SYDNEY: Geophysicists’ current models of how the mantle melts, enters magma chambers and eventually forms the ocean floor can’t explain the high levels of trace elements found in the ocean floor, and are “naïve”, said Australian researchers.
The ocean floor is continually being formed along Earth’s 84,000km-long mid-ocean ridges, as magma upwells and hardens as it meets the ocean.
Until now, it was thought that under the ridge was a closed-system of magma chambers. In this new study, published in the British journal Nature, the authors propose a new model in which the magma chambers are being periodically refilled with fresh magma from below the chambers.
“We think [the old model] is a very naïve view; we think the process is a continuous one where fresh magma from the mantle is mixed with the old magma left behind in the magma chambers,” said lead author Hugh O’Neill, from the Australian National University in Canberra.
Implications for Earth’s mantle, ore deposits
“This has implications for interpreting not only the history and chemical composition of the mantle, but also how elements might build up to high concentration levels and hence the origin of some ore deposits,” he said.
The researchers studied the abundance of trace elements, such as phosphorus, potassium, sodium and titanium, in basaltic glasses collected on ocean ridges worldwide, and kept by the Smithsonian Institute in Washington, DC. These basaltic glasses are formed as the hot, liquid magma meets cold seawater, cooling quickly into glass.
But the basaltic glasses are formed not from the whole of the magma, but from the liquid that’s leftover after much of the magma has solidified into crystals – a process that had been assumed to take place in the closed system of magma chambers, and is called ‘simple fractional crystallisation’.
Observations date back to 1970s
O’Neill was using the widely accepted model of simple fractional crystallisation in magma chambers to extrapolate his analysis of basalts back to the composition of the ‘parental’ magma in the mantle, when “the penny dropped, and it became obvious that these compositions [including trace elements] couldn’t be explained, as has been assumed, by what’s called simple fractional crystallisation.”
The first observations of high levels of trace elements in the basaltic glasses were made as far back as the 1970s, but only on a very local scale. Ascribed to an anomaly, their findings were “swept under the carpet and forgotten about,” said O’Neill.
“Because we were looking at the global pattern of these elements, we realised that this wasn’t just a local anomaly but characteristic of the entire process worldwide,” he said.
Earth’s mantle poorer in rare elements
The differences in the chemistry of ocean floor basalts has, until now, been attributed to the melting process in the mantle. However, O’Neill and Jenner suggest that this is a misinterpretation and that, according to their new model, these differences are in fact due to the processes occurring in the magma chambers.
In addition, “the mantle appears to be poorer in many rare elements than previously thought, because what we see in the basalts are abundances built up over many cycles” O’Neill said.
However, many geochemists and geophysicists are sceptical about the new model.
“The authors have made interesting observations by making use of an important new trace element dataset that they have collected on ocean ridge basalts. While it will take time to fully digest what they propose, I am sceptical,” said geochemist Charles Langmuir, from Harvard University in Massachusetts, USA.
“I do not think the proposed model is consistent with geophysical observation and what we know of magma chambers, [... and] the model interpretation is not consistent with what we know of the chemical consequences of periodically replenished magma chambers,” he said.