Photomicrograph: Mineral Filled Vesicle

I came across this stunning image of a mineral filled vesicle on the September 2023 cover of Geology. The rock sample was collected from the Louisville Seamount Chain in SW Pacific Ocean.

 Source: Elmar Albers et.al. 2023- Timing of carbon uptake by oceanic crust determined by rock reactivity.

Vesicles in igneous rocks are spherical holes formed by expanding gas bubbles. As lava erupts, dissolved gases bubble out. Lava solidifies fairly rapidly on exposure either to air or water. The bubble shape is retained as a small cavity. It gets filled with minerals when magmatic fluids and mineral saturated seawater or groundwater circulate and react with the rock. 

The basalt rock in this study is about 50-74 million years old. The calcite in the vesicle precipitated within 8 million years of eruption. Alteration of undersea basalt is a CO2 sink. Basalt reacts with seawater, trapping carbon in carbonate minerals. The calcium required for formation of carbonate minerals is provided by the alteration of minerals like plagioclase. The study is trying to estimate how long such carbonation reactions continue. Carbonated oceanic crust eventually sinks into the mantle at subduction zones sequestering carbon from the surface for hundreds of millions of years.

This particular vesicle is filled with carbonate (calcite) and clay. Notice the beautiful banding suggestive of pulses of mineral formation. Among the brown and white layers are white bands of faceted saw tooth calcite. And the upper part of the vesicle is filled with large irregular shaped crystals. Surrounding the vesicle is the 'groundmass', made up of tiny crystals of plagioclase feldspar, iron oxide, and volcanic glass. There is no scale in the picture, but my guess is that the vesicle is a few hundred microns across.

In a hand sample a vesicular basalt will look like the example below. This is from the Deccan Traps near Pune. 

The vesicles here are much larger than the first example. Many are empty. Some vesicles have a lining of tiny crystals. Carbonation of terrestrial basalts also constitutes a carbon sink.  Combating global warming and achieving net zero emissions will require, foremost, a steep reduction in emissions, but additionally also removing carbon dioxide from the atmosphere and safely storing it in long term reservoirs. Such carbon removal and sequestration projects are exploring the potential of basalts and related igneous rocks as a long term carbon sink. 

Neglected Children Of The Sea Floor

Carl Simpson and Jeremy B.C. Jackson, in an essay titled Bryozoan Revelations for Science Advances, sketch out the important biological aspects of bryozoans, a type of marine invertebrate. 

From their article: 

"Bryozoans are neglected children of the sea floor. Google corals and you get nearly 4 billion hits, whereas bryozoans get just 4 million. This disparity reflects the enormity and notable beauty of coral reefs and extraordinary diversity of associated species that have long attracted intense scientific research. Yet for all their grandeur, corals occupy less than a tiny fraction of 1% of the global ocean, whereas bryozoans extend from the equator to the poles and intertidal to abyss. Bryozoans are species-rich. As a group, they have at least 10 times more species than corals. They also have a more extensive and continuous fossil record and have been major components of vast seafloor communities for half a billion years".

The enormous diversity of bryozoan skeletal shapes and their abundance as living communities and as fossils from the late Cambrian onwards make it possible to use them to understand evolutionary questions such as timing of origins, rates of change, convergent evolution, and origin of variable form in animal communities. This succinct essay summarizes these lines of research quite well.

As a sedimentologist my interest in these organisms was the role they played as sedimentary particles. Bryozoans, along with echinoids and brachiopods, were among the most common sea floor inhabitants of Paleozoic shallow marine realms. They made up a large proportions of the sediment that formed by the disintegration of shells and skeletons of sea creatures.  

A typical bryozoan skeleton has a trellis like appearance. Bryozoans are colonial organisms building a lattice like structure with the animal living in chambers called zooids. It is these zooids that was my main interest, or rather what I could see inside them. The animal was long gone, decayed away, but the chamber or cavity was filled with different types  of calcite, archiving the process of the transformation of loose sediment into rock. Through geologic history different types of fluids had entered the open spaces within the skeleton and precipitated different types of calcite. Petrologists can infer the changing geochemical conditions as sediments get buried and importantly trace the changes in open spaces, or porosity, as fluids either dissolve sediment or deposit new minerals, information that is useful to the petroleum industry.

I will share a couple of examples of these diagenetic changes observed inside a zooid.

