Partial melting of a phlogopite-clinopyroxenite nodule from south-west Uganda: an experimental study bearing on the origin of highly potassic continental rift ...
Contributions to Mineralogyand Petrology
Contrib Mineral Petrol (1985) 91:321-329
9 Springer-Verlag1985
Partial melting of a phlogopite-clinopyroxenite nodule from south-west Uganda: an experimental study bearing on the origin of highly potassic continental rift volcanics F.E. Lloyd 1, M. Arima 2 and A.D. Edgar 2
1 Department of Geology, University of Reading, Reading, England 2 Department of Geology, University of Western Ontario, London, Canada Abstract. Melting experiments on a mantle-derived nodule
assemblage consisting of clinopyroxene, phlogopite and minor titanomagnetite, sphene and apatite have been done at 20 and 30 kbar between 1,175 and 1,300 ~ C. The nodule composition was selected on the basis of modal and chemical analyses of 84 mantle derived nodules with metasomatic textures from the Katwe-Kikorongo and Bunyaruguru volcanic fields of south-west Uganda. At 30 kbar, 1,225 and 1,250 ~ C, representing 20-30% partial melting, the compositions of glasses compare favourably to those of the average composition of 26 high potassic mafic lavas from the same region. Glasses produced by sufficiently low degrees of partial melting at 20 kbar could not be analysed. Glass compositions obtained for 20 30% melting at 30 kbar have high K20 (3.07-5.05 wt.%), low SiO2 (35.0-39.2 wt.%), high K/K + Na (0.54-0.71), K + Na/A1 (0.99-1.08) and Mg/ Mg + FeT of 0.59 0.62. These results support the suggestion of Lloyd and Bailey (1975) that the nodules represent the source material for the high K-rich lavas of south-west Uganda. If this conclusion is correct it implies that anomalous mantle source of phlogopite clinopyroxenite composition could produced the Ugandan lavas by relatively higher degrees of partial melting than that normally considered for highly alkaline m a f c magmas derived from a pyrolitic mantIe source. Higher degrees of melting are considered likely from such a different source region, rich in alkalis, water and radioactive elements. Steeper geotherms and increased fluxing of sub-rift mantle by degassing would also produce higher degrees of partial melting.
Introduction
Quaternary to Recent volcanism of south-west Uganda, associated with the uplifted and rifted east African craton, is highly potassic and volatile-rich. Along the west branch of the rift, potassium enrichment reaches its peak in the adjacent Katwe-Kikorongo and Bunyaruguru fields. A map of this area is given by Holmes (1950) who recognized a relationship between the strongly alkaline volcanism and the long continued uplift of the east African rift. Bell and Powell (1969) reviewed the many hypotheses to explain these extremely incompatible and LIL element enriched mafic to ultramafic lavas. They distinguished hypotheses involving various types of assimilation of crustal materials Offprint requests to: A.D. Edgar
from those of a mantle origin by partial melting. The problems associated with assimilation hypotheses have been discussed by Bell and Powell (1969). The high Mg, Cr and Ni contents of the lavas are consistent with a mantle origin by partial melting. Two broad categories of partial melting have been suggested involving either a 'normal' peridotitic mantle or a ' m o d i f e d ' mantle. Holmes (1932) proposed partial melting of a 'normal' peridotite source with separation of eclogite and olivine. O'Hara and Yoder (1967) also advocated a 'normal' mantle source with separation of eclogite and Harris (1957) suggested a mechanism of zone-refining. Partial melting of a K and LIL element enriched, 'modified' mantle peridotite has been proposed by Varne (1970), Dawson (1972) and Lloyd and Bailey (1975). The problem of derivation of the lavas from 'normal' mantle is that it requires a mechanism to concentrate K and associated incompatible and LIL elements: a mechanism that is capable of generating an enrichment factor (E.F.) >200 for K in the Ugandan lavas. High pressure experiments (Edgar et al. 1976; Ryabchikov and Green 1978; Barton and Hamilton 1979, 1982; Wendlandt and Eggler 1980a, b; Edgar et al. 1980; Arima and Edgar 1983a, b) support partial melting of a K-enriched 'modified' mantle source at depths around 75-100 kin. Explosive volcanism in south-west Uganda has brought up abundant nodules of alkali clinopyroxenites, rich in phlogopite. Some of these nodules have strain lamellae and granulation features which are absent in nodules with magmatic and cumulate textures and which Lloyd and Bailey (1975) consider to be of metamorphic origin based on the criteria of Den Tex (1971) rather than representing textures produced by interaction of the nodules with magma. The metamorphic textures are consistent with these rocks having a long residence time in the mantle. Many nodules show replacement or metasomatic textures (Holmes 1950; Lloyd and Bailey 1975; Lloyd 1981, 1985). Seismic and gravity studies indicate a region of'lithosphere thinning' or 'a rift cushion' of anomalous low density (Mohr 1981). Replacement of 'normal' mantle by material rich in phlogopite would provide 'a rift cushion' of appropriate density and give the observed uplift of the rift shoulders (Lloyd and Bailey 1975). Olivine is rare in the nodules of Katwe-Kikorongo and Bunyaruguru and neither orthopyroxene nor garnet have been recorded from Ugandan nodules. Ugandan volcanic diatremes have been styled 'kimberlitic' (Holmes 1965), while geochemical and experimental
322 evidence points to a mantle origin for the lavas and therefore the nodule suite might be expected to carry some fragments of original mantle. The composition of the relatively abundant nodules may place some constraint on the nature of the mantle below the rift. The unusual composition of the nodules suggest that a large proportion of the sub-rift mantle may consist of anomalous material. Geochemical studies suggest that partial melting of material represented by the nodule suite could give rise to the K-rich Ugandan lavas (Waters 1953; Bell and Powell 1969; Lloyd 1972; Lloyd and Bailey 1975). To assess if these lavas could be derived by partial melting of anomalous mantle, high pressure melting experiments were done on what we considered to be a representative mantle nodule.
