Survivors - The Geology of Diamonds

From the Spring 2018 issue of Gems&Jewellery magazine, Gem-A assistant tutor Beth West FGA DGA explores the remarkably epic journey of diamonds and how their characteristic strength is rooted in their archaic origins.

"In the deep glens where they lived all things were older than man and they hummed of mystery."
Cormac McCarthy, The Road

Carbon is the fourth most abundant element in the universe. It is one of life’s most important building blocks. We are made up of around 18% carbon. The diamond, however, is formed of pure carbon and bonded in such a way as to make it the earth’s hardest naturally occurring substance.

But these magnificent gems extend far beyond their beauty and durability; they carry whispers of the beginnings of our world.

How old is a diamond?

It has been proven that the oldest diamonds formed around 3.5 billion years ago in an age known as the Archean Eon. At this point, it is believed that the planet had only existed for around 1 billion years. The surface of the earth was still settling.

How are they formed?

The earth is formed in layers. The outer most layer is known as the crust and ranges between 6km and 40km deep. It is divided into the thicker continental crust, making up the landmass, and oceanic crust – the ocean floors. Beneath the crust is the mantle. It accounts for eight tenths of the earth’s volume and is predominantly composed of an igneous rock known as peridotite.

The upper most portion of the mantle and the crust are known as the lithosphere (‘rocky sphere’ in Greek). This part of the earth is rigid, whereas the deeper parts of the mantle are in a permanent state of convection as the rock melts and cools. At the centre of the earth, the liquid core encases the solid inner core – the 5500° C iron and nickel heart.

In the Archean Eon, as the crust shifted and split, portions of it found their place and settled. These archaic, unmoving pieces of crust are known as ‘cratons’. Under each craton is a root or keel of lithospheric mantle that can descend up to 300 kilometres deep. These keels are cooler than the neighbouring convecting mantle, at around 1200° C at its deeper points.

Diamond will start to form at around that temperature if the corresponding pressure is around 40000 atmospheres. That equates to a depth of around 140km. These deep lithospheric keels beneath the cratons, away from the chaos of the convecting mantle, were perfect safe-houses for a growing diamond.

Petra Diamonds' South African Mine, Finsch. Showing open put with subsequent block cave extraction, tunnels visible.
Image by Charles Evans FGA DGA.

Where did the carbon come from?

The carbon would have derived from either a primordial source (extant from the birth of the earth) or from material pulled into the mantle when the ocean floor was pushed beneath colliding continental crust.

The carbon would have been locked into compounds such as methane (CH4) or carbonates (CO3) travelling in melts or fluids around the convecting mantle. When these fluids passed through the mantle keel, they would have reacted to the peridotite, freeing the carbon from their compounds and allowing it to crystallise as diamond. It is in these deep portions of the keel that the oldest diamonds formed.

How did they get to us?

They resided in the keel for millions of years until, around the age of the dinosaurs (between an approximate age of 300 and 80 million years old), they were expelled from their plutonic residence via forceful, violent eruptions of magma produced deep in the mantle.

Kimberlite is the most abundant of the three types of known diamond bearing magma. This powerful magma blasted through to the surface producing a cone shaped hole called a ‘pipe’. Those diamonds that were carried up with the blast were left shaken but intact in the magmatic debris that packed the hole or were forced to travel with the weathering surface of the earth – often for great distances, in rivers or in glaciers, and for millions of years.

From the depths of the earth to the ring on our fingers, these stones have certainly proved their strength. So when we are drenched by the sparkle and marketed bling of these gemstones, it is perhaps worth remembering their remarkable journey.

This article originally appeared in Gems&Jewellery Spring 2018/ Volume 27/ No.1 

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Cover Image: Diamond Crystal. Image by Pat Daly


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サバイバーズ ― ダイヤモンドの地質学

 

これはGem-Aの機関誌「Gems&Jewellery」2018年春号より引用した記事です。Gem-Aのアシスタント・チューター、ベス・ウェスト FGA DGAがダイヤモンドの壮大な旅をたどり、ダイヤモンド特有の強さについてその起源を古代から探ります。

「彼らが棲んでいた深い谷間では、すべてのものが人間より古い存在であり、それらは神秘の歌を静かに口ずさんでいたのだった」
コーマック・マッカーシー著『ザ・ロード』

炭素は宇宙で4番目に豊富な元素であり、生命にとって最も重要なものの一つです。私たちはおよそ18%の炭素で成り立っています。しかしダイヤモンドの場合は、純粋な炭素で形成され、天然に存在する地球上で最も硬い物質となる方法で結合しています。

しかし、この素晴らしい宝石からは、美しさと耐久性のはるか彼方にある、私たちの世界の始まりをささやく声が聞こえます。

ダイヤモンドは何歳?

