Australia: The Land Where Time Began

A biography of the Australian continent 

Older and Hotter Mantle - Geodynamics of the Mantle

Temperatures of the underlying mantle at mid-ocean ridges can be recorded by volcanic rocks that have been erupted at these ridges. Anomalously high temperatures have been found for the mantle, that have been suggested to have been warmed by continental insulation prior to the opening of the ocean basin, in rocks from the bed of the Atlantic Ocean.

According to the author1 a factor that is central to understanding the structure and evolution of the Earth is the distribution and temperature history of the interior of the Earth. This data is pertinent to other areas of study including convection of the mantle, the uplift and subsidence history of the margins of continents, and via volcanism, the composition of the atmosphere and oceans, as well as possible impacts on the biosphere. Brandl et al.(Brandl et al., 2013), writing in Nature Geoscience used basalt from the ancient oceanic crust of the Atlantic and Pacific Oceans to provide constraints on the temperature of the mantle in the past, which the author1 says has been very difficult to find, though for the distribution of mantle temperature at the present there are various ways in which the mantle temperature distribution can be constrained.

Melting of the mantle of the Earth ultimately produces volcanic rocks that have been erupted at the mid-ocean ridges. A greater portion of the mantle is melted when mantle temperatures are higher, which changes the chemical composition of the melts that are erupted. Magma with lower sodium and aluminium and higher iron contents are generated by higher mantle temperatures in the ocean-ridge setting, therefore allowing the temperature of the underlying mantle at the time the rocks were formed to be inferred.

It has been found from variations in the composition of volcanic rocks obtained from ocean ridges of the present that during the generation of lavas mantle temperatures vary from about 1,300oC to about 1,500oC, with the highest of the mantle temperatures being largely found associated with hotspots in the mantle (Klein & Langmuir, 1987; Langmuir, Klein & Plank, 1992). Rocks that formed at a mid-ocean ridge that a now distributed across the ocean basins as a result of plate tectonic movements, have been suggested as being able to provide a record of mantle temperature of the past (Klein & Langmuir, 1987; Keen, Klein & Melson, 1990; Humler, Langmuir  & Daux, 1999). Data from this source have been limited (Humler, Langmuir  & Daux, 1999), as it requires the drilling through hundreds of metres of sediment to reach the volcanic basement rocks of the ocean basins to obtain the older samples.

This problem of obtaining older samples by the resampling of more than 500 basalt glasses ranging in age from 3-165 My, the samples being taken from 45 drill sites in the Atlantic and Pacific oceans as part of the Ocean Drilling Program. To assess the mantle temperatures from these samples at the time they were formed, the authors1 analysed the chemical composition of the samples and compared them with existing data obtained from ocean ridge samples of the present (Lehnert et al., 2000). The oldest samples from the Atlantic Ocean have been shown from their results to have a composition high in sodium and low in iron, in contrast with the younger basalts at the Atlantic mid-ocean ridge that mostly have higher sodium and lower iron. It is suggested by this data that there is a regular, temporal change in temperature beneath the Atlantic ridges. It is suggested by quantifying the effect that the temperature of the mantle was about 100-150oC hotter about 150 Ma, at the time the Atlantic Ocean basin began to open. It is considered that such a large change of temperature over such a relatively short period of time, when compared to the 4.5 Gy history of the Earth, is too large to result from the normal evolution of the interior temperature of the Earth, therefore suggesting it must have a different cause.

It has been suggested by Brandl et al.1 that the high mantle temperatures can be explained by continental insulation. About 180 Ma, prior to the beginning of the breakup of Pangaea, as Africa broke from the Americas and began to rift away, the massive supercontinent would have acted like a blanket insulating the mantle beneath it by preventing it from cooling at the surface which led to a rise in the temperature of the mantle (Anderson, 1982). Once a rift had formed extensive melting and the formation of a mid-ocean ridge where volcanic eruptions produced rocks with chemical compositions characteristic of high mantle temperatures. With the continued rifting apart of the Americas and Africa, rocks characteristic of cooler mantle were erupted at the mid-Atlantic ridge, the composition of the rocks of the ocean floor close to the mid-ocean ridge progressively changing towards the composition of rocks of the present as the excess heat of the underlying mantle was slowly dissipated.

