Naoya Yoshitake *, Shoji Arai **, Yoshito Ishida *** and Akihiro Tamura **** LETTER INTRODUCTION

Transcription

1 156 Journal of Mineralogical N. Yoshitake, and Petrological S. Arai, Y. Sciences, Ishida and Volume A. Tamura 104, page , 2009 LETTER Geochemical characteristics of chloritization of mafic crust from the northern Oman ophiolite: Implications for estimating the chemical budget of hydrothermal alteration of the oceanic lithosphere Naoya Yoshitake *, Shoji Arai **, Yoshito Ishida *** and Akihiro Tamura **** * Division of Earth and Environmental Sciences, Natural Science and Technology Kanazawa University, Kanazawa , Japan ** Earth Science Course, School of Natural System, College of Science and Engineering Kanazawa University, Kanazawa , Japan *** Department of Earth Sciences Kanazawa University, Kanazawa , Japan **** Frontier Science Organization Kanazawa University, Kanazawa , Japan We examined dike like chlorite rocks that replaced isotropic gabbro and dolerite in northern Oman ophiolite in order to understand the chemical budget of hydrothermal alteration of the oceanic lithosphere. During chloritization, the concentrations of Si, Ca, Na, and K decreased, while those of Fe increased. REE (rare earth elements), except Eu, which showed a strong depletion in the chlorite rocks, were immobile during chloritization, which was caused by the downward (recharge) flow of circulated seawater. A portion of Fe was supplied from the overlying mafic extrusives, possibly through the alteration of their plagioclases. We found Ti rich minerals such as rutile and titanite to be the reservoirs of most REE in the chlorite rocks. If the residual fluid, after chloritization, moves upward, it can realize the positive Eu anomaly of the seafloor vent fluids. And, if the fluid is transported to deeper parts of the oceanic lithosphere, rodingites, serpentinites (antigorite rocks), and diposidites with a positive Eu anomaly are formed within gabbros and mantle peridotites. Keywords: Ophiolite, Chloritization, Gabbros, Hydrothermal circulation, Eu anomaly INTRODUCTION Seawater infiltrating the oceanic lithosphere through cracks in the earth s crust hydrothermally alters the lithosphere to a certain depth. Simultaneously, the infiltrating seawater heats up and interacts with wall rocks. The depth of reach and chemical budget of this hydrothermal activity have not been thoroughly understood because of the poor accessibility to the ocean floor. Many authors have extensively studied extrusive rocks such as basalts because of the availability of materials prevailing at the shallow parts of the oceanic lithosphere (Seyfried et al., 1978; Mottl and Holland, 1978; Mottl, 1983). The Oman ophiolite, which provides an excellent exposure to sections ranging from extrusives to peridotites, provides unrivaled information on hydrothermalism in the oceanic lithosphere up to the upper mantle (e.g., Kawahata et al., doi: /jmps b S. Arai, ultrasa@kenroku.kanazawa u.ac.jp Corresponding author 2001; Bosch et al., 2004; Python et al., 2007). In this paper, we describe geochemical and petrological characteristics of unique chlorite rocks that have been replacing the upper crustal isotropic gabbro and dolerite in the northern Oman ophiolite. We believe that our findings will contribute significantly to our understanding of element mobility, chemical budget, and petrological and mineralogical changes resulting from the intense hydrothermal alteration of upper oceanic crust. Miyashita et al. (2007) have also described chlorite rocks from the Oman ophiolite. GEOLOGICAL BACKGROUND The northern Oman ophiolite provides us with a complete ophiolite stratigraphy, which possibly represents a crust upper mantle section of the ocean floor formed at a fast spreading ridge (e.g., Nicolas, 1989). Dark brown chlorite rocks are formed in isotropic gabbros, which are sparsely penetrated by dolerite dikes

