some implications of xenolith glasses for the mantle sources of alkaline mafic magmas

Contrib Mineral Petrol (t991) 108:175-180 Cont r ibu t ions to Mineralogy and Petrology �9 Springer-Verlag 1991

Some implications of xenolith glasses for the mantle sources of alkaline mafic magmas Don Francis

Geological Sciences, McGill University, Montreal, Qu6bec H3A 2A7, Canada

Received June 21, 1990 / Accepted January 14, 1991

Abstract. The assumption that mafic alkaline magmas are derived from mantle sources with a lherzolite mineralogy has become entrenched in the petrologic literature. Although it is commonly assumed that highly alkaline magmas require metasomatised mantle sources, there is little understanding of the spatial relation of such sources with respect to those of associated more Si-rich transitional magmas. Glasses developed in mantle xenoliths represent natural experiments which may provide some insight on this problem. Highly silica undersaturated glasses developed in the amphibole-garnet clinopyroxenite portion of a composite xenolith from Nunivak Island, Alaska, become quartz normative where they penetrate adjacent spinel lherzolite. A comparison of glass compositions in mantle pyroxenite and lherzolite xenoliths reveals that glasses developed in amphibole pyroxenite xenoliths are in general more silica undersaturated than those in lherzolite xenoliths. This suggests that some highly silica undersaturated magmas such as nephelinites may in fact be derived by the preferential melting of amphibole or amphibole-garnet pyroxenite veins and that the spectrum from nephelinite to transitional alkaline basalt that characterizes many individual alkaline volcanic suites is produced by mixing with melt derived from the host lherzolite as the degree of partial melting increases.

Introduction

The compositional spectra of many individual mafic alkaline volcanic suites approximate mixing arrays along which there is a strong correlation between degree of silica undersaturation, enrichment in incompatible trace elements, and in some suites, isotopic depletion (i.e. low STSr/S6Sr and high 143Nd/144Nd) (Chen and Frey 1985; Francis and Ludden 1990; Lum et al. 1989; Pier et al. 1989; Price et al. 1989). These properties have lead to a proliferation of petrogenetic models involving mixing between "lithospheric" and "asthenospheric" mantle

sources to explain the compositional spectra of alkaline mafic lavas (Chen and Frey 1985; Fitton etal. 1988; Leat et al. 1988 ; Lure et al. 1989; Menzies 1989). In most models, the mantle sources are assumed to have a lherzolite to harzburgite mineralogy and the silica-undersaturated magmatic end-member typically is interpreted to represent either extremely small degrees of partial melting or to require a precursor mantle enrichment event. Experimental evidence suggests that increasing degree of silica undersaturation is favoured by melting at higher pressures and/or higher C O S H 2 0 ratios (Edgar 1987). Despite the relative success of such mixing models in reproducing the compositional and isotopic variations in individual alkaline volcanic suites, they provide little information about the nature of the mixing process nor the spatial relationship at the time of melting between the distinct mantle sources involved.

This paper examines the implications of the compositional variation observed in the glasses in mantle xenoliths for the assumption that all silica-undersaturated magmas have lherzolite mantle sources. The compositional variation in such glasses suggests that highly silica undersaturated magmas in many mafic alkaline volcanic suites may be derived by the preferential melting of amphibole-garnet clinopyroxenite veins and that the compositional spectra of some alkaline volcanic suites reflect mixing with melts derived from the enclosing lherzolite host rock.

Xenolith glasses

Interstitial seams and patches of silicate glass are common in the pyroxenite, wehrlite, and lherzolite mantle xenoliths which characterize many alkaline volcanic suites (Edgar et al. 1989; Ellis 1976; Francis 1976a, 1987; Frey and Green 1974; Forbes and Starmer 1974; Gamble and Kyle 1987; Garcia and Presti 1987; Girod et al. 1981; Griffin et al. 1984). There is considerable debate as to whether such glass represents incipient melts produced by decompression as the xenoliths were carried

176

to the surface, infiltrations o f the host magma , or migra- tory melts in the mantle (Garcia and Presti 1987; Edgar et al. 1989). Whichever view is correct, the relatively small volume of glass versus xenolith requires that the composi t ions o f the glasses approach those which would be buffered by the minera logy of their hos t xenolith. As such, these glasses represent rapidly quenched natural experiments which can be used to constra in the composi - tions o f initial melts o f different mant le mineral assem- blages at low pressure. For the purposes o f this paper, I have used mant le xenoliths f rom my collections f rom Nunivak Island, Alaska, and All igator Lake, Yukon. The reader is referred to previous publicat ions for addi- t ional details on their description and composi t ion (Francis 1987; Roden et al. 1984; Francis 1976a, b).

