Evidence for textural and alteration changes in basaltic lava flows using variations in rock magnetic properties (ODP Leg 183)
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Keywords:
Subaerial
Pillow lava
Rock magnetism
Natural remanent magnetization
Subaerial
Pillow lava
Rock magnetism
Natural remanent magnetization
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Ash Mountain, South Tuya, and Tuya Butte are three small basaltic volcanoes in the Stikine volcanic belt of northern British Columbia. The volcanoes rise 700, 500, and 400 m above their bases and are about 3.2, 1.6, and 2.6 km 3 in volume, respectively. They began eruptive activity under several hundred meters of overlying glacial ice, or water in an ice‐impounded lake, and undegassed pillow lava was erupted and forms the bases of all three. Later, as the vents grew into shallow water, explosive phreatomagmatic activity erupted partly degassed glassy tuffs. Finally, when the volcano emerged through the surface of the ice or water (or the water was drained), degassed subaerial lava flows were erupted and were converted to assemblages of foreset‐bedded pillow breccia and pillow lava when subaerial flows crossed a shoreline and flowed into meltwater lakes. The undegassed subglacial pillow base of Ash Mountain is overlain by partly degassed pillows and hyaloclastite tuff cut by dikes; at South Tuya the pillow base is overlain by hyaloclastite tuffs and lenses of pillow lava; at Tuya Butte the pillow base is overlain by foreset‐bedded pillow lava, pillow breccias, and hyaloclastite tuffs, which in turn are overlain by subaerial lava flows composing a small shield volcano. The undegassed basal subglacial pillow lava of the three volcanoes contain 0.10 ± 0.01 wt % sulfur and ∼0.5 wt % H 2 O. The overlying partly degassed assemblages contain 0.06 ± 0.02% sulfur and ∼0.2% H 2 O at Ash Mountain, 0.07±0.01% sulfur at South Tuya, and 0.03±0.01% sulfur at Tuya Butte. The differences in the degree of degassing can be related to the nature of eruption and quenching and the distance of flow of the subaerial lava. When the volcanoes switched from subglacial to shallow water or subaerial eruptions, as shown by change to more explosive activity and then to subaerial lava flows (and by a marked reduction of sulfur in volcanic glass), the magma shifted from tholeiitic to alkalic composition. This transition occurs at each of the three volcanoes. The tholeiitic and alkalic magmas cannot be related by shallow crystal fractionation and apparently originated by differing degrees of deep melting at a mantle source. Prior to eruption the tholeiitic melts overlay alkalic melts in shallow chambers underlying each of the volcanoes because of their lower density and were, therefore, the first to erupt under subglacial conditions. As the volcano grew through the ice (or ice‐impounded water), the volcanic conduit vented to the atmosphere, producing a partial depressurization of the conduit and the subsurface chamber. This sudden reduction in confining pressure caused enhanced vesiculation of volatile saturated melts, particularly of the more volatile‐rich alkalic melts, causing them to rise to the top of the chamber and erupt.
Pillow lava
Subaerial
Breccia
Lava field
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Subaerial
Lava field
Pillow lava
Volcanic plateau
Lava dome
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Intensities of natural remanent magnetization compiled for 177 subaerial basaltic flows ranging from historical time to 10 My and for 204 submarine basalt samples from the North Atlantic basement show that subaerial basalts have a mean intensity, reduced to an equatorial equivalent value, of 36 × 10−4 emu cm−3, about 2·5 times weaker than for submarine basalts (89·6 × 10−4 emu cm−3). Normally magnetized flows have significantly greater NRM than reversed flows owing to a viscous component that represents about one-fifth of the stable remanence. No significant difference appears in the mean intensity between the Brunhes basalts and the older normal basalts or between the intensity, after partial demagnetization, of historic basalts and those ranging from 0·01 to 1, 1 to 2, 2 to 3, or 3 to 4 My. This seems to indicate (i) that no important decrease of the intensity of magnetization of subaerial basalts takes place for periods of several million years, and (ii) that a possible increase of the intensity of the geomagnetic field during the Brunhes epoch is not responsible for the larger amplitude of the axial magnetic anomaly over mid-ocean ridges. For the oceanic basement, the mean intensity decreases by about two-thirds from the basalts of the bottom of the median valley to the basalts from the crests and flanks of the ridge. This diminution, probably produced by intensive low-temperature oxidation of titanomagnetite, is comparable to the decrease of the magnetic anomalies observed in the North Atlantic when going away from the ridge axis.