2 Geochemical characteristics of chloritization of mafic crust 157 Figure 1. Geological map of the Wadi Bani Umar area of the northern Oman ophiolite showing the locality of the outcrop of chlorite rocks ( N, E). After Reuber (1988). exposed along Wadi Bani Umar in the Fizh section of the northern Oman ophiolite (Fig. 1). The isotropic gabbro outcrop ( N, E) is located immediately beneath the sheeted dike complex (Reuber, 1988). A large late intrusive wehrlitic complex intruded the gabbro body along its western margin (Fig. 1). The chlorite rocks are distributed in an area of approximately 100 m across, and form network like dikes or veins (Figs. 2a and 2b) and a plug at the central part. These rocks indiscriminately penetrate the gabbro and dolerite; they are apparently coarse grained when replacing the former and fine grained when replacing the latter (Fig. 2c). This difference in apparent granularity can indicate the replacement nature of chlorite rocks. The chlorite rocks appear to change to wall rocks within a few centimeters from their boundaries (Fig. 2d). Chlorite rocks (chloritites) have been sporadically found around the sheeted dike gabbro transition zone in the northern Oman ophiolite (Miyashita et al., 2007). PETROGRAPHY The textural characteristics of chlorite rocks and their protoliths are shown in Figure 3. Isotropic gabbros consist of plagioclase, clinopyroxene, orthopyroxene, hornblende, actinolite, cummingtonite, ilmenite, magnetite, rutile, and apatite (Fig. 3a). Dolerites show the same mineral assemblage, but are much more fine grained (Fig. 3c). Clinopyroxenes are partially replaced along their rims and cleavages by brownish Mg hornblende to various degrees (Fig. 3a), and orthopyroxenes are also partially replaced along their rims and cleavages by cummingtonite or aggregates of amphiboles and chlorite. This alteration is common to both gabbros and dolerites. Mafic minerals (pyroxenes and amphiboles) are preferentially replaced by chlorite, while plagioclase and opaque minerals remain nearly intact in the wall rocks even at the very thin transition zone around the boundary with chlorite rocks (Figs. 2d and 3c). The plagioclase grains contain many fine chlorite veinlets (Fig. 3e). Aggregates of amphibole, which are the alteration product of pyroxene, remain as rims of chlorite aggregates. Chlorite rocks consist mostly of chlorites (>99 vol%), which show two domains, dark brown and light brown in thin section (Figs. 3b and 3d). The other minerals that are found in these rocks are ilmenite, magnetite, rutile, titanite, and apatite. Chlorites are coarse grained when replacing isotropic gabbro and fine grained when replacing dolerite, thereby inheriting the textures of the original rocks (Figs. 3a 3d). Rutile is commonly found as subhedral small grains, which are surrounded by irregularly shaped titanite grains (Fig. 3f). Possible pseudomorphs after mafic minerals (dark chlorite aggregate) (Fig. 3g) occasionally contain very fine grained (<0.02 mm across) aggregates, which are mainly composed of ilmenite and titanite (Fig. 3h). Apatites may be relics of their protoliths. BULK CHEMISTRY Four sets of samples were collected within a depth of every 50 cm from the chlorite rock/wall contact to a depth of 200 cm within the wall rock (Fig. 4). Two sets were from chlorite rock/gabbro transects, and the other two from chlorite rock/dolerite transects. Major elements The samples were analyzed by XRF (X ray fluorescence spectroscopy) (Rigaku 3270, 50 kv, 200 ma) for estimation of the bulk major element compositions. All wall rock samples have similar bulk element compositions, but dolerite samples have a higher FeO * (total iron) content than the gabbros samples (Fig. 4, Table 1). The chlorite rocks are apparently different from the wall rocks in their chemical composition (Fig. 4, Table 1). The SiO 2, CaO, Na 2 O, and K 2 O contents are lower in the chlorite rocks than in the wall rocks, probably because CaO, Na 2 O, and K 2 O were mostly removed during chloritization (Fig. 4, Table 1). In contrast, the FeO * content is significantly higher in the chlorite rocks than in the wall rocks (Fig. 4). One of the chlorite rock samples which replaced dolerite has preserved some of the plagioclase phenocrysts that