Compound xenolith 14001

Sample 14001 is a compound xenolith from Nunivak Island that consists of an angular fragment of spinel lherzolite sandwiched between two pyroxenites (Fig. 1); one a relatively Fe-rich garnet clinopyroxenite and the other a more magnesian, spinel websterite (Francis 1976 a). All three lithologies contain interstitial amphibole and an amphibole vein cuts the lherzolite fragment. The garnet clinopyroxenite, adjacent spinel lherzolite, and amphibole vein also contain interstitial glass indicating the existence of interstitial melt while the xenolith was entrained in the basanitic magma which carried it to the surface. In the garnet clinopyroxenite, glass occurs as irregular interstitial patches developed at the expense of kaersu- titic (Francis 1976 a, b) amphibole and is characterized by the pres- ence of abundant quench crystals of amphibole (Fig. 2 a). Similarly, glass occurs with euhedral olivine, clinopyroxene, and spinel as fine-grained zones developed in the amphibole vein cutting the lherzolite. In the spinel lherzolite itself, however, glass occurs only as thin seams following fractures and grain boundaries (Fig. 2b) that originate at the garnet clinopyroxenite-lherzolite contact. No glass is observed in the spinel websterite, whose minerals are more magnesian than those of the garnet clinopyroxenite, on the oppo- site side of the lherzolite fragment (Francis 1976 a).

The glasses from the garnet clinopyroxenite and amphibole vein are Ne (nepheline) normative (Table 1) and fall near the olivine

jadeite join in an alkalies versus Si plot (Fig. 3). They are richer

Fig. 2. a Back-scattered electron image of a melt zone in the amphibole-garnet clinopyroxenite portion of xenolith 14001, consisting of quench crystals of amphibole in black glass. The white bar is 10 lain long. b Back-scattered electron image of a seam of glass cutting the spinel lherzolite portion of xenolith 14001. The highly reflective euhedral crystals are apatite. Note the reflectivity contrast along the olivine - glass contact indicating F e - M g exchange between the lherzolite olivine and the glass. The white bar is 20 gin long

in Fe and alkalies and poorer in Si than the glasses in the seams cutting the adjacent lherzolite (Figs. 3, 4, Table 1), which are quartz normative and plot along the plagioclase join. The glass analyses in the amphibole-garnet clinopyroxenite have highly variable P205 contents (Fig. 5), the lower of which may reflect difficulty in avoid- ing quench crystals of amphibole during electron microprobe anal- ysis. The glasses in the lherzolite locally contain euhedral crystals of apatite (Fig. 2b) and have P205 contents which decrease with increasing SiO2, parallel to the apatite-saturation curves deter- mined by Green and Watson (1982).

These textural and chemical relationships suggest that the Fe- rich amphibole of the garnet clinopyroxenite and amphibole vein (Francis 1976a) have experienced preferential partial melting and that some of this melt has infiltrated and reacted with the adjacent more magnesian lherzolite. This reaction has changed a highly silica undersaturated melt into a quartz-normative melt which was supersa- turated in apatite.

Fig. 1. Full thin section image (2.5 cm) of compound xenolith 14001 showing the wedge of clear lherzolite flanked by darker amphibole- garnet clinopyroxenite on the lower right and amphibole websterite on the upper left. A dark amphibole vein cuts the lherzolite and lines the pyroxenite-lherzolite contact

Pyroxenite versus lherzolite xenoliths

The foregoing observat ions suggest that melts in equilib- r ium with amphibole or amphibole-garne t c l inopyroxen- ite are significantly more silica undersa tura ted than