Cpx Apllrn
....
Phi
39 37 35
12
8
N 6
--_
..
r~o
.ll
d I 13-
Composition of the starting material
Typically the Ugandan nodules comprise clinopyroxene (diopside-salite-augite-ferroaugite) and titaniferous phlogopite-biotite+ titanomagnetite, sphene, apatite, calcite, and rarely amphibole. A comprehensive collection, comprising 84 nodules from Katwe-Kikorongo and Bunyaruguru, shows marked inhomogeneity in mineral species and modal proportions (cf. Lloyd and Bailey 1975, Fig. 1 a, b and c) which can not be accounted for by cumulate processes as none of the nodules show textures suggestive of such processes (Lloyd and Bailey 1975; Lloyd 1981). The textures and variability in mineral modes are consistent with those of rocks which have undergone metasomatism of varying intensities. To take account of this variation, the composition of the starting material was based on the modal average of these 84 nodules (Table 1, anal. A), thus emphasizing the volumetric proportions of the nodule minerals rather than average whole-rock chemistry of the nodules. The composition of the average modal mineralogy (Table 1, anal. B 1) has been calculated using the analysis of clinopyroxenes, phlogopites and sphene from nodules in the Katwe-Kokorongo and Bunyaruguru areas given by Lloyd and Bailey (1975) and Lloyd (1981) and assuming stoichiometric compositions for the minor sphene and apatite. A comparison of this recalculated analysis (Table 1, anal. B ~) with that of the wet chemical analyses of a Katwe-Kikorongo nodule with comparable mineralogy (Table 1, anal. B) shows good agreement; the major discrepancies being in SiO2, TiOz, CaO and Na20. These may be readily accounted for by the trace amounts of perovskite, calcite, olivine and glass in the modal nodule mineralogy (Table 1, anal. A) not accounted for in the recalculated composition. In selecting the starting material, it is necessary to consider how completely the nodules may be representative of the mantle source. If the nodules represent partially melted regions, they may show some depletion, but this is difficult to recognise and assess. The 26 samples used to compile the average lava composition (Table 1, anal. C) are all katungite; the most voluminous primitive lava in the area (Holmes 1950). Lavas of extreme composition, e.g. mafurites, are rare and for this reason excluded. If the average nodule composition used in this study represents the source for the katungites, then the liquid composition on some degree of melting of this composition might be expected to approximate that of the katungite, assuming no major differentiation occurred on ascent (Bailey 1982). The large mineralogical variation in the nodules (Lloyd and Bai-
_
~0~
I
0
4
r [2._
2 0
I
I
r -o-i
20
I 40
Degree
No20
- o - -o . . . . I
I
I
60
I 80
I
I00
of M e l t i n g ( w t % )
Fig. l. Determined and calculated melt compositions at 30 kbar with various degrees of partial melting (Table 3). Crystallization sequence is shown at top of diagram. Crystallization intervals experimentally confirmed are shown by solid lines and dashed lines indicate inferred temperature intervals. Abbreviations as in Table 3. Solid symbols connected with solid lines represent microprobe analyses of glasses, and open symbols connected with dashed lines the calculated melt compositions. Vertical dashed line represents interpolated composition for 25% melting ley 1975, Fig. 1) preclude doing experiments on all of the compositions. As shown in Table 1, B, C and E; F appear to indicate that Na is unusually enhanced in the lavas. Enrichment of Na in the lava might be the result of differentiation during ascent but this cannot be assessed. In the nodules the Na content of clinopyroxene is low (Lloyd and Bailey 1975; Lloyd 1981) and it is stored almost entirely in amphibole - a relatively rare mineral in the nodules9 The rarity of amphibole-bearing nodules may indicate that most nodules originate at depths below that of amphibole stability, i.e. 70 km (Olafsson and Eggler 1983), possibly comparable to that of our 30 kbar experiments. If, as textural evidence indicates (Lloyd and Bailey 1975), the nodules have been subjected to mantle derived fluids containing alkalies (Bailey 1982), the studies of Ryabchikov et al. (1982) show that K in such fluids would be fixed in phlogopite at 30 kbar whereas Na would preferentially remain in a fluid phase. On partial melting at these pressures Na would occur in the liquid phase. On ascent from depths equivalent to 30 kbar, Na in the magma could be fixed in amphibole at lower pressures; remain in the liquid, crystallizing at late stages, as melilite or as K-rich nepheline in the ground-
323
Table 1. Average nodule and lava composition from Katwe-Kikorongo and Bunyaruguru, S.W. Uganda (Volume %)
A
Clinopyroxene Phlogopite Amphibole Titanomagnetite Apatite Sphene Perovskite Calcite Olivine Glass
52.5 37.0 0.5 3.5 1.0 4.0 trace trace trace trace
(wt.%)
B
B1
SiO2
FeO MnO MgO CaO Na20 K20 H20 + P205 COz
37.7 7.51 7.33 6.41 7.28 0.15 12.29 15.12 0.42 3.35 1.12 0.14 0.16
4 1 . 3 39.9 1.05 5.71 5.28 0.70 6 . 4 2 7 . 8 4 1.07 13.27 6.81 1.06 5.38 0.74 0.06 0 . 2 1 1.40 11.75 10.15 0.82 14.15 12.35 0.82 0.60 1.95 4.64 3 . 5 4 4.96 1.48 2.27 2.03 0.98 7.00 0.76 4.75
Total
98.98 96.80 98.84
TiO2 A1203
Fe203
C
E.F.
