Magnetic Mineralogy of a Complete Oceanic Crustal Section (IODP Hole 1256D)

Abstract

Oceanic crust is the carrier of the marine magnetic anomalies and is therefore a valuable archive of geomagnetic information. ODP/IODP Hole 1256D was the first to sample an entire sequence of oceanic crust down to the gabbro. We studied the vertical variation of magnetic remanence carriers by means of scanning electron microscopy, microanalysis and rock magnetic measurements. The extrusive layer contains dendritic, low-temperature oxidized titanomagnetites (TMs), i.e. titanomaghemite, with initial compositions close to values previously reported for mid-ocean ridge basalts (MORB). The degree of low-temperature oxidation (maghemitisation) remains fairly constant across the extrusives. We explain the observed increase in Curie temperature with depth by submicron inversion of titanomaghemite to intergrowths of titanomagnetite and nonmagnetic phases, where the Ti-content of titanomagnetite is decreasing with depth. In the underlying sheeted dikes, TMs are again the primary magnetic mineral. Due to slower cooling, they are in most cases oxy-exsolved into lamellar intergrowths of Ti-poor TMs and ilmenite. The magnetominerals are altered to a much higher degree than in the extrusives. In the gabbroic part of the section, TMs reach sizes up to several mm, although the magnetic grain size remains consistently in the pseudo-single-domain range because of grain subdivision by exsolution lamellae. The extrusives carry a thermoremanent magnetisation (TRM), retaining the primary paleomagnetic direction but with a reduced remanence intensity. The sheeted dikes hold a thermo-chemical remanent magnetization (TCRM) or secondary TRM acquired during hydrothermal alteration, whereas the underlying gabbro acquired a TCRM significantly after emplacement due to slow cooling at this depth.

A whole rock absolute paleointensity determination of dacites from the Duffer Formation (ca. 3.467 Ga) of the Pilbara Craton, Australia: An impossible task?

“…A pure 429 TRM is acquired only if oxyexsolution stops above the Curie point of a given magnetic 430 phase. In anycase, the remanence is a thermochemical remanent magnetization (TCRM), 431 produced by simultaneous slow cooling and volume growth of a magnetic phase below 432 its Curie temperature (Nagata and Kobayashi, 1963;Dunlop and Özdemir, 1997, 433 Draeger et al, 2006, Smirnov and Tarduno, 2005, Krasa et al, 2011. As 434 commented by Smirnov and Tarduno, (2005), the formation of TCRM may be a common 435 process in nature, (e.g.…”

Insights into magmatic processes and hydrothermal alteration of in situ superfast spreading ocean crust at ODP/IODP site 1256 from a cluster analysis of rock magnetic properties

“…[] along with scanning electron microscope (SEM) pictures demonstrating (incipient) exsolution in Krása et al . []. Downhole trends of the input parameters are plotted in Figure (adapted from Herrero‐Bervera et al .…”

“…A spinodal exsolution model to explain the increase of the Curie temperature with depth throughout the extrusive section of the core is provided in Krása et al . []. Occasionally high T C values of up to ∼580°C occur, possibly related to later intruded dikes.…”

“…Oxides occur as coarse lamellar intergrowths of magnetite/hemoilmenite solid solutions and rare large magnetite grains with significantly different compositions from the titanomagnetite that occurs in the “fresh” lavas and dikes [ Koepke et al ., ; Krása et al ., ]. The degree of oxidation parameter z for titanomagnetite (unoxidized titanomagnetite has z = 0, completely oxidized or maghemitized titanomagnetite has z =1) is consistently <0.2 for the lowermost dike section (the granoblastic dikes; 1348–1407 mbsf) and gabbros, whereas z ≈ 0.6 for all the igneous section above 1348 mbsf [ Krása et al ., ]. The oxides in this interval also have low ulvöspinel content (8–18 mol % versus 61–74 mol %) and low Al 2 O 3 contents (0.4–0.9 versus 0.9–2.3 wt%) [ Dziony et al ., ].…”

“…The extrusive rock pile comprises 186 samples, the sheeted dike complex 100, and the gabbro 30. Samples were primarily collected for remanent magnetization and paleointensity determinations and accompanying rock magnetic property analysis [ Herrero‐Bervera and Acton , ; Herrero‐Bervera et al ., ; Krása et al ., ].…”