RESOURCE GEOLOGY, 47(6), 319•`330, 1997

Geochemical Behavior of Chromium

Keiichi SHIRAKI*

Abstract: One of the trace elements, chromium, which is incorporated most effectively into the crystallizing minerals with octa- hedral sites from a magma and therefore most sensitive to magmatic fractionation, best indicates the degree of fractionation from the Earth's mantle, and can be used to trace the evolution of the continental crust. Chromium abundances of peridotite constituting the upper mantle are around 3000 ppm,which are slightly lower than those of chondrite. Spinel with Cr as a major constituent, chromite, crystallizes first from most basaltic magmas, and chromite is the only Cr-mineral of economic interest. Chromite orebodies can be divided into layered and podiform types. Chromitite layers in large ultramafic-mafic intrusions, which occupy more than 90 % of world chromite resources, originate from contami- nation of the mafic magma with silicic liquid, because the increase of SiO2 decreases the Cr solubility in basaltic melt. Podiform chromitites, which provide more than half of chromite produced, also form by interaction between basaltic melt and depleted peridotite. Spinels with the highest Cr in terrestrial rocks are found in podiform chromitites, low-Ca boninites and inclusions in diamond. The chromites from low-Ca boninites are higher in Cr/Al than chromites from komatiites with high Cr/Al relative to boninites. The Cr contents of spinels in volcanic rocks tend to increase with increasing SiO2 in primary mag- mas. They also increase with increasing degrees of depletion of basaltic components in the source peridotites. The early crystallized Cr-rich spinels react with the magma to form pyroxenes. Pyroxenes like spinels remove most of the Cr from the magma. In some high-Mg andesites, however, the early formed pyroxenes that coexist with spinels are rather low in Cr, and the Cr contents in the pyroxenes increase with decreasing Mg/Fe in the pyroxenes at the expense of spinels. In primary mafic magmas Cr is reduced relative to mantle rocks by a factor of three to ten through partial melting (1000-300 ppm). However, effects on Cr content of the melting degree are small relative to those of crystal fractionation. Crystal fractionation reduces Cr in the latest felsic differentiates to less than 10 ppm. Chromium contents in clastic sediments are controlled dominantly by the Cr abundances in the source areas, and can be used to estimate the ratio of ultramafic and mafic to felsic rocks in provenance. The Cr abundances of the weathered crust estimated from clastic sediments are 50 to 100 ppm, which are higher than the Cr abundance, 35 ppm, of the exposed Canadian Precambrian shield. Many Archean sediments show elevated Cr concentrations relative to post-Archean sedi- ments, and immature, mafic oceanic terranes have higher Cr abundances than mature, felsic continental crust. The Cr abun- dances of the Earth's surface have decreased with the evolution of the continental crust.

tion in ultramafic rocks, then discuss Cr behavior during 1. Introduction igneous differentiation, and finally consider possible Chromium is the seventh most common element in changes of Cr abundances with continental evolution. the Earth's upper mantle, after O, Mg, Si, Fe, Al and Ca. 2. Chromium Abundance in Ultramafic Rocks Chromium abundances in peridotites constituting the upper mantle cluster around 3000 ppm. In the crustal rocks, The Cr abundances in peridotite nodules in alkali basalt however, Cr is a minor or trace element; the average of and kimberlite or in orogenic peridotites, which are con- basalts, 100-600 ppm Cr, depends mainly on the degree sidered to represent the upper, mantle materials, have been of fractionation or melting, and most granites contain only determined by many investigators. MAALƒÓE and AOKI less than 10 ppm Cr (SHIRAKI,1978). Thus Cr is most (1977) compiled 587 available analyses of lherzolitic nod- sensitive to magmatic fractionation and incorporated most ules and showed that continental and oceanic spinel lher- effectively in the early formed minerals with octahedral zolites have the same Cr abundances of 0.44 % Cr2O3 sites. This highly preferential partitioning of Cr into the (3010 ppm Cr), while continental garnet lherzolites have crystallizing minerals from a magma has best been inter- lower Cr2O3 contents of 0.37 % (2530 ppm Cr). On this preted in terms of the crystal field theory; Cr3+ ions have basis they proposed 0.42 % Cr2O3 (2870 ppm Cr) for Cr by far the largest octahedral site preference energy of all abundances, of the primitive mantle. HARTMANN and the common transition metal ions (CURTIS, 1964; BURNS WEDEPOHL(1993) estimated a lower value of 2500 ppm as and FYFE, 1964). In this paper I first review Cr distribu- Cr concentration of the primitive mantle, while JAGOUTZ et al. (1979) and SUN (1982) cited higher values of 3140 Recieved on September 16, 1997, accepted on November 19, and 3000 ppm, respectively, as Cr abundances of the primi- 1997 tive or fertile mantle. *Department of Earth Sciences , Yamaguchi University, The Cr abundances of 2500-3200 ppm in the Earth's Yamaguchi 753, Japan Keywords: Chromium, Geochemistry, Trace element, Cr- primitive mantle are similar to those in the primitive spinel, Pyroxene, Crustal abundance. bulk Moon, 3140 ppm, but lower than those in Cl car-

