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Keywords:

  • chrome spinel;
  • crystallization course;
  • greenstone;
  • N-MORB;
  • Paleozoic–Mesozoic Mino terrane;
  • zoning of chrome spinel

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological Settings and Nature of Greenstones
  5. Occurrence and Chemistry of Chrome Spinel
  6. Discussion
  7. Acknowlegements
  8. References

Phenocrystic chrome spinel crystallized in normal MORB-type greenstones in the East Takayama area. Associated phenocryst minerals show a crystallization sequence that was olivine first, followed by plagioclase, and finally clinopyroxene. Chrome spinel ranges from 0.54 to 0.77 in Mg/(Mg+Fe2+) and 0.21 to 0.53 in Cr/(Cr+Al); the Fe3+ content varies from 0.07 to 0.22 p.f.u. (O = 4). Significant compositional differences of spinel were observed among the phenocryst mineral assemblages. Chrome spinel in the olivine–spinel assemblage shows a wide range in Cr/(Cr+Al), and is depleted in Fe2+ and Fe3+. Chrome spinel in the olivine–plagioclase–clinopyroxene–spinel assemblage is Fe2+- and Fe3+-rich at relatively high Cr/(Cr+Al) ratios. Basalt with the olivine–plagioclase–spinel assemblage contains both aluminous spinel and Fe2+- and Fe3+-rich spinel. The assumed olivine–spinel equilibrium suggests that chrome spinel in the olivine–spinel assemblage changed in composition from Cr- and Fe2+-rich to Al- and Mg-rich with the progress of fractional crystallization. Chrome spinel in the olivine–plagioclase–clinopyroxene–spinel assemblage, on the other hand, exhibits the reversed variations in Mg/(Mg+Fe2+) and in Cr/(Cr+Al) ratios that decrease and increase with the fractional crystallization, respectively. The entire crystallization course of chrome spinel, projected onto the Mg/(Mg+Fe2+)–Cr/(Cr+Al) diagram, exhibits a U-turn, and appears to be set on a double-lane route. The U-turn point lies in the compositional field of chrome spinel in the olivine–plagioclase–spinel assemblage, and may be explained by plagioclase fractionation that began during the formation of the olivine–plagioclase–spinel assemblage.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological Settings and Nature of Greenstones
  5. Occurrence and Chemistry of Chrome Spinel
  6. Discussion
  7. Acknowlegements
  8. References

Chrome spinel crystallizes over a wide range of compositions as a minor phase from mafic-ultramafic magma, and is considered to be an early-stage crystallization product. The composition of spinel changes according to magma type (Dick & Bullen 1984; Arai 1992; Agata & Hattori 2002), and also varies as a function of magmatic differentiation (Agata 1988; Abzalov 1998). Many authors (e.g. Wilkinson & Hensel 1988; Scowen et al. 1991; Agata 1994) have investigated chrome spinel in basalts and cumulate peridotites, and proposed spinel crystallization courses, such as the trend of Fe- and Ti-enrichment (Barnes & Roeder 2001). Chrome spinel in MORB, however, varies extensively in its Cr/(Cr+Al) and Mg/(Mg+Fe2+) ratios even within a single specimen (Sigurdsson & Schilling 1976; Allan et al. 1989; Natland 1989). The composition may be sensitive to magma composition, coexisting minerals (Henderson & Wood 1981; Bell & Claydon 1992; O'Driscoll et al. 2010), and kinetic processes including cooling rate of magma (Roeder et al. 2003). Spinel composition, therefore, cannot be correlated sufficiently with the stages of spinel crystallization, and the crystallization course of chrome spinel in MORB remains poorly understood.

Chrome spinel occurs in normal (N)-MORB-type greenstones of the Mino terrane, East Takayama area (Agata et al. 2009); it forms phenocrystic grains and is commonly associated with phenocrystic plagioclase and clinopyroxene in addition to olivine. Chrome spinel coexisting with only phenocrystic olivine is occasionally present, and the grains in places show conspicuous compositional zoning. This paper describes the occurrence and chemistry of chrome spinel in the East Takayama area, and discusses the crystallization course of chrome spinel in the East Takayama N-MORB. The discussion herein extends to the genesis of Cr–Al spinel zoning that is common in basalts of oceanic ridge regions (Sigurdsson 1977; Roeder et al. 2006).

The regional geology of the Mt. Hikagedaira district, including the East Takayama area, has been investigated by Adachi and Kojima (1983) and Adachi et al. (1992). These researchers have shown that the Mino terrane consists essentially of Permian greenstones, limestones and cherts, Triassic diamictites, and Jurassic clastic rocks, such as diamictites, shales and sandstones; the rocks comprise an extensive Jurassic mélange. Agata et al. (2009) have studied the stratigraphy and geochemistry of greenstones in the East Takayama area, and have proposed that the volcanic sequence of greenstones was N-MORB first, followed by Iceland-type tholeiite and finally alkalic basalt.

Geological Settings and Nature of Greenstones

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological Settings and Nature of Greenstones
  5. Occurrence and Chemistry of Chrome Spinel
  6. Discussion
  7. Acknowlegements
  8. References

Greenstones that range in age from the Permian to the Triassic are extensively developed in the Mino terrane (Fig. 1), and are typically associated with limestones and cherts; these greenstones are generally considered to form parts of the middle Mesozoic accretionary complex. Greenstone bodies have been understood as fragments of oceanic materials such as plume-influenced ridge basalt (Jones et al. 1993), oceanic island basalt within a plate (Agata & Hattori 2002), and oceanic plateau basalt formed during a super-plume event (Ichiyama & Ishiwatari 2005).

