Alteration of titanomagnetites and its related magnetic properties in the Noya geothermal area, central Kyushu, Japan.

Fujimoto, Koichiro; Kikawa, Eiichi

doi:10.5636/jgg.41.39

Cited by 10 publications

(7 citation statements)

References 19 publications

(22 reference statements)

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“…Several scientists, however, reported that unaltered diabase had magnetization intensity values high enough to produce the anomaly amplitude. A detailed study of hydrothermal alteration of titanomagnetite indicated that this process altered original titanomagnetite and caused recrystalization of Ti-poor magnetite, resulting in a change of primary magnetic direction and loss of sufficient amount of initial thermoremanent magnetization (Fujimoto and Kikawa, 1989). Hydrothermal alteration is a key process in the metamorphism of oceanic rocks.…”

Section: Inclinationmentioning

confidence: 99%

Magnetic Properties of Gabbros from Hole 735B, Southwest Indian Ridge

Kikawa¹,

Pariso²

1991

Proceedings of the Ocean Drilling Program

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A total of 500.7 m of continuous, vertical, oceanic gabbroic section was recovered during Leg 118. The gabbros obtained exhibited various degrees of alteration and deformation, which gave us a good opportunity to study the magnetic properties of oceanic gabbros. Many of these gabbros, which are mainly Fe-Ti oxide gabbros, have strong and unstable secondary magnetic components that were acquired during drilling. Stable inclinations, which are probably in-situ magnetic directions, show a single polarity, with an average value of 66° (±5°), meaning that the studied 501-m oceanic gabbroic block may be a candidate for the source of the marine magnetic anomaly. This may also imply that the metamorphism of oceanic gabbros causing acquisition of magnetization probably occurred within one geomagnetic polarity chron (about 0.3 to 0.7 m.y.) after these gabbros formed at the ridge, leading us to conclude that oceanic gabbros record the so-called Vine-Matthews-Morley type of initial magnetization at the ridge. The average intensity value of stable magnetic components of individual samples, which may be a minimum estimate for remanent magnetizations, is 1.6 A/m. Assuming this magnetic intensity value and a uniform magnetization within an oceanic gabbroic layer having a thickness of 4.5 km (i.e., whole layer 3), it is possible to explain most of the marine magnetic anomaly. If magnetic properties of the samples obtained from Hole 735B are common to oceanic gabbros, layer 3 may contribute more significantly to seafloor spreading magnetic anomalies than previously thought.

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Section: Inclinationmentioning

confidence: 99%

Magnetic Properties of Gabbros from Hole 735B, Southwest Indian Ridge

Kikawa¹,

Pariso²

1991

Proceedings of the Ocean Drilling Program

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“…This has been shown by many experiments and field observations (e.g. Urrutia-Fucugauchi, 1981; Kelso and Banerjee, 1991;Hirt and Gehring, 1991;Walderhaug, 1993;Fujimoto and Kikawa, 1989). Two effects have been reported when rocks (minerals) are heated.…”

Section: Introductionmentioning

confidence: 72%

“…K a in rocks of higher intensity of alteration is lower (Figure 4). This effect is also presented by Fujimoto and Kikawa (1989). The reduction of the value of K a could be due to following factors: (1)The content of TFe 2 O 3 in non-altered protolith reaches 10% but that in altered rocks is < 3% (Wu et al, 1998).…”

Section: Mean Susceptibility (K a )mentioning

confidence: 92%

“…Borradaile and Lagroix (2000) experimented on tectonites from the highgrade granulite facies in the Kapuskasing Structural Zone of northern Ontario and showed that heating enhanced the AARM (anisotropy of anhysteretic remanence) fabric and refined the AMS orientation-distribution, preserving its directions. Fujimoto and Kikawa (1989) showed that magnetic susceptibility and intensity of magnetization of pyroxene andesite lavas in the Noya geothermal area, Japan, generally had lower values of AMS in more intensely altered rocks. This was due to the decomposition of magnetite in intensely altered rocks.…”

Section: Introductionmentioning

confidence: 98%

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Hydrothermal alteration of magnetic fabrics of rocks in the Xiaoban gold-bearing shear belt, Fujian Province, China