3 158 N. Yoshitake, S. Arai, Y. Ishida and A. Tamura Figure 2. Chlorite rocks in the outcrop along Wadi Bani Umar. (a) Chlorite rocks (black portion at the center of the image) replacing gabbro in a complex way. (b) Network of chlorite rocks in gabbro. (c) Coarsegrained and finegrained chlorite rocks replacing gabbro and dolerite, respectively. (d) A closeup of the transition from the gabbro to the coarsegrained chlorite rock. survived chloritization, and thus, has a higher SiO2 content than the others (Fig. 4). REE The gabbro samples were analyzed by ICP MS (inductively coupled plasma mass spectrometry) (ThermoElectron, X7 series ICP MS) for bulk REE estimation. The analysis was carried out according to the procedure used by Shirasaka et al. (2004) (Table 2). Chondrite normalized REE patterns of all the gabbro samples show typical N MORB (normal mid oceanic ridge basalt) like shapes, a slight depletion of LREE (light REE), and flatness in profile from MREE (middle REE) to HREE (heavy REE) (Fig. 5a). The chlorite rocks show a strong negative Eu anomaly, with the abundance for other REE almost similar to that seen in wall rocks; however, one of the chlorite rock samples displays a relatively low value of REE contents (Fig. 5a). This indicates that all REE except Eu are immobile during chloritization, and Eu being highly mo bile, is extensively removed during chloritization. Interestingly, the seafloor hydrothermal vent fluids show a contrasting Eu behavior i.e., they showed a strong positive Eu anomaly (Douville et al., 1999, 2002) (Fig. 5b). MINERAL CHEMISTRY Major elements Microprobe analyses of selected minerals were carried out at Kanazawa University using a JEOL 8800 microprobe. Plagioclases show a wide difference in the An content ranging from 40 to 80 in gabbros and dolerites. Clino pyroxenes indicate the presence of augite, but show a small difference in Mg# [Mg/(Mg + total Fe) atomic ratio] which ranges from 0.75 to Amphiboles that replaced clinopyroxenes are either magnesio hornblende or actinolitic hornblende (Leake, 1978). Mg#, which ranges

4 Geochemical characteristics of chloritization of mafic crust 159

5 160 N. Yoshitake, S. Arai, Y. Ishida and A. Tamura Figure 4. Major element compositional profiles of wall rocks at 50 cm intervals and chlorite rocks. Two transects to the chlorite rock are shown for gabbro and dolerite each. Chl.R., chlorite rock; Cont., contact. from 0.7 to 0.8 in amphiboles, is slightly lower than that in clinopyroxenes. Orthopyroxenes show a relatively low Mg# of around 0.7, while cummingtonites that replaced orthopyroxenes show a slightly lower Mg# (0.65 to 0.70). Chlorites show a wide range of Mg#, from 0.24 to The Mg# tends to be lower in the dark chlorite than in the light colored one (see Figs. 3b and 3d). REE REE compositions of selected minerals were determined in situ by LA ICP MS (laser ablation inductively coupled plasma mass spectrometry) (GEOLASQ+, Agilent 7500 series ICP MS) at Kanazawa University (Morishita et al., 2005). Chlorites show an almost flat pattern with a strong negative Eu anomaly, indicating a variation in the abundance of REE by an order from one point to another in individual samples (Fig. 6a). Opposite to the relatively low abundance of REE of chlorites (Fig. 6a), the other minerals show high REE levels. The REE concentrations in apatites and titanites, which also show a strong negative Eu anomaly, are 1000 to times higher than those in chondrite (Fig. 6a). We also analyzed the fine grained aggregates of Ti rich minerals; however, we