A
Average modal mineralogy of 84 nodules from Katwe-Kikorongo and Bunyaruguru. B Wet chemical analysis of a Katwe-Kikorongo nodule whose mineral mode is closest to the average mineralogy of KatweKikorongo and Bunyaruguru nodules. B1 Calculated composition of nodule based on mineralogy (A). C The average of analyses for 26 representative Katwe-Kikorongo and Bunyaruguru lavas. E.F. The enrichment factors for average lava C, compared with average nodule B. Modal analysis: F.E. Lloyd Wet chemical analyses: D. Bungard, F.E. Lloyd, S. Malik, L. Porteous.
Table 2. Mineralogy and chemistry of starting material (wt.%) Clinopyroxene Phlogopite Apatite Sphene Magnetite Jadeite Total
(wt.%) 52 35 2 4 6.5 0.5 100
Si20 TiO2 A1203 Fe203 FeO MnO MgO CaO Na20 KzO H20 + CO2 P205 F C1 Total
40.20 4.92 7.27 6.69 7.12 0.09 12.60 12.21 1.21 4.18 1.49 0.98 0.70 0.15 < 0.02 99.83
(1) Titanomagnetite proved difficult to separate, necessitating addition of synthetic magnetite instead. The presence of sphene insures that TiO2 is still in significant proportions in the starting chemistry (2) Jadeite (Johannes et al. 1971, page 28) mass; or might escape in the gas phase during eruption. The scarcity of amphibole in the U g a n d a n lavas suggests fixation in amphibole is unlikely. The occurrence of melilite at near solidus conditions and 10 kbar in experiments on katungite composition (Arima and Edgar 1983a) and the presence of a K-rich nepheline in the groundmass (Edgar and A r i m a 1981) suggests that N a is fixed in these late
stage minerals. The nodules present in the lavas are clearly altered by alkali-rich fluids. As these fluids enter the liquid on partial melting the nodules must be depleted in N a relative to their original pre melted condition at depth thus explaining the variation in the E.F. (Table 1). F o r this reason we consider the N a in the nodules does not represent that in the in situ U g a n d a n sub-rift mantle nodules and hence addition of N a to our model nodule composition more realistically represents the actual nodule prior to melting. The modal and chemical compositions of the starting material were obtained by mixing the following minerals in the proportions given in Table 2. Clinopyroxene and phlogopite analysed by Lloyd and Bailey (1975, Table 1 and 2) were separated from $23, 214, a nodule with metasomatic textures. Sphene was separated from a metasomatic sphene clinopyroxenite nodule, $23, 215 (Lloyd and Bailey 1975, Table 7) and for ease of separation apatite was obtained from a relatively rare apatite-rich nodule, $23, 211, which is considered to be a high pressure magmatic crystallate (Lloyd 1981).