319 320 Keiichi SHIRAKI RESOURCE GEOLOGY:

Table 1 Chromium contents (% Cr2O3) in minerals from some ultramafic rocks .

Abbreviations: max, maximum value; Pen, inverted protoenstatite; Ref ., References. 1: ORBERGERet al. (1995); 2: CRAWFORD(1980); 3: MEYER (1975); 4: SOEOLEVet al . (1975); 5: HERVIGet al. (1980); 6: Ross et al. (1954); 7: DICK and BULLEN(1984); 8: OBATAand MORTEN (1987); 9: HARTS et al . (1987); 10: PRESSet al. (1986).

bonaceous chondrite, 3810 ppm, and H-group ordinary that chromite was accumulated is economically mined as chondrite, 3660 ppm (O'NEILL, 1991) . Although Cr is Cr-ore, and chromite ore deposits, in most cases, are present in similar abundances in the Earth's mantle and hosted in ultramafic rocks. the Moon, the Cr/Mg ratios of the Earth's mantle and the Chromite orebodies are classed as either podiform type Moon are 0.49 and 0.36-0.40 of the chondritic value , in alpine-type peridotites or layered type in ultramafic respectively. RINGWOODet al. (1990) attribute the Cr cumulates. Podiform ores have accounted for 57.4 % of depletion in the Earth's mantle to its siderophile behav- all chromite produced until 1984, but chromitite layers in ior during formation of the Earth's core at high pressure . the Bushveld and Great Dyke complexes occupy more Chromium is generally considered to be a refractory than 90 % of world chromite resources (STOWE, 1987). element; in basaltic rocks positive correlations between Cr Chromites in podiform chromitites exhibit a wide and Mg are well known, suggesting that the distribution range of composition from high Cr to high Al . The high- coefficientDcyr-liq between crystal and liquidfor Cr is Cr varieties contain occasionally Cr2O3 greater than 60 %; larger than unity. In general, dunite and harzburgite more they are the most Cr-rich terrestrial chromites along with depleted in basaltic components have more Cr than less those in boninites and diamond inclusions (Table 1). Both depleted lherzolite. In a suite of lherzolite-harzburgite high-Cr and high-Al chromitite pods commonly have xenoliths from Austria, however, KURAT et al . (1980) highly depleted dunite rinds that grade outward into less found that Cr contents decrease systematically with depleted harzburgite. ARAI and YURIMOTO(1994) and increasing Mg/Fe. No correlation between Cr and Mg ZHOU et al. (1994) explained the close association of the was also reported from the Ronda peridotite in southern chromitites with dunites and harzburgites in terms of Spain (FREY et al., 1985). The decrease in Cr with interaction between ascending basaltic melts and harzbur- increasing Mg indicates that the bulk distribution coeffi gites or depleted lherzolites in the upper mantle. cient •¬ between peridotite and liquid can be less Chromites in layered chromitites in differentiated than unity under certain conditions . This is indeed the stratiform complexes have a relatively narrow range of case for ultramafic liquids like komatiites where the Cr Cr and Al concentrations; Cr203 contents of chromites contents tend to be independent of the MgO contents . in the Bushveld and the Great Dyke are 38-55 % (HAT- LIANG and ELTHON (1990) showed that a plot of Cr against TON and VON GRUENEWALDT, 1987) and 52-60 % MgO represents a constant value, 2000-3500 ppm Cr , for (WILSON, 1982), respectively. komatutes with>20 % MgO, and gives a nearly horizontal The Cr contents of spinels in igneous rocks depend regression line for some mantle peridotites. mainly on the Cr/Al ratios in the magmas in which they 3. Chromium Distribution in Ultramafic Rocks crystallized. Generally, lherzolite less depleted in basaltic components contains Al-rich spinel with the Table 1 shows Cr distributions among minerals in lowest Cr, and more depleted harzburgite and dunite some types of ultramafic rocks. A spinel, which contains contain chromite with the highest Cr (ARAI, 1987; Cr as a major constituent, crystallizes first from magmas ROEDER,1994). Spinels in abyssal peridotites are low in with basaltic compositions. The Cr-rich spinel, chromite, Cr (Table 1); their Cr#=Cr/(Cr+Al) does not exceed is the only Cr-mineral of economic interest. Chromitite 0.60. In contrast spinels in alpine-type peridotites show 47(6), 1997 Geochemical behavior of chromium 321

