of the lithospheric upper mantle beneath Western Victoria by infiltrating basaltic melts. The clinopyroxene overgrowth seen in sample A32 probably crystallized from an infiltrating primitive alkalic basalt whereas the crystallization of phlogopite and lack of new clinopyroxene growth in P53 suggests the melt had a more evolved composition than a typical basalt. This metasomatism is a consequence of the extensive basaltic volcanism which has occurred in Western Victoria during the last 4.5 Myr.
References [l] D.R. Cousens, C.G. Ryan, S.H. Sie and W.L. Griffin, Proc. 5th Australian Conf. on Nuclear Techniques of Analysis, Lucas Heights, NSW, ISSN 0811-9422, Australian Inst. Nucl. Sci. Eng. (1987) 58.
PI C.G. Ryan, D.R. Cousens,
S.H. Sie, W.L. Griffin, G.F. Suter and E. Clayton, Nucl. Instr. and Meth. B47 (1990) 55. [31 W.L. Griffin et al., Geochim. Cosmochim. Acta 53 (1989) 561. [41 P. Bertrand and J.C.C. Mercier, Earth Planet. Sci. Lett. 79 (1986) 109. [51 A. Greig and I.A. Nicholls, in Abstracts IAVCEI General Assembly, Santa Fe, New Mexico, Miner. Resour. Bull. 131 (1989) 114. Contrib. Mineral. Petrol. 161 D. Smith and S.N. Ehrenberg, 86 (1984) 274. S.R. Hart and N. Shimuzu, Geochim. 171 M. Sneeringer, Cosmochim. Acta 48 (1984) 1589. b31 D.H. Green, Earth Planet. Sci. Lett. 17 (1973) 456. 191 T.H. Green, S.H. Sie and J. Adam, Proc. 7th Australian Conf. on Nuclear Techniques of Analysis University of Melbourne, ISSN 0811-9422, Australian Inst. Nucl. Sci. Eng. (1991) 135. M.T. McCulloch and S.S. Sun, [lOI W.F. McDonough, Geochim. Acta Cosmochim. 49 (1985) 2051. eds. M.A. Men[Ill D.H. Eggler, in: Mantle Metasomatism, zies and C.J. Hawkesworth (Academic Press, London, 1987) p. 21.