Experimental methods High pressure experiments were done on a 1.27 cm piston-cylinder apparatus (Boyd and England 1960) with talc-pyrex glass as the pressure transmitting media and using the "hot piston out" technique. Pressure and temperature were calibrated at the kyanitesillimanite transition at 22 kbar and 1,300~ C (Richardson et al. 1968) and at the albite=jadeite + quartz reaction at 16.3 kbar and 600~ (Johannes et al. 1971). Both pressures and temperatures were within the accepted range of values. No frictional correction was applied to pressure and no pressure correction to the e.m.f. of the Pt-Pt9oRhlo thermocouples. Because the sample capsules act as semipermeable membranes for hydrogen, the graphite furnace assemblage buffers the fH2 in the experiments giving fo2 values in the capsules probably close to or slightly higher than those of an external NNO buffer (Brey and Green 1977). According to these authors, the fo2 and fR2 in the upper mantle are probably comparable to the NNO buffer. Prior to each run, the crushed crystalline starting material ( 1,250~ C and Ag5oPdso capsules for runs at < 1,225~ C. For each run a constant weight of starting material ( ~ 6 mg) was used to facilitate calculation of Fe loss by alloying with the sample capsules. No volatiles, other than the H20, CO2, F and C1 present in the starting material (Table 2), were added to the experiments. The importance of H20 and CO2 in the source materials for the K-rich magmas has been discussed by many authors (c.f. Wendlandt and Eggler 1980a, b). The possible roles of F and C1 are poorly understood. Insofar as is known, the H20/COz in the starting material in our experiments (CO2/CO2 + H 2 0 Mol. =0.21) is representative of mantle source regions and is close to the range of values used for experiments on katungite composition (Arima and Edgar 1983a). Glass and mineral phases in each run were analysed by an automated MAC-400 electron microprobe using an accelerating voltage of 15 kV. A 0.025 gA sample current was used for the analyses of the minerals and 0.010 gA for the glass analyses. For the latter, short times (5-10 s) and a defocussed beam (10 gm in diameter) were necessary. An average of 4-17 analyses of different glassy areas were made for each run. Areas close to quench crystals were avoided as these crystals affected the glass compositions up to 20 microns away from the analysed spots. Otherwise glass compositions appear to be homogeneous. Glass compositions given in Table 3 are average values with the range being within + 5% of stated values for all oxides excepting K20 and NazO which are within_+ 10%. The low totals of the glass analyses (Table 3) are due to the presence
324 Table 3. Glass composition obtained from melting experiments of average nodule compared to average lava Anal. No."
1 (4)
2
3(8)
4(7) b
5(9)
6(16)
Press (kbar)
30
-
30
30
30
3O
Temp. (~
1,225
-
1,250
1,275~1,250
1,263
1,275
Time (hrs)
5
-
4
1~ 3
5
4
Capsule
AgsoPd5o
-
Pt
Pt
Pt
Pt
Coexisting phases ~
cpx, phl, ilm, ap
-
cpx, phl, ilm
cpx, phl, ilm
cpx, phl, ilm
cpx, phl, ilm
Prop. of melt Calc (wt.%) Mode (vol.%)
20 23
25 -
31 31
31 -
44 -
49 -
SiO2 TiO2 A12Oa FeO d MnO MgO CaO Na20 K20 P205
39.2 3.54 5.76 11.7 0.08 10.6 14.0 1.75 3.07 0.91
37.1 5.55 6.78 11.9 0.18 10.1 12.4 1.56 4.06 1.41
35.0 7.55 7.80 12.0 0.27 9.58 10.9 1.36 5.05 1.91
35.0 7.70 7.88 11.8 0.18 9.89 10.5 1.50 5.69 1.79
37.00 6.30 8.43 10.4 0.21 11.4 10.3 1.40 5.71 1.49
37.7 6.33 8.51 10.3 0.11 11.4 9.88 1.36 5.73 1.45
Total
90.61
91.04
91.42
91.93
92.64
92.77
M g / M g + Fe d K/K + Na K + Na/A1
0.62 0.54 1.08
0.61 0.60 1.08
0.59 0.71 0.99
0.60 0.71 1.09
0.66 0.78 0.94
0.66 0.78 0.93
Anal. No. a
7(17)
8(17)
9(15)
10(9)
11 (12)
12
Press (kbar)
30
20
20
20
20
-
Temp. (~
1,300
1,200
1,225
1,250
1,300
-
Time (hrs)
5
8
8
5
5
Capsule
Pt
AgsoPdso
AgsoPdso
Pt
Pt
-
Coexisting phases c
cpx
cpx, ol, phl, mgt
cpx, ol, phl
cpx, ol
cpx, ol
-
Prop. of melt Calc (wt. %) Mode (vol.%)
67 -
29 27
46 -
76 -
96 -
SiO2 TiO2 A1203 FeO d MnO MgO CaO Na20 K20 P205
38.0 6.68 9.36 9.64 0.19 11.5 10.0 1.64 6.22 1.29
35.4 6.70 8.33 14.6 0.18 8.47 11.0 1.25 5.49 1.78
36.7 6.25 8.41 14.0 0.17 8.85 11.1 1.52 5.74 1.23
40.2 5.47 8.57 8.06 0.10 12.08 11.6 1.16 4.36 1.01
41.0 5.13 7.67 9.16 0.11 13.4 13.1 1.43 4.08 0.72
39.9 5.28 7.84 11.5 0.21 10.2 12.4 1.95 4.96 0.98
Total
94.52
93.20
93.97
92.61
95.80
95.22
Mg/Mg + Fe d K/K + N a K + Na/A1
0.68 0.71 1.00
0.51 0.74 0.96
0.53 0.71 1.03
0.73 0.71 0.77
0.73 0.65 0.78
0.61 0.63 1.09
Figures in brackets indicate number of analyses averaged b Reversal run (see text) ~ Abbreviations are cpx, clinopyroxene; phl, phlogopite; ilm, ilmenite; ap, apatite; ol, olivine; mgt, titanomagnetite d Total iron as FeO or Fe 2 + a
Anal. 2: Interpolated glass composition half way between analyses 1 and 3 Anal. 12: Average composition of Katwe-Kikorongo and Bunyaruguru lavas (see Table 1)
325 of unanalysed volatiles and to problems analysing alkalies caused by volatilization. Taking into account the volatiles (H20 + COz) estimated by the mass balance calculation (see Appendix), most glass analyses are in the range 97-98 wt.%.