a wide variation in Cr2O3 and Cr# (DICK and BULLEN, but its coexisting clinopyroxene has 2.23 % Cr2O3 (SMITH 1984). ARAI (1994) discusses the Fo-Cr# relationships and LEVY,1976). Peridotites often contain emerald-green between peridotite restites and spatially associated Cr-diopsideshaving 0.5-3 % Cr2O3 (e. g., SHIRAKI,1978). lavas. Cr#'s of spinel in peridotite restites are estimated Clinopyroxenes with the highest Cr, up to 15.6 % Cr203, as follows: 0.2-0.6 (mostly 0.4-0.6) for MORB, 0.2-0.5 occur associated with diamond (Table 1). Chromium for intraplate alkali basalts, ca. 0.7 for intraplate tholei- together with Al positively correlates with Na, suggesting ites, ca. 0.8 for oceanic plateau basalts, 0.3-0.5 (mostly replacement of jadeite (NaAlSi2O6) by kosmochlor 0.4-0.5) for back-arc basin basalts, >0.9 for boninites, ca. (NaCrSi2O6) component in the Cr-rich (> 3% Cr2O3) 0.9 for high-Mg arc tholeiites and high-Mg aandesites, clinopyroxenes (SOBOLEVet al., 1975). Amphiboles and 0.1-0.9 (mostly 0.1-0.7) for Quaternary arc magmas of phlogopites contained in ultramafic rocks have high Cr the Northeast Japan arc, and 0.7-0.8 for Precambrian contents reflecting Cr-rich environments in which they layered intrusions. Spinels with the highest Cr are found crystallized. Pargasites up to 2.88 % Cr2O3were reported among subduction-related ultramafics and lavas (e. g., from metasomatized peridotites at Ataq, S. Yemen (MEN- SHIRAKI and KURODA, 1977; BLOOMERand HAWKINS, ZIESet al., 1987). Among phlogopites the highest Cr203 1983; ARAI and TAKAHASHI,1986; ISHIIet al., 1992). 3.08 % was found in a secondary phlogopite from garnet- With increasing pressure spinel lherzolite transforms to phlogopite peridotites in the Kimberley pipes, S. Africa

garnet lherzolite. Because Cr preferentially enters the (ERLANKet al., 1987), although Cr-muscovite (fuchsite) is spinel and Al is concentrated in the garnet, increasing Cr often seen as a low-temperature alteration product of Cr- contents raise the stability field of spinel to higher pres- rich minerals such as chromite (e. g., TRELOAR,1987).

sures (O'NEILL, 1981). In a transitional spinel-garnet,lher- 4. Chromium in Magmatic Fractionation zolite the most Cr-rich spinels are observed in the small

residual grains included in garnets (OBATA and MORTEN, The most remarkable feature of Cr3+ ions is their 1987). At pressures above 10.5 GPa at 1200•Ž the end extremely low solubility in basaltic magmas. Chromium- member Cr-pyrope or knorringite (Mg3Cr2Si3O12) is sta- rich spinels are the first crystallized mineral in most

ble and the solubility of the knorringite molecule in basalts and occur in lavas that contain greater than 260

pyrope is essentially dependent on pressure (IRIFUNE et al., ppm Cr (ALLAN et al., 1988). The Cr solubility in silicate 1982). Chromium-rich pyrope garnets occur characteristi- melts varies with temperature, oxygen fugacity, pressure cally as inclusions in diamond (Table 1). and melt composition. The Cr contents of the melts at Terrestrial olivines usually have low abundances of chromite saturation increase strongly with increasing tem-

