Composite titanomagnetite-ferrian ilmenite grains and correlative magnetic components in a dacite with self-reversed TRM

Kennedy, Lawrence P.; Osborne, Margery D.

doi:10.1016/0012-821x(87)90012-4

Cited by 7 publications

(7 citation statements)

References 15 publications

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“…13. Surprisingly, only one single Generally speaking, the magnetic properties of the Pinatubo dacite are similar to that reported for other volcanic rocks phase was found, with a Curie point close to 530°C from the heating curve, which changes to about 480°C on the cooling exhibiting self-reversal (Uyeda 1958;Kennedy & Osborne 1987;Lawson et al 1987;Haag et al 1990aHaag et al ,b, 1993.…”

Section: Mineralogical Investigationssupporting

confidence: 78%

A detailed magnetic and mineralogical study of self-reversed dacitic pumices from the 1991 Pinatubo eruption (Philippines)

Bina¹,

Tanguy²,

Hoffmann³

et al. 1999

Geophys. J. Int.

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SUMMARY39 dacitic pumice and lithic samples from the 1991 eruption of Mount Pinatubo were investigated through both magnetic and mineralogical means. As in a previous study, natural remanent magnetization (NRM) is found to be reversed for most of the samples, with respect to the direction of the actual geomagnetic field direction. A few samples, amongst them ancient lithics transported by pyroclastic flows, show scattered NRM directions. From thermal demagnetization of these particular samples it is concluded that their orientation changed after emplacement. The emplacement temperature is estimated to be more than 460°C from thermal demagnetization of lithic samples.Two magnetic minerals with large grain sizes are observed under the optical microscope: titanomagnetite (TM) and haemo-ilmenite ( hem-ilm). Microprobe analyses yield x#0.10 for TM and y#0.52 and y#0.54 for two hem-ilm phases, in agreement with the observed Curie temperatures (~480°C for TM and~250°C for hem-ilm). The hem-ilm particles display chemical zonation, which seems to be correlated with a change of the domain structure: typically, a ferrimagnetic (FM) phase with slightly higher titanium content is observed in the central part whilst the crystal margin, which is weakly ferromagnetic (WF, due to spin-canted antiferromagnetism) is slightly poorer in titanium. Two different mechanisms for the origin and formation of the two observed phases are discussed: (1) chemical zonation of hem-ilm crystals due to a change in conditions in the magma chamber shortly before eruption; (2) similar to the microstructures observed from synthetic samples, this zonation in the large natural hem-ilm could be the result of migration of the WF phase towards the grain boundary during residence below the order-disorder transition temperature in the magmatic chamber. The room temperature hysteresis loop, which seems to be dominated by TM, provides multidomain (MD)-like parameters: J rs /J s =0.01 and H cr /H c =20. The large coercivity of remanence (15-40 mT), which is attributed to hem-ilm, may be intrinsic or due to interactions between WF and FM phases. The field dependence of the magnitude of the thermoremanent magnetization (TRM) is not linear: it increases first, reaches a maximum (negative) value for an applied field H close to 0.5 mT, then decreases steadily. By extrapolation, it is estimated that the TRM should be zero for a field of about 12 mT and become positive beyond. This total TRM is in fact the sum of several components. AF demagnetization of TRMs acquired in different fields shows the presence of at least three components: a self-reversed (SR) component that contains both hard and moderately hard components and a soft normal component. Independently of the value of H, the median destructive field of the SR component is of the order of 159

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Section: Mineralogical Investigationssupporting

confidence: 78%

A detailed magnetic and mineralogical study of self-reversed dacitic pumices from the 1991 Pinatubo eruption (Philippines)

Bina¹,

Tanguy²,

Hoffmann³

et al. 1999

Geophys. J. Int.

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“…Self‐reversal by exchange coupling acting between crystallographically different, coexisting titanohematite phases was repeatedly observed in volcanic rocks of dacitic to andesitic composition (e.g. Nagata et al 1953; Heller et al 1986; Kennedy & Osborne 1987) and has been discussed and modelled in various studies (e.g. Uyeda 1958; Hoffman 1975; Prévot et al 2001).…”

Section: Discussionmentioning

confidence: 89%

Low-temperature partial magnetic self-reversal in marine sediments by magnetostatic interaction of titanomagnetite and titanohematite intergrowths

Garming¹,

Dobeneck²,

Franke³

et al. 2007

Geophysical Journal International

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S U M M A R YWith various low-temperature experiments performed on magnetic mineral extracts of marine sedimentary deposits from the Argentine continental slope near the Rio de la Plata estuary, a so far unreported style of partial magnetic self-reversal has been detected. In these sediments the sulphate-methane transition (SMT) zone is situated at depths between 4 and 8 m, where reductive diagenesis severely alters the magnetic mineral assemblage. Throughout the sediment column magnetite and ilmenite are present together with titanomagnetite and titanohematite of varying compositions. In the SMT zone (titano−)magnetite only occurs as inclusions in a siliceous matrix and as intergrowths with lamellar ilmenite and titanium-rich titanohematite, originating from high temperature deuteric oxidation within the volcanic host rocks. These abundant structures were visualized by scanning electron microscopy and analysed by energy dispersive spectroscopy. Warming of field-cooled and zero-field-cooled low-temperature saturation remanence displays magnetic phase transitions of titanium-rich titanohematite below 50 K and the Verwey transition of magnetite. A prominent irreversible decline characterizes zero-field cooling of room temperature saturation remanence. It typically sets out at ∼210 K and is most clearly developed in the lower part of the SMT zone, where low-temperature hysteresis measurements identified ∼210 K as the blocking temperature range of a titanohematite phase with a Curie temperature of around 240 K. The mechanism responsible for the marked loss of remanence is, therefore, sought in partial magnetic self-reversal by magnetostatic interaction of (titano-)magnetite and titanohematite. When titanohematite becomes ferrimagnetic upon cooling, its spontaneous magnetic moments order antiparallel to the (titano-)magnetite remanence causing an drastic initial decrease of global magnetization. The loss of remanence during subsequent further cooling appears to result from two combined effects (1) magnetic interaction between the two phases by which the (titano-)magnetite domain structure is substantially modified and (2) low-temperature demagnetization of (titano-)magnetite due to decreasing magnetocrystalline anisotropy. The depletion of titanomagnetite and superior preservation of titanohematite is characteristic for strongly reducing sedimentary environments. Typical residuals of magnetic mineral assemblages derived from basaltic volcanics will be intergrowths of titanohematite lamellae with titanomagnetite relics. Low-temperature remanence cycling is, therefore, proposed as a diagnostic method to magnetically characterize such alteration (palaeo-)environments.

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“…It is within these compositions that a self‐reversed thermoremanent magnetization can be acquired [ Uyeda , ; Heller et al ., ; Hoffman , ], although a more recent compilation of data suggests that self‐reversal can occur between 0.45 ≤ y ≤ 0.75 [ Fabian et al ., ]. In a majority of models for self‐reversal in the hematite‐ilmenite solid‐solution series, reverse thermoremanent magnetization (RTRM) occurs due to an ordered ferrimagnetic phase being magnetized antiparallel to a magnetizing field as result of interactions with an Fe rich, more disordered phase, i.e., a weakly ferromagnetic phase or intermediate x phase, however the exact cause of the self‐reversing process remains enigmatic (for more details see: Nagata and Uyeda [], Nagata [], Uyeda [], Ishikawa [], Ishikawa and Syono [], Hoffman [], Kennedy and Osborne [], Nord and Lawson [], Haag et al . [], Hoffmann and Fehr [], Bina et al .…”