This thin section of a limestone from the Middle Ordovician strata of the southern Appalachian mountains has been stained using a mixture of Alizarin Red-S and Potassium Ferricyanide. The pink is an iron free calcite. The purple in the center of the zooids is an iron rich calcite. This sequence from an iron free to an iron rich calcite indicates oxygen poor reducing conditions upon burial, a chemical environment in which iron can enter the growing calcite crystal.

And in this thin section the pale bronze colored mineral highlighting the skeletal frame is chert, a variety of silica that has partially replaced the calcite skeleton. The replacement process has been quite delicate, preserving the original structure of the skeleton.

Evolution is not the only story that bryozoans reveal.

Amorphous Precursors To Calcite Cements

Readers of this blog, I am sure, are familiar with terms like Agate, Jasper, Onyx, and Opal. Out of these, Opal is an amorphous variety of silica, where the silica and oxygen atoms are not attached to each other in a regular repeating geometrical pattern. Agate, Jasper, Onyx are varieties of silica that can show gradations from an amorphous form to being cryptocrystalline i.e. made up of tiny crystals. All these substances originate by hardening of a silica gel that congeals out of a silica supersaturated fluid which has separated from a magma, or from hydrothermal groundwater that has become enriched in silica by reaction with surrounding rock or soil. 

They occur as banded siliceous deposits, either as layers or as discrete nodules, in volcanic and sedimentary rocks. Amorphous silica can even be of biogenic origin. Planktonic creatures like Radiolarians have the ability to extract silica from sea water and use it to build its skeleton.

Posted below is a photomicrograph of a cavity in a sandstone filled with banded amorphous silica (center of picture). Notice the regular growth bands (left in plain polarized light)  and the silica fibers (in crossed nicols) that make up the fabric of the amorphous material. I happen to have this rock thin section in my collection , but unfortunately I don't know its provenance!

Transitions from amorphous to a fully crystalline silica (quartz) often occurs within the same rock cavity. Amorphous silica is quite stable and has been found well preserved in rocks hundreds of millions of years old.

In contrast, amorphous naturally occurring varieties of calcium carbonate seem to be exceedingly rare. In my more than two decades of following literature of sedimentary carbonates I have not come across a report of amorphous calcium carbonate cement or shell material. Until recently that is!

In the December 2020 issue of Geology, Sascha Roest-Ellis, Justin V. Strauss and Nicholas J. Tosca suggest that certain types of microspar cements in Tonian age Neoproterozoic limestones (~650 million years old), formed from an amorphous precursor stage. These microspar cements (fine grained calcite) are quite common in rocks of this time period, yet their mineralogical evolution and the geochemical conditions under which they formed is poorly understood. 

In an effort to understand the origin of these cements, synthetic sea water was prepared of a composition that was similar to that measured from fluid inclusions trapped in Neoproterozoic salt deposits. The finding was that the presence of PO4 above a value of 12 micromoles per liter inhibits the nucleation of crystalline forms of calcite and permits deposition of an amorphous Ca-Mg- Carbonate by production of dense liquid droplets once carbonate supersaturation exceeds a threshold value. Neoproterozoic sea water was rich in PO4 as evidenced by the trapped fluid inclusion composition and by calcium phosphate biomineralizing organisms of that age. 

The texture and chemistry of these microspars also suggest an amorphous precursor. The crystals have spheroidal cores which are likely remnants of immiscible liquid/gel particles that would have initially separated out from carbonate saturated sea water. The grain size distribution points to crystal growth by  Ostwald ripening, a process whereby smaller gel particles or droplets disaggregate and the chemicals are reconstituted into larger growing crystals. Furthermore, these calcites have an enhanced strontium content. Usually that occurs if they have originated from an earlier aragonite phase. But there is no sign of relict aragonite in these cements. An alternate explanation is the incorporation of strontium into an amorphous carbonate which also favors intake of strontium.

The amorphous phase does not exist today in these limestones, having recrystallized to a variety of calcite fairly rapidly, perhaps even within a few days or weeks of it forming. The photomicrograph below shows the microspar calcite hypothesized to have recrystallized from an earlier amorphous phase.

Source: Experimental constraints on nonskeletal CaCO3 precipitation from Proterozoic seawater - Sascha Roest-Ellis, Justin V. Strauss and Nicholas J. Tosca, 2020.

As it happens I've had two strikes in the past month. Subir Sarkar and colleagues in their analyses of the Cretaceous age Garudamangalam Sandstone from Ariyalur in Tamil Nadu mention that some cavity filling calcite cements developed from a gel, by which I assume they mean amorphous calcium carbonate.  But they don't pursue this aspect any further in their study. 