Cr (typically <100 ppm), although environments in perature and with decreasing oxygen fugacity (MURCK which the olivines crystallize are rich in Cr. SHIRAKI and CAMPBELL, 1986; BARNES, 1986). At 1300•Ž and 1 (1966) found only 23.5 ppmCr in an olivine from a lher- atm ROEDER and REYNOLDS (1991) showed that the Cr zolite nodule at Ichinomegata Maar of northeast Japan. content of chromite-saturated basaltic melts ranges from This depletion results from the fact that the majority of 0.05 % Cr2O3 at a log fo2=-3 to 1.4 % at a log fo2= Cr are present as Cr3+ in the redox states of the upper -12 .8. They also reported that increasing pressure to 10 mantle. Remarkably high Cr (up to 0.6 % Cr2O3; SMITH, kb has relatively little effect on either the Cr solubility 1974) in lunar olivines was first noted by HAGGERTYet in basaltic melts or on the compositions of coexisting al. (1970), who interpreted this to be Cr2+. This interpre- chromite. However, if chromite and melt coexisted with tation was challenged by BURNSet al. (1973), but has plagioclase, then increasing pressure would decrease the recently been supported by the XANES study of SUTTON stability of plagioclase and thus increase the Al content et al. (1993). High Cr contents (>0.1 % Cr2O3) in ter- of both melt and coexisting chromite (JAQUES and restrial olivines are found in komatiites (up to 0.33 % GREEN, 1980; Fu.III and SCARFE, 1985). Alkali basalts Cr2O3; ARNDTet al., 1977), highly refractory cumulate considered to be of deep-seated origin often contain low from a boninitic magma (up to 0.30 % Cr2O3; Table 1), -Cr , high-Al spinels (e. g., SHIRAKI et al., 1979; FISK and diamond inclusions (up to 0.16 % Cr2O3; Table 1) and BENCE, 1980; ARAI, 1990). ultramafic pseudotachylite (up to 0.28 % Cr2O3; OBATA and KARATO,1995). The common feature of all these environments is the unusually high temperature, which enhances the stability of Cr2+in olivine (LI et al., 1995). Among coexisting ortho- and clinopyroxenes the orthopyroxene has consistently less Cr than the clinopy- roxene. The most Cr-rich orthopyroxene with 1.27 % Cr2O3 in ultramafic rocks is from a websterite cumulate in the Green Knobs kimberlite diatreme of New Mexico, 322 Keiichi SHIRAKI RESOURCE GEOLOGY:

Table 2 Chromium contents in spinel and pyroxene from some igneous rocks

Fig. 1 Cr/(Cr+Al) versus Mg/(Mg+Fe2+) (molar) for spinels in Bonin Islands low- Ca boninite (LCB) (closed circles; SHIRAKI,unpublished data), Guam high- Chromium contents are expressed in wt. % Cr2O3 except for bulk rock in ppm Cr Ca boninite (HCB) (outlined by dashed . line; SHIRAKIet al., 1977),Hahajima calc- *1: highest value . *2: for more Ca-rich rock. alkali basalt (open circles; SHIRAKI, Abbreviations: Ppx, protopyroxene; Opx, orthopyroxene; Pig, pigeonite; Cpx , Ca- unpublished data), Guam island-arc rich pyroxene (diopside and augite); Ref., References. tholeiite (IAT) (outlined by bold line; A: ARNDTet al. (1977); B: ARNDTand FLEET(1979); C: ECHEVERRIA(1980); D: SHIRAKIet al., 1977) and Nanzaki basan- AITKENand ECHEVERRIA(1984); E: RAMSAYet al. (1984); F: SHIRAKIet al . ite (crosses; SHIRAKIet al., 1979) from the (1981); G: SHIRAKI(unpublished data); H: SHIRAKIet al. (1977); I: SHIRAKIet Izu-Bonin-Marianaisland arc, and in mid- al. (1992); J: SHUTOet al. (1985); K: EVANSand MOORS(1968); L: EVANSand ocean ridge basalt (MORB) (dots; DICK WRIGHT(1972); M: SHIRAKIet al. (1979); N:ARAI(1990) . and BULLEN,1984).