Xu¹,

Jianshe

et al. 2003

GeofInt

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La zona de deformación de Xiaoban experimentó alteración hidrotermal a temperaturas moderadas en el Mesozoico (180 Ma). Las muestras estudiadas corresponden a unidades alteradas e inalteradas provenientes de tres niveles en la mina. Después de un estudio sistemático de las fábricas de rocas magnéticas encontramos que: 1) La susceptibilidad se debe principalmente a la presencia de minerales paramagnéticos como la biotita y la pirita. La reducción de la susceptibilidad promedio se debe a la disminución de hierro y al cambio del tamaño del mineral magnético, así como al incremento de minerales diamagnéticos. 2) Decrecimiento de la anisotropía (P) después de la alteración hidrotermal. La tendencia al cambio es la misma que la de la susceptibilidad promedio. 3) El elipsoide de AMS no cambia sustancialmente su forma después de la alteración hidrotermal, y se mantiene una esfera oblada. El eje menor de susceptibilidad axial principal (K3) cae cerca del plano perpendicular al plano de equistosidad de todas las muestras. Sin embargo, el eje mayor de susceptibilidad axial principal está sujeto a grandes cambios. Los resultados indican que los nuevos minerales magnéticos están distribuidos en el plano existente de equistosidad, pero no tienen una orientación preferencial.

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“…A more likely cause for the absence of lineated magnetic anomalies over sedimented ridges is pervasive hydrothermal alteration under the thick sediment blanket [ Levi and Riddihough , 1986]. Hydrothermal alteration results in the replacement of titanomagnetites by nonmagnetic phases (dominant volumetrically) and a smaller amount of magnetite [ Ade‐Hall et al , 1971; Pariso and Johnson , 1991; Fujimoto and Kikawa , 1989]. As a result of this hydrothermal alteration, remanence and saturation magnetization (a direct measure of the proportion of magnetite) can be reduced by an order of magnitude or more [ Woolridge et al , 1990; Pariso and Johnson , 1991] with the remaining magnetic phase being near end‐member magnetite [ Shau et al , 1993].…”

Section: Hydrothermal Effects On Magnetization In Middle Valleymentioning

confidence: 99%

A deep tow magnetic survey of Middle Valley, Juan de Fuca Ridge

Gee

Webb

Ridgway

et al. 2001

Geochem Geophys Geosyst

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[1] Abstract: We report here results from a deep tow magnetic survey over Middle Valley, Juan de Fuca Ridge. A series of track lines are combined to generate a high-resolution map of the magnetic field anomaly within a 10 Â 12 km region surrounding the Bent Hill massive sulfide (BHMS) deposit. A uniformly magnetized body (5 A/m) with a cross section approximating the body inferred from Ocean Drilling Program (ODP) drilling can account for the observed near-bottom magnetic anomaly amplitude. Assuming this magnetization is entirely induced, the average susceptibility (0.11 SI) corresponds to $3.5% magnetite + pyrrhotite by volume, consistent with the abundance of these phases observed in drill core samples. However, this uniform magnetization model significantly underestimates the magnetic anomaly measured a few meters above the seafloor by submersible, indicating that the upper portion of the sulfide mound must have a significantly higher magnetization ($10% magnetite + pyrrhotite) than at deeper levels. On a larger scale, the near-bottom magnetic anomaly data show that basement magnetizations are not uniformly near zero, as had been inferred from analysis of the sea surface anomaly pattern. We interpret this heterogeneity as reflecting primarily differences in the degree of hydrothermal alteration. Our results highlight the potential of magnetic anomaly data for characterizing hydrothermal deposits where extensive drill core sampling is not available.

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Alteration of titanomagnetites and its related magnetic properties in the Noya geothermal area, central Kyushu, Japan.

Cited by 10 publications

References 19 publications

Magnetic Properties of Gabbros from Hole 735B, Southwest Indian Ridge

Magnetic Properties of Gabbros from Hole 735B, Southwest Indian Ridge

Hydrothermal alteration of magnetic fabrics of rocks in the Xiaoban gold-bearing shear belt, Fujian Province, China

A deep tow magnetic survey of Middle Valley, Juan de Fuca Ridge

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