Monday, January 11, 2016

Blueshist Facies And An Exam Written Long Ago

Bear with me.


Emergence of blueschists on Earth linked to secular changes in oceanic crust composition - Richard M. Palin and Richard W. White

The oldest blueschists—metamorphic rocks formed during subduction—are of Neoproterozoic age, and 0.7–0.8 billion years old. Yet, subduction of oceanic crust to mantle depths is thought to have occurred since the Hadean, over 4 billion years ago. Blueschists typically form under cold geothermal gradients of less than 400 °C GPa−1, so their absence in the ancient rock record is typically attributed to hotter pre-Neoproterozoic mantle prohibiting such low-temperature metamorphism; however, modern analogues of Archaean subduction suggest that blueschist-facies metamorphic conditions are attainable at the slab surface. Here we show that the absence of blueschists in the ancient geological record can be attributed to the changing composition of oceanic crust throughout Earth history, which is a consequence of secular cooling of the mantle since the Archaean. Oceanic crust formed on the hot, early Earth would have been rich in magnesium oxide (MgO). We use phase equilibria calculations to show that blueschists do not form in high-MgO rocks under subduction-related geothermal gradients. Instead, the subduction of MgO-rich oceanic crust would have created greenschist-like rocks—metamorphic rocks formed today at low temperatures and pressures. These ancient metamorphic products can hold about 20% more water than younger metamorphosed oceanic crust, implying that the global hydrologic cycle was more efficient in the deep geological past than today.

Blueschist facies metamorphic rocks contains a  very typical mineral assemblage of glaucophane, epidote, jadeite, albite and chlorite. The blue color  is due to Glaucophane which belongs to the sodic amphibole group of minerals.

I had read Miyashiro's book on metamorphic facies during my graduate studies in India, and then  again  as I appeared for the CSIR (Council of Scientific and Industrial Research)  scholarship exam. It served me well. One of the questions in the exam was to explain the absence of blueschist facies metamorphic  rocks from the Archean record.

There could be two explanations I had learnt. One is  that these rocks did  exist in the Archean but had been subsequently re-metamorphosed to a more stable  assemblage. The problem is to explain why there is not even a single instance where the transformation is incomplete and blueschist facies partially preserved at some Archean age subduction complex somewhere on earth. That left the other explanation as the stronger one i.e. such metamorphic rocks did not exist on earth in the Archean due to higher heat flow. I think I wrote a good answer and generally did quite well as I cleared the exam and won the scholarship.

This study also links the high heat flow on early earth to the absence of blueschist facies rocks, but in a different way. High heat flow would have meant greater amounts of partial melting of mantle rocks,  resulting in greater amounts of refractory magnesium partitioning into the magma. Such high magnesium content ocean crust then would have metamorphosed into the Mg rich greenschist type rocks instead of blueschist varieties.

The paper mentions greenschist rocks. There are plenty of them in the Archean terrains of India. The green color is due to minerals like chlorite and hornblende. They form linear belts of low-medium grade metamorphic rocks, formed by the transformation of mafic volcanic and clastic sedimentary rocks formed in an oceanic setting and deformed during events of subduction and orogeny. Some of the best metamorphic mineral collection tours are in such terrains, with rich pickings of garnet, tourmaline, flourite found in the large quartz veins that intrude these greenstone schists.  Many a college geology tours have trampled the greenstone belts of south India near the towns of Dharwad, Chitradurg  and Shimoga.

 Source: Wiki
I never had to dabble in metamorphic petrology again as my research took me a world away into carbonate sedimentology. But Miyashiro's elegantly written book I remember.

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