Ling Chena,b, Jihao Zhu*a, Fengyou Chua,b, Yanhui Dong a, Xing Yua, Zhenggang Lia
aSOA Key Laboratory of Submarine Geoscience, Second Institute of Oceanography, State Oceanic Administration, Hangzhou 310012, PR China;
bDepartment of Earth Sciences, Zhejiang University, Hangzhou 310027, PR China
As one of the slowest spreading ridges of the global ocean ridge system, the Southwest Indian Ridge (SWIR) is characterized by strong segmentation and discontinued magmatism (Cannat et al., 1999, 2003; Sauter and Cannat, 2010; Dick et al., 2003). The crust at the SWIR is thinner compared with fast spreading ridges and highly variable from one segment to another and even within individual segments, leading to exposures of mantle peridotites along the amagmatic axial regions (Dick et al., 2003; Sauter et al., 2004). The 53°E segment between the Gallieni fracture zone (FZ) (52°20′E) and the Gazelle FZ (53°30′E) is a typical amagmatic segment that opens a window to the mantle thus provide a chance to detect the mantle composition directly(Zhou and Dick, 2013).
In this study we examine the mineral compositions of seventeen peridotites samples collected by five dredges and two television-guided grabs (TVG) during the RV Dayang Yihao Cruise 21 (2010) in the 53°E amagmatic segment, to determine the mantle composition and mantle refertilization during melt extraction. The results show that the 53°E peridotites are harzburgites with clinopyroxene content ranging from 0.1% to 6.0%, however they show a great geochemical variability in the ~90km-long ridge segment, with a mineral composition range comparable to the published peridotite data along the whole SWIR (Dick and Bullen, 1984; Dick, 1989; Johnson et al., 1990, 1992; Seyler et al., 2003, 20011). Based on the CI chondrite normalized REE patterns of clinopyroxene, the 53°E peridotites are divided into two groups. The normal group peridotites (13 out of 17) have LREE depleted clinopyroxene patterns that is typical for the abyssal peridotite (e.g. Johnson et al., 1990) thus are thought to be the residue of the mantle melting. The abnormal group peridotites (4 out of 17) obviously enriches in LREE, with flat or near flat REE patterns, thus can not be the pure residue of mantle melting.
Comparison between the normal group peridotites and the mantle melting model calculated with a source of Depleted Mantle (DM) in the spinel stability field shows that the peridotites are more depleted in MREE/HREE (e.g. SmN/YbN: 0.02-0.4), which means that mantle melting should begin in the garnet stability field rather than the spinel stability field. The calculated garnet mantle melting model indicates that ~40% of the melting is presented in the garnet stability field. Despite the normal group peridotites have depleted LREE relative to MREE and HREE, their LREE content are still higher than the melting model. A melt refertilization model with the addition of various melts produced by DM melting to the modelled residual peridotite can explain the slight enrichment of the LREE in the normal group peridotites.
The abnormal group peridotites (Sp-Cr#: 0.34-0.39; Opx-Al2O3: 2.6-3.2 wt.%; Cpx content: 0.1-1.4%; Cpx-Cr#: 0.18-0.21) are obviously more depleted than the normal group peridotites (Sp-Cr#: 0.16-0.27; Opx-Al2O3: 3.2-5.0 wt.%; Cpx content: 1.8-6.0%; Cpx-Cr#: 0.12-0.15), however they are strongly enriched in the incompatible elements such as LREEs, Na and Sr and show flat pattern in the multi-element spider diagram. This kind of enrichment requires melt strongly enriched in the incompatible elements, thus can not be modelled by the refertilization of melt of depleted mantle melting. The melt responsible for the refertilization is probably a low-degree partial melt of a more fertile mantle or a partial melt of an enriched lithology such as pyroxenite beneath the 53°E segment. The enriched mantle component were also inferred to present in other SWIR segments such as the 10-16°E segment, the Atlantis II fracture zone (Salters et al., 2002) and the 63°E segment (Seyler et al., 2004).
Both the normal group and the abnormal group peridotites can be found in one dredge and the abnormal peridotites have mineral compositions even more depleted than peridotites sampled near the Bouvet hot spot (Johnson et al., 1990), which is inconsistent with the ultraslow-spreading rate and the cold mantle of the SWIR, thus implies that the depleted mantle beneath the 53°E segment may be the residue of ancient melting event. This hypothesis is supported by the the low Ol/Opx ratios, coarse grain sizes (>1cm) Opx, and Mg-rich compositions akin to harzburgite xenoliths that sample old continental lithospheric mantle (Kelemen et al., 1998). The presence of both the continental lithospheric-like mantle and the enriched mantle component beneath the 53°E segment indicates that mantle composition is extremely heterogenous beneath the ultraslow-spreading ridge even in a ridge segment scale.
Corresponding author: Jihao Zhu
Email address: zhujh@sio.org.cn
