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Drilling the Mid-Atlantic Ridge

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Artemis
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« on: April 15, 2007, 01:02:17 am »

About the cruise...
Drilling the Mid-Atlantic Ridge
RRS James Cook cruise JC007, 5 March 2007 – 17 April 2007

 

Mid-ocean ridges are a fascinating component of our planet's armour plating. Mid-ocean ridges are the place where new oceanic crust is born, with red-hot lava spewing out along the spreading axis as seafloor spreading progresses. However, the mechanisms by which this occurs are still not well understood by scientists - hardly surprising when you consider that mid-ocean ridges are located thousands of metres below the surface of the ocean.



 Scientists have discovered a large area thousands of square kilometres in extent in the middle of the Atlantic Ocean where the Earth's crust seems to be missing entirely. Instead, the mantle - the deep interior of the Earth, normally covered by crust many kilometres thick - is exposed on the seafloor, 3000m below the surface. It has been described as being like an open wound on the surface of the Earth. What scientists don't know is whether the ocean crust was first developed, and then ripped away by huge geological faults, or whether it never even developed in the first place.

In March-April 2007, a team of scientists from Durham University, Cardiff University and NOCS will board the RRS James Cook to visit this special area of the Mid-Atlantic Ridge, which is called the Fifteen-Twenty Fracture Zone (FTFZ for short - the map on the left shows where this is located).

The scientific team on board the ship is led by Prof. Roger Searle from University of Durham, Dr Chris MacLeod (University of Cardiff) and Dr Bramley Murton (NOCS).

[more on the science behind this cruise...]



Left: Image of the Mid-Atlantic Ridge. You can see how the ridge is broken up into segments by fractures running roughly perpendicular to the ridge axis. The red dot shows the area where the team on board the ship will be working. Bathymetric image courtesy GEBCO.




Equipment

The first step on the expedition is to use TOBI (Towed Ocean Bottom Instrument - right) to build up images of the seafloor surface so that suitable drilling sites can be selected. TOBI images will enable the scientists to identify areas where peridotite is exposed at the seafloor, and give them information on the shape of the ridge, the distribution of faults and identify any areas where magma is erupting onto the seafloor.
[click here for more about TOBI and seafloor surveying]
 

When the team have selected suitable sites for drilling, they will use a robotic rock drill to take samples of the seabed. The drill is mounted on a metal tripod and is lowered onto the seafloor by a special cable. A camera on the tripod allows the scientsts to see exactly where they are placing the drill. The rotating drill bit is diamond-tipped to ensure that it's hard enough to cut through the rock, and produces cylindrical 'rods' of rock (cores, see below). An important feature of this particular drill is that the cores are specially marked to show which way is north so that the scientists know how they are oriented - very important when you're taking lots of samples across an area and you want to know how certain properties vary across space. By taking a series of these cores across the area and analysing them, the scientists will be able to determine how the seafloor spreading process varies across the region, and how the mantle came to be so close to the surface in this area.
[more about rock drilling]

 

Above, from left: the BRIDGE drill being recovered after sampling the seabed; an image from the tripod camera, showing the rig sitting on bare rock, covered in the foreground by a dusting of white sediment. Note the leg of the drill rig, and a sea urchin for scale in the centre of the picture. The dark stripes are mineral-filled cracks in the seafloor; Core recovered from the BRIDGE rock drill. The rock is a gabbro – equivalent to the lavas erupted onto the seabed, but which instead crystallised slowly at a depth of a few kilometres below the seafloor.
 

The team may also carry out dredging - a primitive but still effective way of sampling the seafloor. Methods and equipment have changed little since the pioneering expeditions of HMS Challenger in 1872. A chain-link bag with large metal-jawed opening is lowered to the seabed on a cable, dragged along the bottom for some distance, and then brought to the surface. Although rough and ready, dredging is still a useful way of mapping the broad-scale distribution of rock types on the seafloor. Dredging will only be conducted during the cruise as a backup or in between deployments of the principal tools: the sidescan sonar and seabed rock drill.

 

 

Above left: A rock dredge being deployed from a research ship.
Above right: Chris MacLeod plus full rock dredge. Note how the originally rectangular
mouth to the dredge has been bowed open as the rocks were forced in to the chain bag.

