Journal of Petrology

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Journal of Petrology Volume 42 Number 4 Pages 655-671 2001
© Oxford University Press 2001

Factors Controlling Chemistry of Magmatic Spinel: an Empirical Study of Associated Olivine, Cr-spinel and Melt Inclusions from Primitive Rocks

VADIM S. KAMENETSKY^,*, ANTHONY J. CRAWFORD and SEBASTIEN MEFFRE

SCHOOL OF EARTH SCIENCES AND CENTRE FOR ORE DEPOSIT RESEARCH, UNIVERSITY OF TASMANIA, GPO BOX 252-79, HOBART, TAS. 7001, AUSTRALIA

Received November 5, 1999; Revised typescript accepted June 16, 2000

	ABSTRACT

TOP ABSTRACT INTRODUCTION SAMPLE DESCRIPTION AND... RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Compositions of 2500 spinel–olivine pairs and 400 melt inclusion–spinel pairs have been analysed from 36 igneous suites from oceanic, arc and intraplate tectonic settings. Our data confirm that Cr-spinel mg-number is largely controlled by melt composition, but also influenced by octahedral site substitutions, and rate of cooling. Lavas quenched in submarine environments tend to have higher mg-number at a given cr-number than slowly cooled subaerial lavas and peridotites. Unlike mg-number, Cr-spinel Al₂O₃ and TiO₂ contents show good correlations with melt composition, with only limited post-entrapment modifications. Our data suggest that increased activity of Al₂O₃ decreases the partitioning of TiO₂ into spinels. The Al₂O₃ content of Cr-spinel is a useful guide to the degree of partial melting of mantle peridotites; however, this same relationship is obscured in volcanic rocks. Al₂O₃ contents of volcanic Cr-spinels are mostly determined by melt composition rather than mantle source composition. The data also suggest that most spinels from residual mantle peridotites can be readily differentiated from those hosted in volcanic rocks. Mantle peridotite spinel tend to have lower TiO₂ and higher Fe²⁺/Fe³⁺ ratios than spinel from volcanic rocks. The spinel compositions in our database can be subdivided on the basis of tectonic setting and mode of occurrence using an Al₂O₃ vs TiO₂ diagram. A total of seven fields can be distinguished with varying degrees of overlap. This diagram can then be used to determine the tectonic setting of spinel from altered mafic igneous rocks such as serpentinites or meta-basalts, or detrital spinel in sandstones.

KEY WORDS: mantle; melt inclusions; olivine; spinel; volcanic rocks

	INTRODUCTION

TOP ABSTRACT INTRODUCTION SAMPLE DESCRIPTION AND... RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Roeder (1994) succinctly reviewed detailed studies over the last 20 years of Cr-spinel in mafic and ultramafic rocks. These studies have shown that spinel compositions are a complex function of magma (and source peridotite) composition (e.g. Irvine, 1965, 1967; Evans & Frost, 1975; Fisk & Bence, 1980; Maurel & Maurel, 1982a, 1982b; Sack, 1982; Dick & Bullen, 1984; Allan et al., 1988; Allan, 1992, 1994; Arai, 1992, 1994b), fO₂ (e.g. Hill & Roeder, 1974; Murck & Campbell, 1986; Ballhaus et al., 1991; Roeder & Reynolds, 1991), crystallization temperature and cooling rate (e.g. Fisk & Bence, 1980; Ozawa, 1984; Sack & Ghiorso, 1991; Scowen et al., 1991), and perhaps pressure (e.g. Sigurdsson & Schilling, 1976; Jaques & Green, 1980; Ballhaus et al., 1991; Roeder & Reynolds, 1991). Compared with usually co-liquidus olivine in primitive magmas, Cr-spinel compositions offer the potential to decipher important petrogenetic aspects of such magmas, including information on source peridotite ‘fertility’ (e.g. Dick & Bullen, 1984; Arai, 1987, 1994a; Clynne & Borg, 1997), early stage magma mingling, before or during aggregation of melt batches in subvolcanic magma chambers (e.g. Allan et al., 1988; Allan, 1992; Danyushevsky et al., 1995; Kamenetsky & Crawford, 1998; Kamenetsky et al., 1998) and subsequent shallow-level magma mixing (e.g. Sigurdsson, 1977; Natland et al., 1983; Thy, 1983).

Many of the studies referred to above reported Cr-spinel data in isolation from host olivine compositions. In this study, we summarize a wealth of new compositional data for coexisting chromites and olivines from a diverse range of primitive lavas, including those from large igneous province (LIP) basalts (flood basalts), oceanic intraplate basalts, mid-ocean ridge basalts (MORB), back-arc basin basalts (BABB), intra-oceanic arc basalts (including those of tholeiitic, calc-alkaline and shoshonitic affinities), and several primitive boninite suites. For some suites, we also report analyses of melt inclusions in olivine and Cr-spinel hosts. Having analysed a statistically significant number (usually >50) of olivines from each lava, we are confident that we have identified the most high-Mg olivines crystallizing from each suite, so that the data presented are believed to be representative of the full crystallization range of Cr-spinel in these suites. To facilitate comparison and interpretation of the magmatic Cr-spinel data, we also present new data for several important ophiolitic peridotite suites, including those from the refractory harzburgitic New Caledonian ophiolite, and from in situ oceanic crust of Macquarie Island in the Southern Ocean.

