Silicon and Oxygen Isotopic Composition of Igneous Rocks from the Eastern Manus Basin

ZHAO Huijing, ZENG Zhigang, YIN Xuebo and CHEN Shuai

1) Seafloor Hydrothermal Activity Laboratory of the Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, P. R. China

2)Graduate University of Chinese Academy of Sciences, Beijing 100049, P. R. China

Silicon and Oxygen Isotopic Composition of Igneous Rocks from the Eastern Manus Basin

ZHAO Huijing1),2), ZENG Zhigang1),*, YIN Xuebo1), and CHEN Shuai1)

1) Seafloor Hydrothermal Activity Laboratory of the Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, P. R. China

2)Graduate University of Chinese Academy of Sciences, Beijing 100049, P. R. China

This paper reports silicon and oxygen isotopes of 20 kinds of igneous rocks and their major elements from the eastern Manus Basin. Combining silicon and oxygen isotopic data from other studies, we suppose that both δ30Si and δ18O values increase with the increasing of SiO2content. It means that the fractionation of silicon and oxygen isotopes are affected by the silica content. The positive correlation between CaO/Al2O3ratios and MgO and that between Si/Al and SiO2content indicate that clinopyroxene is the predominant mineral phase in our samples. We suppose that the fractionation of silicon and oxygen isotopes are influenced by mineral fractional crystallization. Probably, it is due to their different silicon and oxygen bridges. In this study, the δ30Simeanvalue= -0.17‰±0.17‰ and δ18Omeanvalue= +6.07‰±0.57‰ are higher than normal δ30Si and δ18O values of mantle, and we propose that these igneous rocks in the eastern Manus Basin are affected by hydrothermal alteration.

Si isotope; O isotope; igneous rocks; eastern Manus Basin

1 Introduction

Silicon, the third most abundant element on the Earth, has three stable isotopes,28Si,29Si and30Si. In nature, silicate (75%) and silica (12%) make up the majority components of the Earth’s crust, which makes silicon a useful element in the geochemical study of the Earth.

Several previous studies have focused on silicon isotope fractionation during biogenic and non-biogenic amorphous silica precipitation, and have shown significant enrichment of28Si during precipitation of silica and clays (Dinget al., 2005, 2004; Opfergeltet al., 2010; Reynoldset al., 2006). Other studies proposed that there is no significant fractionation of silicon isotopes during magmatic-metamorphic rock-forming processes and the variations of silicon isotope composition are mainly due to isotopic fractionation during precipitation of SiO2and biogeochemical processes (Allemanet al., 2005; De La Rochaet al., 2000; Douthitt, 1982; Georget al., 2006; Georget al., 2007b; Méheutet al., 2009). These differences make silicon isotopes useful in distinguishing magmatic, hydrothermal and sedimentary protoliths of strongly metamorphosed silica-rich rocks (Andreet al., 2006). In some other studies, silicon isotopes are used to characterize the geochemistry of igneous rocks (Georget al., 2007a, 2007b; Savageet al., 2010; Ziegleret al., 2005a, 2005b).

Savageet al. (2010) proposed that bulk silicate Earth (BSE) is homogeneous with respect to silicon isotopes because there are no significant relationships existing between δ30Si and age, chemistry or locality of rocks (Savageet al., 2010). However, there are many studies showing a heterogeneity of isotopes and trace elements in mid-ocean ridge basalts (MORB), ocean island basalts (OIB) and back-arc basin basalts (BABB) and providing insights into the dynamics of the Earth’s mantle (Albarede and van der Hilst, 2002; Ben Othmanet al., 1989; Hartet al., 1992; Hofmann, 1997; Parket al., 2010; Sintonet al., 2003). Usually, heterogeneity of BSE develops at convergent plate margins so that components in subduction material might contain sediment and hydrothermal-altered oceanic crust (Bercovici and Karato, 2003; Hawkesworthet al., 1993; Schubertet al., 2001; Stein and Hansen, 2008).

Previous studies about oxygen isotopes displayed that18O variations in submarine basalts are mainly due to low-temperature alteration and hydration (Taylor, 1968), and the higher δ18O value indicates the more extensive alteration (Pineauet al., 1976). Oxygen isotopes might be a useful tool in tracing different types of recycled crustal material in Earth’s mantle. The use of stable isotopes can overcome some limitations when using radiogenic isotopes as tracers of mantle processes (Widom and Farquhar, 2003).

