Basalt

About: Basalt is a research topic. Over the lifetime, 18687 publications have been published within this topic receiving 805136 citations.

Flood Basalts and Hot-Spot Tracks: Plume Heads and Tails

Abstract: Continental flood basalt eruptions have resulted in sudden and massive accumulations of basaltic lavas in excess of any contemporary volcanic processes. The largest flood basalt events mark the earliest volcanic activity of many major hot spots, which are thought to result from deep mantle plumes. The relative volumes of melt and eruption rates of flood basalts and hot spots as well as their temporal and spatial relations can be explained by a model of mantle plume initiation: Flood basalts represent plume "heads" and hot spots represent continuing magmatism associated with the remaining plume conduit or "tail." Continental rifting is not required, although it commonly follows flood basalt volcanism, and flood basalt provinces may occur as a natural consequence of the initiation of hot-spot activity in ocean basins as well as on continents.

Lead isotopic study of young volcanic rocks from mid-ocean ridges, ocean islands and island arcs

Abstract: Lead isotopic compositions of young volcanic rocks from different tectonic environments have distinctive characteristics Their differences are evaluated within the framework of global tectonics and mantle differentiation Ocean island leads are in general more radiogenic than mid-ocean ridge basalt (morb) leads They form linear trends on lead isotopic ratio plots Many of the trends extend toward the field of morb On plots of 207 P b / 204 Pb against 206 Pb / 204 Pb, their slopes are generally close to 01 Island arc leads in general are confined between sediment and morb type leads with slopes of ca 030 on a plot of 207 P b / 204 Pb against 206 Pb / 204 Pb Pb, Sr and Nd isotopic data of Hawaiian volcanics are closely examined Data from each island support a two-component mixing model However, there is a lack of full range correlation between islands, indicating heterogeneity in the end members This mixing model could also be extended to explain data from the Iceland-Reykjanes ridge, and from 45° N on the Atlantic Ridge The observed chemical and isotopic heterogeneity in young volcanic rocks is considered to be a result of long-term as well as short-term mantle differentiation and mixing Lead isotopic data from ocean islands are interpreted in terms of mantle evolution models that involve long-term (more than 2 Ga) mantle chemical and isotopic heterogeneity Incompatible element enriched ‘plume’-type morb have Th/U ratios ca 30 too low and Rb/Sr ratios ca 004 too high to generate the observed 208 Pb and 87 Sr respectively for long periods of time Elemental fractionation in the mantle must have occurred very recently This conclusion also applies to mantle sources for ocean island alkali basalts and nephelinites Depletion of incompatible elements in morb sources is most probably due to continuous extraction of silicate melt and/or fluid phase from the low-velocity zone throughout geological time Data on Pb isotopes, Sr isotopes and trace elements on volcanic rocks from island arcs are evaluated in terms of mixing models involving three components derived from (1) sub-arc mantle wedge, (2) dehydration or partial melting of subducted ocean crust, and (3) continental crust contamination In contrast to the relation between 87 Sr/ 86 Sr and 143 Nd / 144 Nd ratios of ocean volcanics, there is a general lack of correlation between Pb and Sr isotopic ratios except that samples with very radiogenic Pb ( 206 Pb / 204 Pb > 195) have low 87 Sr/ 87 Sr ratios (07028- 07035) These samples also have inferred source Th/U ratios (30-35) not high enough to support long-term growth of 208 Pb Data suggest that their mantle sources have long-term integrated depletion in Rb, Th, U and light ree High 238 U / 204 Pb (y a)values required by the Pb isotopic data are most probably due to depletion of Pb by separation of a sulphide phase Relations between Pb, Sr and Nd isotopic ratios of young volcanic rocks could be explained by simultaneous upward migration of silicate and/or fluid phase and downward migration of a sulphide phase in a differentiating mantleration of a sulphide phase in a differentiating mantle

Generation of sodium-rich magmas from newly underplated basaltic crust

Abstract: SODIUM-RICH rocks of trondhjemite–tonalite–dacite (TTD) or –granodiorite (TTG) suites form much of Precambrian continental crust1. They are thought to have formed by partial melting of subducted oceanic crust2,3—a process that would have been much more widespread early in Earth history than at present, owing to the higher thermal gradients prevailing at that time4. Phanerozoic TTD suites do exist, however, and seem also to relate to subduction zones5. Defant and Drummond6 proposed that these suites form where young (<25 Myr), hot oceanic lithosphere is subducted and melts, thus locally simulating the conditions that led to widespread crustal growth in the Archaean. Here we describe plutonic and volcanic rocks from the Cordillera Blanca complex in Peru, which have characteristics of the high-Al TTD suite but which were produced above a subduction zone containing a 60-Myr-old slab. We present evidence that the complex formed by partial melting of newly underplated basaltic crust, and argue that this mechanism should be considered more generally as an additional way of generating sodium-rich arc magmas.