These new data allow us to re-evaluate the spectrum of chromite compositions recorded from diverse tectonic settings, and to provide a more robust assessment of the significance of Cr-spinel composition variations than is often provided (usually based solely on Cr-spinel Cr/[Cr + Al] vs Fe²⁺/[Mg + Fe²⁺] values). Following the suggestion by Arai (1992), we apply the database and new compositional discriminant plots presented herein to interpret the origin and significance of spinels in sandstones and altered rocks from the Palaeozoic foldbelt in eastern Australia.

	SAMPLE DESCRIPTION AND ANALYTICAL TECHNIQUES

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Petrological criteria for choosing rock suites
Chromian spinel (hereafter ‘spinel’) is mainly present as inclusions in silicate minerals and less commonly it forms phenocrysts. For this study of spinel compositions we have chosen samples that satisfy the following requirements:

1. all rock samples have volcanic origin, i.e. they are relatively rapidly quenched submarine or subaerial lava flows and tuffs or thin dykes with chilled margins. The rocks are fresh to moderately fresh. The choice of samples eliminates, as far as possible, the effects of post-entrapment modification of spinel compositions as a result of re-equilibration or alteration.
   
2. The volcanics are representative of the different magma types occurring in well-constrained tectonic environments, namely, mid-ocean ridges (MORB), back-arc spreading centres (BABB), ocean islands (ocean-island basalt; OIB), continental rifts (LIP) and a variety of island-arc settings. Because of the large geochemical and petrological diversity of island-arc magmatism, different subtypes—boninitic, tholeiitic, calc-alkaline and high-K (shoshonitic) series—are considered separately.
   
3. Within each magmatic series the most primitive compositions (the highest mg-number, i.e. Mg/[Mg + Fe²⁺], and Cr and Ni contents) have been studied to ensure the presence of olivine phenocrysts hosting spinel inclusions. Although other phenocrysts may enclose spinel inclusions during cotectic crystallization (orthopyroxene, clinopyroxene and plagioclase), we favour the study of spinel trapped in olivine. The reasons are as follows: (a) olivine is the earliest phase to crystallize from virtually all mantle-derived melts; (b) olivine remains on the liquidus at least as long as spinel crystallizes; (c) olivine compositions (Fo, Ca, Ni) can be used as an indicator of the degree of magma fractionation; (d) the effects of olivine–spinel post-entrapment Mg–Fe²⁺ exchange can be evaluated; (e) olivine effectively armours enclosed spinel inclusions from other post-entrapment modifications (e.g. Scowen et al., 1991).
   
4. For each magmatic suite the compositional range of olivine was thoroughly studied to guarantee that the most primitive spinels, as well as spinel compositions along the liquid line of descent, are included in the database.
   

A complete set of rock samples used in this study is listed in Table 1 and their locations are shown in Fig. 1. The relevant petrological and geochemical information on the rock suites may be found in publications cited in Table 1. The abundance of olivine phenocrysts in these rocks is rather variable (from a few percent to 40–50 vol. %), and hence the rocks’ MgO contents vary proportionally from 6 to 40 wt %. The range in the composition of olivine cores (Table 1), sometimes very large (10–15 mol % Fo) even within a single sample, suggests that none of the studied rocks represents a true melt composition. Therefore, we regard these porphyritic rocks as a mechanical mixture of olivine and other silicate phenocrysts when present (e.g. plagioclase and clinopyroxene in MORB, clinopyroxene in arc volcanics, and low-Ca pyroxene in boninites) and a residual melt (groundmass).The most Fe-rich olivines are usually in equilibrium with the groundmass composition, whereas more forsteritic olivines crystallized from more primitive melts at higher temperature, and were incorporated in the magma not long before the eruption. Olivine phenocrysts with their spinel and melt inclusions remain largely unequilibrated with the transporting melt, and thus offer snapshots of the preceding magmatic evolution.

View this table: [in this window] [in a new window]   Table 1: Geographic locations of studied samples and main compositional features of coexisting olivine and spinel

 

View larger version (31K): [in this window] [in a new window]   Fig. 1. Map showing the geographic position of studied suites. (For suite numbers see Table 1.)

