Download Petrogenetic Implications

Olivine Phenocryst & Whole-Rock Geochemistry of Basalts from the
Magellan Seamounts, Western Pacific: Petrogenetic Implications
TANG Limei1, 2, MA Weilin1, 2, Yildirim Dilek3, CHU Fengyou1, 2, DONG Yanhui1, 2
1 The Key Laboratory of Submarine Geosciences, SOA, Hangzhou 310012, China
2 The Second Institute of Oceanography, SOA, Hangzhou 310012, China
3Department of Geology, Miami University, Oxford, Ohio 45056
Abstract: Seamounts provide some of the most direct evidence for intraplate volcanism, and samples
from seamounts reveal crucial evidence for the geochemical makeup of oceanic mantle. Olivine
phenocrysts in seamount basalts reveal particularly useful information about the melt source and
magmatic processes during their evolution. We present in this study olivine phenocryst and
major-element geochemistry of 100-70 Ma basaltic rocks from the Magellan Seamounts. These rocks
all belong to OIA type, whose magmas experienced strong olivine fractionation and changed from the
olivine+plagioclase to the olivine+plagioclase+clinopyroxene cotectic in their evolution.
Key words: Olivine Phenocryst, Magmatic evolution, Basalts, Magellan Seamounts
E-mail: [email protected]
1 Introduction
Since the inception of deep-rooted mantle plume hypothesis (Morgan, 1971,1972a, 1972b), and the
elegant explanation of volcanic age progression along the Hawaii-Emperor seamount chains in Pacific
(Clague&Dalrymple,1989), mantles plumes and hotspots have been widely invoked to explain mantle
anomalies within the interiors of lithospheric. In the past two decades, the hotspot hypothesis has been
subjected to critical re-evaluations (Anderson, 2002, 2003, 2007; Hamilton, 2003; Foulger, 2002, 2010;
Foulger &Natland, 2003; Koppers et al., 2003, 2005; Natland and Winterer, 2005; Tarduno et al., 2009).
Therefore, understanding the nature of hotspot volcanism is important, because hotspots trails are used
to obtain the absolute motion of the lithospheric plates through geological time and also because
hotspots are an important means of heat transport between the deep mantle and the Earth’s lithosphere
(Koppers et al., 1998, 2000, 2003, 2004, 2005; White, 2005, Chu Fengyou et al., 2005 ). Seamounts
constitute some of the most direct evidence about intraplate volcanism, when seamounts formed and
into which tectonic setting they erupted is a useful reflection of how the properties of the lithosphere
interact with magma generation in the fluid mantle beneath (Hillier, 2007). It is very important to know
the deep mantle geological progress, rocks from seamount and its minerals give us the chance. Olivine
crystals can be formed directly through magma (Echeverria&Aitken,1986), and they also can be exotic
xenocrysts(Révillon et al., 1999), which include olivine from mantle peridotite (Zhang,2005) as well as
previously magma did not erupt (Thompson&Gibson, 2000). The magmatic olivine can be related to a
mantle melting event (Nisbet et al., 1993), whereas the olivine xenocrysts from the mantle peridotite
can reflect lithospheric mantle compositional characteristics as it was captured from mantle lithosphere
during magma ascent (Zhang,2005). Thus, the research on olivine phenocrysts can provide useful
information about magma source and magma evolution process. In this study, we present the
mineralogy, especially the olivine phenocryst and geochemistry, of basaltic rocks from the Magellan
Seamounts and discuss their petrogenetic evolution.
2 Geological Background
The Magellan Seamount Trail (MST) in the West Pacific Seamount Province (WPSP; Fig. 1) is
defined as a complex, short and discontinuous chain of guyots that formed in the South Pacific Isotopic
and Thermal Anomaly (SOPITA; Fig. 1). The ages of the studied guyots are estimated to range
between 100–70 Ma (Fig.2) based on the orientation of the MST and the existing models of the
absolute Pacific plate motion (Epp, 1984; Duncon, 1985; Engebreston et al., 1985; Lonsdale, 1988;
Henderson,1985; Wessel et al.,1997). The rock samples were obtained from several drill sites in the
Magellan Seamount Trail area during the DayangYihao cruise (2007).
Fig. 1 Location of the West Pacific Seamount Province (WPSP). The South Pacific Isotopic and Thermal Mantle
Anomaly (SOPITA), the DUPAL isotopic mantle anomaly, and the present day active hotspots of the Pacific
Ocean are plotted for reference(after Koppers et al., 2003).