Subaerial
Natural remanent magnetization
Intensity
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Pillow lava
Diamictite
Rift zone
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Natural remanent magnetization
Rock magnetism
Magnetism
Intensity
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The detailed study of 455 basement samples from DSDP Leg 37 reveals magnetic properties, particularly inclinations and intensities, different from those commonly considered representative of Layer 2. Non-dipole inclinations are the most common. The deepest hole (582 m) has a vector average intensity of 24.3 × 10 −4 emu cm −3 (24.3 × 10 −1 A/m) and an inclination of only −14.5°. Induced magnetization never dominates and is usually much less than remanent magnetization, with Q ratio averaging 35 for basalts and 2.6 for plutonic rocks. Viscous magnetization acquisition constant, S, ranges widely from 0.001 to 1 × 10 −4 emu cm −3 (0.001 to 1 × 10 −1 A/m), but is very rarely sufficient to cause VRM to dominate NRM.The major carrier of NRM is cation-deficient titanomagnetite produced by low-temperature oxidation of stoichiometric titantomagnetite. There is no trend of alteration with depth. All the magnetic properties are controlled by conditions within the individual basalt pillows or more massive units. A high degree of cation deficiency is associated with reduced NRM intensity, initial susceptibility, saturation magnetization, and VRM acquisition and increased MDF, Q ratio, and Curie point. Zones of low cation deficiency are presently found only in parts of massive units. With the exception of rare individual samples pillow sequences are highly oxidized throughout.A discussion is given of the kinds of ocean crust drilling and laboratory experiments required to solve the problems of the magnetic structure of Layer 2 as seen at the Leg 37 sites.
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Saturation (graph theory)
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A high‐resolution aeromagnetic survey of the San Francisco Bay area shows prominent positive anomalies over distinctive blue sandstones of Late Miocene age. The total‐field survey was measured at a nominal height of 300 m above the land surface along flight lines spaced 0.5 km apart. Anomalies with amplitudes up to 200 nT correlate with sandstones of the San Pablo Group, and these anomalies are similar in strength to the magnetic signatures of serpentinites and basalts in the surveyed region. Andesitic sandstone of the Neroly Formation, the upper part of the San Pablo Group, has high magnetic susceptibility (0.013 SI units, volume) and relatively strong natural remanent magnetization (0.29 A/m). Total magnetization of the sandstone is two thirds induced and one third remanent magnetization. The presence of coarse‐grained magnetite detritus, low coercivity of remanence, low thermal stability of remanence, and multidomain properties is consistent with the NRM being a viscous remanent magnetization that grew during the Brunhes normal‐polarity chron. The strong magnetic signature of the Upper Miocene sandstones allows their delineation over distances as great as 100 km, through areas where they are concealed by landslides and younger deposits. The sandstones are important structural markers for understanding the complex folding and faulting associated with active fault systems in the San Francisco Bay area.
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Natural remanent magnetization
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Unblocking temperatures of natural remanent magnetization were found to extend well above the dominant Curie points in samples of oceanic basalts from the axis of the East Pacific Rise. This phenomenon is attributed to the natural presence in the basalts of three related magnetic phases: an abundant fine-grained and preferentially oxidized titanomagnetite that carries most of the natural remanent magnetism, a few coarser and less oxidized grains of titanomagnetite that account for most of the high-field magnetic properties, and a small contribution to both the natural remanent magnetism and high-field magnetic properties from magnetite that may be due to the disproportionation of the oxidized titanomagnetite under sea-floor conditions. This model is consistent with evidence from the Central Anomaly magnetic high that the original magnetization acquired by oceanic basalts upon cooling is rapidly altered and accounts for the lack of sensitivity of bulk rock magnetic parameters to the degree of alteration of the remanence carrier in oceanic basalts.
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Rock magnetism
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Long (>100 km) lava flows are relatively common on Mars and Venus and have been identified on the Moon, but they are rarely documented on Earth. However, although ∼75% of the Earth's surface is covered by water, only a small percentage of the ocean floor has been investigated at a resolution sufficient to unequivocally identify the boundaries of long submarine lava flows. Even so, basaltic lava flows as long as 110 km have been identified on the deep (>1500 m) seafloor near Hawaii and the East Pacific Rise. Ambient conditions on the deep ocean floor may favor the development of long lava flows for the following reasons. First, high pressures (>15 MPa) keep volatiles dissolved in basaltic lavas, preventing viscosity increases associated with exsolution and vesiculation. Second, seawater rapidly quenches the surface of submarine basalt flows so that an insulating glass layer, 1–5 cm thick, encases submarine flows within seconds after their emplacement. This glass rind effectively insulates the molten flow interior from additional heat loss, making submarine basalt flows behave as well‐insulated, subaerial tube‐fed flows. Thus, for identical basalt flows emplaced on the deep seafloor and subaerially, a submarine flow could advance farther before stopping. Results of numerical modeling indicate that thin (≤1 m) submarine basalt flows behave similarly to identical subaerial flows, but thicker submarine flows may advance significantly farther than their subaerial counterparts.
Subaerial
Seafloor Spreading
Pillow lava
Seabed
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