could only analyze mixtures of the aggregates and host chlorites because of the small size (<0.02 mm) of the aggregates (Fig. 3h). Chlorites are far lower in Ti content than clinopyroxenes and amphiboles (Fig. 6b), indicating the precipitation of Ti rich phases during their chloritization. Ti clearly co varies with REE (Fig. 6c); their abundance in Ti was ~ 1000 times that in chondrite. We thus concluded that all the minerals in chlorite rocks probably show strong negative Eu anomalies. DISCUSSION Origin of Fe in chlorite rocks: Fe budget of hydrothermalism in the oceanic crust Textural variation during chloritization, such as the transi Figure 3. Photomicrographs of wall mafic rocks and chlorite rocks. OP, plane polarized light. CP, crossed polarized light. Abbreviations: Opx, orthopyroxene; Hbl, hornblende; Cpx, clinopyroxene; Cum, cummingtonite; Pl, plagioclase; Chl, chlorite; Rtl, rutile; Ttn, titanite. All but (g) by transmitted light. (a) Wall isotropic gabbro. Cpx is extensively replaced by Hbl (purple colored interference) (CP). (b) Chlorite rock replacing isotropic gabbro. Note the coarse grained texture (OP). (c) Wall dolerite (CP). (d) Chlorite rock replacing dolerite. Note the fine grained texture (OP). (e) Partially altered gabbro from the transition zone. Note that all mafic minerals are transformed to chlorites, although plagioclase is almost intact (CP). (f) Pseudomorph of a mafic mineral (Mfp) within the chlorite rock replacing gabbro. Note the faint fractures, which are possibly inherited from cleavages in the precursor mineral, in mfp from the upper right to the lower left. The red rectangle represents the panel (h) (OP). (g) Subhedral rutile grains surrounded by anhedral titanite and ilmenite in the chlorite rock (reflected light). (h) Fine grained Ti rich mineral aggregates in coarse grained chlorite possibly replacing a mafic mineral. (g) Ilmenite surrounded by titanite (OP).

6 Geochemical characteristics of chloritization of mafic crust 161 Table 1. Major and minor element bulk rock compositions in gabbros/dolerites and chlorite rocks determined by XRF FeO *, total iron content. Samples were collected from four transects. Table 2. Bulk rock REE compositions (ppm) of gabbros and chlorite rocks determined by solution ICP MS technique Samples were collected from two transects. tion from gabbro (Fig. 3a) to chlorite rocks (Figs. 3b and 3g) via the transition zone (Figs. 2d and 3e) suggests a replacement with an almost constant volume. The high FeO* content of chlorite rocks (Fig. 4) and the low Mg# of chlorites indicates that the addition of Fe to chlorites occurred simultaneously with the removal of Si, Ca, Na, K, and Eu during hydrothermal alteration. Depending on the temperature, ph, and redox potentials, these elements are generally soluble in hydrothermal fluids (Douville et al., 1999, 2002). The hydrothermal fluid infiltrating into the gabbro, which is partly penetrated by dolerite dikes, is strongly reactive with the wall rock. This could possibly be due to the recharge (downward) flow from the shallow parts of the oceanic crust. Fe is probably supplied from the overlying mafic extrusive rocks and is released upon the formation of actinolite and epidote during the alteration of lower basalts and dolerites at low water/rock ratios (Mottl, 1983). Fe can