Meanwhile, Douthitt (1982) noted the positive correlation between δ30Si value of igneous rocks and the silicon content. The similarity between isotopic fractionations of silicon and oxygen have been emphasized, and some studies have indicated that sedimentary processes could result in parallel enrichments between30Si and18O (Douthitt, 1982; Taylor, 1968). Thus, the combination of silicon and oxygen isotopes in the present study would give us new insights into the sources of igneous rocks.

This paper presents major elements data about 20 igneous rocks from eastern Manus Basin and 20 groups of δ30Si and δ18O data. It helps us to study the relationship among silicon, oxygen isotopes and silica content. It also indicates the characteristics of igneous rocks in our study area.

2 Geological Setting

As a quickly-opening back-arc basin (about 10cm yr-1), Manus Basin, which is located in the northeastern Bismarck Sea to the north of Papua New Guinea, is surrounded by Willaumez Rise to the southwest, Manus Island and New Hanover to the north, New Ireland to the east and New Britain to the south (Martinez and Taylor, 1996).

Numerous studies indicate that the site of northern New Guinea is a consequence of island arc – continent collision rather than a doubly subducted piece of oceanic lithosphere (Dewey and Bird, 1970; Johnson and Molnar, 1972; Pascal, 1979; Whitmoreet al., 1999). The ancient fore-arc crust in the eastern Manus Basin is being stretched between the Djaul and Weitin Transforms, resulting in the volcanism along a series of en echelon, sinuous rifts, including East Manus Rifts (ER), Southern Rifts (SR), Manus Spreading Center (MSC) and Extensional Transform Zone (ETZ) (Fig.1) (Martinea and Taylor, 1995; Sintonet al., 2003).

The eastern Manus Basin is thought to have been located above the subduction zone for more than 40 Ma (Kamenetskyet al., 2001). Rock types in Manus Basin vary from MORB-type to arc-type, including a volcanic suite of basalt, basaltic andesite, dacite and rhyodacite, with prevailing hydrothermal alteration (Binns and Scott, 1993; Mosset al., 2001; Sintonet al., 2003; Taylor and Martinez, 2003; Yang and Scott, 2002).

Fig.1 Tectonic setting of the Manus Basin behind the New Britain arc-trench system, Papua New Guinea (modified from Martinez and Taylor, 1996). There are three transform faults (Willaumez, WiT; Djaul, DT; Weitin, WT) bound four extensional zones in the Manus Basin. Our study area is marked.

3 Samples and Analytical Methods

The samples that we analyzed in this study were dredged from the Manus Basin between 151°39′-152°09′ E and 03°40′-03°47′S at sea water depth ranging from 1400 m to 2100 m (Fig.1) during the ‘KX08-973’ cruise in 2008.

These dredged samples, including basaltic andesite, andesite and dacite, have a high SiO2content ranging from 54.26% - 66.75%. Both silicon and oxygen isotopes of these samples were analyzed at the Institute of Mineral Resources, Chinese Academy of Geological Sciences (CAGS), with a MAT253EM-type mass spectrometer. Silicon isotopes were analyzed by the SiF4method for δ30Si (Dinget al., 1988) with a precision of ±0.10‰, the isotopic data being reported relative to the NBS-28 standard. Oxygen isotope compositions were determined by the BrF5method with a precision of ±0.20‰, the data being reported relative to the SMOW standard.

The results of the isotopic ratio analysis of both silicon and oxygen were reported in δ-notation in parts per mil (‰) as follows:

Table 1 δ30Si and δ18O values (in ‰) and major elements (in wt%) of igneous rocks dredged from the Manus Basin.

Major elements were analyzed by Sequential X-ray Fluorescence Spectrometer in the Institute of Geology and Geophysics, Chinese Academy of Sciences, with a precision of ±0.20‰. Data of major elements, δ30Si and δ18O, are given in Table 1.

To better discuss the variations of δ30Si and δ18O in igneous rocks, this paper presents a review of published δ30Si and δ18O data (Table 2) from Douthitt (1982), where δ30SiRQS(‰) values have been converted to δ30SiNBS-28(‰) using the formula δ30SiRQS-NBS-28= -0.28‰ (Molinivelskoet al., 1986).