The absence of amorphous calcium carbonate is limestones, both ancient and recent, is likely because it forms under only very restricted chemical conditions where nucleation of aragonite and high magnesium calcite is inhibited by the presence of ions like PO4, and because its high reactivity results in it transforming quickly to crystalline calcite, erasing itself from the rock record. 

Geological discovery relies heavily on direct observation and measurements of rock/mineral material. But what about ephemeral substances? How does one imagine them and tease out their history? This study highlights the importance of experimental work in geology, where careful laboratory reconstruction of past conditions can throw light on mineralization pathways that have left no physical trace behind.

#Neatrock Entry For Earth Science Week

SciFri Science Club is hosting a #Neatrock challenge as part of Earth Science Week.

Here are my two entries:

Megascopic #neatrock:

This is a migmatitic gneiss from the Greater Himalayan Sequence, Darma Valley, Kumaon Himalaya. Migmatite means a mixed rock made up of a metamorphic host and a newly formed igneous rock. During continental collision, metamorphic rocks buried to great depths and subject to high temperatures may partially melt to form granite magma. The granitic melt segregates into layers. The resultant rock is composed of the original metamorphic host rock such as a gneiss (dark bands)  and granitic igneous layers (lighter bands). This migmatite formed during the Miocene.

Microscopic #neatrock:

This photomicrograph of a Late Ordovician limestone (Fernvale Limestone) from Georgia, U.S.A.  is close to my heart. It formed an important part of my PhD work.  I have stained the thin section with a Potassium Ferricyanide dye. Calcite containing minor amounts of iron (Ferroan calcite Fe+2) is stained blue. Non Ferroan calcite is unstained.  In the center of the photomicrograph is a non ferroan 'dog tooth' spar. It is a calcite crystal with a shape resembling a canine tooth of a dog.

This calcite has a pendant habit. It is hanging from the underside of a particle, in this case a piece of an echinoid shell. Such pendant crystals precipitate in a vadose zone i.e. above the water table.  In this environment, pore spaces are not completely filled with water. Rather, films of water coat grains and form drips. These drips become saturated with calcium carbonate and calcite precipitates from them.  Just like a larger and more familiar stalactite in a cave! Except that this micro-stalactite in tiny..tiny.

Development of a vadose environment indicates that sedimentation was interrupted by a large sea level fall. The sea bed got exposed to rain and a fresh water aquifer developed in the sedimentary deposits.  A tiny 'dog tooth' spar can tell us a fair bit about sedimentary basin evolution and sea level history.

Photomicrograph: Authigenic Feldspars From The Neoproterozoic - South India

Feldspars (plagioclase and alkali feldspars) are the most common minerals in the earth's crust. The vast bulk of them crystallize out of magma and lava. Feldspar also forms during metamorphic reactions. In sedimentary rocks they are commonly seen as detrital grains in sandstones. What is less appreciated is that they can also grow de novo in sediments during diagenesis i.e. during chemical reactions that take place as loose sediment reacts with fluids and gets transformed into rock.

  Authigenic twinned euhedral feldspar cross cutting mud clast

I noticed some lovely examples of such diagenetic or authigenic feldspars from the Neoproterozoic Banganapalli Formation from the Cuddapah Basin in South India during my M.Sc. dissertation project work. I recently got a chance to photograph my old thin sections again and I am posting some more photomicrographs of these authigenic feldspars below.
.
The Banganapalli Formation also termed the Banganapalli Quartzite is made up mostly of conglomerates and sandstones. They rest with an unconformable contact on the Paleoproterozoic Tadpatri Shale. There is spatial variability in the composition of the Banganapalli sediments. In my study area south of the village of Gani in Andhra Pradesh, the conglomerates and sandstone interfinger with limestones. These limestones appear a light purple in outcrop and are made up of carbonate mud with intermittent conglomerate layers and lenses of quartzite and jasper pebbles and cobbles, thin bands and layers of quartz sand along with limestone and siliciclastic mud intraclasts often showing a chaotic fabric (left). Fine clay layers are dispersed through the succession.

Near the contact between the Banganapalli limestones and the overlying Narji Limestone is an  intraclast conglomerate layer (right) made up of carbonate and siliciclastic intraclasts indicating rapid lithification of the sea floor and the subsequent disruption of hardened sea floor crusts during storms and seismic events.