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Figure 1. Distribution of the Mino terrane and the locality of the East Takayama area. Modified after Adachi and Kojima (1983).

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A greenstone–limestone–chert sequence of the Lower Permian age is exposed extensively in the East Takayama area in the Mino terrane (Isomi & Nozawa 1957; Yamada et al. 1985). The margins of the sequence terminate against Triassic diamictites and Jurassic clastic rocks by faults. The contacts of greenstones with limestones appear to be generally conformable; cherts exhibit tectonic contacts with greenstones. The greenstone–limestone–chert sequence is divided into many tectonic blocks by E–W trending faults. Greenstones form pillow lavas commonly and pyroclastic rocks rarely; they strike E–W with nearly vertical dips.

The greenstone body is biostratigraphically divided into two units: Unit I and II in ascending order. Unit I consists of greenstones intercalated with lenses of limestones containing fusuline fossils of the upper Pseudoschwagerina zone. Unit II is overlain conformably by limestones of the lower Parafusulina zone. The greenstones of Unit II are slightly younger than the greenstones of Unit I. The Unit I greenstones predominate in the East Takayama area, and comprise an about 1000 m-thick sequence of tholeiites (Fig. 2). Unit II consists predominantly of alkalic basalts and trachytic rocks; it terminates against the Unit I rocks at faults (Agata & Adachi 2007).

figure

Figure 2. Schematic columnar section of the greenstone sequence.

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Most of the Unit I basalts contain olivine, plagioclase and clinopyroxene as essential phenocryst minerals; olivine basalts without phenocrystic plagioclase and clinopyroxene occur in small amounts. Phenocrystic chrome spinel commonly occurs in the lower portion of the Unit I sequence. Groundmass minerals commonly include olivine, plagioclase and clinopyroxene. Phenocrysts of the Unit II basalts are generally olivine and plagioclase locally with clinopyroxene. The groundmass consists predominantly of olivine, plagioclase and clinopyroxene.

Greenstones in the East Takayama area have undergone a regional metamorphism of pumpellyite–prehnite facies. Secondary minerals such as chlorite, pumpellyite, prehnite, albite, and calcite are widespread in the greenstone body. Olivine was entirely replaced by chlorite. Plagioclase was altered commonly to albite, and locally to pumpellyite and prehnite. Chrome spinel was in places replaced partly by ferritchromit, which is presumably a metamorphic mineral.

The Unit I greenstones were apparently marine basalts. They present a wide range of Ti content with low Zr/Y ratios and in some cases are depleted in Nb. The greenstones are probably basalts that were formed in an oceanic ridge region (Agata et al. 2009). Agata et al. also have classified the Unit I basalts by using a parameter ΔNb {= 1.74 + log(Nb/Y)-1.92log(Zr/Y)}, which discriminates between N-MORB with negative and Iceland-type basalt with positive values (Fitton et al. 1997). Basalts in the lower portion of the Unit I sequence are negative in ΔNb, and are considered to be N-MORB. The upper portion of the sequence consists of ΔNb-positive tholeiites. Transitional-type basalts (ΔNb ≈ 0) occur in the middle portion of the sequence of Unit I.

Occurrence and Chemistry of Chrome Spinel

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological Settings and Nature of Greenstones
  5. Occurrence and Chemistry of Chrome Spinel
  6. Discussion
  7. Acknowlegements
  8. References

Appreciable amounts of phenocrystic chrome spinel generally crystallized in N-MORB of the East Takayama area. Chrome spinel also occurs in some Iceland-type tholeiites and alkalic basalts in trace amounts. Phenocryst mineral assemblages in chrome spinel-bearing N-MORB are commonly olivine–plagioclase–clinopyroxene–spinel, occasionally olivine–spinel, and rarely olivine–plagioclase–spinel (Table 1).

Table 1. Minerals, ΔNb and Cr content of chrome spinel-bearing basalts and textural features of chrome spinel from the East Takayama area
SampleRock typePhenocrystGroundmassΔNbaCr contenta (ppm)Zoning of chrome spinelGlass inclusions in spinel grains
  1. ΔNb = 1.74 + log(Nb/Y)-1.92log(Zr/Y) (Fitton et al. 1997).

  2. I-tholeiite: Iceland-type tholeiite; A-basalt: alkalic basalt;Ol: olivine; Plag: plagioclase; Cpx: clinopyroxene; Sp: spinel; Ilm: ilmenite; No: not observed.

  3. a

    Data is after Agata et al. (2009).

NM-27N-MORBOl SpOl Plag Ilm−0.411072PresentPresent
NM-5N-MORBOl SpOl Plag Ilm−0.49767PresentPresent
NM-6N-MORBOl Plag SpOl Plag Cpx Ilm−0.52590NoPresent
CH-1N-MORBOl Plag Cpx SpOl Plag Cpx Ilm−0.48335NoPresent
CH-2N-MORBOl Plag Cpx SpOl Plag Cpx Ilm−0.49718NoPresent
CH-3N-MORBOl Plag Cpx SpOl Plag Cpx Ilm−0.39253NoPresent
SH-10N-MORBOl Plag Cpx SpOl Plag Cpx Ilm−0.59279NoNo
NI-7I-tholeiiteOl Plag SpOl Plag Ilm+0.24730NoNo
SI-2A-basaltOl Plag Cpx SpOl Plag Cpx Ilm+0.22110NoNo

Chrome spinel occurs as inclusions in olivine phenocryst of N-MORB; it also forms discrete grains scattered in the groundmass. Chrome spinel typically shows octahedral grains commonly with rounded corners, and rarely forms chains of attached octahedra. Chrome spinel grains generally range from 0.1 to 0.5 mm in size, and vary in color from light brown to dark reddish brown in thin sections. Chrome spinel in Iceland-type tholeiite and alkalic basalt occurs as in N-MORB.

Spinel grains in N-MORB commonly contain inclusions (Fig. 3a) that consist of mixtures of fine-grained secondary minerals including chlorite and calcite. The inclusions, reaching 0.2 mm in diameter, generally show octahedral outlines with rounded corners like chrome spinel grains (Fig. 3a); they appear as negative crystals of host spinel and are presumably surrounded by {111} planes of spinel. The morphology is similar to that of inclusions in spinel grains described as hopper crystals in other MORBs (e.g. Allan et al. 1988; Roeder et al. 2001). The inclusions in the East Takayama area presumably were originally of glass as described in other fresh MORBs. Spinel grains containing the inclusions were probably formed as the result of rapid crystallization from super-cooled magma as suggested by Roeder et al. (2001).

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Figure 3. Back-scattered electron (BSE) images of chrome spinel. (a) Chrome spinel showing a euhedral rounded outline. It contains a relatively large inclusion of mixture of fine grained minerals including chlorite and calcite. The outline of the inclusion is similar to that of the chrome spinel grain. Sample CH-1. (b) Zoned chrome spinel. Light-gray core, enriched in Cr, is rimmed by dark-gray margins that are rich in Al. Sample NM-27.

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Conspicuous compositional zoning of chrome spinel is observed in the East Takayama N-MORB with the olivine–spinel phenocryst mineral assemblage. Figure 3b exhibits a BSE (back-scattered electron) image of a zoned chrome spinel grain. The image shows that the light-gray core of spinel, which is enriched in heavy elements including Cr, is rimmed by dark-gray margins, which are rich in light elements such as Al. The dark-gray rim generally ranges from 0.05 to 0.1 mm in width. The occurrence of conspicuous zoning is limited to basalt with the olivine–spinel phenocryst mineral assemblage. The texture of zoned spinel grains is not significantly different from that of unzoned grains; zoned spinel generally forms euhedral grains with rounded corners, mostly ranges from 0.1 to 0.4 mm in size, and occasionally contains glass inclusions.

Chrome spinel was analyzed on a JEOL electron probe X-ray microanalyzer, Model JXA 8800R (JEOL, Tokyo, Japan). The accelerating voltage, specimen current, and beam diameter were 15 kV, 1.2−1.3 × 10−8 A, and 3 μm, respectively. Fe2O3 contents were calculated assuming an ideal spinel chemical formula. Some analyses have been given in the previous paper (Agata et al. 2009). New analysis of spinel in the East Takayama N-MORB was carried out, and representative analyses are listed in Table 2.

Table 2. Representative electron microprobe analyses of chrome spinel in N-MORB from the East Takayama area
SampleNM-27NM-5NM-6NM-6CH-2SH-10
CoreRimCoreRim
  1. a

    Calculated assuming the ideal spinel chemical formula.

  2. Mg#: Mg/(Mg+Fe2+); Cr#: Cr/(Cr+Al); Fe3+#: Fe3+/(Cr+Al+Fe3+).

SiO20.060.120.100.100.070.060.040.08
TiO20.270.260.340.270.190.530.721.00
Al2O328.8243.5834.4142.6243.7629.5324.7223.35
Cr2O336.8420.7831.8023.2821.3133.0237.6636.62
Fe2O3a4.634.804.074.104.746.927.708.69
FeO12.6111.3211.6211.0410.4913.8414.5615.65
MnO0.170.110.200.160.160.210.280.24
MgO15.4117.7916.7518.0518.3214.7314.0413.14
NiO0.190.270.270.270.280.290.190.16
CaO0.030.070.050.020.020.060.040.17
Total99.0399.1099.6199.9199.3499.1999.9599.10
Cation ratios based on O = 4        
Si0.0020.0030.0030.0030.0020.0020.0010.002
Ti0.0060.0050.0070.0060.0040.0120.0160.023
Al1.0131.4271.1681.3901.4251.0390.9030.852
Cr0.8670.4560.7230.5080.4650.7780.8860.895
Fe3+0.1040.1000.0880.0850.0980.1550.1760.202
Fe2+0.3140.2630.2790.2550.2420.3450.3690.405
Mn0.0040.0030.0050.0040.0040.0050.0070.006
Mg0.6840.7350.7180.7430.7530.6550.6350.605
Ni0.0050.0060.0060.0060.0060.0070.0050.004
Ca0.0010.0020.0020.0010.0010.0020.0010.006
Mg#0.6850.7360.7200.7440.7570.6550.6320.599
Cr#0.4610.2420.3820.2680.2460.4280.4950.512
Fe3+#0.0520.0500.0440.0430.0490.0790.0900.104

The composition of chrome spinel varies from grain to grain within a single sample in the N-MORB of Unit I. The extent of grain-to-grain compositional variation within a single sample reaches 0.14 in Mg/(Mg+Fe2+) and 0.27 in Cr/(Cr+Al). No systematic compositional difference was observed between spinel grains forming inclusions in olivine and scattered in the groundmass. Mg/(Mg+Fe2+) and Cr/(Cr+Al) ratios of chrome spinel lie between 0.54 to 0.77 and 0.21 to 0.53, respectively. The Mg/(Mg+Fe2+) ratio appears to increase with decreasing Cr/(Cr+Al) (Fig. 4a). Fe3+ content varies from 0.07 to 0.22 p.f.u. (per formula unit calculated on the basis O = 4), and appears to increase with increasing Cr/(Cr+Al) (Fig. 5a). A similar increase in Fe3+ is common in MORBs of other areas (Dick & Bullen 1984; Barnes & Roeder 2001). Ti content of chrome spinel is low (0.005−0.023 p.f.u.) in the East Takayama area. The compositions of chrome spinel in Iceland-type tholeiite and alkalic basalt of the East Takayama area are shown in Figures 4b and 5b; these basalts, however, contain only trace amounts of spinel, and the details of the chemical nature are unknown.

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Figure 4. Plots of chrome spinel in the Mg/(Mg+Fe2+)–Cr/(Cr+Al) variation diagram. (a) and (b) Chrome spinel in greenstones from the East Takayama area. Closed circle: N-MORB; open square: Iceland-type tholeiite; open circle: alkalic basalt. (c) Compositional field of chrome spinel in other volcanic rocks. MORB (Sigurdsson & Schilling 1976; Dick & Bullen 1984; Roeder & Reynolds 1991); IAT, island-arc tholeiite (Barnes & Roeder 2001); HT, Hawaiian tholeiite (Evans & Wright 1972; Nicholls & Stout 1988; Wilkinson & Hensel 1988).

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figure

Figure 5. Chrome spinel from the East Takayama area plotted on the CrAlFe3+ variation diagram. Chrome spinel in greenstones with (a) N-MORB affinity, and (b) Iceland-type tholeiite and alkalic basalt affinity. Symbols as in Figure 4.

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Core-to-rim compositional variation within zoned chrome spinel grains is shown in Figure 6. Cr/(Cr+Al) ratio considerably decreases from core to rim, whereas Mg/(Mg+Fe2+) ratio moderately increases in most cases. Fe3+ content appears to be generally constant. The extent of core-to-rim compositional variations reach 0.09 in Mg/(Mg+Fe2+) and 0.23 in Cr/(Cr+Al). An appreciable decrease in Cr/(Cr+Al) and an increase in Mg/(Mg+Fe2+) for negative crystal-shaped glass inclusions are rarely observed, but the compositional variation is generally insignificant near inclusions.

figure

Figure 6. Zoned chrome spinel plotted on (a) the Mg/(Mg+Fe2+)–Cr/(Cr+Al) and (b) the Fe3+Cr/(Cr+Al) variation diagram. Arrows indicate the core-to-rim variations of chrome spinel in N-MORB samples with the olivine–spinel phenocryst assemblage (samples NM-5 and NM27).

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Significant compositional differences of chrome spinel in N-MORB are observed among hosts with different phenocryst assemblages (Figs 7, 8), though the tendency of decreasing Cr/(Cr+Al) with an increase in Mg/(Mg+Fe2+) is common to all the assemblages. Chrome spinel in the olivine–spinel assemblage shows a wide range in Cr/(Cr+Al) at relatively high Mg/(Mg+Fe2+) ratios; it is generally depleted in Fe3+. Chrome spinel in the olivine–plagioclase–clinopyroxene–spinel assemblage exhibits relatively high Cr/(Cr+Al) ratios; it is enriched in Fe2+ and Fe3+ relative to Cr/(Cr+Al) compared to the spinel in the olivine–spinel assemblage. Basalt with the olivine–plagioclase–spinel assemblage contains both aluminous spinel and Fe2+- and Fe3+-rich spinel that are characteristic of the olivine–spinel and olivine– plagioclase–clinopyroxene–spinel assemblages, respectively.

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Figure 7. Compositional difference of chrome spinel in N-MORB with different phenocryst assembalges in the Mg/(Mg+Fe2+)–Cr/(Cr+Al) variation diagram; (a) olivine–spinel, (b) olivine–plagioclase–spinel and (c) olivine–plagioclase–clinopyroxene–spinel. Ol, olivine; Plag, plagioclase; Cpx, clinopyroxene; Sp, spinel. Fine curves represent olivine–spinel equipotential curves (Irvine 1965), which were calculated using the spinel thermodynamic model of Sack and Ghiorso (1991); arrows denote the crystallization trend of chrome spinel.

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figure

Figure 8. Compositional difference of chrome spinel in N-MORB with different phenocryst assemblages in the Fe3+Cr/(Cr+Al) variation diagram; (a) olivine–spinel, (b) olivine–plagioclase–spinel and (c) olivine– plagioclase–clinopyroxene–spinel phenocryst assemblage. Symbols as in Figure 7.