Section: Magnetic Characteristics Of Intermediate Titanohematitementioning

confidence: 99%

“…It is within these compositions that a self-reversed thermoremanent magnetization can be acquired [Uyeda, 1958;Heller et al, 1986;Hoffman, 1992], although a more recent compilation of data suggests that self-reversal can occur between 0.45 y 0.75 [Fabian et al, 2011]. In a majority of models for self-reversal in the hematite-ilmenite solidsolution series, reverse thermoremanent magnetization (RTRM) occurs due to an ordered ferrimagnetic phase being magnetized antiparallel to a magnetizing field as result of interactions with an Fe rich, more disordered phase, i.e., a weakly ferromagnetic phase or intermediate x phase, however the exact cause of the self-reversing process remains enigmatic (for more details see: Nagata and Uyeda [1959], Nagata [1961], Uyeda [1957Uyeda [ , 1958, Ishikawa [1962], Ishikawa andSyono [1962, 1963], Hoffman [1975], Kennedy and Osborne [1987], Nord and Lawson [1989], Haag et al [1990a,b], Hoffmann and Fehr [1996], Bina et al [1999] reversal is not suppressed even in fields of 2 T, suggesting that the self-reversal is caused by exchange coupling as only this mechanism can resist such high fields [Dunlop and € Ozdemir, 1997]. The primary control on self-reversal in titanohematite appears to be the coexistence of two phases in the same crystal and their exchange interactions, rather than simple interactions between repeated, highly geometric microstructures.…”

Section: Magnetic Characteristics Of Intermediate Titanohematitementioning

confidence: 99%

Importance of titanohematite in detrital remanent magnetizations of strata spanning the Cretaceous‐Paleogene boundary, Hell Creek region, Montana

Sprain

Feinberg

Renne

et al. 2016

Geochem Geophys Geosyst

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Intermediate composition titanohematite, Fe 2-y Ti y O 3 with 0.5 y 0.7, is seldom the focus of paleomagnetic study and is commonly believed to be rare in nature. While largely overlooked in magnetostratigraphic studies, intermediate titanohematite has been identified as the dominant ferrimagnetic mineral in an array of Late Mesozoic and early Cenozoic Laramide clastic deposits throughout the central United States. Intermediate titanohematite is ferrimagnetic and has similar magnetic properties to titanomagnetite, except its unique self-reversing property. Due to these similarities, and with detrital remanent magnetizations masking its self-reversing nature, intermediate titanohematite is often misidentified in sedimentary deposits. Past studies relied upon nonmagnetic techniques including X-ray diffraction and electron microprobe analysis. While these techniques can identify the presence of intermediate titanohematite, they fail to test whether the mineral is the primary recorder. To facilitate the identification of intermediate titanohematite in sedimentary deposits, we characterize this mineral using low-temperature magnetometry and high-temperature susceptibility experiments, and present a new identification technique based on titanohematite's self-reversing property, for sediments that span the Cretaceous-Paleogene boundary (Hell Creek region, Montana). Results from the self-reversal test indicate that the majority of remanence is held by minerals that become magnetized parallel to an applied field, but that intermediate, selfreversing titanohematite (y 5 0.53-0.63) is an important ancillary carrier of remanence. While earlier literature suggests that intermediate titanohematite is rare in nature, reanalysis using specialized rock magnetic techniques may reveal that it is more abundant in the rock record, particularly within depositional basins adjacent to calc-alkaline volcanics, than previously thought.

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Composite titanomagnetite-ferrian ilmenite grains and correlative magnetic components in a dacite with self-reversed TRM

Cited by 7 publications

References 15 publications

A detailed magnetic and mineralogical study of self-reversed dacitic pumices from the 1991 Pinatubo eruption (Philippines)

A detailed magnetic and mineralogical study of self-reversed dacitic pumices from the 1991 Pinatubo eruption (Philippines)

Low-temperature partial magnetic self-reversal in marine sediments by magnetostatic interaction of titanomagnetite and titanohematite intergrowths

Importance of titanohematite in detrital remanent magnetizations of strata spanning the Cretaceous‐Paleogene boundary, Hell Creek region, Montana

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