4 Results

From Table 1, the δ30Si values vary from -0.4‰ to 0.2‰ with a mean value δ30Simeanvalue= -0.17‰±0.17‰. The average silicon stable isotope value of samples from Manus Basin is has a significantly large positive magnitude than the mean value δ30Si = -0.4‰±0.2‰ of BSE (bulk silicate earth) as reported by Douthitt (1982). The δ30Simeanvaluein this study is lower than the δ30Si of island-arc basalt (IAB) and mid-ocean ridge basalt (MORB) that range from -0.32‰ to -0.25‰ and -0.33‰ to -0.23‰, respectively, and much lower than the value of ultramafic rocks that varies from -0.39‰ to -0.29‰(Savageet al., 2010). On the other hand, the δ18O values of our samples range from 5.1‰ to 6.9‰, with a mean value δ18Omeanvalue= +6.07‰±0.57‰. Obviously, δ18Omeanvaluein this study is higher than δ18Oalkalicbasalts=+5.8‰±0.5‰ and δ18Otholeiiticbasalts=+5.3‰±0.3‰ from OIB (Harmon and Hoefs, 1995) and δ18Oglass= +5.71‰±0.17‰ from MORB (Itoet al., 1987).

The SiO2content of our samples varies from 54.26% -66.75%, and that of MgO ranges from 1.26% to 5.87%. In Fig.2, we can see that SiO2content of our samples displays negative correlation with MgO content, and Na2O+K2O content also shows negative correlation with MgO. All above indicate that these rocks are typically intermediate igneous rocks, including basaltic andesite, andesite and dacite (Table 1).

Fig.2 a) Plot of SiO2(wt%) versus MgO (wt%) of igneous rocks from Manus Basin; b) plot of Na2O+K2O (wt%) versus MgO (wt%) of igneous rocks from Manus Basin.

5 Discussion

5.1 The δ30Si and δ18O of Igneous Rock

In previous studies, some major elements are used to indicate the fractional crystallization of minerals (le Rouxet al., 2002; Peterson and Moore, 1987). In Fig.3a, positive correlation is displayed between CaO and MgO. It means both plagioclase and clinopyroxene crystallization from magma (Peterson and Moore, 1987). Since CaO/ Al2O3ratio is an effective indicator to show the evolution of basalts, a characteristic often connected with crystallization at different pressures (le Roexet al., 1996). And the clinopyroxene fractionation could be invoked by the positive correlation between CaO/Al2O3ratio and MgO content (le Rouxet al., 2002). The positive correlation between CaO/Al2O3and MgO in Fig.3b indicate the crystallization of clinopyroxene. Meanwhile, during magmatic fractionation, the Si/Al ratio of the clinopyroxene increases as the magma becomes progressively more siliceous (Fig.3c)(Kushiro, 1960). Based on Fig.3, we propose that clinopyroxene is the predominant mineral phase in the magma of our study area.

Fig.3 a) The correlation between CaO and MgO; b) The correlation between CaO/Al2O3and MgO; c) The correlation between Si/Al and SiO2. All samples are igneous rocks from Manus Basin.

Table 2 δ30Si and δ18O values (in ‰) of igneous rocks from islands of the MIA, seamounts of the MIA, and the volcanic suite of HM (Douthitt, 1982) (MIA: Mariana Island Arc; HM: Hackberry Mountain)

Based on the data from Douthitt (1982) as cited in Table 2, we calculate the mean value of silicon and oxygen isotopes respectively. For basalts, the mean values are δ30Si= -0.69‰±0.19‰ and δ18O=+6.06‰±0.71‰. For andesite (including basaltic andesite), the average values are δ30Si= -0.60‰±0.31‰ and δ18O=+6.45‰±1.37‰. And for dacite, δ30Si= -0.42‰±0.16‰ and δ18O=+9.00‰±1.58‰ are their mean values.

In fact, in more recent study, the average silicon isotopic value for each rock type group is more precise and becomes heavier as the samples become more evolved with the results: δ30Sibasalt= -0.31‰ ± 0.05‰; δ30Sibasalticandesite= -0.27‰ ± 0.05‰; δ30Siandesite= -0.23‰ ± 0.03‰; δ30Sidacite=-0.19‰ ± 0.04‰ (Savageet al., 2011). Since SiO2increases with the evolution of magma, we can conclude that in the series of basalt-andesite-dacite, both δ30Si and δ18O increase with the increasing of SiO2content. It means that the fractionation of silicon and oxygen isotopes are affected by the content of SiO2(Beucheret al., 2011).