 

The ranges in olivine Fo content in the studied rocks (Table 1) extend to the highest values recorded for a given type of magmatism. The presence in our collection of olivine Fo₉₂ (mid-ocean ridges), Fo₉₀ (ocean islands), Fo₉₃ (continental rifts), Fo_94·5 (island arcs), and Fo_93·5 (back-arc basins) is good evidence for crystallization from high-temperature mantle-derived melts. It is worth noting that the composition of olivine (Fo) may be used as a measure of liquidus temperature and melt MgO only within a given magma series (e.g. MORB), whereas across the various magma series considered here, olivines of similar forsteritic content crystallize at very different temperatures (see Fo vs temperature plot in Sobolev & Nikogosian, 1994; Kamenetsky et al., 1995a). For example, the liquidus temperature of Fo₉₂ in MORB melt (FAMOUS area) is 1300°C (Kamenetsky, 1996), whereas in the Siberian meimechite magma Fo₉₂ crystallized at 1560°C (Sobolev et al., 1992). The MgO content of their parental melts is, therefore, also different (13·5 and 27 wt %, respectively), reflecting different melt Fe²⁺ contents.

To compare compositions of volcanic spinels with those from mantle peridotites we use published mineral analyses from oceanic peridotites worldwide (Dick, 1989), from the Izu–Mariana forearc, Leg 125 (Ishii et al., 1992), and our data for Macquarie Island and New Caledonia peridotites. Information on the composition of melts parental to spinel in a number of samples was also obtained from the study of melt inclusions trapped in either spinel or olivine phenocrysts.

Spinel inclusions in olivine
Spinel inclusions are found in olivine phenocrysts from all magma types, but the abundance of spinel is highly variable. Some phenocrysts may contain numerous swarms of 5–6 to several tens of inclusions (typically in boninites), but more often they host either only occasional individual grains (e.g. North Fiji back-arc basalts), or the majority of olivines are devoid of any inclusions (e.g. Kamchatka high-K picrites).

Spinel inclusions are always octahedral, varying in size from several micrometres (in boninites and arc tholeiites) to 0·5 mm (in MORB and OIB). The colour of spinels in thin sections and brightness in reflected light depend on chemical composition, e.g. Fe–Ti-rich spinels are the brightest and almost opaque, Cr-rich spinels are translucent reddish brown to yellow–brown, and Al-rich spinels are the least reflective, greenish yellow to dull green in colour.

Melt inclusions in spinel and olivine
Melt inclusions, consisting of residual glass and daughter silicate crystals, are very common in spinel and olivine phenocrysts. Homogenization experiments with the melt inclusions were performed using a heating stage designed in the Vernadsky Institute of Geochemistry, Moscow (Sobolev et al., 1980). The details of experimental procedure and compositions of individual inclusions and their host minerals from some suites that are discussed in this paper can be found in our (Zlobin et al., 1991; Sobolev et al., 1992, 1995; Kamenetsky et al., 1993, 1995a, 1995b, 1997, 1998; Kamenetsky, 1996; Kamenetsky & Crawford, 1998) and other (Danyushevsky et al., 1987; Gurenko et al., 1992; Sobolev & Nikogosian, 1994; McNeill, 1997; Gioncada et al., 1998) publications.

Sample preparation and analysis
The samples were crushed and sieved, and 50–200 olivine phenocrysts (0·3–1 mm) from each sample were hand picked, mounted in epoxy mounts, exposed and polished. Spinel inclusions in olivine grains in these mounts were marked for analysis. Similar mounts were prepared with spinel and olivine grains containing homogenized melt inclusions. Spinel and glass inclusions were analysed together with their host minerals by electron microprobe at the University of Tasmania and University of Paris VI (Cameca SX-50) and Vernadsky Institute of Geochemistry, Moscow (Camebax microbeam), using wavelength-dispersive spectrometry at 15 kV. Mineral (San Carlos olivine USNM 111312/444 and Cr-spinel UV-126) and glass (VG2—USNM 111249/52) secondary standards were analysed before and after each run, and 2–5 analytical points were used to calculate average composition. At least 25 grains of olivine and five spinels were studied in each sample. Host olivine was analysed not further than 20 µm from the spinel inclusions. The large number of analyses of individual olivine grains allowed determination of the entire range of olivine compositions, so that analyses of spinels were selectively performed on grains trapped in olivines over a wide range of Fo contents.

	RESULTS AND DISCUSSION

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The main compositional features of studied spinels (mg-number, cr-number = Cr/[Cr + Al], TiO₂, and Fe²⁺/Fe³⁺) are given in Table 1. The compositions of individual spinel inclusions trapped in the most primitive olivines are presented in Table 2. Below we discuss petrological aspects of the spinel compositional range in the lavas studied.