The RRS James Cook will set sail from port in Tenerife on 5 March 2007. This is the RRS James Cook's first scientfic mission....keep up to date with all the latest cruise news and developments by visiting the JC007 cruise dairy from 5 March.

http://www.noc.soton.ac.uk/gg/classroom@sea/JC007/about.html
« Last Edit: April 15, 2007, 01:04:36 am by Artemis » Report Spam   Logged

Artemis
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« Reply #1 on: April 15, 2007, 01:13:15 am »

Cruise diary


Day 3: Wednesday 7 March - Passage to sample area
Position at midnight: 24º20N, 25º29W

The morning watch was quiet with scientists mostly concentrating on setting up computing equipment as well as making records of the ship’s position and speed, and other important readings. At about 02.00GMT depth profile data showed that we were passing over an elevated area, which may have been a submerged volcano, or seamount.  The second watch continued with the recordings and observed the XBT deployment, this instrument measures the sound velocity profile, similarly to the SVP we used yesterday. The XBT has the advantage of being used whilst the ship is in motion whereas the SVP requires the ship to stop. 

We spent much of the afternoon marvelling and being slightly bemused by the huge media coverage our cruise is attracting from the worlds media.  Our three leading scientists Roger, Chris and Bram have been fielding phone calls and interview requests all day, just to mention a few articles…..CNN, Science American.com, Caribbean News, The Times, The Taipei Times, and the list goes on!  There appears to have been some crossed wires along the way as to what the science of the cruise is, primarily that there is a gaping hole in the ocean crust, as exciting as that would be, in fact the upper layers of the ocean crust are missing leaving the lower oceanic crust units and mantle rocks exposed at the seafloor.  We have also received hundreds of questions through the website so please bear with us as we work through them all, unfortunately we won't be able to answer all of them as much as we would like to.  On behalf of all the scientific team we would like to thank everyone who has wished us good luck and a safe journey - so far it seems to be working!

The day ended with some dolphin watching off the aft deck. 

Quote of the Day:
Jack to Kay: “Keep us in suspenders until tomorrow!”

Cruise diary


Day 6: Saturday 10 March - Passage to sampling area
Position at midnight: 18º40N, 37º04W

Michelle writes:


The main task for this morning was to test the rock drill operated by the British Geological Survey (BGS) team (Dave, Davie, Iain and Mike). This involved stopping the ship and lowering the drill into the water until it was submerged under 20m of water. The BGS team checked that the pumps and the communication system were operating correctly, both of which worked fine. The ship was back on course again within 2 hours.

At our afternoon seminar the issue of the watches was raised and it was suggested that the current system of 12 on - 12 hours off was causing some of the team to miss too many meals. After much discussion we decided to change to a 4hrs on-8hrs off watch pattern with Roger, Chris MacLeod and myself on watch 8-12, Jack, Sam and Chris Mallows on 12-4 and Bram and Kay 4-8.

During the evening the crew hosted a barbeque on the aft deck for everyone on board. Despite Sam being left in charge of the BBQ for 5 minutes and cremating the first batch of burgers, the evening was very enjoyable and made a nice break into the (never-ending?!) passage to the sampling area.

Unluckily for the 12midnight -4am shift the clocks went back again to GMT-3 and they had to endure another hour to their shift!


 

Lowering the rock drill into the water...Splashdown!

http://www.noc.soton.ac.uk/gg/classroom@sea/JC007/diary/diary_10-03.html
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« Reply #2 on: April 15, 2007, 01:19:08 am »

Cruise diary


Day 8: Monday 12 March - At the sampling area!
Position at midnight: 13º47N, 44º52W


The majority of the day was spent putting the finishing touches to our survey plans and potential drill sites. The leaders of the scientific team had meetings with the crew to finalise the approach to the sampling area.

Mid morning we crossed the Fifteeen-Twenty fracture zone. By the evening the ship was preparing to approach our first drill site and the excitement among us grew as our destination finally came within sight.

At 1:50GMT we arrived at our first site; quickly surveyed it and prepared to lower the rock drill to its first sight. The drill entered the water shortly before 2:00GMT. Unfortunately at 2:45GMT the BGS team lost communication with the drill and by 3:00 the decision was made to retrieve the drill and continue with a dredge whilst repairs were made.

At 10:00GMT Tuesday the dredge was on board with rock samples! More importantly we now have our first sample of mantle rock-peridotite, exactly what we were aiming to find at this site.

Watch this space for more exciting news and results!

Quote of the Day:
Bram: "Can you write down that boopy thingy?"
Kay: "Sure, wheres the whatsit thingy?"

