Fe-Ti Oxide Mineralogy of DSDP Leg 55 Basalts

Kono, M.; Clague, D. A.; Larson, Edward J.

doi:10.2973/dsdp.proc.55.126.1980

Cited by 12 publications

(4 citation statements)

References 18 publications

(22 reference statements)

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“…Magnetite‐ilmenite intergrowths have been observed in a wide range of rocks [e.g., Schwarz and Symons , 1970; Kono et al , 1980; Hodych , 1996; Kosterov , 2001]. The presence of such intergrowths is usually associated with a highly stable ChRM [e.g., Larson et al , 1969; Tucker and O'Reilly , 1980].…”

Section: Discussionmentioning

confidence: 99%

Thermochemical remanent magnetization in Precambrian rocks: Are we sure the geomagnetic field was weak?

Смирнов

Tarduno

2005

J. Geophys. Res.

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[1] Thellier paleointensity determinations from two dikes of the Early Proterozoic ($2.46 Ga) Matachewan dike swarm (Canada) yield field values of 2.14 ± 0.18 and 9.81 ± 1.65 mT. Corresponding values of virtual axial dipole moment are very low (0.54 ± 0.05 Â 10 22 and 2.49 ± 0.42 Â 10 22 Am 2 , respectively) when compared with the modern field. The characteristic remanent magnetization (ChRM) was isolated over a narrow range of high unblocking temperatures ($520-580°C). Detailed rock magnetic analyses indicate that the ChRM is carried by nearly stoichiometric pseudo single domain magnetite. Scanning electron microscopy (SEM) reveals that the magnetite is in the form of fine intergrowths with ilmenite, formed by oxyexsolution during cooling. The high-temperature oxidation defined in the SEM images could have continued at temperatures below the Curie point of magnetite. In this case, the ChRM would be a thermochemical remanent magnetization (TCRM) rather than a thermal remanent magnetization (TRM). Estimates of the TCRM/TRM ratio show that the Thellier data could underestimate the true field value by a factor of 4 without violating experimental selection criteria. This uncertainty in TRM fidelity translates into a potential range of field values that spans that defined by the modern field ($8 Â 10 22 Am 2 ) and proposed low Precambrian levels ($2 Â 10 22 Am 2 ). Therefore understanding further how TCRM is acquired and records the field represents a major challenge if these and many other similar rocks are to be used in Thellier studies aimed at defining the strength of the Precambrian field.Citation: Smirnov, A. V., and J. A. Tarduno (2005), Thermochemical remanent magnetization in Precambrian rocks: Are we sure the geomagnetic field was weak?,

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Section: Discussionmentioning

confidence: 99%

Thermochemical remanent magnetization in Precambrian rocks: Are we sure the geomagnetic field was weak?

Смирнов

Tarduno

2005

J. Geophys. Res.

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“…Table 6 and Figure 14 show the composition of Fe-Ti oxides of rocks from each of the holes. The ulvospinel content of titanomagnetites from all tholeiites and those from the alkalic upper sill of Hole 524 averages 64.4 mol.%, a typical value for abyssal tholeiitic basalts Kono et al, 1980;Genkin et al, 1980;Johnson and Hall, 1978;Ridley et al, 1976). The titanomagnetites from the alkalic pillows in Hole 524 are distinctive in that they are more Fe 3+ rich and Ti poor (ulvospinel averages 49.8 mol.%).…”

Section: Fe-ti Oxidesmentioning

confidence: 98%

Petrography and Mineral Chemistry of Basalts from Holes 519A, 520, 522B, and 524Leg 73 (South Atlantic) of the Deep Sea Drilling Project

Carman¹,

Dietrich²,

Sommerauer³

1984

Initial Reports of the Deep Sea Drilling Project

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Basalt was encountered under sediments in four holes (519A, 520, 522B, and 524) on Leg 73. The rocks in three holes (519A, 520, and 522B) are typical somewhat evolved mid-ocean-ridge basalt (MORB) tholeiites that occur as pillows, flows, and sills. In Hole 524 there occur (1) an upper alkalic sequence of pillows and a sill enclosed in Cretaceous volcaniclastic sedimentary rocks and (2) a lower, moderately evolved, slightly alkalic tholeiite in a multiple sill, also injected into Cretaceous volcaniclastic rocks.The tholeiites of Holes 519A, 520, and 522B display chemical, mineralogical, and petrographic features that are characteristic of MORB found elsewhere. The chilled margins of flows and pillows display a sequence of quench textures, and the interiors of the thicker flows and sills have the intersertal, intergranular, subophitic, and ophitic textures normally found in such rocks. The rocks are finely microphyric, with pre-emptive Plagioclase and sparse olivine microphenocrysts. Only pillows were encountered in Hole 520, and the rocks of that hole are distinctive in that they show a variolitic structure not seen in any of the other holes of Leg 73.The tholeiites of Hole 524 are petrographically similar to those of Holes 519A to 522B, but they are more strongly microphyric, with all major silicate phases (olivine, Plagioclase, and clinopyroxene) present as abundant to common microphenocrysts. The alkalic rocks of Hole 524 are distinctive in that (1) Fe-Ti oxides are present as pre-eruptive microphenocrysts in addition to the silicate phases, (2) the Plagioclase grains are epitaxially mantled by alkalic feldspar, and (3) biotite and magnesioriebeckite occur as late-stage accessories.The mineral phases have compositions well within the ranges of typical MORB from many locations: Plagioclase microphenocrysts average An 76 _ 80 , olivine Fo 81 _ 87 , clinopyroxene microphenocrysts are close to En 46 Fs 12 Wo 4 4 and zoned toward modest ferrosilite enrichment, titanomagnetites have average 64.4 mol.% ulvospinel, and ilmenites average 8.2 mol.% hematite. Trace elements are likewise well within the ranges reported from other oceanic sites.Clinopyroxenes from the tholeiitic multiple sill display an unusual compositional zoning in which Mj cations (Fe, Mg, Mn, Ti, and Cr) show sympathetic variations from core to rim. There is a distinct drop in iron, manganese, and titanium and a concomitant increase in magnesium and chromium from inner core to outer core and then a strong reversal of these trends in the rims. The temporary reversal of the normal differentiation pattern from inner core to outer core in clinopyroxene occurs in both the early and late phases of the multiple sill and is not explained, although it may be related to variations in oxygen fugacity or the crystallization pattern of the Fe-Ti oxide minerals.

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“…Partial oxidation of titanomagnetite may also take place at high temperatures and be retained as the basalt is quenched (Hauptmann, 1974). Occurrences are also reported in seamounts (Kono et al, 1980) and subaerial volcanic rocks (Akimoto and Kushiro, 1960).…”

Section: [Fe 3+](ve~ 5 M~/~)o~mentioning

confidence: 96%

An occurrence of a fully-oxidized natural titanomaghemite in basalt

et al. 1989

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Titanomaghemite occurs in a relatively fresh doleritic intrusion in an area of Precambrian gneiss in Minas Gerais, Brazil. It hosts exsolution lamellae of ilmenite and contains more than 90% of the iron in the ferric form. It is more resistant to weathering than the ilmenite and is inherited virtually unaltered by the resulting soils. Titanomaghemite, extracted as grains from a weathered rind of the rock, has lattice parameter a0 = 0.8348(3)nm and has a canted spin structure due to substitution of non-magnetic ions on tetrahedral and octahedral sites of the spinel structure. The average canting angle is 32 + 3 ~ and canting occurs predominantly on the octahedral iron sublattice. Its formula, based on microprobe analysis and M6ssbauer spectroscopy may be expressed as:

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Fe-Ti Oxide Mineralogy of DSDP Leg 55 Basalts

Cited by 12 publications

References 18 publications

Thermochemical remanent magnetization in Precambrian rocks: Are we sure the geomagnetic field was weak?

Thermochemical remanent magnetization in Precambrian rocks: Are we sure the geomagnetic field was weak?

Petrography and Mineral Chemistry of Basalts from Holes 519A, 520, 522B, and 524Leg 73 (South Atlantic) of the Deep Sea Drilling Project

An occurrence of a fully-oxidized natural titanomaghemite in basalt

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Fe-Ti Oxide Mineralogy of DSDP Leg 55 Basalts

Cited by 12 publications

References 18 publications

Thermochemical remanent magnetization in Precambrian rocks: Are we sure the geomagnetic field was weak?

Thermochemical remanent magnetization in Precambrian rocks: Are we sure the geomagnetic field was weak?

Petrography and Mineral Chemistry of Basalts from Holes 519A, 520, 522B, and 524Leg 73 (South Atlantic) of the Deep Sea Drilling Project

An occurrence of a fully-oxidized natural titanomaghemite in basalt

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Petrography and Mineral Chemistry of Basalts from Holes 519A, 520, 522B, and 524Leg 73 (South Atlantic) of the Deep Sea Drilling Project