The authigenic feldspars are present in this Banganapalli limestone succession. Feldspars are euhedral (well formed facets) and show contact twinning (separate crystals grow symmetrically forming mirror images across a common plane). They contain inclusions of calcite and clay. Mineral composition studies have shown that authigenic feldspars are either albite (sodium alumimium silicate) or orthoclase (potassium aluminium silicate). At that time (in the late 1980's)  I did not have access to an electron microprobe to accurately ascertain the composition of these feldspars. The twinning exhibited by these feldspars suggests to me that these are albite.

The feldspars cross cut intraclasts (arrow)

and calcite veins (arrow)

They grow in the carbonate mud matrix (arrow)

When did they form?

Feldspar crystals cross cut fractures and veins filled with calcite. This implies that they formed after lithification of the sediment. This could have occurred very early in the sediment history. Sea floor cementation and lithification of carbonate is commonly observed from the Proterozoic, which had calcium carbonate supersaturated oceans. The presence of intraclast layers with chaotically oriented clasts and cracks filled with detrital silica pebbles and sand indicate early lithification and disruption of sea floor. Calcite veins may simply suggest hardening and breakage of lithified sediment layers.  Sea water percolating through cracks in this early lithified sediment would have supplied sodium to the growing feldspars.

Alternatively,  the feldspars formed later under burial conditions. The Banganapalli Formation is made up of immature sandstone bodies containing plagioclase and alkali feldspar detrital grains. They show signs of dissolution and corrosion during diagenesis (in image below arrows point to partially dissolved feldspar grains).

Alkali released from the dissolution of detrital feldspars was transported by groundwater flow and used up in the growth of authigenic feldspars in adjacent limestones.

I'll leave the question open.

There are other interesting diagenetic features too in these sediments. The siliciclastic mud intraclasts show alteration to chlorite and glauconite and there is extensive neomorphic recrystallization of carbonate mud.

Authigenic feldspar are reported from sandstones too. They usually occur as tiny overgrowths on detrital feldspar grains. They are less well known from limestones. So, these unusually large euhedral authigenic feldspars stole the show for me.

Finally, the satellite image below shows the location of the limestone layers (black arrow) containing these authigenic feldspars. They occur on the south dipping limb of the Gani-Kalava anticline near the town of Nandayal.

Photomicrograph: Treasure Inside A Brachiopod Shell

Couldn't help posting this picture. I am currently creating a catalog of carbonate textures and diagenetic fabrics for the geology department at Fergusson College, Pune, which I hope will be used as a teaching aid.

This photomicrograph captures the inside of a Mid Ordovician brachiopod shell. A complex cement sequence is present inside the pore space. The sequence represents passage of the sediment from depositional marine settings to later deep burial depths. During that long journey the sediment encountered fluids of different chemical make up resulting in the precipitation of different cement types.

Pure magic!

Photomicrograph: Super Mature Quartz Arenites From Proterozoic Cuddapah Basin

One of the vivid memories of my Master thesis fieldwork in South India were a series of brightly reflecting hills. In the afternoons, the bare slopes of the hills were a blinding white and you had to wear dark sun glasses to minimize the glare.

These hills were made up of the Paniam Quartzite. This sedimentary sequence is part of the Neoproterozoic Kurnool Group which represents one megacycle of deposition in the long lasting Paleoproterozoic to Neoproterozoic Cuddapah Basin.  In sedimentary petrology terminology these white and bright sediments are quartz arenites, rocks made up mostly of the mineral quartz. In fact, they were super mature quartz arenites, i.e. they were made up of more than 90% quartz. I  point counted several samples and the percentage of quartz was around the 95%-96% mark.

Here is what they look like under the microscope. Notice how rounded the quartz grains are.

The white arrows in the photomicrograph below points to quartz cement which has precipitated between the grains. These cements are called overgrowths. They maintain the same optical orientation as the substrate quartz grain and hence in cross polarized light the detrital grain and the overgrowth appears as a single crystal unit. The detrital quartz grain is outlined by iron oxide dust which helps demarcate the contact between the grain and the later cement.

Here is another example of a super mature quartz arenite. The contact between the detrital grain and cement is again marked by a coat of dust. Notice the planar crystal facets of the quartz cement (white arrow) which contrasts nicely with the rounded detrital particles.

