Magnetization of axial lavas from the southern East Pacific Rise (14°-23°S): Geochemical controls on magnetic properties

Jeff Gee, Dennis Kent

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31 Citations (Scopus)

Abstract

Although the spatial association of iron-rich lavas and high-amplitude magnetic anomalies is well documented, a causal link between enhanced iron content and high remanent magnetization has been difficult to establish. Here we report magnetic data from approximately 250 samples, with 8-16% FeO* (total iron as FeO), from the southern East Pacific Rise (EPR) that provide strong support for the presumed geochemical dependence of remanent intensity. The limited age range (0-6 ka) of axial lavas from this ultrafast spreading ridge (∼150 mm/yr full rate) effectively minimizes variations resulting from time dependent changes in geomagnetic intensity or low-temperature alteration. Systematic sampling relative to the chilled margin illustrates that substantial grain size-related variations in magnetic properties occur on a centimeter scale. Both microprobe data and Curie temperatures suggest that the average groundmass titanomagnetite composition in the southern EPR samples is approximately constant (modal modified ulvöspinel content = 0.67) over a wide range of lava compositions. Saturation magnetization and saturation remanence are highly correlated with FeO* (R = 0.73 and 0.83, respectively), indicating that more iron-rich lavas have higher abundances of otherwise similar titanomagnetite. We show that there is a good correlation between natural remanent magnetization (NRM) and FeO*, provided that sufficient specimens are used to determine the average NRM of a sample (R = 0.63). Because the range of iron contents in mid-ocean ridge basalts is limited, the best fit slope (4.44 A/m per %FeO* in an ambient field of 0.030 mT) should provide reasonable bounds on the equatorial magnetization of submarine lavas (∼10 A/m at 8.5% FeO* and ∼50 A/m at ∼16% FeO*). Finally, we demonstrate that along-axis variations in NRM closely parallel geochemical changes along the southern EPR. Where magnetization values deviate significantly from those predicted from the range of measured FeO* contents, these discrepancies may reflect additional unrecognized geochemical variability.

Original languageEnglish (US)
Pages (from-to)24873-24886
Number of pages14
JournalJournal of Geophysical Research B: Solid Earth
Volume102
Issue number11
StatePublished - Nov 10 1997
Externally publishedYes

Fingerprint

magnetic property
magnetization
Magnetization
Magnetic properties
natural remanent magnetization
magnetic properties
Iron
iron
titanomagnetite
saturation
remanent magnetization
magnetic anomaly
mid-ocean ridge basalt
Remanence
lava
Saturation magnetization
Curie temperature
mid-ocean ridges
Chemical analysis
magnetic anomalies

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

Cite this

@article{bbff65914f02465b9520d9bf3a755e02,
title = "Magnetization of axial lavas from the southern East Pacific Rise (14°-23°S): Geochemical controls on magnetic properties",
abstract = "Although the spatial association of iron-rich lavas and high-amplitude magnetic anomalies is well documented, a causal link between enhanced iron content and high remanent magnetization has been difficult to establish. Here we report magnetic data from approximately 250 samples, with 8-16{\%} FeO* (total iron as FeO), from the southern East Pacific Rise (EPR) that provide strong support for the presumed geochemical dependence of remanent intensity. The limited age range (0-6 ka) of axial lavas from this ultrafast spreading ridge (∼150 mm/yr full rate) effectively minimizes variations resulting from time dependent changes in geomagnetic intensity or low-temperature alteration. Systematic sampling relative to the chilled margin illustrates that substantial grain size-related variations in magnetic properties occur on a centimeter scale. Both microprobe data and Curie temperatures suggest that the average groundmass titanomagnetite composition in the southern EPR samples is approximately constant (modal modified ulv{\"o}spinel content = 0.67) over a wide range of lava compositions. Saturation magnetization and saturation remanence are highly correlated with FeO* (R = 0.73 and 0.83, respectively), indicating that more iron-rich lavas have higher abundances of otherwise similar titanomagnetite. We show that there is a good correlation between natural remanent magnetization (NRM) and FeO*, provided that sufficient specimens are used to determine the average NRM of a sample (R = 0.63). Because the range of iron contents in mid-ocean ridge basalts is limited, the best fit slope (4.44 A/m per {\%}FeO* in an ambient field of 0.030 mT) should provide reasonable bounds on the equatorial magnetization of submarine lavas (∼10 A/m at 8.5{\%} FeO* and ∼50 A/m at ∼16{\%} FeO*). Finally, we demonstrate that along-axis variations in NRM closely parallel geochemical changes along the southern EPR. Where magnetization values deviate significantly from those predicted from the range of measured FeO* contents, these discrepancies may reflect additional unrecognized geochemical variability.",
author = "Jeff Gee and Dennis Kent",
year = "1997",
month = "11",
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language = "English (US)",
volume = "102",
pages = "24873--24886",
journal = "Journal of Geophysical Research: Solid Earth",
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TY - JOUR