The effects of the 4-8am watch are starting to take there toll on the team's conversational abilities!

Cruise diary


Day 9: Tuesday 13th March: Sampling area
Position at midnight: 13º29N 44º54W


After the initial excitement of the dredge this morning we returned to our first drill site which had to be aborted last night due to a loss in communication. The rock drill was successfully deployed and at 14:50GMT the seafloor came into view on the video link. We spent an hour or so searching for an ideal site to drill. An initial attempt to land the drill resulted in the drill toppling over onto the seafloor, the drill was righted and an alternate location was found.

As a marine geologist at the start of my career it was a very exciting moment to see the seafloor 2000m below us by a real time video link, to be able to see what I have been studying remotely was a great start to the day!

Unfortunately our excitement was short lived as after 55 minutes of drilling the drill bit became stuck and we had to retrieve the drill after only penetrating the seafloor by approximately 44cm. Once the core was on board we found it to be heavily degraded and fragmented mantle rock-not what we had hoped for condition wise but what we wanted from a geological perspective.

 

Workstations chart the progress of the rock drill 

 First sight of the seafloor!

   
Chris and the BGS drill team hard at work Deploying 

http://www.noc.soton.ac.uk/gg/classroom@sea/JC007/diary/diary_13-03.html

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« Reply #3 on: April 15, 2007, 01:22:45 am »

Cruise diary


Day 11: Thursday 15 March
Sampling area: TOBI survey
Position at midnight: 13º18N, 45º00W



Today we continued working our way along our first line of the TOBI survey. Just after lunchtime we reached the end of the line and began our first turn whilst towing TOBI. This is a difficult task, similar to towing a trailer behind a car with the added complication of strong winds and moving water! Our expert crew managed to navigate this without difficulty and by 21:00GMT we were on our second TOBI track.

Whilst we are conducting this TOBI survey all the watch keepers are required to record and plot both the ships position and TOBI’s position every half an hour. By doing this we can use our bathymetric map (a map of the topography of the seafloor) to make sure that TOBI is flying above any potential seamounts or peaks. As our TOBI survey is going to last for over a week, we are going to use this time to introduce you all to the crew and technical support staff, without who this cruise would not be possible, check in tomorrow for our first victim!!!!

   
   
The TOBI monitors High resoltuion depth profile Depth 
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« Reply #4 on: April 15, 2007, 01:26:54 am »

Cruise diary


Day 17: Wednesday 21 March
Sampling area: Rock drill
Position at midnight: 13o35N, 44 o 41W


Michelle writes:

Today has been another eventful day starting with disappointment of the rock drill sites.  Two sites were chosen based on the bathymetry and the sonar data from TOBI, in both cases when the drill approached the seafloor it was sedimented and not bare outcrop as we had hoped.  The rock drill has been modified since it was last used and now has a live black and white video camera and a USBL (ultra short baseline line), this allows us to accurately locate the position of the drill in the water.   As a result we are on a steep learning curve as how to maximise the capabilities of these new features, hopefully future drill sites will be more successful.

 
"Can we have some more core please?" Night time deployment of the rock drill

Whilst retrieving the Rock Drill at 11:30GMT the crew noticed that the cable was not feeding onto the winch drum correctly, this delayed us whilst the team fed out the cable again and then hauled it in to try and get it to wrap correctly around the drum.  It is important that the cable is correctly aligned on the winch drum because the cable can become damaged, which could have impacts on our scientific operations.  Once the rock drill was back on board the deck crew decided that in order for operations to continue safely the cable would have to be paid out until the crossed cable on the drum was uncrossed, only then could the cable be re-wound correctly.  This delayed us by eight hours.  By midnight we were on our way to our TOBI survey area and preparing to re-deploy TOBI.




Night time deployment of the rock drill
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« Reply #5 on: April 15, 2007, 01:36:10 am »

Cruise diary


Day 18: Thursday 22 March
Sampling area: TOBI survey
Position at midnight: 13o38N, 44 o 57W

 

Michelle writes:

Since the re-deployment of TOBI operations have been running smoothly.  We have now had the opportunity to look through the first half of our survey and have produced some useful images.  Below is a bathymetric (depth) map of our study area, made by combining the results from several previous surveys conducted by other groups, together with some views of the seafloor derived from our TOBI sidescan data which we have “draped” over the 3-D topography using a specialised software  package called “Fledermaus®”.  See the captions beneath each image for more information about what they are each showing.

