Paleomagnetic constraints on the Archean geomagnetic field intensity obtained from komatiites of the Barberton and Belingwe greenstone belts, South Africa and Zimbabwe

The intensity of the geomagnetic field during Archean is an important source of information about the evolution of the Earth’s core. Hale [Earth Planet. Sci. Lett. 86 (1987b) 354] reported a very low equatorial paleointensity of 5 μT at ca. 3.5 Ga obtained from komatiites of the Barberton greenstone belt, South Africa, and regarded their remanences as thermal overprints due to a metamorphic event. However, this result has a major drawback since remanences of the Barberton komatiites are carried by magnetites formed during serpentinization that typically results in acquisition of chemical remanent magnetization (CRM). We report the results of detailed paleomagnetic, rock magnetic, and paleointensity investigations using komatiite samples from the Barberton (ca. 3.5 Ga) and Belingwe (ca. 2.7 Ga) greenstone belts in South Africa and Zimbabwe, respectively. Paleodirectional data for Barberton are consistent with results of previous work, and the characteristic remanent magnetization (ChRM) of the Belingwe samples reveals a positive fold test, suggesting that samples from both areas record the Archean geomagnetic field. The main carriers of ChRM are Ti-free magnetites presumed to be akin to single domain grains, and their rock magnetic properties are very stable when subjected to laboratory heating up to 600–700 °C. Our microscopic observations provide support that these are secondary minerals formed during serpentinization. Thellier–Thellier paleointensity experiments for the Barberton and Belingwe samples yielded very low mean virtual dipole moment (VDM) estimates of (1.8±1.3)×1022 A m2 and (1.1±0.9)×1022 A m2, respectively. These correspond to about 24 and 15% of the present day value. Considering the low metamorphic grade for the sampling areas, however, it seems to be somewhat difficult to regard the entire ranges of komatiite ChRM as thermal overprints acquired by reheating during metamorphic events. We hence propose as the most likely scenario the possibility that most fractions of ChRM are survivors of primary grain growth CRM. Taking the theoretical and experimental estimates of the ratio of CRM to thermoremanent magnetization (TRM) into account, this suggests that these low mean VDM values can provide constraints on the lower limits of the geomagnetic field intensities in both greenstone belts at ca. 3.5 and 2.7 Ga.