Plagioclase ultraphyric basalts (PUBs) with up to 40% millimeter-sized plagioclase crystals, were sampled from the Mount Jourdanne volcanic massif (~64° E) in the Southwest Indian Ridge. The geochemistry of the host glass, the glassy melt inclusions and their host plagioclase macrocrysts (An60-69) are used to reveal the mantle heterogeneity and to discuss the origin of Mount Jourdanne PUBs. The melt inclusions trapped in plagioclase display low MgO and high SiO2 contents and show rare earth element (REE) patterns resembling enriched mid-ocean ridge basalts (E-MORB). Together with their positive Sr and Eu anomalies, these features indicate that they were derived from an enriched mantle source, likely a refertilized peridotite or a pyroxenite. In contrast to some 61–67° E basalts, there is a lack of negative Eu anomalies in the PUB host glasses, precluding large amounts of plagioclase crystallization from their parental magma. Petrographic observations and the general chemical similarity between melt inclusions and melts equilibrated with the clinopyroxene cores in regional gabbros and/or troctolites suggest that these plagioclase macrocrysts originate from gabbroic mush within the lower crust. The density contrasts allow the effective segregation of plagioclase prior to their incorporation into the host magma. We propose that these plagioclase macrocrysts were entrained when a new batch of magma passed through the crustal mush zone, and resulted in the formation of the PUB. Eruption of Mount Jourdanne PUBs requires a minimum ascending velocity of 5 m d−1 for the host magma, which is not as high as the eruption rate for typical MORB samples. It is likely that the PUB host magma erupts during a period with reduced magma supply, whereas eruption of aphyric lavas correspond to the fast volcanic formation of the Mount Jourdanne massif.
Precambrian banded iron formation (BIF) is one of the most important mineral resources in China, mostly abundant in the North China Craton (NCC) with relatively less common in South China. Since the BIF and siliceous rocks both originated from chemical deposition, the syngenetic BIF and Siliceous rocks can help evaluate their environment of formation. We examine here the mineralogy and geochemistry of siliceous rocks associated with the Tieshanmiao Formation BIF, aiming to decipher the conditions of formation of both BIF and Siliceous rocks in the Wuyang area in the NCC. Analysis of the geochemical characteristics of whole rock shows that the SiO 2 content of the siliceous rock ranges from 90.11% to 94.85% and is relatively high overall. Trace element contents of Ba and U are also high, the Ba/Sr ratio ranges from 3.89 to 25.28 and the U/Th ratio ranges from 0.09 to 0.20. Finally, the ΣREE value of rare earth elements ranges from 57.03 ppm to 152.59 ppm, and these indexes all indicate that siliceous rock resulted from hydrothermal deposition. Plots of Al 2 O 3 ‐SiO 2 , SiO 2 /(K 2 O+Na 2 O)‐MnO 2 /TiO 2 and Mn‐10×(Cu+Co+Ni)‐Fe in discrimination diagrams also verify this interpretation. However, both the MgO content, ranging from 0.16 to 0.32, and the Fe/Ti ratio, ranging from 2.50 to 9.72, suggest that terrigenous material was added during the depositional process. Major and trace element parameters of siliceous rock, such as the Al/(A1+Fe+Mn) ratio (from 0.81 to 0.93), MnO/TiO 2 (from 0.00 to 0.17), Al/(Al+Fe) (from 0.82 to 0.93), Sc/Th ratio (from 0.21 to 0.50), U/Th (from 0.09 to 0.20), (La/Yb) N (from 0.83 to 3.04), and the (La/Ce) N (from 0.01 to 0.02) all imply that the siliceous rock formed in a continental margin. In addition, the Sr/Ba ratio from 0.08 to 0.26, the δ Ce value from 0.31 to 0.90, and the δ Eu value from 0.14 to 0.58, all indicate that the siliceous rock was formed at a relatively deeper water depth and under weak hydrodynamic conditions. Siliceous rock and BIF formed in the same geological setting, with the SiO 2 /(K 2 O+Na 2 O) ratio of siliceous rock ranging from 28.61 to 47.43, the SiO 2 /Al 2 O 3 ratio from 16.53 to 32.37, and the SiO 2 /MgO ratio from 287.28 to 592.81, which are all in agreement with chemical deposition associated with volcanic eruptions. The Al 2 O 3 /TiO 2 ratio from 37.82 to 50.30 indicates that the magma source of siliceous rock was of slightly intermediate composition. During the Late Archean in the Wuyang area, the high concentration and high purity SiO 2 quickly precipitated from hydrothermal fluids to finally result in the accumulation of siliceous rock in a marginal sea, while the input corresponding to iron formation components was deposited to form iron formation layers, and limestone was only the product formed during the deposition intervals of siliceous rock and iron formations. In this study, the synsedimentary siliceous rocks of BIF act as a new way to provide direct evidence to understand the formation environment of BIF due to its high geochemical stability.