Don’t forget our crew profiles at the bottom of the page - today meet Mick our Scientific Systems Manager and Kev our mechnical technician...

 


An “aerial” view of part of our study area.  This is on the Mid-Atlantic Ridge at 14ºSA, looking south.  Warm colours are shallow, cool colours deep. The Mid-Atlantic Ridge’s “median valley” winds through the image from north to south, and marks the boundary between the separating African (left) and (North American (right) tectonic plates.  The “Marathon” transform fault lies in the E-W valley seen in the far distance, about 100 km away.  Images like this are built up from a number of surveys using multi-beam echo-sounders and displayed using special computer software called “Fledermaus®”.




A 3-D view of a large lava flow (centre), about 2 km wide and 5 km long, near the axis of the Mid-Atlantic Ridge.  A gullied, partly eroded fault scarp can be seen on the right.  The light- to mid-grey zone in the centre of the image is the lava flow, filling the floor of the basin below the fault scarp.  The far side of the basin is marked by another fault scarp seen in the distance at top left.  The eruption vent for the flow may be at the small volcano marked by the bright patch, with dark crater in centre, at left of centre foreground.
 


A 3-D view of a fault scarp on seafloor about 1 million years old.  The top of the scarp is probably made of volcanic rocks.  These have been eroded leaving a series of narrow ridges and gullies runn ing down the face of the scarp.  The dark grey area in the right foreground is sediment which has covered the original volcanic seafloor.  Lighter grey at the foot of the scarp represents a fan of material eroded from the scarp. This is very similar to the  erosional topography seen at large escarpments on land.   Scarps such as these represent the pervasive faulting that breaks up newly formed ocean crust.  The scarp is aboput 200 m high and foreground view is about 500 m wide.



A 3-D view of a fault scarp on seafloor about 1 million years old.  The top of the scarp is probably made of volcanic rocks.  These have been eroded leaving a series of narrow ridges and gullies runn ing down the face of the scarp.  The dark grey area in the centre foreground is sediment which has covered the original volcanic seafloor. Scarps such as these represent the pervasive faulting that breaks up newly formed ocean crust.  Scarp is aboput 200 m high and foreground view is about 800 m wide.



A 3-D view of a fault scarp on seafloor about 1 million years old.  The top of the scarp is probably made of volcanic rocks.  These have been eroded leaving a series of narrow ridges and gullies runn ing down the face of the scarp.  The dark grey area to the left is sediment which has covered the original volcanic seafloor. Bright material at the foot of the upper scarp material eroded from the scarp. This is very similar to the  erosional topography seen at large escarpments on land.   Scarps such as these represent the pervasive faulting that breaks up newly formed ocean crust.  The scarp is aboput 200 m high and foreground view is about 500 m wide. 



A 3-D view of hundreds of small volcanoes building up an “axial volcanic riudge” in the centre of the Mid-Atlantic Ridge’s median valley.  These volcanoes are formed from pillow lavas erupted up fissures that mark the precise boundary between the separating tectonic plates.  Most of the volcanoes in this view are 200 m to 500 m in diameter and about 20 m to 50 m high.


http://www.noc.soton.ac.uk/gg/classroom@sea/JC007/diary/diary_22-03.html
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« Reply #6 on: April 15, 2007, 01:40:08 am »

Cruise diary


Day 20: Saturday 24 March
Sampling area: TOBI survey


 

Bramley writes:

Far below the floor of the Atlantic Ocean, deep within the Earth’s crust, a spark of pale-blue light flickers into life, then fades slowly away.  Then another. And another. Until they form a glimmering constellation. Each minute flash, the result of individual crystals that are straining and cracking under enormous stress. A swarm of small earthquakes follows and are the first harbingers of the cataclysmic events that will conclude with the exposure of the Earth’s mantle on the abyssal ocean floor.

Still hot, after its ascent from hundreds of kilometres deep within the interior of the planet, the mantle rock that is now cracking and breaking has recently been depressurised, squeezed and melted. In its ductile state, it cannot support the stresses required to cause earthquakes. But now it is nearer the surface and has cooled down, it has become brittle. The stresses imposed on this rock are the result of the huge tectonic plates of Africa and America moving away from one another at the Mid-Atlantic Ridge. Although slow, only a few centimetres a year, this divergence builds up enormous stresses, resulting in the snapping and cracking of the crust and upwelling of the deep, hot mantle.