Fig. 2 The age of Pacific crust and seamounts chains in WPSP (after Koppers et al., 2003).
3 Samples and Methods
A total of 12 basalt samples were chosen, about 30 g of basalt glass or material near the basalt glassy
margin were prepared by first removing visibly altered material. These samples were crushed into
2-3-mm particles, were then treated with ultrasonic waves in a 0.2 N HCl solution for 15 min and in
distilled water for 45 min. They were then washed with distilled water and left to dry at 60 °C for 12 h
before being pulverized with an agate mortar. The back scatter electron (BSE) images and mineralogy
analysis were obtained using a JOELJXA-8100 microprobe at the Key Laboratory of Submarine
Geosciences, State Oceanic Administration. Major elements were analyzed with an Axios sequential
X-ray Fluorescence Spectrometer and ICP-MS (Agilent 7500) at the Guangzhou Institute of
Geochemistry, Chinese Academy of Sciences.
4 Results
The major element analysis is shown in Table 2. We present the representative EPMA results of
olivine phenocrysts in the basalts from the Magellan Seamounts in Table 2, and show the micrographs
olivine phenocrysts in Fig.3. The MgO contents show a wide range from 2.20 wt.% to 9.29 wt.%, and
the Mg#[Mg/(Mg2++Fe2+)] range from 0.23 to 0.58, indicative of strong fractionation of magmas. The
TiO2 contents are high, mostly higher than 2 wt.%, the highest being 4.28 wt.%. The Fe2O3 content is
high with an average value 13.95 wt.%. The basaltic samples of the Magellan seamount all belong to
alkaline type (Fig.4), and plot in the OIA (Oceanic island alkali basalt) (Fig.4).
Table 1 Major element analysis (wt.% ) of basalts from Magellan Seamounts
Sample
MD53
MJ3
MI20
MP03
MP02
MI23
MP11
MP15
MI24
MP04
MD47
MP01
SiO2
48.48
43.58
41.44
44.03
47.30
41.46
47.46
39.89
40.97
44.91
44.05
47.26
TiO2
2.52
2.75
2.11
2.61
2.96
2.14
4.28
3.11
2.45
3.18
1.74
2.98
Al2O3
16.31
13.74
15.13
15.31
13.80
14.41
13.36
12.63
12.61
14.25
19.50
13.78
Fe2O3
11.63
14.62
16.11
13.62
14.25
14.60
15.53
13.64
13.53
14.58
8.79
14.25
MnO
0.12
0.27
0.12
0.14
0.13
0.20
0.19
0.19
0.18
0.23
0.22
0.12
MgO
3.38
2.20
4.42
5.72
2.90
6.39
3.91
9.29
9.06
5.10
4.14
2.92
CaO
6.11
10.43
10.97
9.81
7.89
12.10
9.07
10.34
12.60
9.64
8.00
7.90
Na2O
3.07
2.53
2.14
2.67
3.23
1.46
3.23
1.51
2.86
2.25
2.31
3.18
K2O
2.11
2.06
1.17
1.25
1.80
1.33
0.89
1.54
0.60
0.80
2.09
1.80
P2O5
1.03
1.10
1.08
0.92
1.99
0.81
0.67
0.43
0.69
0.39
1.55
1.99
Total
99.38
99.40
99.38
99.37
99.37
99.38
99.36
99.41
99.37
99.38
99.41
99.37
Mg#
0.37
0.23
0.35
0.46
0.29
0.47
0.33
0.58
0.57
0.41
0.48
0.29
Fig.3 Photomicrographs olivine phenocrysts
Table 2 Representative EPMA results (wt.% ) of olivine phenocrysts in basalts from the Magellan
Seamounts
Element
MI23-6
MI23-7
MI23-10
MI23-12
MJ3-1
MJ3-2
MJ3-11
MJ3-12
MJ3-14
MJ3-16
MJ3-18
SiO2
40.24
40.14
40.16
39.98
40.24
40.19
39.47
39.71
38.51
0.009
38.80
TiO2
0.00
0.00
0.01
0.06
0.02
0.03
0.01
0.03
0.04
0.007
0.03
Al2O3
0.05
0.07
0.04
0.07
0.06
0.04
0.06
0.07
0.31
0.087
0.09
Cr2O3
0.06
0.00
0.26
0.09
0.14
0.03
0.15
0.29
0.12
45.225
0.49
FeO
12.25
14.51
13.11
14.71
13.77
13.36
13.84
12.28
15.45
0.543
12.27
MnO
0.24
0.24
0.25
0.28
0.13
0.12
0.23
0.15
0.18
0.171
0.19
MgO
47.19
44.91
46.26
45.05
46.03
46.31
44.68
46.37
43.35
0.064
45.59
CaO
0.33
0.39
0.33
0.38
0.25
0.22
0.61
0.20
0.59
0.004
0.20
Na2O
0.02
0.01
0.00
0.02
0.01
0.04
0.00
0.01
0.04
14.417
0.01
K2O
0.00
0.00
0.00
0.01
0.01
0.01
0.01
0.00
0.03
39.058
0.00
NiO
0.27
0.21
0.20
0.15
0.17
0.23
0.10
0.26
0.09
0.116
0.25
Total
100.66
100.49
100.61
100.80
100.83
100.56
99.18
99.37
98.71
99.701
97.92
Fo
87
85
86
85
86
86
85
87
83
85
87
Fig. 4 TAS and TiO2-MnO*10-P2O5*10 diagram of basalts from the Magellan seamounts
