Including those previously published, good paleomagnetic data have been obtained from 48 of 51 sites in 28 separate kimberlites. Radiometric dates (by one or more methods) are available for 17 of these bodies and range from 81 to 145 Ma, with maxima around 90 and 120 Ma. These ages correlate with geochemical contrasts distinguishing what have been designated Group I (younger) and II (older) kimberlites [Smith et al., 1985b]. Stable magnetization appears to be associated with magnetite developed deuterically, after emplacement, but in most bodies, secular variation has not been completely averaged in the remanence acquired. The distribution of “VGPs” calculated is distinctly, but not sequentially, streaked yet correlates broadly with the older and younger petrochemical types. Mean North poles calculated on this basis yield KP1 (N = 14), 81–100 Ma, 64.1°N, 226.1°E, A 95 = 5.2 and KP2 (N = 10), 113–145 Ma, 47.6°N, 269.9°E, A 95 = 9.7. With the exception of one undated pipe (Meltonwold), which was not included in the averages, and the melilititic Male fissure at Swartruggens (in Group II average), all the kimberlites sampled are normally magnetized. Including these new data, 10 of 17 published African Cretaceous poles meet acceptability criteria specified, and define an apparent polar wander (APW) curve from ∼130 to 75 Ma. The curve appears to consist of two segments, trending latitudinally before ∼100 Ma, and meridionally thereafter. The latter trend is evident even in the distribution of the individual KP1 poles, and may be related to the true polar wander speculated by Gordon and Livermore (1987) to have occurred in that interval.
A field and laboratory search for shock-metamorphic features associated with the Bushveld Igneous Complex, South Africa, was made to test the hypothesis that the structure formed by meteorite impact. Samples collected from inliers of basement rock within the complex (the Assen and Marble Hall fragments) and from the Rooiberg series exhibit no megascopic or microscopic shock effects. In particular, shatter cones, Sudbury-type breccias ( = Vredefort pseudotachylite), and planar shock lamellae in quartz are absent. Any shock-metamorphic features originally present have probably been obscured by the emplacement of the widespread Bushveld igneous rocks and by the resulting extensive thermal metamorphism of both the basement rocks and the Rooiberg series. More extensive sampling is needed to test the impact hypothesis adequately.
A good correlation between opaque mineralogy and rock magnetic properties has been established in a study of a small number of oriented samples from four kimberlite dykes and one pipe in the Koidu complex of Sierra Leone. The pipe sample is mineralogically distinctive, in that it alone contains deuteric titanomaghaemite after titanomagnetite, and haematite after chlorite. All of the kimberlites contain xenocrystic, upper-mantle derived ilmenite that has undergone sub-solidus reduction to form Mg—Ti magnetite. Three of the dykes contain serpentinized olivine with microscopic to sub-microscopic magnetite and identifiable metal Ni—Fe alloys, and finely textured Mg titanomagnetite and magnetite as atolls surrounding complexly zoned Cr—Mg—Al spinels. Consistent with these observations, Curie and blocking temperatures range from 520 to 590°C and high alternating field (AF) coercivities imply single domain or pseudo-single domain particles as the bulk carrier of natural remanent magnetization (NRM). In the pipe sample, however, the remanence persisting above 600°C is comparatively strong. Preliminary remanent vectors, isolated by either thermal or AF demagnetization, from four of the five samples are moderately consistent, and yield a mean (n = 4) of D = 323°, I = –17°, k = 18°, and σ95 = 22°.
Research Article| September 01, 1986 Faster spreading or greater ridge length in the Archean? R. B. Hargraves R. B. Hargraves 1Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544 Search for other works by this author on: GSW Google Scholar Author and Article Information R. B. Hargraves 1Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1986) 14 (9): 750–752. https://doi.org/10.1130/0091-7613(1986)14<750:FSOGRL>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation R. B. Hargraves; Faster spreading or greater ridge length in the Archean?. Geology 1986;; 14 (9): 750–752. doi: https://doi.org/10.1130/0091-7613(1986)14<750:FSOGRL>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Dissipation of more heat from a hotter early Earth by the plate-tectonics mechanism would require subduction of, on average, younger lithosphere. In principle, this could be achieved equally well by faster spreading, by having greater ridge length, or some combination thereof. Present-day evidence, however, suggests that younger oceanic lithosphere subducts more slowly than older; if this applied in the Archean, then the "greater ridge length" alternative prevailed. A quantitative expression is derived which indicates that oceanic heat loss is proportional to the cube root of ridge length. This relationship means that if Archean heat flow was 3 times that of the present, 27 times as much ridge would have been required, which would suggest that the Archean Earth was covered by many small plates moving slowly. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
A paleomagnetic study of the Belchertown pluton, a syntectonic Devonian intrusion in the Bronson Hill anticlinorium, Massachusetts, indicates the presence in 23 samples from eight sites in primary quartz monzodiorites of a strong, stable, and consistently oriented remanent magnetization vector (D = 154.9°, I = −13.4°, K = 20.8° and α95 = 12.4°) with a high Q value (7.0). The remanence is inferred to be of thermal and/or thermochemical origin acquired during the original crystallization and cooling of the pluton. Recrystallized equivalents of the primary rocks, which occur as a surrounding border zone, yield weak and inconsistent results (four samples from two sites) due to the absence of opaque minerals. The mean Belchertown vector is discordant with respect to the mean North American Devonian vector (D = 170° and I = +32°) in such a direction as to suggest that the entire pluton has undergone some 30°–60° of rotation to the northwest about a NE-SW axis. The high Q value of the primary rocks and their isolation from magnetic country rocks by the surrounding non-magnetic border zone allow computer modeling of the three-dimensional shape of the pluton using the measured remanent vector and the total field aeromagnetic anomaly. The resulting model is consistent with the proposed amount and direction of tectonic rotation of a once roughly vertically symmetrical funnel-shaped intrusion.
Thermal treatment of nontronite in air, for long periods at 700°C or short periods at 900°C, results in destruction of the nontronite structure, a distinct reddening in color, and a spectacular increase in magnetic susceptibility and saturation magnetization (up to 4.4 Am 2 /Kg). Magnetic property measurements and calculations suggest that the magnetism is due to the presence of ultrafine particles of α or γFe 2 O 3 ; the precise identity has not yet been resolved. The highly magnetic thermally treated nontronite is amorphous to X rays consistent with an ultrafine grain size. Prolonged heating results in the growth of αFe 2 O 3 , as suggested by X ray, IR, and magnetic measurements. Reflectivity spectra of a sample heated for 1 hour at 900°C indicates, in addition to αFe 2 O 3 , the presence of an opaque, magnetite‐like phase. Given the composition of the starting material, and the experimental conditions, the presence of Fe 3 O 4 , is unlikely; the cause of this opacity is as yet unknown. Thermally treated nontronite has chemical, color and magnetic properties akin to those found by Viking on Mars. These results favor an origin for the fine grained Martian surface material by repeated impacts into an Fe‐rich smectite‐charged regolith (Weldon et al., 1980).