View this table: [in this window] [in a new window]   Table 2: Representative compositions of spinel inclusions trapped in the most primitive olivines found in the studied samples (see Table 1)

 

Olivine–spinel mg-number compositional relationships: the effects of crystallization and re-equilibration
The presence of spinel inclusions in olivine over a wide Fo range, including the most evolved olivines (e.g. Fo_75–80 in OIB) within a given sample or magma type, argues for continuous spinel crystallization together with the cotectic silicate phases other than olivine (pyroxenes and plagioclase) over a significant range of temperatures. In some cases (see Table 1 for the suites with the largest ranges of Fo) the crystallization temperature interval may exceed 200–250°C. Given the well-established early crystallization of pyroxenes in many island-arc (e.g. Barsdell, 1988; Eggins, 1993; Sobolev & Danyushevsky, 1994; Della-Pasqua et al., 1995; Kamenetsky et al., 1995a, 1995b, 1997; Della-Pasqua & Varne, 1997) and some MORB magmas (e.g. Kamenetsky et al., 1998), and early crystallization of plagioclase in most MORB (e.g. Allan et al., 1988; McNeill, 1997), as well as presence of spinel inclusions in pyroxene and plagioclase phenocrysts (this study), earlier claims that spinel crystallizes over a narrow temperature interval (Hill & Roeder, 1974; Fisk & Bence, 1980) and ceases to crystallize shortly after the appearance of clinopyroxene and plagioclase (Irvine, 1965, 1967; Dick & Bullen, 1984) seem to be unjustified.

Fractional crystallization (decreasing melt mg-number and temperature) should lead to related change in the mg-number of co-crystallizing olivine and spinel. For example, the observed positive correlation between olivine Fo and spinel mg-number in a given sample or suite (Fig. 2) reflects local equilibrium between these phases. The temperatures of olivine–spinel equilibria, calculated using the Ballhaus et al. (1991) model, are presented in the caption to Fig. 2. However, these temperatures (T_average = 1067 ± 65°C for 650 olivine–spinel pairs shown in Fig. 2) correlate neither with Fo nor with real liquidus temperatures, and are in fact the closure temperatures for the Fe–Mg exchange between host olivines and spinel inclusions. The decrease in primary mg-number values of spinel inclusions in olivine as a result of re-equilibration at temperatures below liquidus (Irvine, 1967; Dick & Bullen, 1984; Ozawa, 1984; Ballhaus et al., 1991; Scowen et al., 1991) does not, however, obliterate correlations between mg-number and Fo in many volcanic suites (Fig. 2). On the other hand, the dispersion in spinel mg-number at a given Fo value, either within a given sample or suite or between suites, is controlled by some other factors. One is the dependence of the Mg and Fe²⁺ partitioning between olivine and spinel on the relative activities of Cr and Al in spinel (Dick & Bullen, 1984) or, in other words, the substitution of Fe²⁺ and Cr for Mg and Al (Allan et al., 1988). For example, Fig. 2 shows that spinel coexisting with olivine becomes more Mg rich with decreasing cr-number. The distribution coefficient K_d = (Mg/Fe)_olivine/(Mg/Fe²⁺)_spinel varies significantly from 2·5 (cr-number 20–30) to 10–12 (cr-number 80–90). At a given spinel cr-number, the distribution coefficient either remains constant (Fig. 2a) or varies within up to 30% of its value (Fig. 2c).

View larger version (35K): [in this window] [in a new window]   Fig. 2. Relationships between host olivine Fo and spinel inclusion mg-number. The compositional trends are presented for 12 populations (I–XII) of spinel compositions. Each population has a limited range in measured cr-number and olivine–spinel blocking temperature, calculated using the Ballhaus et al. (1991) geothermometer. (For suite numbers see Table 1.) I II III IV V VI Spinel cr-number   16–30   50–56   65–75   83–90   30–45   55–63 Tol–sp ± (°C) 1122 ± 50 1114 ± 38 1061 ± 28 1066 ± 27 1108 ± 67 1094 ± 32 Suite number    7–10 1, 2, 4 8, 10, 12   20    1–4   32 VII VIII IX X XI XII Spinel cr-number   64–74   75–81   35–45   50–60   70–80  83–90 Tol–sp ± (°C) 1099 ± 68 1048 ± 71 1096 ± 57 1077 ± 54 1056 ± 49 963 ± 37 Suite number   26   24    7–10    8–10   16  17 Distribution coefficient Kd = (Mg/Fe)olivine/(Mg/Fe2+)spinel values and isopleths of Kd (dashed lines) are shown.

 

To further examine the possible effects of low temperature re-equilibration of spinel in terms of its mg-number we compared the compositions of spinel from quenched (underwater eruption, e.g. MORB, BABB and some arc tholeiites and boninites) and relatively slowly cooled (subaerial eruption, e.g. OIB and high-K arc series) rocks (Fig. 3a and b). The data (grouped according to host olivine Fo composition and plotted on the mg-number–cr-number diagram) demonstrate that at a given spinel cr-number and olivine Fo, spinel mg-number values from subaerial lavas are slightly lower (by up to 10 mol %). This is consistent with statistically lower blocking temperatures for olivine–spinel re-equilibration in subaerial lavas (T_average = 1050 ± 70°C; n = 600) in comparison with quenched volcanics (T_average = 1077 ± 58°C; n = 871). The effects of even slower cooling and lower closure temperature (T_average = 681 ± 44°C; n = 296) are most pronounced in the mg-number of spinel from mantle (ophiolite or abyssal) peridotites. Compositions of spinels associated with olivine Fo_88·5–94 from a number of oceanic (arc- and rift-related) peridotites are significantly offset to lower mg-number (by up to 25–30 mol %) from the trends observed in pillow lavas (Fig. 3c). This shift increases significantly with increasing spinel cr-number.