Munro Township, northeast Ontario, chromitedid not of Hahajima and Guam to LCB of the Bonins; this crystallizeat an early stage until after much olivinehad appears to be an increasing order of SiO2 in the host pri- crystallized, and the Cr203 contents (56.2-48.5 %) in mary magmas. This increase of Cr# in spinels also may chromiteare not very high, even rather low, compared represent increasing degrees of depletion in basaltic with the' very high Cr contents (2000-3200 ppm)and components in the source mantle peridotites (ARAI, Cr/Alratios in its host komatiites(ARNDT et al., 1977). 1994). Table 2 shows Cr distributions in spinels and pyrox- The early crystallized Cr-rich spinel reacts with the enes crystallized from magmas. The most Cr-rich magma at some stage and is replaced by pyroxene , as spinels in volcanic rocks are found in low-Ca boninites BOWEN (1928) and IRVINE (1967) suggested, and HILL (LCB) that are most enriched in SiO2 at a given MgO and ROEDER(1974) showed by experiments. Thus, in (e. g., SHIRAKIand KURODA,1977; CAMERONet al., more differentiated basalts and andesites where Cr- 1980). High-Ca boninites (HCB), sanukitoids and high- spinel disappears, pyroxenes become a principal Cr- Mg andesites have spinels with the highest Cr next to bearing mineral. Pyroxenes, like spinels, remove most those in LCB (e. g., SUZUKiand SHIRAKI,1980;TATSUMI of the Cr from the magma because of its very strong and ISHIZAICA,1981; ARAI, 1994). The Cr contents in octahedral site preference energy (SEWARD,1971; HUEB- spinels tend to increase with increasing SiO2 in the NER et al., 1976; SCHREIBERand HASKIN,1976; BARNES, mantle-derived primary magmas. In Fig. 1 are shown 1986). On the other hand, at high oxygen fugacities Cr#s of spinels in some volcanic rocks from the Izu- spinels continue crystallizing to Cr-poor titanomagnetite Bonin-Mariana island arc and in mid-ocean ridge (HILL and ROEDER,1974). basalts (MORB). The Cr#'s increase from Nanzaki In some high-Mg andesites SHIRAKIet al. (1986) found basanite of Izu Peninsula through MORB, island-arc that the early formed pyroxenes that coexist with spinels tholeiiite (IAT) of Guam and calc-alkali basalt or HCB are rather low in Cr, and the Cr contents in the pyroxenes 47(6), 1997 Geochemical behavior of chromium 323 increase with decreasing Mg/ Fe in the pyroxenes. The typ- ical example is shown in Fig, 2. The clinoenstatite boninite cry- stallizes pyroxenes in order of protopyroxene, orthopyroxene, pigeonite and augite, following olivine with Fo93.2-90.5(SHIRAKI et al., 1989). The earliest proto- pyroxene, now inverted to cli- noenstatite with Mg#=100Mg/ (Mg+Fe)> 90, has rather low Cr due to preferential parti- tioning of Cr into coexisting spinel. Some of the later ortho- pyroxenes that crystallized after spinel had ceased to crys- tallize reveal increasing Cr Fig. 2 Cr2O3 (wt%) versus 100Mg/(Mg+total Fe) (molar) for clinoenstatites (open cir- contents at the expense of Cr- cles), orthopyroxenes (closed circles), pigeonites (triangle) and augites (cross) in a cli- rich spinels. The highest Cr2O3 noenstatite boninite from Ani-jima, Bonin Islands (SHIRAKIet al., 1989), Large sym- content (1.36%) among pyrox- bols are phenocryst cores, and small symbols phenocryst rims and smaller grains. enes in boninites is found in an orthopyroxene with Mg# 87.8 (Table 2). In high-Mgandesites with relativelylow CaO, orthopy- Table 3 Chromium abundances of igneous rocks, sedimentary rocks and continental crust roxene has higher Cr than coexisting clinopyrox- ene, indicating that orthopyroxene crystallized prior to clinopyroxene. Differentiated sanukites with>60 % SiO2where spineldoes not exist, con- tain bronzitewith high Cr contentsattaining to 1.25 % Cr2O3 (Table 2). Some of the bronzite phe- nocrysts in high-Mg andesiteshave higher Cr than enstatitesin ultramaficrocks. In tholeiiticand alkali basalts-where spinelreacts with magmas,clinopyroxenes (diopside and augite) have more than half of the cr. present in the rocks. The Cr contentsin their diopside.or diopsidicaugite oftenexceed 1 % Cr2O3.With advancingfractiona- tion Cr contents in pyroxenes decrease steadily (Fig. 2), because the distributioncoefficients of Cr betweenpyroxene and melt are usually greater than unity. Crystallizationof amphiboleand mica also removesCr from the magma (e. g., SHIRAKI,1966). Thus Cr in an igneousrock is the best indicatorof the fractionationstage of the-magma. 5. Chromium Abundance and Continental Evolution Chromium abundances in various geological units are summarized in Table 3. Chromium is most concentrated in ultramafic rocks constituting the Earth's upper mantle, and decreases with increasing degrees of fractionation from the man- tle. In mafic rocks Cr is reduced relative to mantle rocks by a factor of three to ten through partial 324 Keiichi SHIRAKI RESOURCE GEOLOGY:

Fig. 3 Plot of Cr contents in sedimentary rocks versus their depositional ages. Data sources are as follows. N. America: CAMIREet al. (1993), CONDIEand DEMALAS(1985), CRICHTONand CONDIE(1993), DIA et al. (1990), FENGand KERRICK(1990), GIBBSet al. (1986), JENNERet al. (1981), MCGREGORand MASON(1977), MCLENNANet al. (1984, 1995), NUTMANet al. (1984), SMITHet al. (1992); S. Africa: DANCHIN,(1967), FEDOet al. (1996), LASKOWSKIand KRONER(1985), MCLENNANet al. (1983b), WRONKIEWICZand CONDIE(1987, 1989, 1990); Australia: BAVINGTONand TAYLOR (1980), BHATIAand CROOK(1986), ERIKSONet al. (1992), LEAKE(1996), MAASand MCCULLOCH(1991), MCLENNANet al. (1983a); India: ARORAet al. (1994), NAQVIand HUSSAIN(1972), NAQVIet al. (1983, 1988), SUBRAMANIANet al. (1985); other areas:CONDIEet al. (1991), FLOYDet al. (1989), GIBBSet al. (1986), GU (1994), MCLENNANet al. (1990), MONGELLIet al. (1996), SHERATON(1980), SHIRAKI(1978), VANDE KAMI, et al. (1994), WINCHESTERand MAX(1989). melting. Many primary magmas in equibrium with mantle the latest felsic differentiates to less than 10 ppm. Unusually peridotite appear to have 300-1000 ppm Cr (Table 2). high Cr in granitic rocks suggests that they were not formed Chromium in the primary magmas varies with the degree by crystal fractionation alone, but other processes such of melting, but the effects on Cr content of the melting as interaction and mixing of mafic and felsic materials, degree are small relative to those of crystal fractionation. remelting of crustal rocks, metasomatism etc. were A wide dispersion of Cr values has often been found in involved in their genesis (e. g., JAYANANDAet al., 1995). basaltic rocks erupted mostly on the continents (e. g., I-type granites derived from igneous material are general- PRINZ,1967). In abyssal basalts, however, Cr shows a rel- ly lower in Cr than S-type granites from sedimentary pro- atively uniform distribution (SHIRAKI,1978). Tholeiitic tolith (Table 3). basalts from island arcs have generally low Cr, reflect- Although Cr contents show a large variation in igneous ing their low Mg/Fe ratio. At the same Mg/Fe there is rocks ranging from 3000 ppm in ultramafic rocks to less no difference in Cr content among present-day basalts than 10 ppm in felsic rocks, there are no main sedimenta- from mid-ocean ridges, subduction zones and within- ry rock types concentrating Cr to a great extent. Black plate environments, but Archean basalts are enriched in shales are often only slightly higher in Cr than ordinary Cr compared to post-Archean basalts (CONDIE, 1989). shales, but their Cr contents very rarely exceed 1000 Andesites, like basalts, show a wide range of Cr contents ppm (SHIRAKI,1978). In general shales have higher Cr depending on the fractionation stage of the magma. High than sandstones. However, Cr is occasionally enriched Cr in high-Mg andesites (boninite and sanukitoid) indi- in sands containing chromite; in black sands and placers, cates that they represent a primary magma derived from a level of several percent Cr2O3 may be attained. Because an ultramafic source. Crystal fractionation reduces Cr in Cr-spinel is chemically and mechanically more durable 47(6), 1997 Geochemical behavior of chromium 325 than any other mafic minerals, detrital Cr-spinel in clas- compositions of the exposed sections of the Talkeetna tic sediments provides strong evidence for denudation island arc, Alaska, and the island arc-related Canyon of ultramafic rocks, and has been used to elucidate the Mountain Complex, Oregon, range from high-Mg basalt petrological characteristics and tectonic setting of the to basaltic andesite, and their bulk Cr contents are source ultramafic rocks (e. g., PRESS, 1986; ARAI and remarkably high, 696 ppm and 1106 ppm, respectively OKADA, 1991; COOKENBOOet al., 1997; NANAYAMA, (PEARLYet al., 1990). These immature, mafic oceanic 1997; ARAI et al., 1997). Therefore, the bulk Cr contents terranes produce Cr-rich sediments at an early stage, but in clastic sedimentary rocks are controlled most impor- later they evolve into mature continental crust consist- tantly by the proportion of ultramafic (and mafic) rocks ing dominantly of Cr-poor, granitic rocks. Thus the in the source areas, and can monitor changes in source crustal abundance of Cr has decreased with the