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Plutonic Rocks
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
New oceanic crust forms at mid-ocean ridges as magma rises to fill space created when plates diverge. The spreading lithosphere, composed of crust and some underlying mantle, cools and becomes denser during spreading. So, the youngest seafloor and highest elevations are at the ridges, and the oldest seafloor and lowest elevations are at ocean margins. The ocean lithosphere may not be the same composition and structure everywhere, but all evidence suggests that there are some standard components. The evidence comes from many sources including drill core, seismic studies, laboratory experiments, grab sampling from the ocean floor, and dredging. The best information, however, may come from studies of ophiolites. Ophiolites are parts of oceanic crust and mantle that were uplifted and added to continental margins or are exposed in islands. The term ophiolite derives from the Greek words ophio (snake) and lite (stone), referring to the commonly green color of the rocks that make up ophiolites. There are many ophiolites around the world, but most are small or very fragmented; Table 6.4 lists some of the best-known and studied ones.
Distribution of rocks at and below the surface
Published in A.C. McLean, C. D. Gribble, Geology for Civil Engineers, 2017
Plates are bounded by active ridges and trenches. Ridges or mid-ocean ridges are the centres of divergence or of spreading-apart of two plates. At such a mid-ocean ridge, new oceanic basalt is extruded and added to each plate. Such a process was recognised by examining the magnetic properties of the basalts at each side of a mid-ocean ridge over a distance of several hundred kilometres. Within igneous rocks, small crystals of magnetite act like magnets and align themselves in the Earth’s magnetic field as the rock cools, eventually retaining a strong permanent magnetisation indicating the position of the Earth’s magnetic poles at that time. Studies in this branch of the subject (called palaeomagnetism) have shown that the Earth’s magnetic poles have moved throughout geological time and that the polarity of the Earth’s magnetic field is not constant but that the north and south poles alternate at regular intervals, in the order of half a million years. That is, at regular intervals, the Earth’s magnetism is reversed.
Petroleum Geological Survey
Published in Muhammad Abdul Quddus, Petroleum Science and Technology, 2021
The mid-ocean ridges are under sea mountain ranges, linked together in a chain. The ridges are formed along a divergent tectonic plate boundary. A trench is a long, narrow pit in the ocean bottom typically found parallel to the plate boundary. Crust spreading is the fresh addition or formation of new ocean crustal rock.
Decoding the marine biogeochemical cycling of mercury by stable mercury isotopes
Published in Critical Reviews in Environmental Science and Technology, 2023
Lin Yang, Ben Yu, Deming Han, Kun Zhang, Hongwei Liu, Cailing Xiao, Ligang Hu, Yongguang Yin, Jianbo Shi, Guibin Jiang
A recycling process of oceanic Hg into the mantle was revealed by the consistent positive values of Δ199Hg observed in island arc basalts and mid-ocean ridge basalts with marine sediments and seawater (Yin et al., 2022). Large amounts of Hg in marine sediment can be delivered to the mantle in subduction zones. The majority of subducted Hg can be recycled back to the surface environment through arc volcanism, while the remaining Hg entered the upper mantle and was released at mid-ocean ridges by mid-ocean ridge basalt volcanism. Additionally, the Hg isotopic composition in marine sedimentary rocks has emerged as a proxy for discerning photic zone euxinia (PZE), a phenomenon involving H2S-rich waters occurring in photic zones, having passive impacts on biological evolution in ancient oceans (Zheng et al., 2018). A significant negative MIF and a positive MDF were observed during euxinic intervals (Zheng et al., 2018). Two possible mechanisms were suggested as the photoreduction of Hg2+ dominated by reduced sulfur ligands resulting in depletion of odd isotopes in Hg2+ (Motta, Kritee, et al., 2020; Zheng & Hintelmann, 2010), and the enhanced sequestration of atmospheric Hg0 to the sediments by thiols and sulfide in PZE conditions.
Mid-Devonian basaltic magmatism and associated sedimentation: the Ooloo Hill Formation, central-eastern South Australia
Published in Australian Journal of Earth Sciences, 2023
C. Wade, A. J. Reid, E. A. Jagodzinski, M. J. Sheard
Ooloo Hill Formation basalts and andesite sit above the mid-ocean ridge basalt (MORB)–ocean island basalt (OIB) array and below the metasomatised mantle array on the Th/Yb–Nb/Yb diagram (Figure 12a). Eruptive sequences #1, #2 and #4 form a trend parallel to the MORB–OIB array, consistent with within-plate evolution of mantle sources (e.g. Liu et al., 2021), whereas eruptive sequence #3 has an apparent, although tenuous, vertical trend toward the metasomatised mantle array (Elliott et al., 1997; Hawkesworth et al., 1997). Differing evolutionary trends are also highlighted on Figure 12b, suggesting eruptive sequences #1 and #2 had an asthenospheric mantle source, possibly derived from OIB, and potentially share a common source region to eruptive sequence #4 that underwent differentiation. The separate but parallel offset in eruptive sequence #3 suggests it was derived from a more enriched OIB-like intraplate parent (Figure 12b). The lithospheric component in this eruptive sequence may be linked to slab-melt contributions into the mantle source or lithospheric contamination of a plume-derived source (Xia & Li, 2019). The mantle source components for Ooloo Hill Formation were therefore likely to be a heterogeneous OIB-like intraplate parent magma derived from the asthenospheric mantle. Different magma sources for Ooloo Hill Formation are demonstrated by incompatible element ratios (Figure 12b, c) suggesting a dynamic magmatic plumbing system.
The Alpha-Mendeleev ridge, a large igneous province with continental affinities
Published in GFF, 2019
The samples on the Northwind Rise and north of it comprised three groups with 40Ar/39Ar ages of ca. 112 Ma, ca. 100 Ma and ca. 83 Ma. Two younger samples of rock are (low-Ti tholeiitic basalts – LT, and high-Ti alkaline basalts – HT). Such a distribution of low- and high-Ti rock varieties is typical for plume-related continental flood basalts (CFB). Trace element compositions for this group and the Alpha Ridge sample are also similar to CFB (Mukasa et al. 2009). All the studied volcanic rocks belong to the intraplate volcanic associations whose petrological, geochemical and isotopic parameters are consistent with their emplacement in a continental setting (Table 2). Taken together, geochemical and isotopic attributes of the studied volcanic rocks are similar to those of CFB provinces originating from deep-seated hot spot sources and often associated with continental break-up (Andronikov et al. 2008). Furthermore, the chemical affinity of the basalt dredged from the Alpha Ridge (Fig.1) (Williamson & Larsen 2007) is consistent with the alkaline compositions reported for igneous rocks of the onshore Sverdrup Basin. It is noteworthy that the data from the dredged rocks have compositions that exclude typical ocean-island basalt (OIB), normal mid-ocean-ridge basalts (N-MORB), or even Gakkel Ridge enriched mid-ocean-ridge basalts (E-MORB) (Mukasa et al. 2009).