View larger version (35K): [in this window] [in a new window]   Fig. 3. Relationships between mg-number and cr-number in spinels from recent and modern submarine (a) and subaerial (b) rocks, and mantle peridotites (c). Peridotitic samples: I, Izu–Mariana forearc, Leg 125 (Ishii et al. (1992); II, New Caledonia; III, Macquarie Island; IV, variety of abyssal peridotites (Dick, 1989). Isopleths of olivine Fo (dashed lines) are calculated as a linear regression for olivine–spinel data from recent and modern submarine volcanics shown in (a).

 
Spinel mg-number should be interpreted with caution, as it is a complex function of a number of factors, the most important of which are (1) mg-number of the parental melt; (2) partitioning of Al and Cr in spinel, and hence Al₂O₃ in the melt (see below); (3) Fe²⁺/Fe³⁺ in the melt, and hence fO₂; (4) post-entrapment re-equilibration with silicate minerals, and hence the cooling rate and spinel grain size. Consideration of the kinetics of olivine–spinel Mg–Fe²⁺ interdiffusion (Ozawa, 1984) precludes significant subsolidus re-equilibration for rapidly cooled, submarine volcanics, as their cooling rates of 100–100,000°C/h provide insufficient time for re-equilibration at near-magmatic temperatures. This contrasts with the rates of 0·1–0·001°C/h for slowly cooled subaerial thick lava flows, lava lakes, dykes and intrusive rocks, and thus the variable and extensive re-equilibration observed (Fig. 3b and c; Scowen et al., 1991; Barnes, 1998). With respect to re-equilibration, Fo–mg-number or mg-number–cr-number correlations, even in submarine volcanics (Figs 2 and 3a), are not true liquidus relationships, as spinels trapped in more evolved olivine are likely to be re-equilibrated to a lesser extent than spinels hosted by more primitive, higher-temperature olivine. However, in this case Mg and Fe²⁺ exchange is considered to be minimal <5 mol % mg-number (Dick & Bullen, 1984; Allan et al., 1988; Allan, 1994; Clynne & Borg, 1997), and the decrease in spinel mg-number, along with olivine Fo, is still a reflection of fractional crystallization. Figure 3a shows the isopleths of minimal Fo values of olivines hosting spinel of any cr-number value. This provides a useful estimate of olivine compositions in rocks where olivine is pervasively altered.

Spinel Al₂O₃ vs TiO₂: a guide to magma chemistry and tectonic provenance
Unlike Mg and Fe²⁺ in spinel trapped in olivine, magmatic abundances of trivalent (Al, Cr) and tetravalent (Ti) cations experience very little, if any, change during post-entrapment re-equilibration because of their low diffusivity in olivine (Roeder & Campbell, 1985; Scowen et al., 1991; Barnes, 1998). To examine the effects of melt composition on the abundances of Al₂O₃ and TiO₂ in spinel, we use compositional pairs of spinel and glass coexisting as: (1) melt inclusions in spinels; (2) melt and spinel inclusions in the same olivine or olivines with similar Fo content; (3) spinel microphenocrysts in natural glass (Table 3).

View this table: [in this window] [in a new window]   Table 3: Average spinel and coexisting melt inclusion–glass compositions

 

A positive correlation between Al₂O₃ and TiO₂ contents in spinel and coexisting melt is demonstrated over significant intervals of averaged spinel and melt compositions (e.g. 3–39 and 4·6–18 wt % Al₂O₃, and 0·04–3·9 and 0·07–3·9 wt % TiO₂, respectively, see Fig. 4a and b and Table 3) sampled from a variety of magmatic types and tectonic environments. Similar covariations also exist even if more restricted compositional intervals represented by the individual samples or suites are considered (Fig. 4c and d). Our interpretation, concurring with other studies of natural rocks (e.g. Crawford, 1980; Dick & Bullen, 1984; Allan et al., 1988; Arai, 1992; Della-Pasqua et al., 1995; Kamenetsky, 1996) and compilation of experimental data (Danyushevsky, 1995), is that these relationships are primarily controlled by magmatic Al₂O₃ and TiO₂ abundances. The correlation of spinel and basaltic melt Al₂O₃ abundances, similar to that in Fig. 4a, has also been shown by experimental studies (Maurel & Maurel, 1982a; Roeder & Reynolds, 1991). The magma compositional control on spinel Al₂O₃, and the presence of Al-rich rims around Cr-rich cores in some spinel phenocrysts and inclusions in olivine (this study and Allan et al., 1988; Allan, 1992; Della-Pasqua et al., 1995) argue against the relatively high pressure of crystallization, which has been suggested to explain the rather aluminous (Al₂O₃ >40 wt %) composition of some spinels in MORB (e.g. Irvine, 1967; Sigurdsson & Schilling, 1976; Sigurdsson, 1977; Fisk & Bence, 1980). We also note that despite a good correlation between TiO₂ in coexisting melt and spinel, low-Al spinel from the island-arc series and LIP are systematically more Ti rich at a given melt TiO₂. This implies that increasing Al activity in the system melt–spinel reduces the partitioning of Ti into spinel.