evolu- composition (e. g., SHIRAKI, 1966; GIBBS et al., 1986; tion of the continental crust. CONDIE,1993). Acknowledgement: I thank Prof. Shoji ARAIof Kanazawa TAYLORand MCLENNAN(1985) showed that the abun- University and Dr. Shigeko TOGASHIof the Geological dances of some elements (rare earth elements, Th, Sc, Survey of Japan for reviewing the manuscript. etc.) in clastic sedimentary rocks can provide an index References of upper crustal composition, because these elements are transferred virtually quantitatively from the crust AITKEN, B. G. and ECHEVERRIA,L. M. (1984): Petrology and into clastic sediments during erosion of the upper crust geochemistry of komatiites and tholeiites from Gorgona Island, Columbia. Contrib. Mineral. Petrol., 86, 94-105. and are not fractionated significantly during the process ALLAN, J. F., SACK, R. O. and BATIZA, R. (1988): Cr-rich of sedimentation and diagenesis. The Cr contents in spinels as petrogenetic indicators: MORB-type lavas from clastic sediments also can be used to estimate Cr abun- the Lamont seamount chain, eastern Pacific. Am. Mineral., dances in provenance. The average of shale and sand- 73, 741-753. stone is 83 ppm Cr and 27 ppm Cr, respectively (Table 3). ARAI, S. (1987): An estimation of the least depleted spinel peri- SHIRAKI(1978) estimated the Cr abundance of the weath- dotite on the basis of olivine-spinel mantle array. N. Jb. ered continental crust to be 55 ppm, assuming a ratio of Miner. Mh., 1987, 347-354. shale to sandstone of 1 : 1. Greywacke, considered to ARAI, S. (1990): Chemical compositions of chromian spinel represent an average sample of the exposed crust, has and olivine in some alkaline basalts from Japan. Sci. Rep. an average Cr value of 88 ppm (WEDEPOHL,1995). These Kanazawa Univ., 35, 25-38. ARAI, S. (1994): Compositional variation of olivine-chromian values are high, compared to the Cr abundance, 35 ppm, of spinel in Mg-rich magmas as a guide to their residual spinel the upper continental crust by TAYLORand MCLENNAN peridotites. J. Volcanol. Geotherm. Res., 59, 279-293. (1985), which is based on that of the Canadian Pre- ARAI, S., KADOSHIMA,K., MANJOORSA, M. V., DAVID, C. P. cambrian shield estimated from composite samples and and KIDA, M. (1997): Chemistry of detrital chromian rock proportions. This would result from temporal and spinels as an insight into petrological characteristics of their spatial changes of Cr abundances associated with conti- source peridotites: an example from the Ilocos Norte ophio- nental evolution. lite, northern Luzon, Philippines. J. Min. Petr. Econ. Geol., Relationships between Cr contents in sedimentary 97, 137-141. rocks and their depositional ages are shown in Fig. 3. ARAI, S. and OKADA, H. (1991): Petrology of serpentine sand- Temporal changes appear to have occurred in upper stone as a key to tectonic development of serpentine belts. crustal contents of Cr (see also Table 3). Many Archean Tectonophys., 195, 65-81. ARAI, S. and TAKAHASHI,N. (1986): Petrographical notes on clastic metasedimentary rocks of both continental and deep-seated and related rocks (4). 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S., SOBOLEV, N. V. and LAVRENT'EV, YU. G. and provenance of sediments from the Pongola Supergroup, (1975): Chrome-rich clinopyroxenes from the kimberlites South Africa: Evidence for a 3.0-Ga-old continental cra- of Yakutia. N. Jb. Miner. Abh., 123, 213-218. ton. Geochim. Cosmochim. Acta. 53, 1537-1549. STOWE, C. W. (1987): The mineral chromite. In Evolution of WRONKIEWICZ,D. J. and CONDIE, K. C. (1990): Geochemistry Chromium Ore Fields (STOWE, C. W. ed.), Van Nostrand, and mineralogy of sediments from the Ventersdorp and New York, 1-22. Transvaal Supergroups, South Africa: Cratonic evolution SUBRAMANIAN,V., VAN'T DACK, L. and VAN GRIEKEN, R. during the early Proterozoic. Geochim. Cosmochim. (1985): Chemical composition of river sediments from the Acta, 54, 343-354. Indian sub-continent. Chem. Geol., 48, 271-279. ZHOU, M.-F., ROBINSON,P. T. and BAI, W.-J. (1994): Formation SUN, S.-S. (1982): Chemical composition and origin of the of podiform chromitites by melt/rock interaction in the earth's primitive mantle. Geochim. Cosmochim. Acta, 46, upper mantle. Mineral. Deposita, 29, 98-101. 330 Keiichi SHIRAKI RESOURCE GEOLOGY:

クロムの地球化 学的挙動

白 木 敬 一

要 旨:マ グマか ら晶出す る八面体席 を もつ鉱物 に最 も効 た起源 マ ン トルにおける玄武 岩成分 の枯 渇程度 の反映で 果的 に取 り込 まれ,マ グマの分化作用 に最 も敏感 に反応 もある. す る微量元素 ク ロムは,マ ン トル か らの分化 の程度 を最 早期 に晶 出 したCrに 富 むス ピネルはマ グマ と反応 し もよ く表 し,そ れを使 って大陸地殻の進化 を辿 るこ とが て輝石 をつ くる.輝 石 もス ピネ ル同様マ グマ か らCrの で きる. 大 部分 を取 り去 る.し か し,い くつか の高Mg安 山岩 に 上部 マ ン トル をつ くるかん らん岩 のCr存 在度 は コン お いて は,ス ピネル と共存す る早期晶出輝石が む しろCr ドライ トよ りやや低い約3000ppmと 見積 られ る.苦 鉄 質 に乏 しく,輝 石 中のCrが ス ピネルの減少 とと もに増加 マ グマ か ら最初 に晶 出す るCrを 主要構 成成分の一 つ と す るこ ともあ る.初 生苦鉄 質マ グマのCr含 有量 は部分 す るス ピネル,ク ロマ イ トは唯一 の経済価値 を もつCr 溶融 によってマ ン トル より1/3～1/10に 減少す る(1000～ 鉱物 である.Cr鉱 床 は層状 とポデ ィフォーム型 に分 け ら 300ppm).し か し部分 溶融の効果 は結 晶分化作 用 よ り小 れる.世 界 のCr資 源の90%以 上 を占め る層状 分化 岩体 さい.結 晶分化作用 に より最終分化物 である珪長質岩 の のク ロミタイ ト層 は,苦 鉄 質マグマが珪酸分 を混成 した Cr含 有量は10ppm以 下 となる. 結 果つ くられた.珪 酸分 の増加 はマグマのCr溶 解度 を 砕屑性堆積 岩のCr含 有 量はお もに供給源地のCr量, 減少 させ るか らであ る.現 在 のCr生 産量 の半ば以上 を 即 ち超苦鉄質 ～苦鉄質岩 と珪長質岩の比 によって決定 さ 供給 す るポディフ ォーム型 鉱床 もまた,苦 鉄質マ グマ と れ,堆 積岩 のCr含 有量か ら供給源地のCr存 在度 を求め かん らん岩 の反 応 に よ り生 じた.ポ デ ィフ ォー ム型鉱 る ことが で きる.堆 積岩か ら求 めた地殻 上部のCr存 在 床 ・低Caボ ニ ナイ ト ・ダイアモ ン ドの包有 物 中のス ピ 度は50～100ppmと な り,カ ナダ楯状地 の35ppmよ り高 ネルは地球上 のス ピネルのなかで最 もCrが 高い.特 に い.始 生代 の堆積 岩は後の時代 の もの よ り一般 にCrに ボニナイ トは コマ チアイ トよ り全岩Cr/Alが 低い に もか 富 む.海 洋地殻上 に生 じた若 い島弧 は苦鉄 質岩が多 く, かわ らず,そ のス ピネルのCr/Alは コマチ アイ トのス ピ 珪長質岩 の多 い成熟 した大陸地殻 よりCr存 在 度が高 い. ネル よ り高 い,火 山岩 中のス ピネ ルのCrは 初 生マ グマ 地球表層部 のCr存 在度 は大陸の成長 ・進化 とともに減 のSiO2の 増加 と ともに増 加す る傾 向があ る.こ れは ま 少 して きた.