Results The results of this study must be considered in terms of the limitations and constraints of experiments involved in determining compositions of melts. Among the more important of these are: 1. Determination of equilibrium. 2. Distinction between primary and quench (potential liquid) minerals of which phlogopite is the most important as it often occurs as both primary and quench crystals. 3. Determination of the proportion of melt in the charge and hence the degree of partial melting. 4. Loss of Fe to the Pt and AgsoPdso capsules in runs at high temperatures and for long durations. In the following sections, we attempt to assess the affects of constraints 2-4 either by experiment, observation, calculation or extrapolation. Experiments designed to demonstrate equilibrium conditions are discussed in a later section.
Distinction between primary and quench materials Although experiments were done under conditions in which no H 2 0 was added, the presence of phlogopite in the starting material produced sufficient H20 due to its breakdown to cause quench phlogopite to occur in the run products. Textural and chemical criteria to distinguish primary and quench phlogopites are well documented (eg. Yoder and Kushiro 1969; Edgar et al. 1976). Similar criteria were used in this study - quench phlogopites having accicular habit, higher TiO2 and lower Mg/Mg + FeT (FeT = total iron) than tabular and/or platy primary varieties. Glassy material often contained feathery aggregates and microcrystalline patches of quench material which increased in proportion in lower temperature experiments. In assessing the degree of partial melting, as indicated by the proportion of modal glass, all quench material was counted as glass.
Degree of partial melting In runs with low degrees of melting ( 4 0 % glass, the occurrence of crystal settling prohibited the observational method and only the calculation method was used. For both methods the degree of melting is believed to be within+ 20% of the values given.
Loss of iron to the sample capsules The problem of iron loss to the Pt and AgsoPds0 capsules is the most serious limitation of this study. The necessity of using long run times under conditions of low degrees
of partial melting increases the amount of Fe loss. The exact amounts lost are very difficult to assess, particularly for compositions such as our starting material with high Fe + 3/Fe + 2. Losses of Fe to the sample capsules were estimated by the least squares mass balance calculations and by microprobe analyses of the capsules for Fe after each run. The microprobe analyses were done along lines perpendicular to the capsule walls. The results showed that Pt capsules gained up to 3.6 wt.% Fe and AgsoPd5o capsules up to 1.1 wt.% Fe at the contact between the inner part of capsules and the sample charges. The penetration depth of Fe is 75__ 5 gm into the Pt capsules at 1,250 ~ C, 30 kbar and 110_+ 5 gm into the AgsoPdso capsules at 1,225 ~ C and 30 kbar. Based on these analyses, loss of FeO from the sample charge due to alloying with the capsules may be approximately 40 wt.% FeO to Pt capsules in runs at 1,250-1,300 ~ C for 4-5 h at 30 kbar and 20 wt.% FeO to AgsoPdso capsules at 1,225 ~ C in runs for 5 h at the same pressure. These values are in good agreement with those estimated by the mass balance calculations (see Appendix).
Experimental data Compositions of glasses and coexisting crystalline phases are given in Tables 3 and 4 respectively. At 30 kbar and 1,300 ~ C only clinopyroxene is present and is the probable liquidus phase. At 1,275 ~ C, phlogopite and ilmenite appear and are joined by apatite at 1,225 ~ C. The lowest temperature run at 1,175 ~ C and 30 kbar has the same phase assemblage as the 1,225 ~ C run but still contains minor amounts of glass that could not be analysed. Variation in the compositions of the glasses at 30 kbar with different degrees of melting are shown in Fig. 1 in which the value for complete melting (100%) is assumed to be the same as that of the average nodule starting material (Table 2). Changes in the glass compositions with decreasing degrees of melting are in agreement with the appearance of incoming minerals as temperature decreases. Figure 1 shows an increase in K20, TiO2 and AlzO3 and a decrease in CaO, MgO, and SiOz by increasing crystallization of clinopyroxene between 100 and 67% melting. Although increasing proportions of clinopyroxene might be expected to increase the FeOT in the glass, Fig. 1 shows an initial decrease with decreasing amounts of melting, probably due to the loss of iron to the sample capsules at high temperatures. With incoming of phlogopite along with clinopyroxene between 67-20% melting, there is an increase in CaO and FeO~ and a considerable decrease in KzO and A1203 in the glasses (Fig. 1). With increasing crystallization, the P205 content of glass shows a systematic increase between 100 and 30% melting (Fig. 1), whereas at 20% melting the glass has a lower P205 than that at 30% melting. This is attributed to crystallization of apatite which may occur between 20 and 30% melting based on its presence in 20% and absence in 30% melting experiments. Fig. 2 shows the calculated modal proportions of all phases in run products at various temperatures and 30 kbar. Details of this calculation are given in the Appendix. With decreasing temperature, the proportion of phlogopite increases from 12% at 1,275 ~ C to 35% at 1,225 ~ C whereas that of clinopyroxene varies in a relatively narrow range (30 to 40%) (Fig. 2). Phlogopite and ilmenite crystallize between 1,275 and 1,300 ~ C.