7 162 N. Yoshitake, S. Arai, Y. Ishida and A. Tamura Firgure 5. Chondrite normalized REE patterns. Chondrite values after Mc Donough and Sun (1995). (a) Wall rock gabbro and chlorite rocks. Note the strongly negative Eu anomaly of chlorite rocks (Chl.Rock). (b) Comparison of seafloor (S.F.) hydrothermal fluid (Douville et al., 2002) with gabbro and chlorite rock. Note the strongly positive Eu anomaly, which is in contrast to the negative Eu anomaly of the cholorite rock (a). Figure 6. Chemical characteristics of minerals. (a) Chondrite normalized REE patterns of several minerals in the chlorite rock, as analyzed by LA ICP MS. Note that chlorites show low REE abundances relative to the chlorite rock. Both titanite and apatite are rich in REE but show negative Eu anomalies. Chondrite values after McDonough and Sun (1995). (b) Plot of Al 2 O 3 vs. TiO 2 in clinopyroxenes, hornblendes, and chlorites. Note the formation of Ti poor chlorites and Ti rich minerals (rutile, titanite, and ilmenite) during chloritization. (c) Change in intensity of signals with time for Ti and Ce during LA ICP MS analysis of fine grained Ti rich aggregates in the chlorite rock (Fig. 3h). Ti rich minerals are the main reservoir of REE in the chlorite rock. also be supplied from plagioclase during albitization or anorthositization (e.g., Vanko and Laverne, 1998). The a mount of Fe introduced in plagioclase by the hydrothermal fluids within dolerites or basalts, though initially low, possibly increases with a decrease in the water/rock ratio downsection in the oceanic crust. The very limited number of chlorite rocks in the northern Oman ophiolite (e.g., Miyashita et al., 2007) indicates the low water/rock ratios in chloritization, as described in this paper. Preservation of REE (except Eu) in chlorite rocks: REE budget of hydrothermalism in the oceanic crust Ti rich minerals are expected to play an important role in the preservation of REE in chlorite rocks; the REE content in chlorite, which is the main constituent of the rock, is too low to balance the bulk REE content (Figs. 6a and 6c). The REE content in apatite is 1000 to times higher than that in chondrite; however, apatite occupies only 0.1 vol% of the rock. Similarly, the volume of titanite is too low to replicate the bulk REE abundances in the chlorite rocks. Fine grained Ti rich mineral aggregates (Fig. 3h), whose volumes are sufficiently high in dark chlorites (Fig. 3h), are possibly the main reservoirs of REE in the chlorite rocks (Fig. 6c). It is noteworthy that all minerals in the chlorite rocks show strongly negative Eu anomalies (Fig. 6a), possibly indicating the effective removal of Eu during chloritization. Eu is expected to be divalent in hydrothermal solutions at high temperatures (>250 C) (Sverjensky, 1984). Eu 2+ possibly forms stable complexes with Cl in hydrothermal solutions (e.g., Humphris and Bach, 2005) and is

8 Geochemical characteristics of chloritization of mafic crust 163 thus effectively removed from the mafic rocks during chloritization. Other REE have been retained in the chlorite rocks and are contained in newly formed Ti rich minerals such as titanite and apatite (Fig. 6a). Termination of hydrothermal circulation: Major elements and REE budget of the altered mantle harzburgite Successive hydration of rocks causes a decrease in the volume of hydrothermal fluids that flow downwards to the upper mantle, where their circulation may cease. Diopsidites and related antigorite rocks (high temperature serpentinites) (Python et al., 2007) are possibly formed from the fluid that reaches the uppermost mantle of the Oman ophiolite. Diopsidite is characterized by a high Ca content, high Mg#, and a positive Eu anomaly (Python et al., 2007). The high Mg# (>0.95) of the minerals found in diopsidites (Python et al., 2007) can be explained by the precipitation of Fe rich chlorites (subtraction of Fe) from the fluid before it reaches the peridotite. The precipitation of diopside with a positive Eu anomaly (Python et al., 2007) is comparable with the gain of Ca, Si, and Eu in the fluid during the chloritization of gabbros and dolerites. ACKNOWLEDGMENTS We are grateful to J. Uesugi, T. Suzuki, H. Okamura, and Y. Takemoto for their help in sampling of the Oman ophiolite. We thank M. Python for her support in our field work in Oman. We thank S. Ishimaru for her help in the ICP MS analysis of the solution and in the preparation of the manuscript. We appreciate S. Miyashita, A. Ishiwatari, and