James M. Hall

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

The Fe-Ti magnetic carriers of the dike-rich second kilometer of oceanic crust at Ocean Drilling Program Site 504 are shown to be secondary in origin, that is, their formation is likely to considerably postdate initial igneous crustal formation. An important consequence of the delay in formation of carriers is that the polarity of their magnetization has a significant probability of opposing that of the overlying, little-altered extrusives. If this is the case, the linear magnetic anomalies in the area have sources crust, which, at least locally, is magnetized with successive layers of opposite polarities. The magnitude of the contribution of deeper layers to anomalies will depend both on their thickness and average magnetization, which are not as yet well known. Two types of secondary magnetite, SMI and SMII, separated spatially and by the steps leading to their formation, are recognized in the dikes of the second kilometer of the crust at Site 504. Both were formed in conditions of hydrothermal greenschist metamorphism. Since all newly formed oceanic crust is thought to experience this type of alteration at an early stage, polarity reversal with depth as a result of the delayed formation of magnetic carriers is likely to be widespread.

Viscous magnetization at 300 K in a profile through Troodos type oceanic crust

Abstract This study examines the question of the relative importance of viscous magnetism, (VM), at 300 K, to the total magnetization of older oceanic crust. A section through the Troodos, Cyprus, ophiolite, now recognized as a good proxy for in-situ oceanic crust, has been used as the source of samples. The section extends downwards continuously from little altered submarine extrusives through greenschist facies sheeted dikes to mafic and variously serpentinized ultramafic cumulate intrusives. The principal result of the investigation is that VM is not expected to be the dominant magnetization, and is often relatively negligible, at all crustal levels. VM acquisition varies irregularly with depth, with predicted maximum values equivalent to about one-third of the total magnetization, in the extrusives and sheeted dikes. The contribution of VM in two samples of serpentinized ultramafics is insignificant. VM acquisition does not show any simple relationships with primary lithology, alteration history, or magnetic properties or history. The occurrence of two-stage VM growth mechanisms is widespread. The second stage typically has an onset time of about 103 min and an acquisition rate three times that of the initial growth mechanism. The physical origin of this two-stage mechanism, and its significance in predicting VM growth over geologic time intervals, are considered to be important problems for future work.

Constructional features of Troodos type oceanic crust: Relationships between dike density, alteration, magnetization, and ore body distribution and their implications for in situ oceanic crust

Quantitative relationships are demonstrated between dike density and hydrothermal alteration, magnetization, location of volcanic massive sulfide (VMS) ore bodies and a number of other properties in a 40 by 15 km segment of the Extrusive Series of the Troodos Ophiolite. Low-temperature alteration extends from the sediment-extrusive interface to close to the 25% dike density surface, while the onset of greenstone type alteration is closely associated with the 50% dike density surface. These associations are explained in terms of the regional scale variations in the dike density contour surfaces following the form of the top of the gabbroic layer of the ophiolite. An upper magnetic zone, characterized by stable high remanence, terminates just below the 25% dike density surface. A deeper magnetic zone, characterized by high induced magnetization, straddles the top of the Sheeted Complex. Volcanic massive sulfide bodies are concentrated in a narrow depth range centered about 0.1 km above the 25% dike density surface. A model for the postridge crest evolution of ore bodies accounts for this optimum depth range for preservation. Physical property changes with dike density, and the depth distribution of VMS ore bodies and prospects for the ophiolite as a whole support this model. Similar relationships appear to be present in the Samail, Macquarie Island, Newfoundland and Chilean ophiolites and Icelandic crust. As a result of relationships found in the Troodos ophiolite, an alternative is proposed to the current lithologic profile for Ocean Drilling Program Hole 504B.

Interpretation of aeromagnetic anomalies in terms of hydrothermal alteration in Cretaceous normal polarity superchron extrusives of the Troodos, Cyprus, ophiolite

One hundred and twenty–two meters altitude aeromagnetics and ground data over the Cretaceous normal polarity extrusives of the Troodos ophiolite have allowed source modeling. These data have been digitized and recontoured in gridded and reduced to the pole (RTP) formats. The principal result is confirmation of large anomalies with amplitudes of several thousand nanoteslas and dimensions of 1–6 km. Since the extrusives are everywhere normally magnetized, anomalies must have an explanation other than polarity reversals. Strong magnetization of overlying little altered extrusives and dikes, and weak magnetization of underlying hydrothermalty altered material, allows forward modeling in terms of source layer thickness. Detailed modeling was carried out for ∼50 km2 subarea containing two highs separated, with 3000 nT contrast, by a low, for which ground information had been specifically acquired. The low coincides with a thick series of sheet flows containing former hydrothermal flow-ways (gossans). Modeling of a former reservoir of hydrothermal fluid beneath the low, with reduction of the magnetic source layer from 1000 to 250 m, is consistent with observations. For the extrusives as a whole, gossans are weakly associated with relatively low RTP field. Although individual gossans are too small to perturb the kilometer-scale anomalies, they do show several relationships with anomalies: Gossans are absent from areas of very high (>6500 nT) and very low (<1500 nT) RTP field and, within the intervening range, they are bimodally distributed. Reconsideration of the nature of gossans accounts for some of these features.

Crustal Research Drilling Project

The drilling component of the Cyprus Crustal Study Project will begin with site preparation in February 1982. Rig assembly will be completed by the end of February, and drilling will start during the first week of March 1982. The first drill hole (CY-1) will penetrate a 2-km vertical section of pillow lavas extending from the base of the overlying Cretaceous sediments to the upper part of the sheeted dike complex. This hole will be located at the mouth of the Akaki River Canyon, about 1.5 km northwest of the village of Malounda. Drilling of this hole will be completed about June 1, 1982.