L. V. Dmitriev1, S. Yu. Sokolov2, and N. S. Sokolov3
1Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian
Academy of Sciences (GEOKhI)
2Geological Institute, Russian Academy of Sciences
3Lomonosov State University, Moscow
Among burning, albeit unresolved, problems of ocean-floor geology one faces the issue of mantle magmatism evolving during the formation of oceanic lithosphere from the initial continental breakup to lithospheric accretion in present-day mid-ocean ridges (MORs). Direct and reliable evidence on magmatism is only available from axial and crestal MOR portions, i.e., it portrays events no farther back than ca. 1 Ma. Deciphering these events from petrologic and geochemical data points to global and local heterogeneities in mantle sources of basalts, variations in the scale of mantle upwelling and magmatic productivity, etc. These data are indispensable to the statistical quantification of geodynamic settings in which oceanic lithosphere is generated. Sampling coverage of magmatic rocks up to 5-10 m.y. old along outer MOR flanks is rather sparse, and the only evidence on compositions of oceanic basement rocks as old as 150-180 m.y. is that recovered by deep-sea drilling. Judging from these scanty data, abyssal basalts are similar in their compositional range to MORBs. This affords the sole, albeit crucial, conclusion that, on a global scale, throughout the formation time of the oceanic lithosphere, geodynamic conditions of mantle magmatism have generally remained within the range established for the modern MOR structures. This evidence, however, is clearly insufficient to form a judgment on how magmatism and geodynamics evolved through time and space.
This issue can be approached by correlating data on magmatic petrology, tectonics, topography, and geophysical fields of one or another feature or region, etc. For MORs, such correlations have been documented on a qualitative level in many recent publications.
Lately, high resolution data from geophysical fields in the oceans have become available, and new raw data on MOR magmatism have been amassed. This affords a quantification of the correlation between petrologic and geophysical parameters for the zero-age phase of MOR evolution. A quantification like this was carried out over the best understood northern part of the Mid-Atlantic Ridge [Dmitriev et al., 1999]. The latter study established a persistent correlation between the principal parameters of mantle magmatism, tectono-magmatic segmentation of the ridge, anomalous gravity field, the geoid surface, seismicity, and tomography data. This correlation was viewed as resulting from the interplay of processes of various levels, such as mantle upwelling, tectonics, and magmatism, involved in the formation of MORs, accessible to observation now and not yet obscured by imminent geological events.
Another finding from that study is that mantle magmatism occurs here in two geodynamic settings, clearly distinct in space, sharply contrasting, and linked with independently generated spreading- and plume-related basalt associations. These associations are identified reliably based on major-element variations in basalts. The above provided an incentive for us to venture a reconstruction of how petrologic parameters of ocean floor magmatism have been evolving through time, based on gravity variations along magnetic lineations of various ages, and assuming the correlation between these values to remain essentially unchanged over time. This study sets out to attempt such a reconstruction.
(1) The selected segment is the most ancient fragment of the Atlantic lithosphere (opening time of ca. 170 Ma).
(2) The entire period of formation of this segment was controlled by steady slow spreading, whose rate remained virtually constant (ca. 3 mm/yr.).
(3) The area is least affected by faulting. The number of transform faults per unit area is considerably smaller here compared to other segments of the Atlantic. Offsets along these transforms are smaller as well.
(4) The central part of the area (the EW-trending zone between 20oN and 35oN) is virtually free of the impact of subsequent magmatic processes. Intraplate magmatic manifestations are pronounced clearly only near the boundaries of the area.
(5) Most part of the MAR, from the Cape Verde FZ at 15o N to 30o N, is composed of spreading-related basalts. The Azores superplume is centered at ca. 45o N, outside the study area. Transition from the spreading association to plume association occurs between 30o N and 40o N, i.e., within the selected segment.
As mentioned above, in currently forming MORs, mantle magmatism is taking place in two fundamentally dissimilar, plume- and spreading-related, geodynamic settings. Detailed petrologic and geochemical characteristics of the MOR magmatism and the underlying causes of the correlation between geodynamic settings and specific basalt associations being generated are addressed in [Dmitriev, 1998; Dmitriev et al., 1999]. Here, our concern is only with the information needed to solve the problem posed.
The plume basalt association is generated during intense upwelling of mantle material from depths between 400 to 700 km or more, possibly including the lower mantle, and its partial melting at elevated (over 1400o C) temperatures with the resultant basaltic magmatism. This association includes three ORT (ocean rift tholeiite basalt) groups: The ORT 1, the most widespread group, whose parental melts segregate from mantle sources at relatively high PT parameters. Geochemically, most of these basalts are classed with the N-MORB type of [Wilson, 1989], showing a slight enrichment. The ORT-Fe group is comprised of Fe-enriched basalts derived through in-chamber differentiation of ORT 1. The rare ORT-K group includes basalts generated at the greatest depths and highest temperatures. These basalts are considerably enriched geochemically and correspond to the T-MORB type of [Wilson, 1989]. The development of the plume association involves voluminous volcanism and the formation of a basaltic layer of increased thickness and of positive topographic forms.
The spreading basalt association is generated through slow upwelling of mantle material from relatively shallow, less than 400 km, depths. Its parental melts originate at temperatures below 1400o C. The spreading association is typified by small volumes of volcanism and a thin to absent basaltic layer. The association includes the most widespread group of depleted basalts, ORT 2 (N-MORB), and the ORT-Na group, the less widely spread variety of the most depleted ORT 2 with elevated Na abundances.
The plume and spreading basalt associations are identified reliably enough based on their chilled glass compositions using the 8-component discriminant D1, whose value depends on PT parameters of the formation of parental melts and of their evolution. Increased spreading rates result in a greater magmatic productivity, an expanded range of PT parameters of cotectic crystallization and ORT compositions, and a greater proportion of spreading to plume basalt associations.