 
The example I have presented show only one generation of quartz overgrowth cement. There are instances where two generations of quartz overgrowth cements are present. Like the detrital grain, the first generation overgrowth has a coating of iron oxide or clay and is abraded. This indicates that the quartz grains have been derived from the erosion of older silica cemented sandstones. The original source of the quartz in these older sandstones were igneous or metamorphic rocks. After being eroded from these rocks and then transported and deposited, the quartz grains were overlain by silica cement (the first generation cement) and lithified into a sandstone.

Later (perhaps tens of millions of years later), this sandstone was uplifted and eroded. Disaggregation of grains during weathering broke of quartz sand particles along with attached fragments of cement. This cement overgrowth then got abraded and rounded during transport and acquired a dust coat. In its final site of deposition it was overlain by new silica overgrowth (the second generation cement). Abhijit Basu and colleagues present an interesting example of such "second cycle" or "recycled" quartz arenites from the sedimentary sequences of the Bastar Craton from Eastern India (Image to left: source Basu et al 2013).

A careful examination of quartz arenites and generations of silica cements can reveal a lot of useful information about uplift, erosion and recycling history of the earth's crust.

Quartz arenites are not restricted to the Proterozoic. They are common in younger age Paleozoic, Mesozoic and Cenozoic deposits too. They occur only sporadically in Archean age deposits. Thick sequences of quartz arenites become more common in the Proterozoic. This increase in the occurrence of quartz arenites in the Proterozoic has to do with the changing tectonics of the earth.

Among the common rock forming minerals, quartz is relatively chemically inert and is more resistant to physical breakdown during weathering and transport. In the Archean, sedimentary basins were generally linear troughs formed in front of island arcs. Due to these tectonically active conditions, the basin floor subsided quickly and detritus derived from weathering of igneous and metamorphic source rocks was deposited and buried before physical attrition and chemical dissolution could remove unstable minerals. The result was a mineralogically "immature" sandstone with the framework of the rock made  up of  quartz, feldpsars and volcanic and metamorphic rock fragments in different proportions . The sediments and associated volcanic material frequently got metamorphosed to a low grade "green mineral" assemblage of chlorite, actinolite and epidote. These deformed and metamorphosed successions embedded in Archean gneiss terrains are known as greenstone belts.

There are a few instances of quartz arenites in the Archean from terrains of the Canadian Province, the Baltic Shield in Russia and from the Bababudhan Group of the Dharwar Greenstone belt in South India. Many of these have been interpreted as a product of intense chemical weathering in Archean soils, wherein unstable pyroxenes, feldspars and meta-igneous and meta-sedimentary rock fragments were leached away, leaving behind a quartz rich residue. Sedimentary structures like cross bedding and ripple marks indicate shallow water environments of deposition where the sand was further subjected to physical attrition leaving behind a quartz rich sand deposit.

Such conditions of longer residence time and more intense chemical weathering in soil profiles and long periods of attrition and physical sorting by wave and tidal action became more common in basins of Proterozoic age. Phases of prolonged magmatism and heat loss from around 3 billion years ago to 2 billion years ago resulted in a cooler earth and one that now was made up of large rafts of granite/granodiorite crust which was buoyant and tectonically stable. Although the boundary between the Archean and the Proterozoic is pegged at around 2.5 billion  years ago, basin tectonic styles do not change abruptly. These were evolving conditions.

In Peninsular India, Proterozoic age sediments were deposited in two types of basins manifesting different tectonic styles. "Mobile Belts" are reminiscent of the older Archean greenstone belts in that they were tectonically active elements of the crust, perhaps forming in subducting settings at the boundary between two cratonic blocks. They are depressions which contain abundant volcano-sedimentary successions made up of volcanic flow and ash beds interlayered with  immature sand and mud and chemically precipitated silica and iron oxide layers.  These are interpreted as deeper water deposits. Some basins contain stromatolite limestone/dolomite. There are a few quartzite deposits too.  These may be the metamorphosed equivalents of quartz arenites which were deposited in shallow water environments.  These successions were subjected to metamorphism, deformation and intrusion by granitic plutons during orogenic episodes forming the "mobile belts" or fold belts.  The Aravalli and Delhi Group of sediments which make up the Aravalli mountain ranges in Rajasthan are a good example of these Early to Mid Proterozoic mobile belts.