In the Red Sea the rifting environment of the present, marking the continuing breakup of Africa and Arabia, was used by Brandl (Brandl et al., 2013) to test this scenario. According to the author1 the temperature of the mantle beneath the Red Sea also appear to be higher than normal. Moving from the Red Sea to the Indian Ocean, where the mid-ocean ridge is much older, the temperatures of the mantle appear to decline, which is consistent with the proposed continental insulation.

Statistical significance is a question with respect to the ancient samples, as in the Atlantic Ocean there are only 4 drill holes that produced samples of the ancient crust, compared to more than 1,000 drill holes producing samples along the mid-Atlantic Ridge of the present, which suggests the apparently high mantle temperatures could simply have occurred by chance. This argument is countered by the finding that all of the 4 samples dating to more than 100 Ma overlap with the extreme, hot end of the compositional data array of mid-ocean ridges of the present from around the world. Ridge segments above the Iceland hotspot are the only places where samples of rock are erupted that record such high mantle temperatures at the present.

Of the ridge segments sampled around the world, apart from Iceland region, less than 5 % overlap in compositional space with the ancient samples. It is seen as statistically significant that all of the 4 ancient sites are on the extreme portion of the present-day array. The higher temperatures cannot be attributed to an ancient hotspot as occurs in Iceland, as most of the ancient samples were not located near the tracks of ancient hotspots. There are remaining questions as to whether the ancient samples from Atlantic Ocean are representative of conditions on a hemisphere-wide scale. To confirm the result of high temperatures being associated with the rifting of the entire Atlantic basin requires a much larger sample coverage.

The fact that 3 of the 4 oldest sites from the Pacific Ocean have a similar chemical signature to the old Atlantic sites is an enigmatic feature of this new data. It is implied by these data that there are also high mantle temperatures beneath the Pacific basin, though the samples from the Pacific Ocean were formed several thousand kilometres from the nearest margin of a continent. There is no systematic mantle temperature change over time so the raised temperatures of the mantle cannot have arisen from continental insulation, unlike the case with the samples from the Atlantic. This suggests that there must be other factors that also contribute to ancient variations in temperature beneath the ocean basins.

Debate is continuing as to whether chemical composition variations of basalts from the mid-ocean ridges do actually reflect mantle temperature variations, with some insisting that the variations in composition of the mantle are actually the ultimate cause (Niu & O'Hara, 2008), at least in some regions (Zhou & Dick, 2013). The author1 suggests that the abundant evidence of high mantle temperatures beneath most hotspots is not consistent with this interpretation, though the relative roles of mantle temperature and mantle composition are still to be resolved fully.

It has been suggested by Brandl et al. that in the Atlantic Ocean ancient oceanic crust formed from mantle that was anomalously hot, having been warmed by the insulating effect of the continents. It is shown by the data that the present view of the temperature and composition of the mantle that is based on mid-ocean ridge magmas of the present is not the whole story, and much could be learned by sampling the composition of old oceanic crust more systematically. Most of the ancient oceanic crust appears to have been generated under conditions that are rare beneath ridges of the present, based on the available data.

 

Sources & Further reading

  1. Langmuir, Charles. "Mantle Geodynamics: Older and Hotter." Nature Geosci 6, no. 5 (05//print 2013): 332-33.

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated 20/05/2013
Home
Journey Back Through Time
Geology
Biology
     Fauna
     Flora
Climate
Hydrology
Environment
Experience Australia
Aboriginal Australia
National Parks
Photo Galleries
Site Map
                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading