1Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC),Yokosuka 237-0061, Japan
2Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa 920-1192, Japan3Graduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan
(Received February 9, 2007; Accepted March 1, 2007)
This paper reports the first discovery of lanthanide tetrad effect in an oceanic plagiogranite recovered from an OceanCore Complex at 25°S along the Central Indian Ridge. The plagiogranite consists mainly of sodic plagioclase, quartz, andamphibole with accessory minerals of epidote, titanite, zircon, apatite, and allanite. The chondrite-normalized REE pat-tern of the studied sample exhibits noticeably high REE abundances with a conspicuous negative Eu-anomaly, indicativeof very high degrees of fractional crystallization. In addition, the convex (M-type) tetrad effect is clearly recognizable inthe REE pattern, implying an unusually volatile-rich (e.g., H2O, CO2, Li, B, F, and/or Cl) parent magma for the plagiogranite.Because MORB sources are known to have significantly low volatile content, interaction with an external fluid originat-ing from seawater is suggested to be responsible for the presence of the tetrad effect in the oceanic plagiogranite.
in H2O, CO2, Li, B, F, P, and/or Cl) at a later stage ofmagma differentiation (e.g., Bau, 1996, 1997; Irber, 1999;Zhao et al., 2002). Although little is known about the ori-gin of the fluid as well as the melt-fluid interaction proc-esses, the lanthanide tetrad effect is considered to be im-portant in order to understand the chemical evolution ofmagma system.
Here, we report the first discovery of the lanthanidetetrad effect in an oceanic plagiogranite from an OceanCore Complex (OCC) at 25°S along the Central IndianRidge (CIR) (Fig. 1).
GEOLOGICAL BACKGROUND AND STUDIED SAMPLE
Granitic rocks constitute a major part of continentalcrust. In contrast, oceanic crust is mostly composed ofbasaltic rocks. However, plagioclase-rich leucocraticrocks termed “oceanic plagiogranite” have been observedin minor amounts, but ubiquitously in mid-ocean ridgesand ophiolite complexes (e.g., Aldiss, 1981). In mid oceanridges, plagiogranites have been mostly recovered fromfracture zones and OCCs where the lower part of the oce-anic crust (gabbro~ultramafics) is tectonically exposedat the seafloor. Geological studies on ophiolite sectionsindicate that the plagiogranite generally intruded intouppermost part of the gabbro sequence (e.g., Gillis and
INTRODUCTION
It is well known that the irregular distribution patternsof rare earth elements (REE), termed the “lanthanide tet-rad effect”, are found in highly evolved granitic rocks(e.g., Masuda et al., 1987; Masuda and Akagi, 1989).Kawabe (1992) has shown that the presence of the lan-thanide tetrad effect has been theoretically explained us-ing refined spin-pairing energy theory in terms of thequantum mechanical energietics of 4f electrons. However,it is still uncertain how the tetrad REE pattern formed innatural geological samples (e.g., Kawabe, 1995; Irber,1999; Zhao et al., 2002; Monecke et al., 2007).
Previously, the tetrad effect has been attributed to ana-lytical artifacts (e.g., McLennan, 1994) or fractional crys-tallization of REE-rich accessory minerals such asmonazite and garnet (e.g., Pan, 1997). Subsequent inves-tigations have, however, clearly demonstrated that thesefactors cannot account for the tetrad REE pattern in highlyevolved granitic rocks (Kawabe, 1995; Bau, 1997; Irber,1999). Recently, the tetrad effect has been attributed tothe interaction of silicate melt with a coexisting fluid (rich
136 K. Nakamura et al.
Coogan, 2002). Typical oceanic plagiogranites consist ofalbitic plagioclase, quartz, and hornblende with acces-sory minerals (e.g., Aldiss, 1981). The presence of horn-blende indicates that these rocks were formed under el-evated water activities (Koepke et al., 2004). Two mecha-nisms have generally been proposed for the plagiogranitegeneration: (1) extreme crystal fractionation of MORBmagma (e.g., Rao et al., 2004; Dilek and Thy, 2006), and(2) anatexis of lower oceanic crust (e.g., Gillis andCoogan, 2002; Koepke et al., 2004, 2007). Despite themany geological and geochemical studies, thepetrogenesis of the oceanic plagiogranite is still unclear.