Non-transform discontinuity (NTD) is one category of tectonic units along slow- and ultraslow-spreading ridges. Some NTD-related hydrothermal fields that may reflect different driving mechanisms have been documented along slow-spreading ridges, but the discrete survey strategy makes it hard to evaluate the incidence of hydrothermal activity. On ultraslow-spreading ridges, fewer NTD-related hydrothermal activities were reported. Factors contributing to the occurrence of hydrothermal activities at NTDs and whether they could be potential targets for hydrothermal exploration are poorly known. Combining turbidity and oxidation reduction potential (ORP) sensors with a near-bottom camera, Chinese Dayang cruises from 2014 to 2018 have conducted systematic towed surveys for hydrothermal activity around a large NTD along the ultraslow-spreading Southwest Indian Ridge (SWIR, 48.1–48.7° E). Five new potential hydrothermal anomaly sites (2 inferred and 3 suspected) of high or low temperature and the previously inferred Sudi hydrothermal field occurred in diverse morphotectonic settings along a 78 km long ridge axis. The calculated vent frequency (Fs, sites/100 km) was ~7.7 over the entire study area, higher than the modified value (Fs ≈ 6.5) between 48 and 52° E of SWIR. Even only for the 54 km long large NTD, three hydrothermal anomaly sites yielded an Fs of ~5.6, which is higher than that of most ridge sections and is comparable to some fast-spreading ridges with high-resolution surveys. This indicates that NTDs along ultraslow-spreading ridges could be promising areas in fertilizing hydrothermal activities. Moreover, the deeply penetrating faults on the rift valley walls and strain-focused areas may contribute to the formation of NTD-related hydrothermal circulations. We suggest that NTDs along ultraslow-spreading ridges may be potential targets for further exploration of hydrothermal activities and seafloor sulfide deposits.
As one of the regional, magmatic, robust, axial ridge segments along the ultraslow-spreading Southwest Indian Ridge (SWIR), the magmatic process and mantle composition of the axial high relief at 64° E is still unclear. Here, we present major and trace elements and Sr-Nd-Pb isotope data of mid-ocean ridge basalts (MORBs) from 64° E. The basalts show higher contents of Al2O3, SiO2, and Na2O and lower contents of TiO2, CaO, and FeO for a given MgO content, and depletion in heavy rare-earth elements (HREE), enrichment in large-ion lithophile elements, and lower 87Sr/86Sr, 143Nd/144Nd and higher radiogenic Pb isotopes than the depleted MORB mantle (DMM). The high Zr/Nb (24–43) and low Ba/Nb (3.8–7.0) ratios are consistent with typical, normal MORB (N-MORB). Extensive plagioclase fractional crystallization during magma evolution was indicated, while fractionation of olivine and clinopyroxene is not significant, which is consistent with petrographic observations. Incompatible trace elements and isotopic characteristics show that the basaltic melt was formed by the lower partial melting degree of spinel lherzolite than that of segment #27 (i.e., Duanqiao Seamount, 50.5° E), Joseph Mayes Mountain (11.5° E), etc. The samples with a DMM end-member are unevenly mixed with the lower continental crust (LCC)- and the enriched mantle end-member (EM2)-like components, genetically related to the Gondwana breakup and contaminated by upper and lower continental crust (or continental mantle) components.
We present the major and trace elements and Sr, Nd, and Pb isotopes in mid-ocean ridge basalts (MORB) from the East Pacific Rise (EPR) at 2.6–3.1°S. These samples are low-K tholeiites and show significant variation in their major element compositions (e.g. 4.60–8.18 wt% MgO, 8.34–12.12 wt% CaO, 9.78–14.25 wt% Fe2O3, and 0.06–0.34 K2O wt%). Trace element abundances of the 2.6–3.1°S MORB are variably depleted (e.g. (La/Sm), N = 0.51–0.78, Zr/Y = 2.35–3.42, Th/La = 0.035–0.056, and Ce/Yb = 2.38–3.96) but closely resemble the average N-MORB. In the compatible elements (Ni and Cr) against incompatible element Zr plots, the 2.6–3.1°S MORB show well-defined negative correlations, together with a liquid line of descent (LLD) modelling and petrographic observations, implying a significant role of olivine, plagioclase and clinopyroxene fractionation during magma evolution. When compared to global MORB and peridotites, the 2.6–3.1°S MORB and most of the other axial lavas from the South EPR show similar Zn/Fe, Zn/Mn, and Fe/Mn ratios, attesting to a peridotite-dominated mantle lithology. However, the relationships between incompatible trace element ratios, such as Zr/Rb and Nb/Sm, and the negative correlation between Zr/Nb and 87Sr/86Sr indicate a geochemically heterogeneous mantle source. The mantle beneath the South EPR likely consists of two components, with the enriched component residing as physically distinct domains (e.g. veins or dikes) in the depleted peridotite matrix. In the Sr–Nd–Pb isotope space, the South EPR MORB lie along the mixing lines between the depleted MORB mantle (DMM) and the ‘C’-like Pukapuka endmember. We infer that low-F melts derived from these enriched materials may cause localized mantle heterogeneity (veins or dikes) via an infiltration process. Subsequent melting of the refertilized mantle may impart an isotopically distinct characteristic to South EPR MORB.
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