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Chrome spinel in the East Takayama N-MORB is relatively low in Cr/(Cr+Al) and characteristically depleted in Fe3+ and Ti. It is comparable to spinel in other MORBs (e.g. Bryan & Moore 1977). Similar chrome spinel has been reported in some late-Paleozoic greenstones in the Tamba terrane adjoining the Mino terrane (Koizumi & Ishiwatari 2006), but spinel in most greenstones in the Mino-Tamba belt is higher in Cr/(Cr+Al) and richer in Fe3+ and Ti (Agata & Hattori 2002; Ichiyama et al. 2008).

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological Settings and Nature of Greenstones
  5. Occurrence and Chemistry of Chrome Spinel
  6. Discussion
  7. Acknowlegements
  8. References

Crystallization Course of Chrome Spinel in the East Takayama N-MORB

Olivine–spinel equipotential curves in the Mg/(Mg+Fe2+)–Cr/(Cr+Al) diagram (Irvine 1965) were calculated at a fixed temperature for a given Fo mole % of olivine and Fe3+ in spinel. We used the spinel thermodynamic model of Sack and Ghiorso (1991) in our calculation, and the results are shown in Figure 7. Equipotential curves in the diagram shift to the left nearly in parallel as temperature or Fo mole % of olivine decreases or when Fe3+ content of spinel rises.

Spinel in the East Takayama N-MORB with the olivine–spinel assemblage is plotted along a curve in the Mg/(Mg+Fe2+)–Cr/(Cr+Al) diagram in Figure 7a. The curve intersects the equipotential curves. The Fe3+ content of the spinel is relatively constant (Fig. 8a). The intersection implies a change in temperature and/or Fo during crystallization of the chrome spinel. The variation of spinel compositions in Figure 7a indicates that chrome spinel decreases in Cr/(Cr+Al) and increases in Mg/(Mg+Fe2+) with decreasing temperature or Fo in olivine or both. The change in composition from Cr- and Fe2+-rich to Al- and Mg-rich spinel probably occurred as the result of fractional crystallization. The decreasing trend of Cr/(Cr+Al) during the crystallization of olivine and spinel has been shown in partial melting experiments of MORB (Fisk & Bence 1980). Similar decreases have been reported in ultramafic cumulates of ophiolite sequences (Himmerberg & Loney 1980; Ozawa 1983; Agata 1988).

Plots of chrome spinel in the East Takayama N-MORB with the olivine–plagioclase–clinopyroxene–spinel assemblage form a curve that intersects the equipotential curves in the Mg/(Mg+Fe2+)–Cr/(Cr+Al) diagram (Fig. 7c). The curve of the plots shows an increase in Cr/(Cr+Al) and a decrease in Mg/(Mg+Fe2+) with lowering temperature and/or lowering Fo and/or rising Fe3+ content of spinel. The Fe3+ content of chrome spinel increases with increasing Cr/(Cr+Al) (Fig. 8c). Fe3+-rich chrome spinel is generally considered to form at late stages of chrome spinel crystallization (Eales & Marsh 1983; Wilkinson & Hensel 1988; Agata 1994); the Fe3+-increase accompanies reducing temperature and Fo in olivine. The intersection with the equipotential curves in Figure 7c seems to be the result of fractional crystallization. The crystallization trend of chrome spinel probably takes place from Al- and Mg-rich to Cr- and Fe2+-rich spinel in the olivine–plagioclase–clinopyroxene–spinel assemblage. The trend appears to be opposite to that of spinel in the olivine–spinel assemblage. Similar Al- and Mg-decreasing trends have been reported in other MORBs (Fisk & Bence 1980; Roeder et al. 2003), and experimental results have shown that chrome spinel from MORB increases its Cr/(Cr+Al) ratio with fractionation after plagioclase begins to crystallize (Roeder & Reynolds 1991).

The ΔNb value, which discriminates between magma sources of marine basalts (Fitton et al. 1997), is negative in the East Takayama chrome spinel-bearing N-MORBs (−0.4 to −0.6) (Table 1). These N-MORBs probably have a common mantle source chemically identical to that of modern N-MORBs (Agata et al. 2009). Sample NM-27, most primitive among the East Takayama N-MORBs, contains exclusively olivine and spinel as phenocrysts, and has the highest Cr content (Table 1). The variation in phenocryst assemblage, combined with Cr content of whole-rock, suggests that the crystallization sequence of minerals was olivine and chrome spinel first, followed by plagioclase, and finally clinopyroxene. The crystallization sequence is identical to that of other N-MORBs (e.g. Irvine 1979). The spinel compositional difference between the phenocryst assemblages (Figs 7, 8) also suggests that the phenocryst assemblage changes from olivine–spinel through olivine–plagioclase–spinel to olivine–plagioclase–clinopyroxene–spinel as the temperature and/or Fo decreases. The Al-increasing trend in the olivine–spinel assemblage probably represents early-stage crystallization course of spinel, and the Cr-increasing trend in the olivine–plagioclase–clinopyroxene–spinel assemblage is for late-stage crystallization course. The entire crystallization course of chrome spinel in the Mg/(Mg+Fe2+)–Cr/(Cr+Al) and Cr–Al–Fe3+ diagrams is depicted in Figure 9. The crystallization course has a U-turn in the diagrams and appears as if it were set on a double-lane route. The U-turn point is located near the most aluminous end in the compositional field of chrome spinel in the olivine–spinel assemblage; it also lies in the field of chrome spinel in the olivine–plagioclase–spinel assemblage (Figs 7-9). The U-turn probably took place when plagioclase began to crystallize. Chrome spinel formed at the early stage of crystallization is similar in Cr/(Cr+Al) to spinel at the late stage but is more enriched in Mg and depleted in Fe2+ and Fe3+. Chrome spinel is most aluminous in the middle stage of spinel crystallization. Similar results have been obtained from melting experiments in the system Mg2SiO4–CaMgSi2O6–CaAl2Si2O8–MgCr2O4–SiO2 (Irvine 1977).