The plagiogranite sample studied here was recoveredfrom an OCC at the CIR 25°S by the manned submers-ible Shinkai 6500 during the YK05-16 Cruise. In the dive,peridotite, gabbro, dolerite, and basalt were also sampledalong with the plagiogranite. The plagiogranite isleucocratic and fine to medium grained. The followingobservations indicate that the plagiogranite is a part ofleucocratic vein within gabbroic rocks: (1) the sample hasa platy shape (Fig. 2(a)), and (2) only gabbroic rocks weresampled in close proximity to the sampling point of the
Fig. 1. Index maps showing the dive track of Shinkai 6500 dive6K#919 and sampling locality of the studied plagiogranite sam-ple at an OCC located at 25°S along the Central Indian Ridge.
Fig. 2. (a) Photograph of studied plagiogranite sample. Noteits platy shape and leucocratic and fine-grained charactershown on the sawed surfaces. (b), (c) Photomicrographs of theplagiogranite sample. Abbreviations: Pl = plagioclase, Qtz =quartz, Amp = amphibole.
Discovery of lanthanide tetrad effect in an oceanic plagiogranite 137
plagiogranite. Occurrences of the studied sample(leucocratic vein within gabbroic rock) are generally con-sistent with those of plagiogranites in ophiolites (e.g.,Nicolas, 1989) and other OCCs (e.g., Dick et al., 2000).On the microscopic scale, the plagiogranite consistsmainly of sodic plagioclase and quartz with small amountsof amphibole (Figs. 2(b) and (c)), which is typical of pre-viously reported oceanic plagiogranites (Aldiss, 1981).Trace amounts of epidote, titanite, zircon, apatite, andallanite are also observed in the sample as accessory min-erals.
ANALYTICAL PROCEDURE
About 50 g of whole rock sample was crushed intosmall chips in a tungsten carbide mortar and pulverizedin a tungsten carbide ball mill. 200 mg of the powderedsample was weighed into a 95% Pt-5% Au crucible. Af-ter wetting the sample with a few drops of de-ionizedwater purified by the Milli-Q system (Millipore Corpo-ration, MA, USA), 3 ml 38% w/w HF (TAMAPURE-AA-10 grade, Tama Chemicals, Tokyo, Japan) was added andthen heated and evaporated to dryness at 120°C. Afterthe HF treatment, 300 mg of Li2B4O7 (GR grade, Merck,Darmstadt, Germany) was weighed into the crucible withseveral mg of LiBr (E grade, Kanto Chemical, Tokyo,
Table 1. REE data for two geochemical reference materials ofBCR-2 and JG-2, and the studied plagiogranite sample(6K#919R-07)
1)The normalization is based on the C1 chondrite values given by Andersand Grevesse (1989).2)The Eu-anomaly is defined as Eu/Eu* = EuCN/(SmCN × GdCN)1/2.3)The degree of the tetrad effects, calculated according to the methodof Irber (1999), was expressed by the quantification factor TE1,3 = (t1
× t3)1/2 where t1 = [CeCN/(LaCN2/3 × NdCN
1/3) × PrCN/(LaCN1/3 ×
NdCN2/3)]1/2, t3 = [TbCN/(GdCN
2/3 × HoCN1/3) × DyCN/(GdCN
1/3 ×HoCN
2/3)]1/2.
10
100
1000
1
10
100
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Ch
on
dri
te-n
orm
aliz
ed
(a) BCR-2
(b) JG-2
This study
Raczek et al. (2001)Willbold & Jochum (2005)
This study
Ujiie & Imai (1995)Dulski (2001)
Kawabe (1995)
Japan). The mixture was fused at 1180°C for 8 min in amuffle furnace (5 min fusion plus 3 min agitation), andthen cooled to room temperature. Finally, the fused glassbead was transferred to a 50 ml polypropylene bottle andthen progressively dissolved with 1 mol l–1 HNO3(TAMAPURE-AA-10 grade, Tama Chemicals, Tokyo,Japan) in an ultrasonic bath.
REE concentrations were measured using Agilent7500ce ICP-MS (Agilent Technology, Tokyo, Japan). Thesensitivity at the typical ICP-MS run conditions was ~150Mcps ppm–1 for 140Ce. Oxide and hydroxide formationrates were maintained at less than 1.0% for CeO+/Ce+ andapproximately 0.1% for CeOH+/Ce+. Under these operat-ing conditions, interference corrections are only neces-sary for BaO and BaOH on Eu and PrO and CeOH on Gdby applying constant correction factors determined usingsingle element monitor solutions prior to sample analy-sis. External calibration standards were prepared by gravi-metric serial dilution of 10 µg ml–1 mixed standard solu-tion supplied by SPEX CertiPrep (NJ, USA), which werematrix matched to sample solutions using the fusion flux.115In and 209Bi were used as internal standards. Based on
Fig. 3. Chondrite-normalized REE plots of (a) BCR-2 and (b)JG-2 to compare new data from this study with the publishedvalues. Note that the analytical uncertainties in this study aresmaller than the plot marks. Normalizing C1 chondrite valuesare taken from Anders and Grevesse (1989).
duplicate runs of blank solutions prepared following ex-actly the same method as was used to digest the rock sam-ples, total procedural blanks for REE were calculated tobe generally lower than 100 pg, and the blank contribu-tions to the sample analysis were mostly less than 0.01%,which is negligible.