T. Morishita for fruitful discussions. Comments by N. Shikazono, K. Fujimoto, and an anonymous reviewer were very helpful in revising the manuscript. This study was partly supported by a Grant in Aid for Creative Scientific Research (19GS0211). REFERENCES Bosch, D., Jamais, M., Boudier, F., Nicolas, A., Dautria, J. M. and Agrinier, P. (2004) Deep and high temperature hydrothermal circulation in the Oman ophiolite Petrological and isotopic evidence. Journal of Petrology, 45, Douville, E., Bienvenu, P., Charlou, J.L., Donval, J.P., Fouquet, Y., Appriou, P. and Gamo, T. (1999) Yttrium and rare earth elements in fluids from various deep sea hydrothermal systems. Geochimica et Cosmochimica Acta. 63, Douville, E., Charlou J.L., Oelkers, E.H., Bienvenu, P., Jove Colon, C.F., Donval, J.P., Fouquet, Y., Prieur, D. and Appriou, P. (2002) The Rainbow Vent fluids (36º14 N, MAR): the influence of ultramafic rocks and phase separation on trace metal content in Mid Atlantic Ridge hydrothermal fluids. Chemical Geology, 184, Humphris, S.E. and Bach, W. (2005) On the Sr isotope and REE compositions of anhydrites from the TAG seafloor hydrothermal system. Geochimica et Cosmochimica Acta, 69, Kawahata, H., Nohara, M., Ishizuka, H., Hasebe, S. and Chiba, H. (2001) Sr isotope geochemistry and hydrothermal alteration of the Oman ophiolite. Journal of Geophysical Research, 106, Leake, B.E. (1978) Nomenclature of amphiboles. Canadian Mineralogist, 16, McDonough, W. F. and Sun, S S. (1995) The composition of the earth. Chemical Geology, 120, Miyashita, S., Adachi, Y., Neo, N. and Tanaka, S. (2007) Significance of chloritite bodies found from the dike gabbro transition of the Oman ophiolite. Geochimica et Cosmochimica Acta, 71, A674. Morishita, T., Ishida, Y., Arai, S. and Shirasaka, M. (2005) Determination of multiple trace element compositions in thin (<30 µm) layers of NIST SRM 614 and 616 using laser ablation inductively coupled plasma mass spectrometry (LA ICP MS). Geostandards and Geoanalytical Research, 29, Mottl, M.J. (1983) Metabasalts, axial hot springs, and the structure of hydrothermal systems at mid ocean ridges. Geological Society of America Bulletin, 94, Mottle, M.J. and Holland, H.D. (1978) Chemical exchange during hydrothermal alteration of basalt by seawater I. Experimental results for major and minor components of seawater. Geochimica et Cosmochimica Acta, 42, Nicolas, A. (1989) Structure of ophiolites and dynamics of the oceanic lithosphere. In Petrology and Structural Geology, 4, pp. 380, Kluwer Academic Publishers, MA.USA. Python, M., Ceuleneer, G., Ishida, Y., Barrat, J. A. and Arai, S. (2007) Oman diopsidites: a new lithology diagnostic of very high temperature hydrothermal circulation in mantle peridotite below oceanic spreading centres. Earth and Planetary Science Letters, 255, Reuber, I. (1988) Complexity of the crustal sequence in the northern Oman ophiolite (Fizh and southern Aswad blocks): The effect of early slicing? Tectonophysics, 151, Seyfried, W.S.Jr., Mottl, M.J. and Bischoff, J.L. (1978) Seawater/ basalt ratio effects on the chemistry and mineralogy of spilites from the ocean floor. Nature, 275, Shirasaka, M., Arai, S., Ishimaru, S., Ishida, Y., Shimizu, Y. and Morishita, T. (2004) The solution introduction ICP MS technique to trace element analysis of rocks. The Science Reports of Kanazawa University, 48, Sverjensky, D.A. (1984) Europium redox equilibria in aqueous solution. Earth and Planetary Science Letters, 67, Vanko, D. A. and Laverne, C. (1998) Hydrothermal anorthitization of pagioclase within the magmatig/hydrothermal transition at mid ocean ridges: examples from deep sheeted dikes (Hole 504B, Costa Rica Rift) and a sheeted dike root zone (Oman ophiolite). Earthe and Planetary Science Letters, 162, Manuscript received October 22, 2008 Manuscript accepted January 5, 2009 Published online March 10, 2009 Manuscript handled by Koichiro Fujimoto