In this study, δ30Simeanvalue= -0.17‰±0.17‰ and δ18Omeanvalue= +6.07‰±0.57‰, both of them are higher than the value of basalts (see Section 4). It agrees with the previous suggestion that silica content influences the silicon and oxygen isotope fractionation. Since olivine and plagioclase usually co-exist in basalt (Groveet al., 1992), and clinopyroxene is the predominant mineral phase in our samples, we suppose that the fractionation of silicon and oxygen isotopes have a relationship with the mineral fractional crystallization. Ding (1994) had a similar conclusion that the δ30Si value of quartz-feldspar-mica in granite decreases along with the oxygen isotope variation. Perhaps,30Si and18O are easily to be fractionated from clinopyroxene than from olivine because of their differentsilicon and oxygen bridges.

5.2 The Information from δ30Si and δ18O in Manus Basin

It has been long known that back-arc lavas are likely to be influenced by H2O and other components derived from the adjacent subducting slab (Kelleyet al., 2006; Langmuiret al., 2006; Pearceet al., 1994; Pearce and Stern, 2006; Taylor and Martinez, 2003), and so does the Manus Basin (Sintonet al., 2003). Normally, two major sources of back-arc magma are: 1) mantle peridotite, and 2) subduction-related crustal material containing in fluids and/or melts derived from the subducting slab (Kamenetskyet al., 2001).

The result δ30Simeanvalue= -0.17‰±0.17‰ in this study is higher than the mean value of BSE (δ30Simeanvalue= -0.29‰± 0.08‰) (Savageet al., 2010), and higher than either δ30Siandesite=-0.23‰±0.03‰ or δ30Sidacite=-0.19‰± 0.04‰. The result δ18Omeanvalue= +6.07‰±0.57‰ in this study is also higher than mantle values (δ18O=+5.5‰± 0.7‰) (Matteyet al., 1994). Considering the prevailing hydrothermal alteration in Manus Basin (Binns and Scott, 1993; Mosset al., 2001; Sintonet al., 2003; Taylor and Martinez, 2003; Yang and Scott, 2002), the higher δ18O value indicates the more extensive alteration (Pineauet al., 1976). We propose that both the higher δ30Si and δ18O of our samples suggest the contamination by a source affected by hydrothermal alteration. It means that the igneous rocks in Manus Basin are contaminated by hydrothermal fluid, and hydrothermal alteration makes rocks enriced in30Si and18O.

6 Conclusion

In this study, the igneous rocks from Manus Basin are intermediate rocks including basaltic andesite, andesite and dacite. Combining silicon and oxygen isotope data from other studies, we suppose that both δ30Si and δ18O increase with the increasing of SiO2content. It means that the fractionation of silicon and oxygen isotopes are affected by the silica content. The positive correlation between CaO/Al2O3ratios and MgO, and that between Si/Al and SiO2content indicate that clinopyroxene is crystallized in these igneous rocks. We suppose that the fractionation of silicon and oxygen isotopes has a relationship with the mineral fractional crystallization. Probably, due to their different silicon and oxygen bridges,30Si and18O are easier to be fractionated from clinopyroxene than from olivine.

Since the values δ30Simeanvalue= -0.17‰±0.17‰ and δ18Omeanvalue= +6.07‰±0.57‰ are higher than normal δ30Si and δ18O values of mantle, we propose that these igneous rocks in Manus Basin are affected by hydrothermal alteration.

Acknowledgements

This work was supported by the National Key Basic Research Program of China (Grant No. 2013CB429700), National Natural Science Foundation of China (Grant Nos. 40976027, 40830849 and 40906029), and Shandong Province Natural Science Foundation for Distinguished Young Scholars (Grant No. JQ200913).

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(Edited by Ji Dechun)

(Received November 2, 2012; revised April 9, 2013; accepted May 4, 2013)

? Ocean University of China, Science Press and Springer-Verlag Berlin Heidelberg 2014

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E-mail: zgzeng@ms.qdio.ac.cn