Duplicate analyses of five different digestions forgeochemical reference materials of BCR-2 (a basalt stand-ard provided by USGS) and JG-2 (a granite standard is-sued by GSJ) demonstrate that the reproducibility of ourmethod was generally better than 1% for BCR-2 and 3%for JG-2 (Table 1). Chondrite-normalized REE pattern ofour BCR-2 data is quite smooth and coherent (Fig. 3(a)),which agrees well with the recently published high-precision data obtained by ID-TIMS/MIC-SSMS (Raczeket al., 2001) and magnetic sector field ID-ICP-MS(Willbold and Jochum, 2005). This clearly indicates thehigh accuracy of our REE data. The REE pattern for JG-2 is also smooth and coherent, except for a conspicuousnegative Eu-anomaly (Fig. 3(b)). The REE abundancesare generally in good agreement with the published val-ues measured by ICP-AES (Kawabe, 1995) and ICP-MS(Ujiie and Imai, 1995; Dulski, 2001). It should be notedthat the convex (M-type) tetrad effect in JG-2, recognizedby Kawabe (1995), is also observed in our data.
RESULTS AND DISCUSSION
The studied plagiogranite sample exhibits a relativelyflat REE pattern with slight enrichment of light REE(LREE). Noticeably high REE abundances with a con-spicuous negative Eu-anomaly (Fig. 4) indicate consid-erably high degrees of fractional crystallization. This, in
Fig. 5. Comparison of chondrite-normalized REE patterns be-tween the studied plagiogranite sample and other oceanicplagiogranites from the Mid-Atlantic Ridge (Aldiss, 1981) andthe Troodos (Cyprus) and Semail (Oman) ophiolites (Aldiss,1981; Gillis and Coogan, 2002). Normalizing C1 chondritevalues are taken from Anders and Grevesse (1989).
10
100
1000
Ch
on
dri
te-n
orm
aliz
ed
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
1. tetrad 2. tetrad 3. tetrad 4. tetrad
10
100
1
Ch
on
dri
te-n
orm
aliz
ed
This study
Mid-Atlantic Ridge
Troodos OphioliteSemail Ophiolite
La Ce Pr Nd SmEu Gd Tb Dy Ho Er TmYb Lu
300
Fig. 4. Chondrite-normalized REE pattern of the plagiogranite.It shows an irregular REE pattern with four convex segmentsdisplaying the lanthanide tetrad effect. Note that the analyticaluncertainties in this study are smaller than the plot marks. Nor-malizing C1 chondrite values are taken from Anders andGrevesse (1989).
turn, suggests that crystal fractionation played a signifi-cant role in the formation of plagiogranite. In addition,the clear M-type tetrad effect is recognized in theplagiogranite. While tetrad REE patterns have previouslybeen found in highly evolved granitic rocks from conti-nents and island arcs (e.g., Bau, 1996; Irber, 1999; Zhaoet al., 2002), this is the first discovery of the lanthanidetetrad effect in oceanic plagiogranite.
It is known that a conspicuous negative Eu-anomalyis a typical REE feature of tetrad granites (e.g., Irber,1999). Crystal fractionation of plagioclase, which has avery high partition coefficient for Eu, is commonly re-garded to cause a negative Eu-anomaly in igneous rocks(e.g., Möller and Muecke, 1984). However, Irber (1999)has shown that strong Eu depletions (Eu/Eu* < 0.06) withtetrad REE patterns cannot be accounted for only byplagioclase fractionation. Instead of being caused by par-titioning of Eu into plagioclase, the Eu depletion is at-tributed to a preferential partitioning of Eu into a co-existing aqueous fluid phase (Flynn and Burnham, 1978;Candela, 1991). Our sample exhibits a conspicuous nega-tive Eu-anomaly (Fig. 4). However, the size of the nega-tive Eu-anomaly (Eu/Eu* = 0.32) is significantly smallerthan those reported for other highly evolved tetrad gran-ites (cf., JG-2 in Fig. 3(b): Eu/Eu* = 0.03), in spite of theclear M-type lanthanide tetrad effect in the sample (TE1,3= 1.08, see Table 1), which is comparable to that of JG-2(TE1,3 = 1.07). This may suggest that the fractional crys-tallization of plagioclase, rather than partitioning into
Discovery of lanthanide tetrad effect in an oceanic plagiogranite 139
coexisting fluid, was responsible for the negative Eu-anomaly in the plagiogranite sample.