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Figure 9. Summary of crystallization course of chrome spinel in (a) the Mg/(Mg+Fe2+)–Cr/(Cr+Al) and (b) the CrAlFe3+ variation diagram. Open arrow: crystallization course before plagioclase crystallization; closed arrow: crystallization course with crystallization of plagioclase.

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The Al content of spinel is controlled presumably by Al distribution between spinel and magma. Partial melting experiments of MORB (Maurel & Maurel 1982) have shown that the Alspinel/Almagma distribution coefficient increases with increasing Al2O3 content of melt and exhibits little dependence on temperature. The Al2O3 content of magma in the East Takayama N-MORB apparently increased with fractionation before plagioclase crystallization; it presumably decreased after plagioclase began to crystallize, as is usual in MORB (Irvine 1979). The distribution coefficient appears to have increased in the early–middle stage of spinel crystallization and decreased in the middle–late stage. The U-turn in the crystallization course of chrome spinel may be understood in terms of the compositional change of MORB magma with fractionation.

Implications to the Genesis of Zoning of Chrome Spinel

Conspicuously zoned spinel grains occur in the olivine–spinel assemblage of the East Takayama N-MORB. The zoning of chrome spinel generally shows considerable decrease in Cr/(Cr+Al) and moderate increase in Mg/(Mg+Fe2+) from core to rim (Fig. 6a); the Fe3+ content, in most cases, is relatively constant (Fig. 6b). Similar zoning has been reported in other MORBs (Allan et al. 1989; Roeder et al. 2006). Cr–Al zoning of spinel also occurs in basalts from the Yakuno ophiolite, Japan (Ishiwatari 1985). The zoning of increasing Mg and Al toward the rim in MORB is generally not thought to be the result of simple fractional crystallization, because the general spinel crystallization trend is considered to be Cr-increasing (e.g. Allan et al. 1988; Roeder et al. 2001).

The causes of the chrome spinel zoning, except for fractional crystallization, include reaction of xenocrystic spinel with the host magma (Fisk & Bence 1980; Agata & Hattori 2002), magma mixing (Allan et al. 1988), and kinetic effects (Thy 1983; Longhi et al. 1993). Chrome spinel in the East Takayama area shows no fragmental textures; there does not appear to have been a reaction of xenocrystic spinel with magma. The view of magma mixing rests on the premise that chrome spinel shows reverse zoning (Allan et al. 1989). The core-to-rim compositional variation in the East Takayama chrome spinel, which occurs in the olivine–spinel assemblage, is concordant with the spinel crystallization trend before plagioclase begins to crystallize (Figs 6a,9). The core-to-rim variation is not reverse zoning; there is no evidence for magma mixing. The kinetic effects that result in zoning include diffusion-controlled rapid growth of spinel from super-cooled magma (Roeder et al. 2001, 2003). The East Takayama chrome spinel occasionally contains negative crystal-shaped glass inclusions, which are indicative of rapid crystallization; the kinetic effects may have caused the core-to-rim compositional variation. There is, however, no difference in crystal habit that suggests difference in crystal growth between zoned and unzoned grains: these grains are similar in size, generally show euhedral outlines with rounded corners, and occasionally contain glass inclusions. The kinetic effects on zoning formation seem to have been generally not large.

The crystallization course of chrome spinel in the East Takayama area comprises two crystallization trends: one is Mg- and Al-increasing, and the other is Mg- and Al-decreasing (Fig. 9). Zoned chrome spinel, coexisting with olivine alone, generally shows no core-to-rim inconsistency with the crystallization course prior to the plagioclase crystallization. The zoning of Mg- and Al-increase toward the rim probably formed mainly as the result of fractional crystallization. Certain measured core-to-rim variations show a trend that is somewhat different from that of other variations. The chrome spinel decreases in Cr/(Cr+Al) from core to rim, but is relatively constant in Mg/(Mg+Fe2+) (Fig. 6). It also exhibits significant decrease in Fe3+, which is inconsistent with a common view that the Fe3+ content of chrome spinel increases with fractionation or in some cases remains constant (e.g. Ozawa 1983; Agata 1994; Roeder et al. 2003). We think that the somewhat different trend may be the result of rather high compositional modification by diffusion-controlled crystallization.