177

Table 1. Average glass analyses, 14001

Host Amphibole Amphibole Spinel pyroxenite vein lherzolite

No. 9 3 21

SiOz 49.02 48.33 52.55 TiOz 1.69 3.26 2.78 A1203 18.69 19.93 18.97 F%O3 1.76 1.28 0.76 Cr203 0.02 0.03 0.04 MgO 2.65 2.35 2.85 FeO 8.99 6.52 3.85 MnO 0.18 0.17 0.10 CaO 5.12 8.91 8.03 Na20 6.74 5.i6 3.79 K20 2.47 2.84 3.04 PzO5 2.36 1.87 2.27

Total 99.69 100.65

Normative mineralogy (weight percent)

99.03

Quartz 0.00 0.00 2.88

Feldspar 62.33 64.78 75.00 Orthoclase 14.60 16.74 17.94 Albite 37.76 25.20 32.05 Anorthite 9.98 22.85 25.02

Nepheline 10.43 9.98 0.00

Clinopyroxene 0.00 7.57 0.00 Diopside 0.00 4.05 0.00 Hedenbergite 0.00 3.51 0.00

Orthopyroxene 0.00 0.00 9.08 Enstatite 0.00 0.00 7.08 Ferrosilite 0.00 0.00 2.00

Olivine 14.37 5.86 0.00 Forsterite 4.64 2.79 0.00 Fayalite 9.73 3.07 0.00

Accessories 10.96 12. I 7 11.41 Chromite 0.02 0.04 0.07 Magnetite 2.56 1.86 1.10 Itmenite 3.21 6.19 5.30 Apatite 5.16 4.08 4.95

Corundum 1.27 0.00 0.27

Electron microprobe analyses performed with a Cameca CAME- BAX/Micro System at McGill University with an accelerating volt- age of 15 kV, a current of 5 nA, and counting times of 30 s per element. Fe 3 +/(total Fe) is taken to be 0.15. Average for amphibole pyroxenite glasses includes only analyses with P205 greater than 1.5 wt% to exclude analyses which represent mixtures of glass and quench crystals of amphibole

those in equ i l ib r ium wi th o l i v i n e - d o m i n a t e d spinel lherzol i te u n d e r s imi lar t e m p e r a t u r e cond i t ions at low pressure. This conc lus ion is s u p p o r t e d by a c o m p a r i s o n o f glass c o m p o s i t i o n s in d i f ferent xenol i th types (Fig. 6a). The c o m p o s i t i o n s o f xenol i th glasses become sys temat i - cal ly m o r e silica u n d e r s a t u r a t e d go ing f rom (1) anhy- d rous spinel lherzol i tes f rom A l l i g a t o r Lake (Franc i s 1987), t h r o u g h (2) a m p h i b o l e a n d / o r f ine -gra ined zones af ter a m p h i b o l e in spinel lherzol i tes to ha rzburg i t e s f rom N u n i v a k Is land, A l a s k a (Franc i s 1976b), to (3) a m p h i - bo l e -bea r ing ga rne t a n d / o r spinel pyroxen i tes f rom N u n - ivak Is land, A la ska . A c o m p a r i s o n o f pub l i shed c o m p o -

30

o 20

D ~)

+

Z

10

J i i

/

~176 / 0,. AoJ

0 W W ~ I

3O L0 5O 6O 7O Si eotions

Fig. 3. Plot of total alkalies vs Si in cation units showing the compositions of glass in compound xenolith 14001: open circles, glass in the amphibole-garnet clinopyroxenite portion; triangles, glass in fine-grained zones developed in amphibole vein; open squares, glass seams in the spinel lherzolite portion. The smaller symbols represent a reference compilation of mafic lavas (> 7.5 wt% MgO) from alkaline volcanic suites containing foidite lavas classified on the basis of the C.I.P.W. Norm: open circles, foidites containing more than 15 wt% normative Ne (nepheline) + Le (leucite) ; crosses, basanites with 5-15 wt% normative Ne; open squares, alkaline olivine basalts (0 5 wt% normative Ne) to transitional hypersthene- normative basalts. See Francis and Ludden (1990) for data sources

15 : / / 0. 2

8

0 I i

0 5 10 15 Fe cGtions

Fig. 4. Plot of Mg vs total Fe in cation units of the glass in compound xenolith 14001 with symbols as in Fig. 2. Solid lines, represent compositions of liquids that would coexist with the olivine (Mg No. =0.895) in the spinel lherzolite for Kds (partition coeffi- cients) of 0.2 and 0.3, whereas the dashed lines, represent the compositions of liquids that would coexist with the clinopyroxene (Mg No. =0.723) of the amphibole-garnet clinopyroxenite for Kds of 0.2 and 0.3 assuming Fe3 +/total Fe = 0.15)

s i t ions o f o the r xenol i th glasses (Fig. 6b) shows tha t glasses in a m p h i b o l e - b e a r i n g pyroxen i t e s and wehrl i tes are sys temat ica l ly m o r e silica u n d e r s a t u r a t e d t han glasses in o l i v i n e - d o m i n a t e d lherzol i te or ha rzburg i t e xenol i ths .