5 Discussion
Magmatic olivine phenocrysts and mantle peridotite-derived olivine xenocrysts have distinct
differences in chemical compositions. The CaO content of the former is significantly higher than that of
the latter (Francis, 1985; Gurenko et al., 1996; Thompson & Gibson, 2000); the CaO content of olivine
xenocrysts derived from a mantle peridotite source is substantially less than 0.1wt% (Gurenko et al.,
1996; Thompson & Gibson, 2000; Hirano et al., 2004) because the CaO content of the olivine and the
crystallization pressure, temperature and oxygen fugacity are mainly controlled by the CaO and FeO
contents of magmatic melts (Jurewicz & Watson, 1998; Libourel, 1999). The low CaO content and the
high Mg # of mantle peridotites result in very low CaO contents in their olivine. Because the CaO
contents of olivine phenocrysts in the basalt samples from the Magellan Seamounts are mostly higher
than 0.1wt% (Fig.5a), we can rule out the possibility of a mantle peridotite source. The relationship
between the CaO and NiO contents and Fo values also suggests magmatic origin of the olivine. The
NiO content is consistent with the decrease of Fo, which deviates significantly from the trend of mantle
peridotites, in line with a magmatic fractionation trend (Fig.5b) (Sato, 1977).
The partition coefficients of Fe-Mg between olivine and magmatic melt are commonly used to
investigate whether the olivine phenocrysts are balanced to whole rock Mg# values (Nisbet et al., 1993；
Thompson & Gibson, 2000). If magmas do not undergo fractional crystallization or heap of
crystallization, the olivine composition and the whole-rock Mg# would constitute an equilibrium curve,
the Mg-richest olivine crystallized equilibrium from initial magma should fall on the balance curve
range, and thus olivine crystallized from magma evolution should fall below the balance curve, while
olivine xenocrysts can fall above or below the equilibrium curve according to its source characteristics
(Révillon et al., 1999).The olivine phenocrysts of the basalts from the Magellan Seamounts fall below
the balance curve (Fig.6a), suggesting that they are the result of crystal fractionation of balanced whole
rock.
The basalts from the Magellan Seamounts show positive correlations of CaO with MgO at
MgO<8.0% (Fig.6b), suggesting that magmas of the Magellan Seamounts changed from the
olivine+plagioclase to the olivine+plagioclase+clinopyroxene cotectic, because the much higher CaO
content of clinopyroxene than in olivine and plagioclase clinopyroxene fractionation would produce
decreasing CaO.
Fig.5 Correlation diagram of Fo-CaO(a) and Fo-NiO(b)
Fig.6 Correlation diagram of Fo-MgO(a) (Shaded area is olivine composition range of balanced with whole-rock
MgO content )(Sato, 1977)and CaO-MgO(b)
6 Conclusions
OIA-type magmas of the 100-70 Ma basalts from the Magellan Seamount experienced strong olivine
fractionation, and changed from the olivine+plagioclase to the olivine+plagioclase+clinopyroxene
cotectic during their evolution.
Acknowledgements
This work is supported by the National Basic Research Program of China "973" (Grant No.
2013CB429705), the State Oceanic Administration Foundation of China (Grant No. 2013335) and the
Basic Scientific Research Foundation of the Second Institute of Oceanography (Grant No. JT1304). We
appreciate the help of professor Liuying of Guangzhou Institute of Geochemistry, Chinese Academy of
Sciences for the major elements analysis.
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Tang Limei, Postdoctoral, Assistant Researcher, The main research direction: Marine Geology, Petrology and
Geochemistry, Tel: 0571-81963165,13735427786, Email: [email protected]