View larger version (39K): [in this window] [in a new window]   Fig. 4. Spinel–melt compositional relationships expressed in terms of Al2O3 and TiO2 abundances (in wt %). (a, b) average compositions from Table 3. The error bar is equal to 1. Continuous line in (a) is a power best fit through all data; dashed arrowed line shows the correlation from the experimental study by Maurel & Maurel (1982a). Continuous and dashed lines in (b) are power best fits through the high-Al (Al2O3 in melt >14 wt % and in spinel >19 wt %) and low-Al (Al2O3 in melt <14 wt % and in spinel <15 wt %) compositions, respectively. (c, d) Melt inclusions and host spinel from individual suites: I, Macquarie Island; II, FAMOUS area, MAR; III, 43°N, MAR; IV, 54°S, MAR; V, Cape Vogel, Papua New Guinea.

 
The dependence of spinel Al₂O₃ and TiO₂ concentrations on the parental melt composition (Fig. 4) suggests the use of an Al₂O₃ vs TiO₂ diagram to discriminate between spinels that crystallized from different magmas in different geodynamic settings (Fig. 5). Compositions of spinel coexisting with primitive olivine (Fo >84 mol %) from LIP, OIB, MORB [excluding uncommon for MORB high-Ti spinel from 43°N, Mid-Atlantic Ridge (Kamenetsky & Crawford, 1998)] and island-arc magmas form distinct fields with little overlap (Fig. 5a). The exception is spinel from modern back-arc environments, such as Valu Fa Ridge (Lau Basin), Vanuatu and Okinawa troughs, North Fiji, Manus and Woodlark basins. As expected by their transitional and transient setting, back-arc spinel compositions spread across island-arc and MORB fields (Fig. 5b), reflecting the presence of subduction-related and MORB mantle and magma components and diverse melting conditions in this complex setting.

View larger version (24K): [in this window] [in a new window]   Fig. 5. Al2O3 vs TiO2 compositional relationships in spinel inclusions trapped in primitive olivine Fo >84: (a) discrimination between mid-ocean ridge basalt (MORB, grey circles), ocean-island basalt (OIB, grey squares), large igneous province (LIP, and island-arc magmas (ARC, {utri}); (b) spinels from modern back-arc settings show both island-arc and MORB affinities; (c) discrimination between lower-Ti (boninites and tholeiites) and higher-Ti (calc-alkaline and high-K) island-arc series.

 

Island-arc spinels show far more significant overall and intra-sample variations in TiO₂ content than spinels in other magmas (Fig. 5c). These variations probably reflect the fact that parental magmas are also variable in TiO₂ as a result of mixing [e.g. with high-Ti OIB melts (Danyushevsky et al., 1995)]. In general, arc spinel TiO₂ systematics is consistent with whole-rock chemistry and helps to distinguish between boninites–arc tholeiites, and calc-alkaline–high-K series, using a boundary at 0·3–0·4 wt % TiO₂ (Fig. 5c).