178

5

1120~ Apotite �9 [] saturation

d c 2

88oz 0 I I I

35 L5 55 65 75 SiO 2 wt.%

Fig. 5. Plot of P2Os vs SiO2 in wt% of the compositions of glasses in compound xenolith 14001 with apatite saturation curves at 7.5 kbar from Green and Watson (1982). Symbols as in Fig. 3

Implications for the mantle sources of alkaline magmas

The range of silica contents of glasses in mantle xenoliths from amphibole-garnet clinopyroxenite to anhydrous lherzolite mimics that of the spectrum of alkaline mafic magmas (Fig. 3). These glasses have, however, low Mg contents (Table 1) and relatively evolved compositions; the majority ranging from phono-tephrites to andesites if classified according to conventional classification schemes (LeBas et al. 1986). These compositions pre- sumably reflect the fact that the xenolith glasses have been quenched at approximately 1 atmosphere pressure. At mantle temperatures and pressures, and at larger degrees of partial melting, however, the compositions of these initial melts would (to a first approximation) shift towards that of olivine because of the pronounced decrease in the olivine liquidus volume with increasing pressure (Falloon et al. 1988; Presnall and Hoover 1987; Stolper 1980; Takahashi and Kushiro 1983). If enough olivine is added to give these glasses the Mg content of the mafic alkaline magmas, the spectrum of glass compositions from pyroxenite to lherzolite xenoliths is essen- tially coincident with that of the Earth's mafic alkaline magmas (Fig. 3).

These observations lead to a relatively simple model for the mantle source region for mafic alkaline magmas and the origin of their compositional spectra. Small numbers of black pyroxenite xenoliths with or without amphibole or garnet are characteristic of lherzolite xenoliths suites in alkaline basalts from around the world. Studies of compound xenoliths (Irving 1980; Kornprobst and Conquer6 1972; Wilshire and Shervais 1975) have suggested that pyroxenite xenoliths represent relatively Fe-rich veins in a more magnesian lherzolite upper mantle. Most consider such veins to be crystal segregates introduced by the passage of alkaline magmas through a lherzolite mantle (Irving 1980; Wilshire et al. 1980). The development of such pyroxenite veins is an expected result of the rise of asthenosphere melts into a lherzolite

, , i

30 N ~

o = 20 ~3 O

2x2 +

~3

0 ~ ~

a 30 40 50 60 70 Si cations

i i ,

t 20 O

o Z

10

/ _ / [][] I

/ :~ 1 0l Di 0

b 30 40 50 60 70 Si cations

Fig. 6. a Plot of total alkalies vs Si in cation units for glass in anhydrous spinel lherzolite xenoliths from Alligator Lake, open squares; glass in spinel lherzolite xenoliths from Nunivak Island with amphibole or fine-grained melt zones interpreted to be after amphibole, solid squares; glass in amphibole-garnet pyroxenite xenoliths from Nunivak Island, open circles. Nunivak lherzolite xenoliths with the most Si-poor glass retain abundant amphibole, while no amphibole remains in the lherzolite xenoliths with the most Si-rich glasses, b Plot of total alkalies vs Si in cation units for glass reported in ultramafic xenoliths in alkaline volcanic rocks. Half filled squares, glass in spinel lherzolite and harzburgite xenoliths; circles, glass in wehrlite and garnet pyroxenite xenoliths. Data sources : Edgar et al. 1989; Ellis 1976; Frey and Green 1974; Gam- ble and Kyle 1987; Girod et al. 1981; Griffin et al. 1984; Irving 1974; Kuo and Essene 1986; MacRae 1979

to harzburgite lithosphere because of the step in the soli- dus of the peridotite-HzO-CO2 system (Meen et at. 1989; Olafsson and Eggler 1983). With the exception of potassic suites (Bailey, 1987), models for the origin of the compositional spectra of mafic alkaline magmas, have generally ignored the fact that any mantle source region is likely to contain a percentage of such veins.

