Synchrotron X-ray Tomography: A New Technology for Studying the Earth's Mantle

When one thinks of the standard tools geologists use to study the Earth, a particle accelerator is unlikely to come into mind. Nevertheless, Dr. Wenlu Zhu of the University of Maryland is utilizing the X-ray radiation given off by a synchrotron, which is a type of particle accelerator, to image the melt structures of synthetic partially molten mantle rocks. Zhu proposes that by imaging the melt distribution in three-dimensions, one can more completely quantify the transport properties of the partially molten region beneath ocean ridges, where plates are created. Zhu is the lead researcher on the project, which is a collaborative effort, involving scientists from the University of Maryland, Woods Hole Oceanographic Institute, and Argonne National Labs.

Experiments have already been conducted on similar synthetic systems. However, these studies rely on 2D measurements to infer the transport properties and therefore, paint an incomplete picture of the melt structure. At the Advanced Photon Source, Zhu et al. rotate their cylindrical, partially molten samples 180° through an x-ray beam in .12° increments to build digital 3D representations of their partially molten samples. Using the digital melt data, one can perform a so-called numerical experiment to simulate magma flowing through their samples. By doing this, Zhu et al. are able to quantify flow properties such as permeability. Permeability is defined as the capacity for the material to allow fluid, in this case magma, to pass through it. It is a fundamental property of the rock.

Figure 1: Diagram of imaging technique.

Figure 2: 3D image of melt distribution of olivine-melt from Zhu et al. (2011).

Preliminary results look promising. However, extrapolating these to the Earth may be challenging. So far, the only experiments that have been conducted have used the simplest system, which is olivine-melt. However, in the mantle, there is an assortment of minerals. Adding different mineral types may significantly alter the flow properties of the rock. In addition to having a compositional variety, the Earth’s mantle is moving at a rate of about 10 cm yr^-1. Although slow, this motion causes deformation that may give way to drastically different melt structures. The experiments currently being conducted by Zhu and her team are performed in isostatic equilibrium, and therefore, do not take in account the motion of the mantle. However, Zhu et al. have made plans to investigate the role of composition as well as deformation on the flow properties of partially molten mantle rocks.

Synchrotron radiation is a useful tool for studying magmatic processes in the Earth. So far, research performed by Zhu et al. has led to the first direct measurements of permeability for synthetic, partially molten mantle rocks. If their results help better constrain the melt transport properties at ocean ridges, they may help geologists better understand the mantle melting elsewhere on the Earth, such as at subduction zones where volcanoes usually form.

Zhu, W., Gaetani, G.A., Fusseis, F., Montési, L.G.J., De Carlo, F. (2011), Microtomography of Partially Molten Rocks: Three-Dimensional Melt Distribution in Mantle Peridotite, Science: 332 (6025) 88-91, [DOI:10.1126/science.1202221].

Tracking Potential Water on Europa

Europa's surface in true color (left) and false color (right). Courtesy of NASA/JPL.

Europa is a moon with a surface predominantly covered by ice orbiting the gas giant Jupiter.   The Voyager mission imaged a majority of Europa’s surface in the late 1970s.  The Galileo mission was launched in 1989 to orbit the Jovian system.    It obtained higher resolution of sections of Europa’s surface.  These missions revealed an ice-covered surface that resembled a ball of twine, covered with linear and curvilinear ridges.   Data from these missions also revealed that Europa generates its own magnetic field, which provides evidence that Europa has a briny ocean beneath the surface.

Image of a ridge on Europa courtesy of NASA/JPL.

Europan ridges are of specific scientific interest because they not only reveal information about geologic processes but also the potential for the existence of shallow water.  The interaction of water on Europa with the surface could have implications for the astrobiological potential of the icy satellite.  The reactions of water and the radiation that bombards the surface of Europa could cause chemical reactions that may be beneficial for the existence of life in the water.

One hypothesis for Europan ridge formation is that shallow water intrusions within the ice shell could freeze and expand to create the surface features of the ridges.  As the water freezes and expands it exerts a significant amount of pressure on the surrounding ice and may create cracks within the ice.   These cracks may serve as a conduit for water to reach the surface of Europa.

As the water intrusion freezes it exerts pressure in all 

directions and can cause upward flexure (red) and sideways compression and flexure (blue) to form a ridge.

In order to explore the water intrusion hypothesis several questions must be answered.  How much pressure can the freezing water exert on the surrounding ice?   Is this pressure enough to crack the ice?  How deep can the water intrusion be and still be able to create these ridge features.  These questions are being evaluated currently through the use of numerical models and may provide the answer to where is the water on Europa.

References and Further Reading: 

Cassidy, T., Coll, P., Raulin, F., Carlson, R. W., Johnson, R. E., Loeffler, M. J., Hand, K. P., et al. (2010). Radiolysis and Photolysis of Icy Satellite Surfaces: Experiments and Theory. Space Science Reviews, 153(1-4), 299-315. doi:10.1007/s11214-009-9625-3

Christopher F. Chyba and Cynthia B. Phillips. (2002). Europa as an Abode of Life. Origins of Life and Evolution of Biospheres (Vol. 32, pp. 46-47). doi:10.1111/j.1744-618X.2010.01158.x

Han, L., & Showman, A. P. (2008). Implications of shear heating and fracture zones for ridge formation on Europa. Geophysical Research Letters, 35(3), 6-10. doi:10.1029/2007GL031957

The Melt Redemption

The spreading center axis of a mid-ocean ridge is always believed to be the only exit for the upwelling melts beneath it to erupt. However, recent observations suggest that those seemingly abiding melts have probably already found a secret passage to escape. Throughout the global mid-ocean ridge system, mid-ocean ridges are offset by transform faults. These transform faults, with efficient conductive cooling, may produce a thickened lithosphere that directs melts away, but recent research showed that, the thickened crust along transform faults might indicate efficient melt extraction toward transforms.

Laura and Laurent (2011) apply a 3D model of melt migration and extraction to investigate the unusual thickened crust along an oceanic transform at fast spreading ridges, and suggest that a melt extraction zone (MEZ) should be responsible for the melt redemption. Melt migration at mid-ocean ridge system can be modeled by a 2-D process with two steps: (1) vertical upwelling under buoyancy within the asthenosphere; (2) lateral migration along a low-permeability boundary (permeability barrier) inclined towards the ridge axis within the thermal lithosphere. However, in 3-D, the thermomechanical  structure of the transform will affect the melt pathway to the ridge axis and result in thickened crust. Laura and Laurent solve 3-D model based on a thermal structure that incorporates rheological feedbacks associated with brittle deformation and hydrothermal cooling, and emphasize the potential structural controls on melt extraction. Their results show that an MEZ, interpreted as structural damage such as faults and dikes, intersects the permeability barrier, redistributes excess ridge crust to the transform domain, and accounts well for the unusual crustal thickness at intermediate and fast slipping transform faults observed.

Conceptual geometry of melt extraction zone (MEZ) (A) ssociated with a fast-spreading mid-ocean ridge, (B) associated with a transform fault.

(A) Isoviscous model results for crustal thickness for the simplified ridge-transform cases and varying the distance of lateral extraction; (B) schematic representation of crustal accretion at ridge segments with no shunting; (C) Schematic representation of crustal redistribution into the transform domain when shunting is allowed.

However, at ridges with slow and ultraslow spreading rate, no thickened crust has been observed along transform faults. Thus, more questions emerge. If there is any MEZ around slow to ultraslow slipping transform? What prevents these MEZs from acting efficiently? What’s the relationship between the depths of MEZ & permeability barrier and the spreading rate? My research will be focusing on these questions and I will be modeling the mid-ocean ridge systems with different spreading rate, and finally find out the reasons for the different crustal thicknesses among transform faults with different slipping rates.

Reference

Hebert, L. B., and L. G. J. Montési (2011), Melt extraction pathways at seg- mented oceanic ridges: Application to the East Pacific Rise at the Siqueiros transform, Geophys. Res. Lett., 38, L11306, doi:10.1029/2011GL047206.

The Strength of Mineral Growth

Salt crystal growth has the power to destroy cinderblocks, roadways and building walls from the inside out.  Surprised well take a look at this:

Figure 1: Sample of Cordoba Cream limestone exposed to Sodium 

Sulfate solution for 47 days. [Image source: G. Scherer]

This cinder block was infiltrated by a sodium sulfate solution through capillary action.    Tiny grains of salt crystals grew both on the inside and outside of this block.  The ones that grew from the internal pore spaces caused all of the damage seen.  The engineers performing this experiment were determining the conditions for which crystal growth will occur within the blocks causing instability and ultimately failure.  As you can see they definitely achieved their goal.  It was determined that the presence of a supersaturated fluid alone would not allow the crystals to continue their growth beyond simply filling the pore spaces of the material.  There must be a disjoining force to prevent the crystal from coming in direct contact with the pore space walls, allowing continued replenishment of the source fluid and thus continued crystal growth.

Figure 2: Views of typical crystallization patterns [Image source: C. Noiriel et al.]

  

So how does this knowledge help a geologist?

            Well directly it doesn’t.  Beyond the immediate crystal growth occurs within pore spaces of material.  But with a deeper look into their study you can find applications for it that may have a great deal of interest for geologists; particularly in the fields of carbon sequestration and possibly even in the oil industry regarding fracking.  In both of these fields instead of allowing a fluid (oil) to rise to the surface we are artificially forcing fluids into a confined environment.  The pore spaces in this environment are not filled with air as one might think instead they are usually filled with a brine solution with a very complex chemistry.  So what happens when this brine solution gets exposed to mixtures that are foreign to the confining pressures of such an environment?  Well that is not fully understood but one scenario that may occur is the dehydration of the brine resulting in crystal growth within pore spaces….  That sounds familiar.

Figure 3: X-ray tomography of sample ADA-1 depicting

internal crystal growth[Image source: C. Noiriel et al.]

            However simply translating the engineers work to a completely different set of conditions is not practical as there are numerous variables that do not match with their study.  One such variable is the confining pressure under which these crystals may grow.  Fortunately for us we have the ability to simulate these pressures here at Maryland.  If crystal growth does occur to the point of cracking the pore space walls could this lead to instability within the reservoir?  Well with any luck we will be able to begin to better understand at least this part of the question.

Image Sources and Further Reading

Scherer, G.W., 2004, Stress from crystallization of salt, Cement and Concrete Research, 34(9): 1613-1624.

Noiriel, C., Renard, F., Doan, M-L., Gratier, J-P., 2010, Intense fracturing and fracture sealing induced 

by mineral growth in porous rocks, Chemical Geology, 269(3-4): 197-209.

Measuring both major and trace element concentrations using LA-ICP-MS

    All mass spectrometry can only measure isotopic ratios. To obtain element abundances from isotopic ratios, we need two kinds of standards–external standards and internal standards. External standards, with known composition, provide the coefficients that convert signal strength (counts per second) to chemical concentration. These coefficients are then applied to samples to determine the concentrations of unknowns. However, these coefficients may vary from standards to samples. For LA-ICP-MS, such variation is caused by the inconstant laser ablation rate on different matrix and instrumental drift. This is why we need internal standards, which correct the variation (Fig. 1). An internal standard is usually an element with known concentration in both standards and samples. To learn the concentration of an internal standard, EMPA is applied prior to LA-ICP-MS analysis.

Fig. 1 A schematic flow of LA-ICP-MS analysis.

    Following Liu et al. (2008), I am now developing a new LA-ICP-MS analytical method that replaces the conventional internal standard, which is one element, with the sum of major element oxides. This enables us to measure both major and trace elements by LA-ICP-MS alone in that the sum of major element oxides is approximately 100% (assuming H₂O, CO₂, halogens, etc. contribute little). Since the major difference lies in the internal standard, my task is to examine whether this new internal standard can effectively correct the variation in signal-strength/concentration coefficients, and which external standard would be the best with regard to both accuracy and precision.

Fig. 2 Plot of deviation using BHVO as external standard for major and rare earth elements. The pink shaded area denotes 10% deviation area from referred values.

    Five reference glass materials, including BHVO, BCR-2G, BIR-1G, KL-2G and ML-3B, were analyzed in our work. In each session, one of the glasses was assigned as the external standard while the rest as samples. As for precision, 1 σ uncertainty is better than 4% for most elements. Accuracy, however, is not as good. In Fig. 2, deviation relative to referred values frequently falls outside the ballpark (±10%), especially for rare earth elements. In order to monitor potential isobaric interferences, many elements were measured by more than one isotope. Isotopic ratios were plotted in Fig. 3, with true values plotted at the end of each series. Obviously, isotopic ratios obtained in my experiments agree well with the true values, which indicates that isobaric interference may not be a significant issue in my experiments. So far, it remains unknown what caused the accuracy problem in this work.

Fig. 3 Plot of isotopic ratios as monitors of isobaric interference.

Reference

Liu, Y-S., Hu, Z-C., Gao, S., Gϋnther, D., Xu, J., Gao, C-G., Chen, H-H., (2008) In situ analysis of major and trace elements of anhydrous minerals by LA-ICP-MS without applying an internal standard. Chem. Geol. 269, 237-251.

JOURNEY THROUGH TIME WITHOUT OXYGEN or HOW FAR IT CAN BE TO GET DEEP KNOWLEDGE

             

As everybody knows, oxygen is an essential gas both for plants and human or animals to live. “Is it so important to know?” – maybe someone asks. Really, YES, because if without food human can live about 1 month, without water up to 1 week but without oxygen just 5 minutes. So, our dependence on oxygen makes to think about its distribution in air and causes of its appearance in an atmosphere. “Is it important to know why oxygen appears?” – maybe asks other body. Apparently, yes, because knowing reasons of oxygen inception we can predict its future and naturally fortune of mankind. 

As history of Earth numbers 4,6 billions years scientists need to travel through time using records (sedimentary and volcanic rocks) remained from deep time to understand processes occurred on the Earth’s surface. For this purpose in summertime 2007 a group of people set off for a time travel called FAR-DEEP (The Fennoscandia Arctic Russia - Drilling Early Earth Project) in Arctic Russia as place where records are well-preserved. This journey was conceived by Professor Victor A. Melezhik from Geological Survey of Norway, Trondheim and University of Bergen. The main idea was to understand processes and events which occurred during the Archean- Palaeoproterozoic (AR-PR) transition period (2,500-2,000 million years ago (Ma)) when as believed the oxygen level was dramatically increased. On the Figure 1 you can see places of drilling where volcanic-sedimentary consequences are presented. This is Murmansk region and Karelia located on the Fennoscandian shield where summer is mostly like winter in Washington, DC. Latitudes of these regions equal to latitudes of Alaska in the USA and geologists needed to not only survive but work very hard to get core material (Figure 2).

Well, FAR-DEEP fieldwork started and finished in 2007, where I also took part for 2.5 months to carry out the documentation of the core. There was a great experience of fieldwork and a collective collaboration. During the fieldwork stage our group faced many difficulties needed to be overcome. We worked 7/24 in our field camp (Figure 3) to receive the core material and if your turn was to work at night be prepared to fight with unsparing wind and perishing cold.

  Even change the place was a big trouble because of absent of a road and slash. Eventually 15 holes were drilled to get 3650 meters core what sounds like a distance between the Lincoln Memorial and the US Capitol in DC but reflects 500 million years as 1/9 part of the all Earth’s history. After summer 2007 the core was brought to Trondheim for further sampling among collaborators and multidisciplinary studying.

During those 500 millions years many events and changes occurred in and on the Earth. Among them are:

1.           Global rifting (simply said huge cracks in the ground with the size of more then New York state) with the worldwide spread LIP (Large Igneous provinces) related with the richest deposits of copper, nickel, chromium, platinum etc.

2.       The GOE (Great Oxidation Event) when oxygen changed from almost 0% to 1% of present atmospheric level (now the oxygen concentration is 21%) that means that oxygen is presented in the atmosphere just for half of its history. And question is: “What trigger was to make oxygen to appear?”

3.             Signatures of the first petroleum generation (so-called Shunga Event). Petroleum which we use for human needs (e.g. plastic, gasoline, clothes and even cosmetics) and which price is a crucial factor in current economy as large deposits appeared about 600 million years ago but scientists found its trace 2,000 million years ago.

            Picture 4 represents other global events which like a mystery occurred at the same time near AR-PR boundary. Scientists want to understand which one was a cause and which one was a consequence.   

Figure 4. δ¹³C - time plot with main events happened at Palaeproterozoic time

          The collaborators involved in the project are from many countries (Russia, the USA, Norway, Germany, Scotland etc.) and they are doing research ranging from palaebiology to geochronology. Currently, the core samples are analyzing and comparing with similar succession from Gabon.

           Here are some links and references about this challenging Project:
Official page of the FAR-DEEP

One of the recent posts with nice pictures

Melezhik, V.A., Fallick, A.E., Hanski, E.J., Kump, L.R., Lepland, A., Prave, A.R., and Strauss, H., 2005. Emergence of the aerobic biosphere during the Archean-Proterozoic transition: Challenges of future research. Geol. Soc. Am. Today, 15:4–11. full text of the paper

Reconstructing the sedimentary diagenetic paleogeofluids!

As the petroleum exploration continues in Tarim basin, recently it has been found that the Cambrian and Ordovician petroliferous carbonate reservoir rocks develop abundant vugs and veins (Fig 1), resulting in good porosity for the hydrocarbon storage. Where do the fluids come from? What’s the source and geochemical composition of the diagenetic fluids? In which kind of mechanism did they alter the carbonate rocks? Can this kind of diagenesis result in considerable petroliferous carbonate reservoir rocks? These questions are still up in the air. “This is our focus right now. To figure out the cause of this kind of carbonate diagenesis and reconstruct the properties of the according diagenetic fluid is very important for our ongoing petroleum exploration!”, Wenqing Pan, the executive of  the petroleum exploration department of Tarim PetroChina Corporation, says in the departmental routine meeting.

Fig. 1 Various phenomena found in the outcrops. A, diagenetic altered dolostone; B, hand specimen of saddle dolomite, which is normally regarded as a typical product of hydrothermal dolomitization; C, silicified dolostone with quartz vugs in the hole; D, calcite vugs with clay inside; E, calcite and dedolostone; F, vugs and according thin section view under polarized light microscope.

The Peking University sedimentary geochemistry research group(PKU-SGRG) led by Professor Ping Guan, the chief scientist in Institute of oil and gas-PKU , decided to meet this challenge. Huan Cui, his graduate student, conducted most of this research project as part of his master degree thesis. In the field, they chose several outcrops (Fig. 2) and collected many samples for further indoor geochemistry analysis.

Fig. 2 The locations of studied outcrops in Tarim basin, NW China.

Through the petrographic studies with microscopes in the lab, it is found that the carbonate rocks in northern Tarim basin have developed a diagenetic paragenetic sequence of silicification, hydrothermal dolomitization and calcitization (Fig. 3). These three major types of diagenesis greatly altered the studied carbonate rocks.

Fig. 3 Thin sections of major diagenesis that caused the alternation of the studied carbonate rocks in Tarim basin. A-B are typical thin sections showing silicification diagenesis, C-D are thin sections showing hydrothermal dolomitization diagenesis and E-F are thin sections showing calcitization diagenesis, stained by Alizarin Red S. Dol-dolomite, SD- saddle dolomite, MD-matrix dolomite, Cal-calcite, Qz-quartz, DeDol-dedolomite.

To reconstruct the temperature and salinity of the paleofluids is important for our understanding of the diagenetic alternation. Using heating and freezing stage, Cui measured the melting and homogenization temperatures of fluid inclusions in the calcite, quartz and fluorite crystals and reconstructed the salinity and temperature of the diagenetic fluids that caused the alternation of the carbonate rocks (Fig. 4). Most of the silicification fluids show high salinity and high temperature (120-140℃), indicating a hydrothermal feature compared with the normal geothermal temperature. The calcitization fluids show high temperature but a little bit low salinity, indicating a mixture of high-salinity fluid and low-salinity meteoric water or formation water.

Fig.4 The measuring process of melting and homogenization temperatures of fluid inclusions. The measured homogenization temperature represents the simultaneous temperature of the fluids at that time. With the measured melting temperatures we can calculate the salinity of the fluids.

Many lines of evidences, such as the quartz fluid inclusion with a high salinity and homogenization temperature, the positive Eu anomaly in REE pattern of the saddle dolomite, and the calculated δ18OSMOW (+5.5‰–+12‰) (Fig. 5) of the fluid which precipitated the calcite crystals, all indicate that there exist magmatic hydrothermal fluids. 

Fig. 5 Using the fluid temperature got from the thermometric analysis of fulid inclusions in the calcite crystals and the oxygen isotopic values of the precipitated calcite crystals, we can calculate that the oxygen isotope of the fluids that precipitate the calcite crystals. The calculated value indicates that the equilibrium fluid is magmatic hydrothermal fluids.

Things revealed are far more complicated than our initial expectations. With carbon, oxygen and strontium isotopic analysis, it is also found that the hydrothermal diagenetic fluids have been mixed with organic carbon, meteoric water or the 87Sr rich formation water(Fig. 6) in different degree. 

Fig. 6 Altered carbonate rocks and calcite veins in different sections all show 87Sr/86Sr increase compared with the unaltered carbonate counterpart, indicating the diagenetic fulids are 87Sr rich, which may be an imprint of formation water source. LST-limestone, MD-matrix dolomite, SD-saddle dolomite, Cal-calcite veins, Dol-dolostone.

Based on the geochemical results, it is proposed that the carbonate rocks in the research area have an according diagenetic response to the “magmatic hydrothermal fluids-formation water compound fluid system”. The magmatic hydrothermal fluid is an important Si and REE source of the diagenetic fluid flow and is also a heat engine that drives the magmatic hydrothermal fluids and formation water cycle underground, consequently result in large scale of diagenetic alternations in the carbonate strata. The hydrothermal fluid dissolution indicates that the open fault area is a promising target for the petroleum reservoir exploration in the future. “The fault area in the seismic profiles is now our future target to find more oils!” Wenqing Pan says.

However, this is not an end, but a second new start! How to use the 3-D seismic data to predict this kind of reservoir rocks in a larger scale? Is there any appreciable difference in geophysical properties between the altered reservoir rocks and the surrounding unaltered rocks? Can we use seismic data to detect them deep underground? Is there any 3-D distribution pattern of different kinds of diagenetic alternations? These still need to be further studied. Together with the colleagues in the Institute of Geology and Geophysics, Chinese Academy of Science, funded as part of the National Key Research Project, the members in Professor Guan’s PKU-SGRG group are still on the exploring way!