Data Report: Magnetic Properties and Magnetic Oxide Mineralogy of Upper Crustal Rocks from Holes 504B and 896A

Abstract

We present the results of studies of magnetic mineralogy and magnetic properties of Holes 504B and 896A upper crustal rocks recovered during Leg 148. The ratio of saturation remanence to saturation magnetization (J RS /J S ) ranges from 0.012 to 0.154 (mean = 0.06) in samples from Hole 504B, consistent with pseudo-single-domain behavior. Hole 896A samples display a variable ratio of J RS /J S that ranges from 0.012 to 0.154 (mean = 0.076), reflecting the differences in grain morphologies and magnetic properties between pillow and massive units.Intensity of natural remanent magnetization (NRM) ranges from 0.01 to 0.24 A/m (mean = 0.09 A/m) in Hole 504B samples, less than that measured in samples recovered during other legs because the drilling-induced remanence imparted by the bottom-hole assembly was substantially less during Leg 148. Volume susceptibility averages 0.032 ± 0.029 SI, similar to that from upper parts of the dike sequence. In all samples examined from Hole 504B, high-temperature oxidation of primary titanomagnetite has produced lamellae or pods of magnetite and ilmenite. Hydrothermal alteration has further altered the minerals in some samples to a mixture of magnetite, ilmenite, titanite, and a high-titanium mineral (either rutile or anatase). Electron microprobe analyses show that magnetite lamellae are enriched in the trivalent oxides Cr 2 O 3 (mean -0.076 wt%), A1 2 O 3 (mean = 0.566 wt%), and V 2 O 3 (mean = 0.757 wt%), whereas divalent oxides are concentrated in ilmenite lamellae (mean MnO = 1.997 wt% and mean MgO = 0.073 wt%).In samples from Hole 896A, NRM intensity ranges from 0.14 to 22.59 A/m (mean = 5.06 ± 4.91 A/m). Volume susceptibility averages 0.0272 ± 0.0254 SI. Low-temperature oxidation of primary titanomagnetite has produced titanomaghemite exhibiting shrinkage cracks. Ulvöspinel content averages 0.508 and ranges from 0.217 to 0.647. Titanomaghemite appears slightly enriched in A1 2 O 3 (mean = 1.697 wt%), Cr 2 O 3 (mean = 0.064 wt%), V 2 O 3 (mean = 0.673 wt%), MnO (1.257 wt%), and MgO (0.577 wt%).

Subsolidus evolution and alteration of titanomagnetite in ocean ridge basalts from Deep Sea Drilling Project/Ocean Drilling Program Hole 504B9 Leg 83: Implications for the timing of magnetization

“…They found that the degree of deuteric oxidation increases with increasing depth and suggested that the higher degree of oxidation is due to a slower cooling rate of the deeper sheeted dike basalts. Therefore hightemperature oxidation is apparently a common subsolidus process affecting the titanomagnetite in the lower sheeted dikes at Hole 504B (down to 2100 m bsf for Leg 148 samples [Stokking et al, 1996] ............................ alteration are therefore key processes in subsolidus evolution and modification of titanomagnetite and account for the properties of the magnetic carrier in the sheeted dike basalts. The primary titanomagnetite in the upper sheeted dike basalts may appear to be homogeneous when observed with reflectedlight microsopy and even SEM or EMPA, even though it has been subjected to pervasive subsolidus oxidation and exsolution.…”

Delayed magnetization of the deeper kilometer of oceanic crust at Ocean Drilling Project Site 504

“…This involves the redeposition of near-pure magnetite, which is the second type of secondary magnetite, or SMII. Redeposition strongly favors nucleation on ilmenite lamellae that have survived in leached primary grains, leading in some instances to products that closely mimic and were formerly mistaken [Pariso et al, 1995;Pariso and Johnson, 1991;Stokking et al, 1996] for the magnetites containing lattices of ilmenite lamellae often produced during the initial cooling of basaltic flows and dikes [Haggerty, 1976]. The stages in the production of SMII are illustrated in Figure 1.…”

Three‐dimensional inversion of marine magnetic anomalies on the equatorial Atlantic Ridge (St. Paul Fracture Zone): Delayed magnetization in a magmatically starved spreading center?

“…The efficient remanent magnetization of deeper layers could have been delayed during their thermochemical history. Ocean Drilling Program (ODP) results from holes 504B [ Kinoshita et al , 1989; Pariso and Johnson , 1989; Pariso et al , 1995; Smith and Banerjee , 1986; Stokking et al , 1996; Hall and Muzzatti , 1999], 735B [ Pariso and Johnson , 1993], and 896A [ Stokking et al , 1996] have been used to lend support to the concept of the contribution of deeper crustal source layers to surface anomalies.…”