Sleep's (1974) proposal that melting in a mantle source will initiate and tend to concentrate in heterogen- eities with the lowest melting point is supported by the melt development observed in compound xenolith 14001. In a veined mantle source, melting will begin in

179

the relatively Fe-rich pyroxenite veins with breakdown of amphibole to olivine, clinopyroxene and a melt that is likely to be olivine nephelinite in composi t ion (Francis and Ludden 1990; Olafsson and Eggler 1983). With increasing temperature and/or decreasing pressure, the more magnesian interstitial amphibole in the host lherzolite and finally the lherzolite itself will begin to melt, and the composit ion of the aggregate melt will become increasingly rich in silica, evolving towards a hyper- s thene-normative composition. In this model, the spec- t rum of mafic alkaline melts is produced as a mixing array of liquids derived f rom a common amphibole pyroxenite-veined lherzolite source without resorting to extremely small degrees of partial melting or mixing between spatially isolated sources.

The model presented above is not unique and presents the reader with a classic chicken versus egg dilemma. Are amphibole pyroxenite veins the source or the crystal accumulates of highly alkaline magmas passing through a lherzolite mantle? This question can not be answered on the basis o f phase equilibria alone and both possibili- ties are probably correct on a global scale. Francis and Ludden (1990) have proposed that the chemical and isotopic evolution f rom olivine nephelinite to transitional hypersthene-normative basalts in small-volume alkaline volcanic centres, such as Fort Selkirk in the central Yukon, are best explained by the progressive melting of a mantle source comprised of lherzolite containing amphibole-garnet pyroxenite veins. Such veins would have to have been previously introduced into the lherzolite mantle by precursor alkaline magmas rising f rom deeper mantle regions in the manner proposed by Meen et al. (1989).

Regardless of the above dilemma, the composit ional systematics of xenoliths glasses clearly indicate that melts equilibrated with amphibole or amphibole pyroxenite are systematically more silica undersaturated than those equilibrated with lherzolite at low pressures. Al- though more studies are needed, the existing experimental data suggest that this relationship is likely to persist at the elevated pressures at which mafic alkaline magmas are produced in the Ear th 's mantle. This demonstrat ion of the sensitivity of the degree of silica saturation of an initial melt to the mineral assemblage of possible mantle sources points out the possible dangers of models in which intensive variables are calculated assuming a lherzolite mineralogy for the mantle sources of all magmas in an alkaline volcanic suite.

Even if the derivation of highly silica undersaturated magmas f rom pyroxenitic mantle sources proves to be a restricted phenomenon, the development of apatite in the glass veins cutting the lherzolite of compount xenolith 14001 has impor tant implications for models of the reaction between basaltic melts and lherzolite mantle. The composit ions of the glasses in xenolith 14001 agree with predictions that such reactions will result in an in- crease in Si and decrease in Fe in the magmas involved and are possibly capable of converting tholeiitic magmas to calc-alkaline magmas (Kelemen 1990; Kelemen et al.

�9 1990 a) and alkaline magmas to tholeiitic magmas (Fran- cis 1987). For such reaction models to explain the trace-

element characteristics of calc-alkaline or tholeiitic magmas, however, relatively large mantle/melt ratios (ca. 50- 100) are required (Kelemen et al. 1990b) if only the ma- jor silicate phases of the mantle are involved. I f the trace- element characteristics of magmas reacting with a lherzolite mantle are controlled by the crystallization of ac- cessory phases, such as apatite, because of the decrease in their solubility with increasing Si content (Green and Watson 1982), then the mantle/melt ratios required to explain the trace-element characteristics of natural suites would decrease dramatically.

Acknowledgements. This paper has benefited from discussions with numerous colleagues and graduate students. The manuscript has improved because of reviews by Mike Roden, Bill Luhr, Antony Irving and an anonymous reviewer. I am also indebted to Richard Yates for photography and Jim Mungall for assistance on the electron microscope.

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some implications of xenolith glasses for the mantle sources of alkaline mafic magmas

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