Acknowlegements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological Settings and Nature of Greenstones
  5. Occurrence and Chemistry of Chrome Spinel
  6. Discussion
  7. Acknowlegements
  8. References

We thank Prof. K. Suzuki of Nagoya University for discussion and advice. The electron probe microanalysis was conducted at the Department of Earth and Planetary Sciences, Nagoya University. We also acknowledge two referees, Profs. A. Ishiwatari of Tohoku University and S. Arai of Kanazawa University, who have given helpful advice. This work was partly supported by a Grant for Scientific Research and Publication from Nagoya Keizai University.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Geological Settings and Nature of Greenstones
  5. Occurrence and Chemistry of Chrome Spinel
  6. Discussion
  7. Acknowlegements
  8. References
  • Abzalov M. Z. 1998. Chrome-spinels in gabbro-wehrlite intrusions of the Pechenga area, Kola Peninnsula, Russia: Emphasis on alteration features. Lithos 43, 109134.
  • Adachi M. & Kojima S. 1983. Geology of the Mt. Hikagedaira area, east of Takayama, Gifu Prefecture, central Japan. Journal of Earth Sciences, Nagoya University 3, 3767.
  • Adachi M., Kojima S., Wakita K., Suzuki K. & Tanaka T. 1992. Transect of central Japan: From Hida to Shimanto. In Adachi M. and Suzuki K. (eds.) 29th IGC Field Trip Guidebook 1, Paleozoic and Mesozoic Terranes:Basement of the Japanese Island Arc, pp. 148178, Nagoya University, Nagoya.
  • Agata T. 1988. Chrome spinels from the Oura layered igneous complex, central Japan. Lithos 21, 97108.
  • Agata T. 1994. The Asama igneous complex, central Japan: An ultramafic-mafic layered intrusion in the Mikabu greenstone belt, Sambagawa metamorphic terrain. Lithos 33, 241263.
  • Agata T. & Adachi M. 2007. Chromite-bearing alkalic basalt in the greenstone body, Mino Paleozoic-Mesozoic terrane in the east Takayama area, central Japan. Journal of Natural Science Society, Nagoya Keizai University 41, 718 (in Japanese with English abstract).
  • Agata T., Adachi M. & Tsuboi M. 2009. Greenstones in the Mino Paleozoic-Mesozic terrane of the east Takayama area, central Japan: Evidence for magmatism evolution from normal ridge to plume volcanism. Journal of Geology 117, 415427.
  • Agata T. & Hattori I. 2002. Chromite in greenstone lavas from the Kanakasu area, Nanjo Massif of the Mesozoic Mino terrane, central Japan. Mineralogical Magazine 66, 575590.
  • Allan J. F., Batiza R., Perfit M. R., Fornari D. J. & Sack R. O. 1989. Petrology of lavas from the Lamont seamount chain and adjacent East Pacific rise, 10°N. Journal of Petrology 30, 12451298.
  • Allan J. F., Sack R. O. & Batiza R. 1988. Cr-rich spinels as petrologenetic indicator: MORB-type lavas from the Lamont seamount chain, eastern Pacific. American Mineralogist 73, 741753.
  • Arai S. 1992. Chemistry of chromian spinel in volcanic rocks as a potential guide to magma chemistry. Mineralogical Magazine 56, 173184.
  • Barnes S. J. & Roeder P. L. 2001. The range of spinel compositions in terrestrial mafic and ultramafic rocks. Journal of Petrology 42, 22792303.
  • Bell B. R. & Claydon R. V. 1992. The cumulus and postcumulus evolution of chrome-spinels in ultrabasic layered intrusions: Evidence from the Cuillin Igneous complex, Isle Skye, Scotland. Contributions to Mineralogy and Petrology 112, 242253.
  • Bryan W. B. & Moore J. G. 1977. Compositional variations of young basalts in the Mid-Atlantic Ridge rift valley near la. 36°49′N. Geological Society of America Bulletin 88, 556570.
  • Dick H. J. B. & Bullen T. 1984. Chromian spinels as a petrogenetic indicator in abyssal and Alpine-type peridotites and spatially associated lavas. Contributions to Mineralogy and Petrology 86, 5476.
  • Eales H. V. & Marsh J. S. 1983. Al/Cr ratios of coexisting pyroxenes and spinellids in some ultramfic rocks. Chemical Geology 38, 5774.
  • Evans B. W. & Wright T. L. 1972. Compositions of liquidus chromite from the 1959 (Kilauea Iki) and 1965 (Mokapopuli) eruptions of Kilauea volcano, Hawaii. American Mineralogists 57, 217230.
  • Fisk M. R. & Bence A. E. 1980. Experimental crystallization of chromite spinel in FAMOUS basalt 527-1-1. Earth and Planetary Science Letters 48, 111123.
  • Fitton J. G., Saunfers A. D., Norry M. J., Harderson B. S. & Taylor R. N. 1997. Thermal and chemical structure of the Iceland plume. Earth and Planetary Science Letters 153, 174208.
  • Henderson P. & Wood R. J. 1981. Reaction relationships of chrome-spinels in igneous rocks – further evidence from the layered intrusions of Rhum and Mull, Inner Hebrides, Scotland. Contributions to Mineralogy and Petrology 78, 225229.
  • Himmerberg G. R. & Loney R. A. 1980. Petrology of ultramafic and gabbroic rocks of the Canyon mountain ophiolite, Oregon. American Journal of Science 280A, 232268.