Spinel Al₂O₃ (cr-number): inferences for mantle source and melting
Spinel cr-number (or Al₂O₃) values are commonly used to constrain the nature of the mantle peridotite source and the degree of partial melting (e.g. Jaques & Green, 1980; Duncan & Green, 1987; Bonatti & Michael, 1989). Although this is largely correct for mantle peridotites, as the compositional features of other minerals (e.g. olivine Fo, Al₂O₃ in pyroxenes) support trends in the spinel cr-number, the inferences for the parental mantle based on the cr-number of liquidus spinel could be misleading. The projection of cr-number in volcanic spinels onto the so-called Olivine–Spinel Mantle Array (Fo vs cr-number diagram; see Arai, 1987, 1994a) has been used in a number of petrological studies (e.g. Sigurdsson et al., 1993; Arai, 1994b; Sobolev & Danyushevsky, 1994; Sobolev & Nikogosian, 1994; Kamenetsky & Clocchiatti, 1996). However, if this method is applied to compare mantle sources of magmas from flood basalt provinces with boninites or intraplate ocean-island magmas with island-arc tholeiites that all crystallized spinel with similar low Al₂O₃ (high cr-number), the conclusion should be that they have similar refractory, harzburgitic sources. On the other hand, mid-ocean ridge magmas that crystallize high-Al spinel, and are interpreted as products of multistage high-degree partial melting (Danyushevsky et al., 1987; Sobolev & Shimizu, 1993), should, according to Arai’s method, be derived from an undepleted lherzolitic mantle source. This is clearly in conflict with other petrological and geochemical data, including TiO₂ abundances in spinel. Spinel Al₂O₃ abundances depend on the melt composition (Fig. 4), which is a function of pressure, temperature, and degree of partial melting, as well as source chemical and phase compositions. We emphasize here that the compositions of spinel from LIP, OIB and MORB form a continuum on Al₂O₃ vs TiO₂ diagram (Fig. 5a), which extends from the high-Ti, low-Al corner (LIP) to the low-Ti, high-Al end (MORB). This trend is coincident with pronounced changes in the mantle petrological–geochemical characteristics (fertile–enriched to refractory–depleted) and conditions (pressure and temperature) of mantle melting. Pressure and temperature vary from high [e.g. >60 kbar and >1800°C for Siberian meimechites (Sobolev et al., 1992)] through moderate [e.g. 20–40 kar and 1450–1600°C for Hawaiian and West Greenland magmas (Eggins, 1992; Sobolev & Nikogosian, 1994; Gill et al., 1995)] to low values [e.g. 2–22 kbar and 1200–1450°C for mid-ocean ridge primary melts (Danyushevsky et al., 1987; Falloon & Green, 1988; Kinzler & Grove, 1992; Sobolev & Shimizu, 1993)]. As expected, these variations in conditions of melting coincide with changes in magma chemistry, with abundances of Mg, Fe and Ti decreasing, and Si and Al increasing along the trend.

Using the analogy with LIP and OIB spinel, low Al₂O₃ abundance in island-arc spinels may suggest that their parental magmas were also high-Mg high-temperature liquids formed at elevated pressures. This is in agreement with estimates of pressure and temperature of the mantle source for a number of island-arc magmas: for example, East Kamchatka [30–50 kbar, 1500–1700°C (Kamenetsky et al., 1995b)], Tonga [20–25 kbar, 1450–1550°C (Sobolev & Danyushevsky, 1994)], Troodos Upper Pillow Lavas [20–30 kbar, 1380–1550°C (Sobolev et al., 1993)], and Vanuatu [30 kbar, 1300°C (Eggins, 1993)].

The use of spinel chemistry: examples from volcanisedimentary and ancient rocks
Volcaniclastic rocks may have multiple and variable sources. In these rocks detrital spinel may be the only primary igneous mineral not affected by alteration and, therefore, it can provide valuable petrological information. Compositions of detrital spinel may help to recognize and identify the chemical affinity and tectonic provenance of the source rocks.

The discrimination between ‘mantle’ and volcanic spinel is the first step in constraining the spinel compositional populations. We have shown above that spinels from mantle peridotites tend to have statistically lower mg-number than spinel coexisting with primitive olivine (Fo >88) from volcanic rocks (Fig. 3c). However, this criterion is effectively irrelevant for sediments, as olivine is rarely preserved. The more useful variables are TiO₂ and Fe²⁺/Fe³⁺ in spinel. ‘Mantle’ spinels have statistically lower TiO₂ (<0·2 wt %) and higher Fe²⁺/Fe³⁺ (>2) over the whole interval in Al₂O₃ (6–56 wt %) than volcanic spinels, though low-Ti compositions (TiO₂ <0·2 wt %) high Fe²⁺/Fe³⁺ (up to four) compositions also exist among volcanic boninitic and arc tholeiitic spinels (Fig. 6). Compositions of ‘mantle’ spinels can be subdivided into two broadly overlapping fields (Fig. 6) represented by spinel from suprasubduction zone peridotites (lower Al₂O₃) and spinel from MORB-type peridotites (higher Al₂O₃). The compositional affinities of volcanic spinel can be evaluated using the data from Figs 4 and 5.

View larger version (49K): [in this window] [in a new window]   Fig. 6. Discrimination between volcanic and ‘mantle’ spinel using TiO2 (a) and Fe2+/Fe3+ (b). Symbols as in Fig. 3c. Continuous and dashed lines enclose compositions from suprasubduction zone and MORB-type peridotites, respectively.

 

To demonstrate how spinel composition can be used to unravel the tectonic evolution and petrogenesis of altered volcanic and sedimentary rocks in ancient fold belts we present a summary of a study where spinel chemistry has provided valuable information. The Lachlan Fold Belt (LFB) in southeastern Australia contains Cambrian to Devonian volcanic and sedimentary rocks intruded by Silurian and Devonian granitoids (Coney, 1992). The Ordovician rocks of the LFB consist of two contrasting assemblages, quartz-rich continental-derived turbidites, and medium- to high-K volcanics and volcaniclastics. The genetic relationship between these two sequences has long been a problem as the majority of their contacts are fault bounded. The Snowy Mountains terrain is located in the central LFB. It contains a thin block of Ordovician volcaniclastic rocks interbedded with minor volcanics in faulted contact with Ordovician quartz-rich sandstones. The faults on either side of the volcaniclastic rocks contain ophiolitic fragments. The fault to the east contains altered pillow basalts, cherts, meta-dolerite and hyaloclastites. The fault to the west contains serpentinite, meta-dolerite and altered basalts.