Overlapping in time with the mobile belts but extending into the Neoproterozoic are the epicratonic basins, also known as the "Purana" basins. These basins experienced less volcanic activity and were subjected to less deformation and metamorphism than that seen in the mobile belts. They contain thick sequences of quartz arenites and limestones.  These basins were initiated by extension and rifting of the continental crust resulting in extensive shallow marine shelf areas.  Low relief Archean to Early Proterozoic source terrains made up of granitic and metamorphic rocks were subjected to intense chemical weathering. Since the basin floor subsided slowly in these passive margin basins, shallow water conditions prevailed for long periods of time. Quartz rich residues were transported and deposited as sand sheets in beach and tidal flat settings and as sand shoals in more open waters away from the shorelines. Wave action further sorted them into a texturally mature sand.

The Paniam Quartzite, whose afternoon glare I tried in vain to avoid, is a remnant of one of these vast sand sheets that occur across many epicratonic Proterozoic basins in Peninsular India. Other examples of this stable cratonic style of deposition include the Vindhyan Basin in Central India and the Kaladgi and Bhima Basins of South India.

The satellite image below shows the brightly reflecting slopes (white arrows) of this quartz arenite deposit around the village of Gani in Andhra Pradesh. The black dotted line is the contact between the Archean basement and the overlying Proterozoic Cuddapah Basin sediments. The linear structure is the left lateral Gani Kalava fault offsetting the Cuddapah Basin.

And here is one final photomicrograph of the Paniam quartz arenite showing well rounded detrital grains with faceted quartz overgrowths meeting in planar contact in the pore spaces.

Photomicrograph- Ooid Growth Over A Foraminifera Nucleus

This week's microscopic view is from the Mississippian limestones exposed in the southern Appalachian mountains of Alabama.

Ooid cortices made up of tiny radial crystals of calcite growing around a foraminifera nucleus.

I will be putting up a longer post on ooids and oolite facies as important contributors of carbonate sand through geologic time and their significance as indicators of sea water chemistry and ecologic disruptions.

Stay tuned.

#thinsectionthursday

Photomicrograph: Botryoidal Silica And Dolomite Cement In Proterozoic Sandstone

This week, a gorgeous example of botryoidal and banded silica cement filling pore spaces in Proterozoic sandstones from Central India.

The sandstone has a complex history of cementation. Pore spaces are filled with dolomite or siderite, chalcedony and calcite.

Isolated dolomite rhombs (image above) were the first mineral to precipitate around quartz grains, growing inwards into pore spaces. Another strong possibility is that the rhombs are the mineral siderite which is the iron carbonate FeCO3. Siderite often alters into a mixture of hydrated iron oxides known as limonite which preserves the shape of the original siderite forming pseudomorphs.

Silica precipitation was either contemporaneous or succeeding the dolomite/siderite cements. Occasionally, silica cements cross cut the iron carbonate (image above, white arrow), indicating that at least some silica was introduced after the dolomite/siderite.

Finally, calcite cement filled the remaining open spaces.

 
It replaces the dolomite/siderite cement (top image, white arrows) but retains the iron oxide bands thus preserving the original shape of the dolomite/siderite crystals.

Calcite also cuts across (bottom image, white arrows) the silica geodes.

#ThinSectionThursday

Photomicrograph- Micro Fault Displacing Proterozoic Stromatolite Laminae

From the Paleoproterozoic Vempalle Dolomite near the village of Gani, Cuddapah Basin, South India,

This was my M.Sc dissertation area. Vempalle Dolomites got me fascinated with carbonate rock textures and diagenesis.

The image shows a micro fault displacing stromatolite laminae. Stromatolites are biosedimentary structures formed when sediment is either trapped within microbial sheets or when CaCO3 minerals like aragonite precipitate around the sheets that cover the sea floor. The microbial colonies grow in a variety of shapes and structures in response to the wave energy conditions. Flat sheet like structures like the one seen in outcrop from where I sampled this rock indicates a low energy regime.

Of interest here:

a) The presence of oolites associated with these lamellar stromatolites. Oolites form in high energy conditions where sediment grains are constantly rolled around and held in suspension for periods of time. This allows layers of calcium carbonate to precipitate around a nucleus resulting in a coated grain containing concentric rings of CaCO3. The presence of layers of oolites in a lamellar stromatolite rock suggests that oolites forming in high energy tidal channels and shoals were transported by storms onto adjacent lower energy settings such as these microbial covered tidal flats.

b) There is variation in the shape and size of dolomite crystals. This variation is not randomly distributed but is fabric selective. The fine grained stromatolite laminae has been replaced by fine grained dolomite. There is some patchy neomorphic (recrystallization) growth of this dolomitized mud into coarser irregular dolomite.  Pore spaces and sheet cracks and fractures are filled with coarser irregular shaped dolomite crystals.  Rhomb shaped dolomite crystals are associated with oolites. This suggests that the rock underwent multiple episodes of dolomitization. The fine grained stromatolite aragonite mud got replaced early by very fine grained dolomite crystals. Contemporaneously, sheet cracks and pores filled with a coarse irregular shaped dolomite crystals.  Both the saturation levels of the replacing fluid and the abundance of nucleation sites affect dolomite crystal shape and size. Finer grained substrates offer abundant nucleation sites resulting in finer grained dolomite. Crystals growing from supersaturated fluids form quickly and interfere with adjacent crystals resulting in irregular shaped interlocking textures.

Oolites made up of either aragonite or high Mg calcite crystals were replaced by rhomb shaped crystals. Rhombic shapes form when dolomite replaces coarser grained substrates or precipitates from fluids which are mildly saturated. In such instances there are fewer nucleation sites and individual crystals have a degree of freedom to grow crystal facets.


There is also chert (microcrystalline silica) in this rock. Its replaces oolites and is present in pores spaces and in fractures.

#ThinSectionThursday

Photomicrograph- Marine, Meteoric And Burial Carbonate Cements

JSR Paper Clips in their "A Look Back" series highlights an influential paper by J.A.D. Dickson on the use of staining of carbonate rocks to differentiate in a thin section the different mineral phases of calcium carbonate.

A staining procedure consisting of preliminary etching with dilute hydrochloric acid, treatment with a mixed solution of alizarin red-S and potassium ferricyanide, and a final treatment with alizarin red-S alone (Dickson, 1965) permits the distinction of orthorhombic carbonates and of calcite from other trigonal carbonates. The potassium ferricyanide stain reveals the distribution of iron in both calcite and dolomite. The use of the stains is illustrated by a discussion of the petrography of selected specimens and interpretations of the origin of various petrographic entities.

I am heartily thankful for this technique. I stained literally hundreds of thin sections of Ordovician carbonates for my PhD work. It helped me understand the changes in cement types and their chemical composition as the limestones passed from a marine setting to becoming a freshwater aquifer during sea level drops to their ultimate burial to depths of hundreds of feet where they encountered Mg rich brines from which precipitated the mineral dolomite.

Here is that sequence brought out so clearly by a mix of Alizarin Red S and Potassium Ferricyanide.

1) Bladed crystals of non ferroan marine calcite nucleated on a brachiopod shell (stained pink)
2) Equant crystals of ferroan calcite precipitated in a confined fresh water aquifer that formed during a late Ordovician sea level drop (stained purple)
3) Rhombic crystals of a non-ferroan dolomite precipitated during deep burial (not stained). This dolomite cuts across the early marine and later ferroan calcite cements.

... my series on photomicrographs of carbonates will continue...

Photomicrograph- Late Ordovician Calcite Cement Stratigraphy In Cathodoluminescence

Cathodoluminescence (CL) brings out beautifully the hidden growth history of calcite crystals. This photomicrograph is of a Late Ordovician pore space from the Fernvale Limestone, Georgia, Southern Appalachians. It is showing calcite cement grown syntaxially over echinoid fragments. Echinoid skeletons are monocrystalline. A syntaxial overgrowth means that pore filling precipitated calcite has maintained the same crystallographic orientation over this monocrystalline substrate. As a result, successive crystal masses even if precipitated at different times under different conditions appear to be one continuous block under polarized light and under crossed nicols. It takes CL to reveal these different growth phases.

The black growth zones were precipitated in oxidizing conditions by fresh water in the vadose zone (above the groundwater table). The black zones are pendant, hanging on the underside of skeletal grains. They are in essence micro-stalactites.

This was followed by another growth phase in suboxic conditions with the incorporation of divalent Mn(+2) in the calcite lattice. Divalent Mn is an activator of CL, hence the bright yellow growth bands interspersed with a thin black bands indicating periodic return to Mn poor oxidizing conditions.

The last phase is a pore filling phreatic ferroan calcite cement precipitated by reducing meteoric fluids in deeper burial conditions. Fe+2 is a quencher of CL. The cement appears dull brown.

The pore space is a couple of millimeters across.

#ThinSectionThursday