  • Ichiyama Y. & Ishiwatari A. 2005. HFSE-rich picritic rocks from the Mino accretionary complex, southwestern Japan. Contributions to Mineralogy and Petrology 149, 373387.
  • Ichiyama Y., Ishiwatari A. & Koizumu K. 2008. Ptrogenesis of greenstones from the Mino-Tamba belt, SW Japan: Evidence for an accreted Permian oceanic plateau. Lithos 100, 127146.
  • Irvine T. N. 1965. Chromian spinel as a petrogenetic indicator, I. Theory. Canadian Journal of Earth Sciences 2, 648672.
  • Irvine T. N. 1977. Chromite crystallization in the Join Mg2SiO4–CaMgSi2O6–CaAl2Si2O8–MgCr2O4–SiO2. Carnegie Institute of Washington Yearbook 76, 465472.
  • Irvine T. N. 1979. Rocks whose composition is determined by crystal accumulation and sorting. In Yorder H. S. Jr (ed.) The Evolution of the Igneous Rocks, pp. 245306, Princeton University Press, Princeton, NJ.
  • Ishiwatari A. 1985. Igneous petrogenesis of the Yakuno ophiolite (Japan) in the context of the diversity of ophiolites. Contributions to Mineralogy and Petrology 89, 155167.
  • Isomi H. & Nozawa T. 1957. 1:50,000 Geological Map of the Funatsu Area and its Explanatory Text. Kanazawa (10) No. 44, Geological Survey of Japan (in Japanese with English abstract).
  • Jones G., Sano H. & Valsami-Jones E. 1993. Nature and tectonic setting of accreted basalts from the Mino terrane, central Japan. Journal of Geological Society 150, 11671181.
  • Koizumi K. & Ishiwatari A. 2006. Oceanic plateau accretion inferred from Late Paleozoic greenstones in the Jurassic Tamba accretionary complex, southwest Japan. Island Arc 15, 5883.
  • Longhi J., Fram M. S., Auwer J. V. & Montieth J. N. 1993. Pressure effects, kinetics, and rheology of anorthositic and related magmas. American Mineralogist 78, 10161030.
  • Maurel C. & Maurel P. 1982. Étude expérimetale de la distribution de l'aluminium entre bain silicaté basuque et spinelle chromifère – Implications pétrogénétiques: Tenur en chrome des spinelles. Bulletin de Minéralogie 105, 197202.
  • Natland J. H. 1989. Partial melting of a lithologically heterogeneous mantle: Inferences from crystallization histories of magnesian abyssal tholeiites from the Siqueiros Fracture Zone. In Saunders A. D. and Norry M. J. (eds.) Magmatism in the Ocean Basins. Geological Society of London, Special Publication 42, pp. 4170.
  • Nicholls J. & Stout M. Z. 1988. Picritic melts in Kilauea: Evidence from the 1967–1968 Halemaumau and Hiiaka eruptions. Journal of Petrology 29, 10311057.
  • O'Driscoll B., Emeleus C. H., Donaldson C. H. & Daly J. S. 2010. Cr-spinel seam petrogenesis in the Rum layered suite, NW Scotland: Cumulate assimilation and in situ crystallization in a deforming crystal mush. Journal of Petrology 51, 11711201.
  • Ozawa K. 1983. Relationships between tectonite and cumulate in ophiolites: The Miyamori ultramafic complex, Kitakami mountains, northeast Japan. Lithos 16, 116.
  • Roeder P. L., Gofton E. & Thornber C. 2006. Cotectic proportions of olivine and spinel in olivine-tholeiitic basalt and evaluation of pre-eruptive processes. Journal of Petrology 47, 883900.
  • Roeder P. L., Poustoveov A. & Oskarsson N. 2001. Growth forms and composition of chromian spinel in MORB magma: Diffusion-controlled crystallization of chromian spinel. Canadian Mineralogist 39, 397416.
  • Roeder P. L. & Reynolds I. 1991. Crystallization of chromite and chromium solubility in basaltic melts. Journal of Petrology 32, 909934.
  • Roeder P. L., Thornber C., Poustoveov A. & Grant A. 2003. Morphology and composition of spinel in Pu'u'O'o lava (1996–1998), Kilauea volcano, Hawaii. Journal of Volcanology and Geothermal Research 123, 245265.
  • Sack R. O. & Ghiorso M. S. 1991. Chromian spinels as petrogenetic indicators: Thermodynamics and petrological applications. American Mineralogist 76, 827847.
  • Scowen P. H., Roeder P. L. & Helz R. T. 1991. Reequilibration of chromite within Kilauea Iki lava lake, Hawaii. Contributions to Mineralogy and Petrology 107, 820.
  • Sigurdsson H. 1977. Spinels in Leg 37 basalts and peridotites: Phase chemistry and zoning. Initial Reports. Deep-sea Drilling Project 3, 883891.
  • Sigurdsson H. & Schilling J.-G. 1976. Spinels in Mid-Atlantic Ridge Basalts. Earth and Planetary Science Letters 29, 720.
  • Thy P. 1983. Spinel minerals in transitional and alkali basaltic glasses from Iceland. Contributions to Mineralogy and Petrology 38, 141149.
  • Wilkinson J. F. G. & Hensel H. D. 1988. The petrology of some picrites from Mauna Loa and Kilauea volcanoes, Hawaii. Journal of Petrology 98, 326345.
  • Yamada N., Adachi M., Kojima S., Yamazaki H. & Bunno A. 1985. 1:50,000 Geological Map of the Takayama Area and its Explanatory Text. Kanazawa (10) No. 52, Geological Survey of Japan (in Japanese with English abstract).