At low spreading rates, plume development occurs as an independent process superimposed on spreading. Plume- and spreading-related ORT associations are then distinctly apart in space. In the North Atlantic, this is demonstrated spectacularly by how sharply the Iceland and Azores superplumes, as well as microplumes south of the Cape Verde FZ, at 22o N and 25o N, are delimited. With increasing spreading rates, boundaries between plume and spreading associations are erased.
Solving the problem posed included two stages.
Figure 9 reveals the following characteristics of the free-air anomalous pattern in the study area:
(1) Along the axial rift zone, the general transition from negative to positive free-air anomalies, in terms of means rather than individual spikes, begins north of 30oN. This is also the area of transition from spreading to plume basalt associations, the south terminus of the Azores megaplume (see above).
(2) Transition from negative to positive values in a S-N direction occurs along each pair of isochron profiles, with elevated anomalous values forming a symmetric V-shaped pattern: along M5, on both sides of the ridge, transition to positive values occurs near 23oN, and along M13, at ca. 30oN. A similar pattern is displayed by the western M21 and M30 lineations, their positive values migrating gradually northward as their age increases. With the eastern M21 and M30 lineations, this pattern is upset, because their negative values give way to positive ones further south here, at 25o N and 28o N, respectively. The V-pattern just mentioned of negative vales giving way to positive ones is perturbed in the MAR axial zone. This is due to the fact that the magnitude of free-air gravity anomalies is affected by the overall topographic level, chiefly determined by the degree of lithospheric cooling away from the MAR axis. Introducing due corrections to the magnitude of free-air anomalies from profile to profile would eliminate the differences in mean background values on the profiles in the segment under study without affecting the position of the V-pattern formed by dissimilar anomalous values along the profiles. For this reason, no corrections were calculated.
(3) The smoothest gravity field with the most symmetric distribution along isochron profile pairs is maintained between 20o N and 30o N, or, roughly, between the Kane and Atlantis fracture zones.
Assuming that the correlation of zero-age petrologic and geophysical parameters has persisted throughout the lithosphere formation time in the study region (see above), the increase in free-air anomaly values can be inferred to mark the transition from spreading-to plume-related basalt associations. Hence, the northward widening V-shaped pattern of positive anomalies might be due to the plume having gradually migrated over the past 67 m.y. from north to south in the course of formation of the region's lithosphere.
Upset gravity field symmetry in the northeast part of the region, along the eastern profiles 21 and 30, might be due to a local volcanic overprint giving rise to the Atlantis-Meteor seamount system. Complications in the free-air anomaly pattern in the southwest part of the region might also be due to superimposed geologic processes, related in this case to the formation of features such as the roughly EW-trending Royal Trough on the MAR's west flank.
By and large, data presented here are not at variance with the known concepts of plume formation scenarios and plume-spreading interaction in the mid-ocean ridge system, of migrating plumes, and of results of this process, as recorded in the petrologic parameters of magmatism and in the geophysical and geomorphologic lithospheric signatures; e.g., [Grachev, 1987; Ito and Lin, 1995; Ribe et al., 1995; Schilling, 1991; Sleep, 1996; Vogt, 1976; White et al., 1995; Yale and Phipps Morgan, 1998]. A study of the latest 10 m.y. migration of the Azores megaplume, drawing on a detailed correlation of the MAR morphology, tectonics, gravity field, tomography, and magnetic versus bathymetric data between 36oN and 40oN on the one hand and petrologic modeling data on the other, is presented in [Cannat et al., 1999]. According to this study, the Azores megaplume has been migrating at 60 mm/yr for several million years. If it is taken for a proved fact that the V-shaped pattern of free-air anomalies under discussion results from a plume magmatism zone migrating southward throughout the Cenozoic, then the angle of inclination of this V-pattern in the space vs. time, or along-MAR distance vs. anomaly age, reference frame yields the average migration rate. Assuming that the plume front, established from the V-shaped pattern of free-air anomalies, has migrated approximately from the Oceanographer FZ to Kane FZ, i.e., ca. 11o, or 1221 km, since M30 (67 Ma) to the present, we obtain a migration rate of ca. 18 mm/yr, an average value for the Cenozoic. Considering that global tectonic processes on the earth are non-linear, i.e., that they occur in a pulse mode, it can be ascertained with confidence that rates averaged over periods long enough will invariably be lower than peak rates from certain particular epochs. Therefore, the time behavior of the Azores plume, inferred from the totality of extremely diverse data by many researchers, is defined in an internally consistent manner as a non-steady state southward migration involving the formation of structures superimposed on a standard oceanic crust.
Our study shows that, in the course of formation of the Atlantic lithosphere, within the chosen study area the geodynamic setting giving rise to mantle plumes has existed for at least 60 or 70 m.y. The fact that plume-related basalt associations have migrated over this time interval a distance of at least 1200 km here, with spreading rate remaining comparatively steady, implies that plume formation does not depend on spreading and is overprinted on it. This corollary further supports the inference of independence of the two processes, advanced by [Dmitriev et al., 1999].
The new approach here proposed to reconstruct petrologic parameters of magmatism from isochron gravity anomaly profiling data holds promise for solving the issue of magmatic evolution during the formation of oceanic lithosphere through the Earth's history. While exercising this approach, the choice of one or another study area should take due account of a number of constraints imposed by (i) inconsistencies in sampling coverage of magmatic rocks, (ii) non-uniform reliability of magnetic data, and (iii) tectonic histories varying in complexity from area to area. To further refine this approach, one can make use of Bouguer anomalies and heat flow data.
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