The REE characteristics of our plagiogranite samplefrom the Indian Ocean OCC are significantly differentfrom the other oceanic plagiogranites reported from Mid-Atlantic Ridge (MAR) and ophiolites (Fig. 5). Comparedto plagiogranites from Cyprus and Oman ophiolites, REEabundances of our plagiogranite sample are noticeablyhigher, reflecting greater degrees of differentiation of theparental magma. Moreover, plagiogranites from ophiolitesexhibit slightly LREE-depleted patterns, similar toMORB, whereas our plagiogranite show exactly the op-posite LREE-enriched pattern. Crystal fractionation ofamphibole or partial melting of amphibolite can cause theenrichment of LREE in plagiogranite melt, becauseamphibole has high partition coefficients for heavy REE(HREE) (Green, 1994). In comparison with ophioliteplagiogranites, the REE patterns of the MARplagiogranites are more closely resemble that of ours (Fig.5). Compared to our sample, however, HREE abundancesof the MAR plagiogranites are significantly low and thenegative Eu-anomalies are generally weaker. This prob-ably reflects differences in the degree of fractional crys-tallization including that of plagioclase.
The tetrad effect is generally ascribed to partitioningof REE between silicate melt and water-dominated fluid.Thus, the tetrad REE pattern of the plagiogranite impliesthat the parent magma was very rich in H2O, CO2 andelements such as Li, B, F, P and/or Cl. If this is the case,there are two possible sources of the volatile elements inthe plagiogranite magma: (1) MORB magma, or (2) ex-ternal fluid. The former is less likely, because MORBmagma is well known to be significantly depleted involatiles compared to typical arc magmas (Saal et al.,2002). This is inconsistent with the unusually volatile-rich nature of the plagiogranite, the final product ofMORB differentiation, suggested by the tetrad REE pat-tern. This leads us to propose that the external fluids, prob-ably derived from seawater, were incorporated into theoceanic plagiogranite magma and played a significant rolein the formation of the tetrad REE pattern during the fi-nal stage of magma evolution. In fact, incorporation ofseawater-derived fluid into plagiogranite magma has pre-viously been pointed out by many researchers based onisotopic studies, e.g., Sr (Lanphere and Coleman, 1981;Kawahata et al., 1997; Bosch et al., 2004). Anatexis ofamphibolites that were altered by seawater-derived hy-drothermal fluid (Gillis and Coogan, 2002) is also likelyto generate such fluid-rich oceanic plagiogranite magma.Moreover, partial melting of gabbros triggered by the in-filtration of seawater-derived water-rich fluids has re-cently been proposed as the origin of fluid-rich oceanicplagiogranite magma (Koepke et al., 2004, 2007). Thesegeological and geochemical lines of evidence are con-
sistent with our hypothesis that the oceanic plagiogranite,with the tetrad REE pattern, was produced by crystalfractionation and fluid-melt interaction of plagiogranitemagma enriched in fluids after the incorporation ofseawater.
In conclusion, this discovery of the tetrad effect inoceanic plagiogranite indicates that fluid-melt interactiontook place during late stage MOR magmatism, as in arcmagmatism. The incorporation of external fluids, prob-ably originating from seawater, and high-degrees of frac-tional crystallization could play an important role in thegeneration of the oceanic plagiogranite exhibiting the lan-thanide tetrad effect.
Acknowledgments—The authors wish to thank Y. Imai and theShinkai 6500 operation team and M. Ishiwata and the crew ofR/V Yokosuka for their skillful support during the YK05-16Cruise. Thanks are also due to the shipboard scientific partyfor their invaluable collaboration, and A. R. L. Nichols for im-proving the manuscript. We also thank K. Okino for permittingus to use the unpublished bathymetry data acquired by theYK05-16 Cruise. The constructive and helpful reviews by T.Akagi and an anonymous reviewer are gratefully acknowledged.This study is financially supported, in part, by the Grants-in-Aid for Scientific Research (No. 14253003) from the Ministryof Education, Culture, Sports, Science and Technology of Ja-pan (MEXT), the Grant-in-Aid for JSPS Fellows from MEXT,and JAMSTEC multi-disciplinary research promotion grantentitled “An interaction mantle and life through Earth history”.The cruise expedition was conducted as a part of the Deep-SeaResearch Program funded by MEXT.
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