326 Table 4. Representative microprobe analysis of crystalline phases Press (kbar) Temp(~
30 1,225
30 t,250
30 1,263
30 1,275
Time (hrs)
5
4
5
4
Phases a
cpx
phi
ilm
cpx
phl
cpx
phi
cpx
phl
SiO2 TiO2 A1203 Cr203 FeO b MnO MgO CaO Na20 K20
50.94 1.68 2.64 0.08 6.90 0.07 13.08 23.27 t.73 0.04
37.72 6.54 14.11 0.03 11.69 0.05 15.72 0.23 0.22 10.02
43.00 0.00 0.27 48.61 0.29 6.00 -
50.84 t.32 3.52 0.07 6.23 0.06 14.10 22.21 2.18 0.09
38.30 6.20 14.80 0.00 7.98 0.00 17.90 0.00 0.03 10.01
51.39 t.10 4.32 0.25 5.37 0.01 15.75 21.47 0.67 0.05
38.96 5.86 15.16 0.00 7.30 0.00 18.85 0.00 0.00 10.10
51.25 1.45 3.34 0.25 5.36 0.It 16.14 20.73 0.71 0.04
39.54 5.88 15.36 0.00 6.80 0.00 19.91 0.00 0.00 10.09
Total
100.43
96.33
98.17
100.62
95.22
100.38
96.23
99.38
97.58
0.77
0.71
0.18
0.80
0.80
0.84
0.82
0.84
0.84
Mg/Mg + Feb Press (kbar) Temp (~
30 1,300
20 1,200
20 1,225
20 1,250
Time (hrs)
5
8
8
5
Phases a
cpx
cpx
ol
phl
cpx
phi
ol
cpx
ol
SiO2 TiO2 A1203 CrzO3 FeO b MnO MgO CaO Na20 KzO
50.09 1.00 4.33 0.36 5.15 0.06 16.34 21.69 0.62 0.03
49.40 t.62 4.16 0.00 6.91 0.07 14.38 21.94 0.50 0.09
39.83 0.03 0.00 0.00 19.89 0.00 41.01 0.32 0.00 0.00
36.81 8.63 13.95 0.37 10.01 0.t0 0.12 0.00 9.96
51.05 1.30 4.69 0.20 5.94 0.02 14.95 20.93 0.25 0.04
38.10 8.36 14.85 0.05 9.71 0.07 15.27 0.00 0.00 10.11
39.52 0.08 0.00 0.00 19.10 0.07 41.84 0.37 0.00 0.00
52.00 0.88 2.39 0.18 4.32 0.01 19.31 20.02 0.39 0.00
41.02 0.00 0.00 0.04 9.65 0.05 48.59 0.21 0.00 0.00
Total
99.67
99.07
101.08
96.06
99.37
96.52
100.98
99.50
99.56
0.85
0.79
0.78
0.74
0.82
0.74
0.80
0.89
0.90
Mg/Mg + Fe b
16.11
a Abbreviations are given in Table 3 b Total iron as FeO or Fe 2+
1300'
~. 1250
"
\l
1200-
2b
4'o Modal P r o p o r t i o n
do
80
(wt.~)
Fig. 2. Modal proportions of phases in experiments at 30 kbar. The modes were calculated by the least squares method using microprobe analyses of all phases (see Appendix)
Using the modal proportions of all phases (Fig. 2) and compositions of crystalline phases (Table 4), the melt compositions at 30 kbar with various degrees of melting were calculated. As shown in Fig. 1, the values obtained by mass balance calculations are in reasonable agreement with the glass analyses for all of the oxides, excepting MgO which shows considerable deviations in runs with 44 and 49% melting. This discrepancy is much less between 10 and 20% melting which is the range of interest of the present study. R u n s at 20 kbar are listed in Table 3, the compositions of the glass on various degrees of partial melting and the modal proportions of the phases shown in Figs. 3 and 4 respectively. Clinopyroxene and olivine crystallize at 1,300 and 1,250 ~ C. A t 1,225 ~ C phlogopite appears and is joined by titanomagnetite at 1,200 ~ C (Fig. 3, Table 3). A trace a m o u n t of olivine is present in the r u n at 1,200~ C but could not be identified in the 1,175 ~ C run. These results suggest the presence of a reaction relationship: phlogopite + c l i n o p y r o x e n e ~ olivine + melt, such as observed between 10-22 kbar in experiments on katungite (Arima and
327 30% is considered to be most likely we cannot draw any conclusions from the 20 k b a r experiments.