最も古いダイヤモンドはおよそ35億年前の太古代に形成されたと証明されています。この時点では、惑星はおよそ10億年間のみ存在していたと考えられていました。そして、地表の状態は安定していました。

ダイヤモンドはどのように生成されるか?

地球はいくつかの層から成ります。最も外側の層は地殻と呼ばれ、6km~40kmの深さがあります。そしてさらに大陸で構成される厚い大陸地殻と海洋地殻 ―つまり海底― に分けられます。地殻の下にはマントルがあり、地球全体の10分の8を占め、主に橄欖岩(かんらんがん)という火成岩で構成されています。

マントルの最上部と地殻はリソスフィア(ギリシャ語で「岩石が多い領域」の意味、岩石圏ともいう)と呼ばれます。この部分は堅いものの、マントルの深い部分では岩石が熔融したり冷えたりして、永久に対流の状態にあります。地球の中心では、液体の外核が固体の内核 ―5500°Cの鉄とニッケルでなる中心部― を覆っています。

太古代に地殻は動いて分裂し、その切り離された部分はそれぞれの場所に落ちつきました。古い動かない地殻の部分を「クラトン」と呼びます。それぞれのクラトンの下、最大300キロメートルの深さにリソスフィア・マントル・キールがあります。このキールは深い部分でもおよそ1200°Cであり、対流によって熱を循環させているマントルよりも温度の低い場所です。

およそ40000気圧に相当する圧力があれば、ダイヤモンドの生成が始まります。これは、およそ140kmの深さと同じです。クラトンの下の深いリソスフィア・キールは、対流によって熱を循環している混沌としたマントルから離れているため、ダイヤモンドが成長するのに最適な場所となりました。

ペトラ・ダイヤモンド社の南アフリカ、フィンシュ(Finsch)鉱山。露天掘りとその後に行われるブロック・ケイビング法(トンネルが見える)。
写真: Charles Evans FGA DGA.

炭素はどこから来たのか?

炭素は、原始時代の(地球の出生から存在した)物質から生じた、または海底が大陸地殻に衝突して押し下げられた時にマントルに引き込まれた物質から生じたと考えられています。

炭素は、マントル付近の流体の中でメタン(CH₄)や炭酸塩(CO₃)等の化合物に閉じ込められています。この流体がマントル・キールを通過すると、橄欖(かんらん)岩に反応し、炭素は化合物から離れてダイヤモンドとして結晶します。最も古いダイヤモンドが生成されたのは、キールの深部です。

ダイヤモンドはどのようにして私たちに届いたのか?

ダイヤモンドは、マントルの深部で生じるマグマの力強く激しい噴火によって深部から噴出されるまで、何千万年もの間、恐竜時代(およそ3億年~8000万年前)頃までキールに留まっていました。

3種類のマグマの中でキンバーライトが最も豊富にダイヤモンドを含んでいます。この強力なマグマが噴火によって「パイプ」と呼ばれる円錐形の穴をつくり、地表まで到達します。噴火で上部に運ばれたダイヤモンドは、穴に閉じ込められたマグマの砕屑物の中にとどまるか、あるいは地表で風化され ―何百万年の間、川や氷河の中で長距離を旅します。

地球の深部から私たちが身に着ける指輪までの道のりをたどると、ダイヤモンドの強さがはっきりとわかります。私たちがダイヤモンドの輝きに包まれるとき、この驚くべき旅を思い出してみてください。

この記事はGems&Jewellery Spring 2018/ Volume 27/ No.1  より引用したものです。

表紙:ダイヤモンドの結晶。写真: Pat Daly

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