T1 - Magnetization of axial lavas from the southern East Pacific Rise (14°-23°S)

T2 - Geochemical controls on magnetic properties

AU - Gee, Jeff

AU - Kent, Dennis

PY - 1997/11/10

Y1 - 1997/11/10

N2 - Although the spatial association of iron-rich lavas and high-amplitude magnetic anomalies is well documented, a causal link between enhanced iron content and high remanent magnetization has been difficult to establish. Here we report magnetic data from approximately 250 samples, with 8-16% FeO* (total iron as FeO), from the southern East Pacific Rise (EPR) that provide strong support for the presumed geochemical dependence of remanent intensity. The limited age range (0-6 ka) of axial lavas from this ultrafast spreading ridge (∼150 mm/yr full rate) effectively minimizes variations resulting from time dependent changes in geomagnetic intensity or low-temperature alteration. Systematic sampling relative to the chilled margin illustrates that substantial grain size-related variations in magnetic properties occur on a centimeter scale. Both microprobe data and Curie temperatures suggest that the average groundmass titanomagnetite composition in the southern EPR samples is approximately constant (modal modified ulvöspinel content = 0.67) over a wide range of lava compositions. Saturation magnetization and saturation remanence are highly correlated with FeO* (R = 0.73 and 0.83, respectively), indicating that more iron-rich lavas have higher abundances of otherwise similar titanomagnetite. We show that there is a good correlation between natural remanent magnetization (NRM) and FeO*, provided that sufficient specimens are used to determine the average NRM of a sample (R = 0.63). Because the range of iron contents in mid-ocean ridge basalts is limited, the best fit slope (4.44 A/m per %FeO* in an ambient field of 0.030 mT) should provide reasonable bounds on the equatorial magnetization of submarine lavas (∼10 A/m at 8.5% FeO* and ∼50 A/m at ∼16% FeO*). Finally, we demonstrate that along-axis variations in NRM closely parallel geochemical changes along the southern EPR. Where magnetization values deviate significantly from those predicted from the range of measured FeO* contents, these discrepancies may reflect additional unrecognized geochemical variability.

AB - Although the spatial association of iron-rich lavas and high-amplitude magnetic anomalies is well documented, a causal link between enhanced iron content and high remanent magnetization has been difficult to establish. Here we report magnetic data from approximately 250 samples, with 8-16% FeO* (total iron as FeO), from the southern East Pacific Rise (EPR) that provide strong support for the presumed geochemical dependence of remanent intensity. The limited age range (0-6 ka) of axial lavas from this ultrafast spreading ridge (∼150 mm/yr full rate) effectively minimizes variations resulting from time dependent changes in geomagnetic intensity or low-temperature alteration. Systematic sampling relative to the chilled margin illustrates that substantial grain size-related variations in magnetic properties occur on a centimeter scale. Both microprobe data and Curie temperatures suggest that the average groundmass titanomagnetite composition in the southern EPR samples is approximately constant (modal modified ulvöspinel content = 0.67) over a wide range of lava compositions. Saturation magnetization and saturation remanence are highly correlated with FeO* (R = 0.73 and 0.83, respectively), indicating that more iron-rich lavas have higher abundances of otherwise similar titanomagnetite. We show that there is a good correlation between natural remanent magnetization (NRM) and FeO*, provided that sufficient specimens are used to determine the average NRM of a sample (R = 0.63). Because the range of iron contents in mid-ocean ridge basalts is limited, the best fit slope (4.44 A/m per %FeO* in an ambient field of 0.030 mT) should provide reasonable bounds on the equatorial magnetization of submarine lavas (∼10 A/m at 8.5% FeO* and ∼50 A/m at ∼16% FeO*). Finally, we demonstrate that along-axis variations in NRM closely parallel geochemical changes along the southern EPR. Where magnetization values deviate significantly from those predicted from the range of measured FeO* contents, these discrepancies may reflect additional unrecognized geochemical variability.

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