Rock-magnetic and remanence properties of synthetic Fe-rich basalts: Implications for Mars crustal anomalies

Stefanie Brachfeld, Julia Hammer

Research output: Contribution to journalArticle

16 Citations (Scopus)

Abstract

We characterized the magnetic mineral assemblage and remanence properties of a set of synthetic samples patterned on the meteorite-derived basalt composition A*, which contains 18.9% total FeO. Basalts were synthesized at conditions that track 4 oxygen fugacity (fO2) buffer curves, from 3.4 log units below the quartz-fayalite-magnetite (QFM) buffer to 5 log units above QFM, and 6 cooling rates from 105 to 3 °C/h. The resulting array of samples was characterized using magnetic hysteresis loops, temperature dependence of saturation magnetization and saturation remanence (10 to 1000 K), and the acquisition and demagnetization of anhysteretic remanent magnetization (ARM) and thermoremanent magnetization (TRM). The magnetic mineral assemblage characteristics are strongly dependent on fO2. Samples synthesized at the iron-wüstite (IW) buffer have a very low concentration of remanence-carrying grains, which are likely near the superparamagnetic-stable-single-domain boundary. Samples synthesized at the QFM and nickel-nickel oxide (NNO) buffers contain a slightly higher concentration of remanence-carrying grains, which are stable-single-domain to fine pseudo-single-domain particles, respectively. Samples synthesized at the manganese oxide (MNO) buffer contain the highest concentration of magnetic grains, which are up to 100 μm in diameter. The dominant Fe-Ti oxide produced is an Mg- and Al-bearing titanomagnetite with 2.4-2.7 Fe cations per formula unit. The Curie temperatures of the QFM samples are consistent with their electron-microprobe derived compositions. Those of the NNO sample set are very slightly elevated with respect to their electron microprobe derived compositions. The Curie temperatures of the MNO samples are elevated up to 200 °C above what they should be for their composition. We attribute the Curie temperature elevation to high-temperature nonstoichiometry of the titanomagnetite. The IW sample set acquired very weak TRMs with intensities of 0.02 to 0.5 A/m. This intensity of remanence is a factor of 50-500 too low to generate the observed 1000 nT anomalies detected on Mars by the Mars Global Surveyor MAG-ER experiment. The QFM, NNO, and MNO samples acquired TRMs up to 40 A/m in a 10-μT applied field, and up to 200 A/m in a 50-μT field, with little or no dependence on cooling rate. Our results suggest that Fe-rich melts that crystallize extensive titanomagnetite can generate an intensely magnetized layer in the Martian crust, even if the remanence was acquired in a weak field. The QFM sample set can easily account for the observed 1000-nT Mars magnetic anomalies, even in a magnetized layer as thin as 15-30 km.

Original languageEnglish
Pages (from-to)599-617
Number of pages19
JournalEarth and Planetary Science Letters
Volume248
Issue number3-4
DOIs
StatePublished - 30 Aug 2006

Fingerprint

Ferrosoferric Oxide
fayalite
Quartz
Remanence
remanence
basalt
mars
Mars
magnetite
nickel
Rocks
rocks
anomalies
quartz
Buffers
magnetic properties
anomaly
titanomagnetite
manganese oxide
Curie temperature

Keywords

  • Mars
  • Mars Global Surveyor
  • TRM
  • magnetic anomalies
  • planetary magnetism
  • rock magnetism
  • titanomagnetite

Cite this

@article{5f07844a369247d98e932541c5db33ec,
title = "Rock-magnetic and remanence properties of synthetic Fe-rich basalts: Implications for Mars crustal anomalies",
abstract = "We characterized the magnetic mineral assemblage and remanence properties of a set of synthetic samples patterned on the meteorite-derived basalt composition A*, which contains 18.9{\%} total FeO. Basalts were synthesized at conditions that track 4 oxygen fugacity (fO2) buffer curves, from 3.4 log units below the quartz-fayalite-magnetite (QFM) buffer to 5 log units above QFM, and 6 cooling rates from 105 to 3 °C/h. The resulting array of samples was characterized using magnetic hysteresis loops, temperature dependence of saturation magnetization and saturation remanence (10 to 1000 K), and the acquisition and demagnetization of anhysteretic remanent magnetization (ARM) and thermoremanent magnetization (TRM). The magnetic mineral assemblage characteristics are strongly dependent on fO2. Samples synthesized at the iron-w{\"u}stite (IW) buffer have a very low concentration of remanence-carrying grains, which are likely near the superparamagnetic-stable-single-domain boundary. Samples synthesized at the QFM and nickel-nickel oxide (NNO) buffers contain a slightly higher concentration of remanence-carrying grains, which are stable-single-domain to fine pseudo-single-domain particles, respectively. Samples synthesized at the manganese oxide (MNO) buffer contain the highest concentration of magnetic grains, which are up to 100 μm in diameter. The dominant Fe-Ti oxide produced is an Mg- and Al-bearing titanomagnetite with 2.4-2.7 Fe cations per formula unit. The Curie temperatures of the QFM samples are consistent with their electron-microprobe derived compositions. Those of the NNO sample set are very slightly elevated with respect to their electron microprobe derived compositions. The Curie temperatures of the MNO samples are elevated up to 200 °C above what they should be for their composition. We attribute the Curie temperature elevation to high-temperature nonstoichiometry of the titanomagnetite. The IW sample set acquired very weak TRMs with intensities of 0.02 to 0.5 A/m. This intensity of remanence is a factor of 50-500 too low to generate the observed 1000 nT anomalies detected on Mars by the Mars Global Surveyor MAG-ER experiment. The QFM, NNO, and MNO samples acquired TRMs up to 40 A/m in a 10-μT applied field, and up to 200 A/m in a 50-μT field, with little or no dependence on cooling rate. Our results suggest that Fe-rich melts that crystallize extensive titanomagnetite can generate an intensely magnetized layer in the Martian crust, even if the remanence was acquired in a weak field. The QFM sample set can easily account for the observed 1000-nT Mars magnetic anomalies, even in a magnetized layer as thin as 15-30 km.",
keywords = "Mars, Mars Global Surveyor, TRM, magnetic anomalies, planetary magnetism, rock magnetism, titanomagnetite",
author = "Stefanie Brachfeld and Julia Hammer",
year = "2006",
month = "8",
day = "30",
doi = "10.1016/j.epsl.2006.04.015",
language = "English",
volume = "248",
pages = "599--617",
journal = "Earth and Planetary Science Letters",
issn = "0012-821X",
publisher = "Elsevier BV",
number = "3-4",

}

Rock-magnetic and remanence properties of synthetic Fe-rich basalts : Implications for Mars crustal anomalies. / Brachfeld, Stefanie; Hammer, Julia.

In: Earth and Planetary Science Letters, Vol. 248, No. 3-4, 30.08.2006, p. 599-617.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Rock-magnetic and remanence properties of synthetic Fe-rich basalts

T2 - Implications for Mars crustal anomalies

AU - Brachfeld, Stefanie

AU - Hammer, Julia

PY - 2006/8/30

Y1 - 2006/8/30

N2 - We characterized the magnetic mineral assemblage and remanence properties of a set of synthetic samples patterned on the meteorite-derived basalt composition A*, which contains 18.9% total FeO. Basalts were synthesized at conditions that track 4 oxygen fugacity (fO2) buffer curves, from 3.4 log units below the quartz-fayalite-magnetite (QFM) buffer to 5 log units above QFM, and 6 cooling rates from 105 to 3 °C/h. The resulting array of samples was characterized using magnetic hysteresis loops, temperature dependence of saturation magnetization and saturation remanence (10 to 1000 K), and the acquisition and demagnetization of anhysteretic remanent magnetization (ARM) and thermoremanent magnetization (TRM). The magnetic mineral assemblage characteristics are strongly dependent on fO2. Samples synthesized at the iron-wüstite (IW) buffer have a very low concentration of remanence-carrying grains, which are likely near the superparamagnetic-stable-single-domain boundary. Samples synthesized at the QFM and nickel-nickel oxide (NNO) buffers contain a slightly higher concentration of remanence-carrying grains, which are stable-single-domain to fine pseudo-single-domain particles, respectively. Samples synthesized at the manganese oxide (MNO) buffer contain the highest concentration of magnetic grains, which are up to 100 μm in diameter. The dominant Fe-Ti oxide produced is an Mg- and Al-bearing titanomagnetite with 2.4-2.7 Fe cations per formula unit. The Curie temperatures of the QFM samples are consistent with their electron-microprobe derived compositions. Those of the NNO sample set are very slightly elevated with respect to their electron microprobe derived compositions. The Curie temperatures of the MNO samples are elevated up to 200 °C above what they should be for their composition. We attribute the Curie temperature elevation to high-temperature nonstoichiometry of the titanomagnetite. The IW sample set acquired very weak TRMs with intensities of 0.02 to 0.5 A/m. This intensity of remanence is a factor of 50-500 too low to generate the observed 1000 nT anomalies detected on Mars by the Mars Global Surveyor MAG-ER experiment. The QFM, NNO, and MNO samples acquired TRMs up to 40 A/m in a 10-μT applied field, and up to 200 A/m in a 50-μT field, with little or no dependence on cooling rate. Our results suggest that Fe-rich melts that crystallize extensive titanomagnetite can generate an intensely magnetized layer in the Martian crust, even if the remanence was acquired in a weak field. The QFM sample set can easily account for the observed 1000-nT Mars magnetic anomalies, even in a magnetized layer as thin as 15-30 km.

AB - We characterized the magnetic mineral assemblage and remanence properties of a set of synthetic samples patterned on the meteorite-derived basalt composition A*, which contains 18.9% total FeO. Basalts were synthesized at conditions that track 4 oxygen fugacity (fO2) buffer curves, from 3.4 log units below the quartz-fayalite-magnetite (QFM) buffer to 5 log units above QFM, and 6 cooling rates from 105 to 3 °C/h. The resulting array of samples was characterized using magnetic hysteresis loops, temperature dependence of saturation magnetization and saturation remanence (10 to 1000 K), and the acquisition and demagnetization of anhysteretic remanent magnetization (ARM) and thermoremanent magnetization (TRM). The magnetic mineral assemblage characteristics are strongly dependent on fO2. Samples synthesized at the iron-wüstite (IW) buffer have a very low concentration of remanence-carrying grains, which are likely near the superparamagnetic-stable-single-domain boundary. Samples synthesized at the QFM and nickel-nickel oxide (NNO) buffers contain a slightly higher concentration of remanence-carrying grains, which are stable-single-domain to fine pseudo-single-domain particles, respectively. Samples synthesized at the manganese oxide (MNO) buffer contain the highest concentration of magnetic grains, which are up to 100 μm in diameter. The dominant Fe-Ti oxide produced is an Mg- and Al-bearing titanomagnetite with 2.4-2.7 Fe cations per formula unit. The Curie temperatures of the QFM samples are consistent with their electron-microprobe derived compositions. Those of the NNO sample set are very slightly elevated with respect to their electron microprobe derived compositions. The Curie temperatures of the MNO samples are elevated up to 200 °C above what they should be for their composition. We attribute the Curie temperature elevation to high-temperature nonstoichiometry of the titanomagnetite. The IW sample set acquired very weak TRMs with intensities of 0.02 to 0.5 A/m. This intensity of remanence is a factor of 50-500 too low to generate the observed 1000 nT anomalies detected on Mars by the Mars Global Surveyor MAG-ER experiment. The QFM, NNO, and MNO samples acquired TRMs up to 40 A/m in a 10-μT applied field, and up to 200 A/m in a 50-μT field, with little or no dependence on cooling rate. Our results suggest that Fe-rich melts that crystallize extensive titanomagnetite can generate an intensely magnetized layer in the Martian crust, even if the remanence was acquired in a weak field. The QFM sample set can easily account for the observed 1000-nT Mars magnetic anomalies, even in a magnetized layer as thin as 15-30 km.

KW - Mars

KW - Mars Global Surveyor

KW - TRM

KW - magnetic anomalies

KW - planetary magnetism

KW - rock magnetism

KW - titanomagnetite

UR - http://www.scopus.com/inward/record.url?scp=33747080290&partnerID=8YFLogxK

U2 - 10.1016/j.epsl.2006.04.015

DO - 10.1016/j.epsl.2006.04.015

M3 - Article

AN - SCOPUS:33747080290

VL - 248

SP - 599

EP - 617

JO - Earth and Planetary Science Letters

JF - Earth and Planetary Science Letters

SN - 0012-821X

IS - 3-4

ER -