The chemistry of spinels from the ophiolitic fragments and volcaniclastic rocks provides important constraints on the relationship between the Ordovician quartz-rich and volcaniclastics sequences. Spinels from a hyaloclastite from the eastern fault have high Al₂O₃ similar to those from MORB or MORB-type back-arc rocks, indicating that they crystallized from a melt containing between 14 and 16 wt % Al₂O₃ (Fig. 7). In contrast, spinels from serpentinites in the western fault zone are low in both TiO₂ and Al₂O₃, indicating that these rocks are fragments of suprasubduction zone mantle peridotites. It is therefore unlikely that the rocks in both fault zones could be derived from a single ophiolite. Spinels from the volcaniclastic sequence have low Al₂O₃ and moderate TiO₂ (Fig. 7), indicating that they crystallized from a melt containing between 10 and 11 wt % Al₂O₃. This melt composition is similar to that of the most primitive shoshonitic basalts, which are interbedded with the volcaniclastic rocks. The spinels were therefore derived locally from the erosion of the basalts and not from erosion of the ophiolitic rocks within the fault zones on either side.

View larger version (32K): [in this window] [in a new window]   Fig. 7. Al2O3 vs TiO2 compositional relationships in spinel from the Snowy Mountains rocks (Ordovician Lachlan Fold Belt, SE Australia). Spinels are from volcaniclastic sandstone (grey crosses), hyaloclastite (grey diamonds) and serpentinites (grey triangles) are compared with compositional fields of spinel from volcanic rocks and mantle peridotites (see Figs 5 and 6a).

 

The chemistry of the spinels from the Snowy Mountains shows that the Late Ordovician volcaniclastics are separated from the Early Ordovician quartz-rich turbidites by both MORB-type and SSZ-type ophiolites. This suggests that the sedimentary rocks originated from shoshonitic volcanics in a separate tectonic environment to the continental turbidites and were juxtaposed during either strike-slip tectonics and/or the closure of major oceanic basins.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION SAMPLE DESCRIPTION AND... RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
At magmatic conditions, spinel mg-number is a function of melt mg-number and Al₂O₃, whereas at near- and post-magmatic conditions it is controlled by the rate of cooling and re-equilibration with the silicates. Within a given volcanic suite, spinel mg-number can be used as a measure of the extent of crystal fractionation. The mg-number–cr-number relationships in spinel from altered volcanic rocks or sediments can help to approximate compositions of olivine, which once coexisted with spinel.

Abundances of Al₂O₃ and TiO₂ in magmatic spinel are mainly controlled by contents of these oxides in the parental melts, and thus can be used to discriminate between different magma types, their tectonic affinities and mantle sources. The application of spinel Al₂O₃ (cr-number) alone to directly constrain the mantle source composition is not confirmed in this study.

The compositional features of spinel and the differences in TiO₂ and Fe²⁺/Fe³⁺ between magmatic and mantle spinel can be used in characterizing altered magmatic rocks, and the sources of detrital spinels in clastic sedimentary rocks, thus contributing to the understanding of the tectonic development of source geological terrains.

	ACKNOWLEDGEMENTS

 
The authors are indebted to Maya Kamenetsky, who made this study possible by patiently separating, mounting and polishing a myriad of olivine and spinel grains. Microprobe work was assisted by I. Sigurdsson, N. Kononkova, W. Jablonski, and D. Steele. We would like to thank H. Bougault, J. W. Bristow, W. E. Cameron, S.-L. Chung, R. Clocchiatti, D. H. Green, B. J. Griffin, S. Eggins, R. Hekinian, N. Métrich, R. Mühe, A. K. Pedersen, A. P. le Roex, R. Shinjo, A. Sobolev, S. Zlobin, and many others for providing samples of spinel-bearing rocks. Special thanks go to K. Stait for keeping the rockstore of the University of Tasmania in perfect order. Hugh O’Neill and Greg Yaxley helped with some heating experiments. We appreciate numerous discussions with M. Portnyagin, L. Danyushevsky, A. Sobolev, F. Della-Pasqua, A. Gurenko, G. Yaxley and M. Gasparon. Steve Eggins and Leonid Danyushevsky refined our understanding of spinel–olivine re-equilibration. We thank Leonid Danyushevsky for comments on an earlier version of this paper, and Steve Barnes and Brent McInnes for formal reviews. This work was funded by ARC Large Grants to A.J.C., an ARC Research Fellowship Grant to V.S.K. and funding from the Australian Research Council’s Research Centres Program.

	FOOTNOTES

 
^*Corresponding author. Fax: 61-3-62232547. E-mail: Dima.Kamenetsky{at}utas.edu.au

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