Cpx' OI Phi
--Mgt
Determination of equilibria and reversal experiments 40 38 36 12 10 8
~6 4 2 0 14
~
~O
12 10 8 6 4 2 [
I
20
I
I
I
I
I
I
40 60 80 Degree of melting (wt.~
100
Fig. 3. Determined and calculated melt compositions at 20 kbar. Crystallization sequence is shown at top of diagram. Ol-olivine, Mgt-magnetite. Other symbols and abbreviations as in Fig. 1
/ /
1300
Table 5. Composition of the katungite used in the reversal experiments and near liquidus mineral compositions
s
~
In order to determine whether these experiments app r o a c h e d equilibrium conditions, a reversal experiment at 30 k b a r was done in which the temperature was lowered isobarically from 1,275~ C to 1,250 ~ C. In reversal experiments o f long d u r a t i o n with Fe-bearing compositions, there is a p r o b l e m between maintaining the run time o f the nonreversed experiments and the effect such increased run times have on the increased loss of iron from the charge to the capsule. To minimise the loss of iron on both the composition o f the melt and that of the coexisting phases, the total time o f the reversal experiment was kept at 4 hours (1 h at 1,275 ~ C, 3 h at 1,250 ~ C). Analysis of glass in the reversal run is in excellent agreement with that o f the original run (Table 3). Additionally, the mineral assemblage was identical and compositions of mineral phases were very c o m p a r a b l e to that o f the original run. This reversal strongly suggests that our experiments represent equilibrium conditions. A second m e t h o d was used to show that equilibrium had been achieved in our experiments by determining the liquidus temperature and near liquidus phases at 30 k b a r for a lava whose composition (Table 5) was similar to that o f the average lava (Table 1) and to the calculated melt composition at 25% melting o f the nodule at 30 k b a r (Table 3, Fig. 1). If our results represent equilibrium, the liquidus temperature, near liquidus phases and their chemistry should be similar to those o f the runs on the nodule at 1,225 to 1,250 ~ C. Two wt.% H 2 0 was a d d e d to the lava to give a value a p p r o x i m a t i n g that o f the calculated glass composition at 25% melting. Results of the experiments on the lava are given in Table 5. A t 1,265 ~ C, 30 k b a r only quench material and glass are present whereas at 1,250~ C c l i n o p y r o x e n e + g l a s s are
1250-
w
~4/
1200-
2'o
4'o
6'0 Modal
Proportion
go
16o
[wt %1
Fig. 4. Modal proportions of phases at 20 kbar. The modes were calculated by the least squares methods using microprobe analyses of all phases (see Appendix)
E d g a r 1983 a). The glass compositions p r o d u c e d in all runs at 20 k b a r are highly potassic (Table 3, Fig. 3). Glass in runs at less than 30% melting was too small to analyze. The calculated modes of all phases at 20 k b a r are shown in Fig. 4. Based on experiments with 30% and higher degrees o f melting, the tendency, but not the absolute values, of the glasses at 20 k b a r (Fig. 3) are c o m p a r a b l e to those at 30 k b a r (Fig. 1). As partial melting at values less than
Press (kbar) Temp (~ Time (hrs)
-
30 1,235 1.0
Phase a
katungite
cpx
phl
cpx
SiO2 TiO2 A1203 FezO3 FeO MnO MgO CaO NazO KzO H2O+ P205 CO2
42.3 5.19 8.58 6.58 5.96 0.19 9.32 13.07 1.75 4.63 1.07 0.79 0.12
51.09 2.20 2.91 5.54 b 0.16 13.96 22.49 2.21 0.00 -
36.15 6.68 14.27 10.44b 0.07 16.07 0.13 0.13 9.92
50.49 0.94 4.87 5.44 b 0.05 14.73 22.49 1.10 0.00 -
Total
99.55
100.56
Abbreviations are given in Table 3 b Total iron as FeO
30 1,250 0.7
93.86
100.11
328 present and at 1,235~ clinopyroxene+phlogopite+ilmenite+ glass occur. These experiments indicate the liquidus temperature of the lava at 30kbar was between 1,250~1,265 ~ C; values 13-30~ higher than that of the extrapolated temperature for the 25% melting run of the nodule. Compositions of the near liquidus clinopyroxene and phlogopite in the lava (Table 5) are in excellent agreement with those obtained from the nodule under comparable conditions (Table 4). Considering the slight discrepancies in the compositions of the lava and the estimated composition of glass produced on 25% melting of the nodule, the agreement is very reasonable and provides support for the suggested equilibrium condition based on the reversal experiments.