On a normal mid-ocean ridge, this upwelling causes the mantle to melt and form new oceanic crust. But here, between 15.5°N and 13°N, the melt is in short supply. As a result, the gap caused by the plate divergence is not being filled by molten rock. Instead, the mantle itself must fulfil this role. Over the next few thousand years, earthquakes will split the oceanic crust right through. The resulting crack will then begin to slip faster and faster, eventually taking up the divergent plate motion. Beneath the ocean, as the crack breaks through the crust, seawater penetrates downwards, finally infiltrating into the mantle. Unlike the crust, which is made of volcanic rock, the mantle is predominately formed from crystals of olivine. In its dry state, olivine forms extremely strong rock (called peridotite), much stronger than the crust. But when seawater reacts with it, a new mineral is formed. This is serpentine, an extremely weak material that slips and yields easily. It is along this serpentinised slip-plane that the mantle now begins to emerge.

What causes the mantle to yield less melt than usual, and how the overlying crust can be torn away, remains a mystery. Is it because the mantle is cold and unable to melt as much, or is it ‘infertile’, having already partially melted many eons ago? Does the mantle become deeply serpentinised, and is that in part responsible for its exhumation as the crust tears away? What secrets of its history, when buried deep within the Earth, does the exposed mantle rock yield?

These are the questions that we hope to answer during our voyage and after, having spent long hours in the laboratory analysing our samples. The images we are getting from our unique sonar, black and white pictures of the ocean floor that resemble those from Venus or the Moon, will tell us much about how the crust is torn away and the mantle exhumed. The chemistry of the rocks, both the peridotites and the lavas, will also tell us about the deformation and melting history of the mantle. From this evidence we hope to unravel the mystery of this rent in the Earth’s crust and thereby get closer to understanding the workings of our planet....
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« Reply #7 on: April 15, 2007, 01:42:02 am »

Cruise diary


Day 22: Monday 26 March
Sampling area: TOBI survey
Ship's position at midnight: 13 21.6376N, 44 54.91477W

 

Chris writes:

Today I finished processing the raw sidescan sonar data from the southern part of our TOBI survey area using a software package called PRISM. In general the data were of good quality, although I had to make corrections for various things such as sea-surface reflections and ping drop-outs.

After the TOBI passes had been processed and re-located to their correct geographical position, I combined them together and created a digital mosaic of the sea-floor, which I then overlaid onto the bathymetry data. This allowed the senior scientists to have a virtual 3D roam about along the sea-floor, looking at areas of particular geological interest where we will later on take cores and dredges from.

Below is an example of the 3D map I created using Fledermaus software. The high back-scatter (white areas) indicate bare rock outcrops, and the low back-scatter (dark areas) are typical of sediments. This image shows a volcanic ridge several hundred metres high that runs from north to south at the very edge of our survey area.

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« Reply #8 on: April 15, 2007, 01:48:20 am »

Cruise diary


Day 23: Satruday 31 March
Sampling area: TOBI survey
Ship's position at midnight: 13 21.6376N, 44 54.91477W


Bramley writes:

What’s in a piece of rock?

Since our earliest involvement with the Sea, Mankind has imagined an ancient realm of unfavomable darkness, where nothing stirs and nothing changes. A place locked in time from which mythical gods emerge to torment those navigators foolish or brave enough to venture across its endless surface. Even today, when we think of the deep ocean, we think of processes that are infinitely slow: the fine rain of sediment accumulating at one centimetre per thousand years on the seafloor, layers of mud dating back to the birth of the ocean itself, and the creeping motion of the Earth’s crustal plates as they crawl across the surface of the globe. Yet in this abyssal darkness, volcanoes burst into life, showering white-hot molten lava across the seabed, incinerating everything in its path and causing gysers of superheated water to gush upwards into the depths above.  Elsewhere, as we are now discovering, the Earth’s mantle these volcanoes extrudes onto the surface odf the sea floor forming large corrugated mountains.

But what of these volcanoes and the mantle rocks: what are they made of, and what do their composition tell us about their formation? Ironically, the ocean crust forms the youngest areas of the Earth’s surface. What seems like an ancient environment dating back to the beginnings of our planet is actually in a constant state of renewal. Whereas the ancient interiors of the continents contain rock that dates back over 3,500 million years, the oldest ocean crust is a mere 160 million years old. Why? Because new ocean crust is continuously created at the mid-ocean ridges, a line of volcanoes that encircles the global ocean, stretching for over 70,000 km. The rate of crustal growth is between 20 and 150mm per year. So, after one million years, an ocean like the Atlantic will widen by about 20 km. But this doesn’t mean that the Earth is expanding. For every millimetre by which the ocean crust grows, an equivalent amount is destroyed. The destruction occurs along subduction zones, the antithesis to mid-ocean ridges, where the world’s most destructive earthquakes and tsunamis are born. And it is this history of crustal recycling, stretching back to the birth of the planet 4,600 million years ago, that the volcanoes of the mid-ocean ridges record in their lavas.

“Why not use lavas from volcanoes on land?” you may ask. Because they erupt through hundreds of kilometres of ancient continental crust, they become contaminated on route to the surface. But the mid-ocean ridges are erupting through a few kilometres of their own lava crust, and hence preserve a clean record of their origins.

To unlock their mystery, all that is required are small fragments of glass and crystals from the mantle rock. Held within are the elements that tell the story of cooling and crystallization of the molten rock, its origin deep below the crust during the melting of the mantle, and further back in time, over hundreds of millions of years to the fate of ancient oceans past, the formation of the continents and the origin of the early Earth.

Before we can begin this geological journey, we first have to collect our fragments of rock. Although seemingly the simplest of operations, it is often the most tricky, and certainly the most expensive. Finding and recovering rocks from 3 or 4km below the ocean surface requires an ocean-going research ship with accurate navigation, precision station keeping, sonar imaging and detailed multibeam-bathymetry. This is team work:  the skills of the officers on the Bridge holding the ship in position, often to within 200m of the volcanic target; the technical and engineering support team ensuring the sonar is giving the best and clearest images of the seafloor; and the deck crew running out the winches at their maximum speed whilst keeping a keen eye on the wire tension and proximity of the sampler to the bottom.

Perhaps the least technical aspect of the operation is the rock sampler itself. Affectionately referred to as the ‘dredge’, this devise is simply a heavily weighted steels bucket with a chain-mail bag. Just think of it as an ocean-going geological hammer! Once on station, the dredge is deployed on the a steel rope and the whole package accelerated downwards at 60 metres per minute. It is then drasgged over a few hundred metres of seafloor. Recovery is made at a similar rate, and the round trip to the seafloor in 3500m of water takes about 90 minutes.

Once on board, the dredge is emptied and the rocks taken to the wet lab, a laboratory full of sinks, saws, bright lights and sample tables . The rocks are washed, sawn in half, sorted, described, numbered, curated and bagged for analysis back at the NOC.

For the rock samples, this is just the beginning of its journey back through time. In the laboratory in Southampton, the freshest fragments are picked from each sample. Once mounted on a glass slide, the fragments are polished to 120 microns thick. The analysis for major elements involves placing the sample in an electron micro-probe where a 20,000 volt electron beam is fired at a spot only 15 microns in diameter. This causes secondary X-Rays whose energy, wavelength and amplitude indicate the concentration of the major elemental constituents of the rock or mineral. These are measured as a percentage of the total mass of the material and tell the history of cooling and crystallization of the magma before it erupted, or the melting and history of the mantle. The trace elements are far lower in concentrations, and measured in parts per million. To analyse these, we vaporize a 20 micron spot of the sample using a high-powered laser beam. The vaporised rock is then sucked into an argon plasma at 8,000 degrees centigrade. At this temperature, the molecules of rock or mineral are striped to their individual elements, then these are stripped of their outer electrons. The resulting charged particle beam is passed through an intense magnetic field that separates out each elemental mass. The number of atoms of each mass is then counted, often at a rate of several million per second, and their abundances and ratios measured. The trace element concentrations tell us about the melting history of the mantle, while the isotopic ratios of elements like strontium, neodymium, hafnium and lead tell us about the recycling of earlier ocean crust into the mantle and the origin and evolution of the Earth itself. All this from a tiny piece of rock.
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« Reply #9 on: April 15, 2007, 01:52:30 am »

Cruise diary


Day 28: Sunday 1 April 2007
Sampling area: Dredging
Ship's position at midnight: 12º57N, 44º56W


Michelle writes:

The last week has been really busy on board the James Cook; especially for the geologists who have been working non-stop on all the seafloor rocks we are now happily in possession of!!  We’ve had both success and failure with the rock drill and dredging.  After some initial teething problems with the dredging the scientists and the deck crew have a hopefully now foolproof technique which is working well and giving excellent results.  The dredges generally take around 3 hours from initial deployment to retrieving the dredge.  The dredge consists of a metal net with an open front with a pipe attached behind it as a second trap for samples.  Once on board, both parts of the dredge are emptied of all the samples and sediment and taken into the deck lab to be processed.

We begin by washing the sediment off the samples and having an initial look to determine if the rock is mantle or volcanic in origin.  Samples are then selected to be cut into two on the rock saw, we cut all the large samples and a representative selection of the smaller samples as well as any which catch our eye.  We need to cut the rocks so that we have a fresh surface to look at the different minerals in the rocks because they are often coated on the outside by manganese and other seafloor weathering minerals.  Each type of rock has a distinctive mineral assemblage and texture and these features are used to name and identify the rocks.  These key features of the rocks are described and the sample labelled and bagged up ready to be analysed in more detail back at our different universities.  The PhD students, especially Sam and myself have been benefiting greatly from the vast pools of knowledge that Bram, Chris and Jack have about seafloor rocks and have been learning continuously from them.  It has been hard work with dredges coming up all the time and needing to completely process the previous dredge samples before the next arrives on deck, however it has been worth it and has been an invaluable experience for those of us at the start of our careers.  Although I must admit when I started my PhD I never expected that I would spend my time cut, washing and blow drying rocks, and I definitely never expected to say that sentence!!

So what have we found?  So far we have retrieved a range of mantle rocks and a range of volcanic rocks.  The mantle rocks we’ve retrieved, called peridotites, have a broad range of textures and compositions with a range from quite fresh to completely serpentinised. Serpentinisation is the alteration process that happens to mantle rocks because they are unstable at seafloor temperatures and pressures and they react with seawater to create a new mineral called serpentine.  This mineral has a range of colours and forms and usually creates an attractive looking mesh-like network of veins throughout the rock.  Serpentine is very soft and may help to lubricate the faults that bring these mantle rocks to the seafloor.  This mineral can also be found on land where pieces of ocean floor have been emplaced onto the land (called an ophiolite), an example of this is The Lizard, Cornwall where serpentinite has been mined in the past and used to carve decorative pieces.


Pillow lava fragments

   

Serpentinised peridotite


http://www.noc.soton.ac.uk/gg/classroom@sea/JC007/diary/diary_01-04.html
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Artemis
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« Reply #10 on: April 15, 2007, 01:54:05 am »

Cruise diary


Day 28: Monday 2 April 2007
Sampling area: Dredging
Ship's position at midnight: 12º57N, 44º56W

Bramley writes:

The Start of a New Month Brings a Gem of a Find

Diamonds may be a ‘girls best friend’ but they are also the shape of pure carbon while under the enormous pressures encountered deep within the Earth’s mantle. Most diamonds are found in volcanic ‘kimberlite’ pipes that cut through ancient continental crust. But we weren’t expecting to find any diamonds in the mantle that is exposed on the seafloor out here, at 13°N on the Mid-Atlantic Ridge.

This is because the mantle pressures are too low and the carbon concentrations too scarce to make diamond. However, as dawn broke at the start of our second month at sea, we were amazed to find small, clear crystalline shapes in the gravel from the latest dredge. In amongst fragments of fresh mantle peridotite, sparkles and glints of blue-white light caught the eye of Bob Spencer, the Deck PO on the 4-8 watch. Photographed below are the small, clear crystal sifted from the gravel. Without access to geochemical analyses, we cannot be sure of the nature of these crystals. However, a carefully controlled scratch test by Glyn Collard, the 2nd Engineer, proved them considerably harder than a pint beer-glass. Their refractive index is much higher than either water or gin, and their shape is also reminiscent of the cubic crystal structure of diamond. In a subsequent dredge we found more of the same, and even some quite large ones (although these are a bit worn and dirty on the outside). All of the crystals are clear, inclusion free, and white in colour; so only of the best quality. While we await the outcome of chemical analyses, the ‘samples’ are being kept locked up in the ship’s safe – just in case the gleam of ‘promised riches’ causes the ship’s company to mutiny.....

   


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Artemis
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« Reply #11 on: April 15, 2007, 01:56:53 am »


That was the last diary from the crew exploring the Mid-Atlantic Ridge, the last one dating April 2, 2007.


That was nearly two weeks ago.

I hope that they are onto something and, of course, make it known to the public.
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Using rocks and minerals to heal the earth and us.


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« Reply #12 on: April 15, 2007, 04:04:24 am »

Just loved this Artemis!  Thanks for posting it all here.  By the way, my sister-in-law used to be married to Dan Pomeroy, the guy that invented the drill bit for core drilling  back in the early 80s.  I just now found 6 other patents that use his bit too.  Anywhere in the world where cores are drilled, they use Pomeroy bits.

Loved those diamonds they dredged up too.
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ILLIGITIMI NON CARBORUNDUM

Thus ye may find in thy mental and spiritual self, ye can make thyself just as happy or just as miserable as ye like. How miserable do ye want to be?......For you GROW to heaven, you don't GO to heaven. It is within thine own conscience that ye grow there.

Edgar Cayce
Artemis
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« Reply #13 on: April 15, 2007, 07:38:52 pm »

Thanks, Rockessence, my you do get around! 

I find the whole field of underwater archaeology fascinating.  I edited the diary so that only the substantative parts remained, but even then, you can see how much trouble they have on these expeditions. With all that trouble, I find it hard to believe that they can make any judgments of what lies below with absolute certainty.

They are a bit negligent about making entries for this expedition but once they do, I will post them here.
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« Reply #14 on: May 19, 2007, 01:42:22 am »

Cruise diary


Day 38: Friday 12 April 2007
Sampling area: Dredging
Ship's position at midnight: 12º57N, 44º56W
Bramley writes:

Gems, Lies and Video-diaries.


While some people seek solace at the bottom of a glass, we found ‘Truth’. Or rather, the absence of the bottom of a beer-glass led to the discovery of the truth about the source of our ‘diamonds’. But before that, as the First of April passed into the Second, and then to the Third, our delight turned from scientific wonder, to greed, and finally to suspicion concerning the amazingly prolific haul of gems from the abyss. Questions were raised and answers sought, but not before ‘diamond fever’ had spread through the ship’s company like an over-ripe banana through a chimpanzee. Whereas the rock-sample laboratory had once been the exclusive haunt of gaunt, exhausted and goggle-eyed petrologists, it had suddenly become ‘The Place’ for deck-crew, engineers and officers alike to hang-out. Such was their new-found fascination for all things Geological that we began drawing up a lecture series, starting with ‘Hutton’s Unconformity’ and the “Abyss of Time”, progressing through ‘Plate Tectonic Theory’ and concluding with ‘What Isotope Geochemistry Can Do For You’. Yet despite their obvious delight in our science, the plan of a lecture series met with little enthusiasm.

No, the interest of the ship’s company strictly lay in a ‘hands-on’ approach. So much so, that a photograph taken discreetly by a hidden camera caught them rummaging through the dredged gravel with eyes sparkling like magpies, picking at glittering gems dotted here and there. At least this explained the sudden and apparent decrease in the numbers of ‘diamonds’ we were recovering. Or so we thought.

In an attempt to recover our scientific samples, a polite notice was put-up around the ship. The emphasis was not to seek blame (we already had our suspicions), but rather to encourage the return of the material for the benefit of the scientific community. And so we went on, each eyeing the other, no-one mentioning what was on everyone’s mind. “Was that a change in ship’s course towards the Caymen islands?”. “Did you hear footsteps in the sample-store last night?”. “Who’s got the key to the safe?”. That was, until this morning. As dawn broke over a tranquil sea, the sun rising into a crystal-clear blue sky, a cry echoed out from the rock-saw lab. Tucked away at the back of a cupboard was a heavy, clear-glass beer-mug: a typically British, clear-glass beer-mug. A facetted, barrel-shaped glass, with a looped handle at its side, and a thick
glass base. Or rather more idiosyncratically, this beer mug was without its thick glass base. A fact that rendered it inadequate for its originally intended purpose. Suspiciously, the base had been sawn off. On close inspection, fragments of the sawn glass base were found lying in the dirt beside the saw. Some if these had been crudely fashioned into diamond-shaped crystals, but discarded because of some flaw or other in their manufacture. A quick comparison with our ‘gems’ revealed the ghastly truth. On reflection, it was obvious really. The fact that it deceived such an experienced and sceptical geological team like ours was embarrassing. Obviously, someone had swapped our real gems for these fakes!


So, as we approach Tenerife, on behalf of us all on the maiden science voyage of the RRS James Cook, its been great having you along with us to share in our adventure. Take care.

 

The infamous beer mug





JC007 bids you farewell!

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