Petrological implications If the Ugandan alkali clinopyroxenite nodules represent the source of the highly potassic Ugandan volcanism, then partial melting of such nodules under the conditions suggested by Lloyd and Bailey (1975, Fig. 5) may be expected to yield melts whose compositions are broadly comparable to the Ugandan lavas. As shown in Table 3, all of the glasses produced in this study have a highly potassic nature characterized by high K / K + N a ratios ( > 0.5). Interpolation of the glass composition at 25% melting (Fig. 1) is broadly comparable to the average Ugandan lava (Tables 1 and 3). In Table 3 and Fig. 1, no correction has been made to FeOT to account for iron loss to the capsules. However such loss may affect the proportions of phases and possibly the phase assemblages (cf. Stern and Wyllie 1975). Effects of iron loss could not be assessed in this study. If our estimate of 20% FeOx loss is realistic for the temperatures of these runs then the FeOT in the glass compositions at 20 and 30% partial melting will increase by about 1-2 wt.% FeOT, making the agreement not quite so favourable with respect to FeOx but very reasonable with respect to the other oxides. Extrapolation of the glass compositions at 30 kbar to less than 20% melting suggests that the trends shown in Fig. 1 will not change appreciably assuming no other phases appear between the solidus and 20% melting. This assumption is supported by a run at 1,175 ~ C which consists mainly of clinopyroxene and phlogopite with minor amounts of ilmenite, apatite, and glass. The glass analyses and the calculated melt compositions show similar trends with decreasing temperature (Fig. 1). The melt compositions at lower degrees of melting than 20% may have lower K 2 0 and A1203 and higher CaO and SiO2 contents than those of the average Ugandan lava (Fig. 1). As shown in Fig. 1, the highest KzO content and K / K + N a ratio of melt would be attained near a temperature where phlogopite completely melted out. The N a 2 0 content of the melt is almost constant for all degrees of partial melting (Fig. 1). Clinopyroxenes coexisting with melt at 1,225 and 1,250 ~ C at 30 kbar have higher N a 2 0 than those in higher temperature runs (Table 4) and those in the nodule itself (Lloyd and Bailey 1975; Lloyd 1981). These results suggest that a highly potassic melt, with K / K + N a > 1, could be formed by partial melting of a mantle source in which the N a 2 0 is higher than that of the average nodule composition used in this study. The possibility that Na is preferentially incorporated in a vapour phase rather than the clinopyroxene at subsolidus conditions (Bailey 1982) cannot be assessed from the
present study as our experiments were all done under suprasolidus conditions. The melt with a relatively higher potassic character (K/K + Na > 1) could be produced at 30 kbar by greater than 20% melting of the phlogopite-clinopyroxenite used in this study. Based on the model pyrolite mantle composition (Green 1973) and on the available heat sources for such a composition (Yoder 1976), highly alkali-rich lavas such as the average lava (Table 1, anal. C) are products of much lower degrees of melting than we propose. This is also suggested by the melting experiments on the peridotite-COz-H20 system (Olafsson and Eggler 1983). As phlogopite is an extremely minor constituent in the peridotitic mantle used by Olafsson and Eggler (1983), it is an early melting phase and melts out near the solidus (1,100 ~ C at 30 kbar under vapour absent conditions). The substantial amount of K20 will be attained only in the melts produced near the condition where phlogopite melts out. In the present study, using a more phlogopite enriched mantle source, the melting interval of phlogopite is extended relative to that proposed by Olafsson and Eggler (1983). Therefore it is possible to produce highly potassic magmas from a phlogopite enriched clinopyroxenite with higher degrees of partial melting than from a peridotitic source. Our experiments suggest that the lavas of south-west Uganda could be partial melts of an anomalous source of composition comparable to that of the average nodule used. Continental rift areas, such as south-west Uganda, have been described as sites of mantle degassing (Bailey 1980), leading to steeper geotherms and increased fluxing of subrift mantle (Bailey 1970). Additional heat for melting may come from the higher K and other radioactive elements which phlogopite enriched mantle may contain. Bailey (1972) proposed that mantle volatiles flow into arches and tensional regions of the lithosphere, locally metasomatising the mantle to a composition of alkali pyroxenite, causing uplift and rifting. The composition and metasomatic textures of nodules from continental rift volcanism support this hypothesis (Lloyd and Bailey 1975). The present experiments strongly suggest that melting of this anomalous, metasomatised mantle could result in highly potassic volcanism.
Appendix Using the least squares method (Stormer and Nicholls 1978) in which the "best fit" for mass balance equations is indicated by minimal sums of "squares of residual", mass balance calculations were done to determine the modal proportions of all phases for each run. The compositions of phases used in the calculation are given in Tables 3 and 4. Except for the run with 20% melting, ilmenite in the runs at higher degrees of melting could not be analyzed because of its fine grain size. Fine grained apatite was also difficult to analyse. In the calculation ilmenite analysis at 20% melting (Table 4) and apatite of composition [Cas (PO4)3OH] were used. Iron loss from the sample charge to the capsule was estimated by the calculations. As shown in Table 3 and Figs. 1 and 3, these calculated amounts of melts are believed to be reasonable estimates as they all have low sums of "squares of residuals" (< 3) and the results are in good agreement with the amounts determined optically. For example, in the run at t,225 ~ C and 30 kbar glass, clinopyroxene, phlogopite, ilmenite, and apatite occur. Excepting the very minor apatite, analyses of these phases (Tables 3 and 4) yielded by least squares methods 19.7 wt.% glass, with a sum of "squares of residual" of 1.6. In this run, the calculated iron loss is about 20 wt.% FeO of the sample charge. The proportion of glass given
329 in Table 3 is a normalized value after excluding the amount of iron loss. This value is in reasonable agreement with the 23 volume % of glass obtained by optical observation. Conversion of the volume % to wt. %, using the appropriate densities of the coexisting minerals and calculation of the density of the melt (Bottinga and Weill 1970; Mo et al., 1982) gave 19 wt.% glass; in excellent agreement with the calculated least squares value. In the calculation, the pressure effect on the melt density was not taken into account.
Acknowledgements. This research was supported by the Natural Science and Engineering Research Council of Canada. FEL and ADE are grateful to the British Council for travel grants to facilitate this inter university research program. We thank J. Forth, R.L. Barnett and R. Shirran for technical assistance.
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