Secondary Mineralization In Deccan Basalts: Timing And Precipitation Environments

When and how did these minerals form?

Gargoti Museum

These are the world famous zeolites and other secondary minerals (green apophyllite) that fill cavities and cracks in Deccan Basalt lava flows.  Zeolites and other secondary minerals like apophyllite are calcium, sodium, potassium bearing alumino-silicates with varying amounts of water and other volatile elements like fluorine (apophyllite). They are prized by mineral collectors and by petrologists who study them to understand the geologic conditions that affected the lava after their eruption. This has broader implications for understanding the initiation and evolution of fluid circulation systems during the burial and exhumation of the lava pile.

A recent study has taken a step towards understanding the timing and precipitation conditions of these secondary minerals in basalts.

Exceptional Multi Stage Mineralization of Secondary Minerals in Cavities of Flood Basalts from the Deccan Volcanic Province, India - Berthold Ottens, Jens Götze, Ralf Schuster, Kurt Krenn, Christoph Hauzenberger, Benkó Zsolt and Torsten Vennemann. 

The most exciting part of this study is the publication of  absolute ages of mineralization of apophyllite using Rubidium-Strontium (Rb-Sr) and Potassium-Argon (K-Ar) radiometric methods. As far as I know , these are the first ever published absolute dates of secondary mineralization in Deccan lavas.

The scientists studied lava flows from the famous Savda quarries near the town of Jalgaon and also from quarries near the town of Nasik, both in the state of Maharashtra.The photograph below shows a portion of the lava sequence that was studied in one Savda quarry. The occurrence of secondary minerals in lava cavities is shown in the lava profile to the right. All the following images in this post are from the paper linked to above.

This study concentrated on examining the mineral sequences found in the large cavities in the central portions of a lava flow. Smaller amygdules (fully filled cavities) and vesicles (partially filled small cavities) occurring at the bottom and top of lava flows won't contain the entire sequence of minerals.

The investigation revealed three broad stages of mineralization. Stage 1 sequence precipitated first. It consists of an iron-magnesium and potassium bearing clay layer (containing celadonite and smectites) coating the walls of cavities and microbial films and filaments.  Stage 2 sequence consists of calcite (calcite 1), fine grained zeolites (zeolite 1), and plagioclase, followed by a layer of chalcedony and quartz.  This is followed by a second generation of larger calcite (calcite 2) and zeolite (zeolite 2) crystals. The common zeolites, both in zeolite 1 and zeolite 2 stages are heulandite, and stilbite with additional mordenite observed in zeolite 1 assemblage. Stage 3 sequence is made up of a third generation of calcite (calcite 3) , along with apophyllite and rare powellite (calcium molybdate). 

This multi-generation mineralization sequence could only be ascertained by carefully noting down the mineral sequences appearing in hundreds of different large cavities. Such a broad examination is necessary since the entire sequence may not have crystallized in any one cavity. The two pictures below demonstrate this problem.

On the left is a cavity which shows two generations of calcite (Stage 2) grown on a clay-chalcedony substrate (Stage 1). On the right is apophyllite (Stage 3) directly overlying Stage 1 mineral encrusted bio-filaments. In both these cavities, the Stage 2 zeolites have not precipitated. In some other cavity one might encounter a sequence of Stage 2 large zeolites overlain by Stage 3 calcite, but not the earlier calcite and zeolites, nor the Stage 3 apophyllite. Reconstruction of the entire sequence thus requires an examination of a large number of cavities in order to work out the true order of mineral succession.

This reconstructed mineral paragenetic sequence is presented in the graphic below.

The researchers used fluid inclusion studies and carbon and oxygen isotope analysis to narrow down the temperature conditions during precipitation (fluid inclusion) and the source of mineralizing fluids (isotope analysis).

Fluid inclusions are tiny volumes of fluid that get trapped during crystal growth. The fluid may be a liquid or a vapor or both. For ascertaining temperature of entrapment, a bi-phase inclusion is selected. The sample is heated until it reaches a temperature where the inclusions change from a heterogeneous (bi-phase) state to a homogeneous (one phase) state. This homogenization temperature (range) is taken to be the temperature of fluid entrapment i.e. initiation of crystal growth.

Fluid inclusion analysis of calcite 1, calcite 2 and quartz indicated temperatures between 94 deg C to 173 deg C. Inclusions in stage 3 calcite and apophyllites indicated higher temperatures between about 140 deg C to 244 deg C.

The carbon isotopes of calcites (presented as the ratio of C13 to C12) showed depleted values due to enrichment of the lighter C12 isotope. This was taken to indicate a substantial biogenic contribution for the carbon (the lighter isotope is preferentially taken up by organisms during photosynthesis). The oxygen isotope values for calcite 1 and calcite 2 were enriched in the heavier isotope O18. Values of delO18 were + 14 - +15 for calcite 1 and +19 - +27 for calcite 2. DelO18 is a measure of the ratio of the two isotopes of oxygen O18 and O16. Such enriched values indicate magmatic source fluids, although the slightly lighter values of calcite 1 suggests mixing with meteoric water, which is enriched in the lighter isotope O16 as compared to magma and sea water (meteoric water is derived from rainfall and ends up percolating through rock as groundwater). Isotope analysis of calcite 3 is not presented in this study.

Rb-Sr and K-Ar geochronology of apophyllites shows that precipitation of apophyllite took place repeatedly as discrete events spread over a large time span. The Nasik sample yielded ages of 58 mya (million years ago) and 21 mya. Apophyllites from Savda quarry near Jalgaon yielded ages of 45 mya and 27 mya.

The observed mineral sequence along with data from fluid inclusions, isotopes and geochronology have enabled the researchers to propose the following sequence of events depicted in the schematic below.

After the eruption of a lava flow, interaction of the hot lava surface with meteoric water resulted in filling up of cracks and cavities with water, accompanied by the alteration of mineral olivine, plagioclase and volcanic glass.  Fe, Mg, Si, Al released from the rock was recombined to form clay mineral and iron hydroxide coatings on the walls of cavities and on microbial films and filaments.

As the lava layer got buried under younger lavas, the composition and temperature of the fluids evolved resulting in the precipitation of calcite 1, fine grained zeolites 1 and chalcedonay and quartz. The oxygen isotope values of calcite 1 indicates a mix of magmatic residual fluids and meteoric water. Continued burial resulted in a diminishing contribution from meteoric water. Zeolite 2 and calcite 2 phases precipitated from magmatic residual fluids as indicated by the oxygen isotope values which are enriched in the heavier isotope. Maximum burial temperatures at this stage have been estimated to be about 150 deg C.

This interpretation of Stage 1 and Stage 2 mineral assemblages having formed immediately following volcanism and during burial is in line with previous thinking regarding secondary mineralization in Deccan basalts. Data about the timing of Stage 3 however has thrown up a surprise.

Geochronology indicates that precipitation of stage 3 minerals like calcite 3 and apophyllite took place much later. Deccan volcanism ended by 64 mya across most of the province. Mineralization ages of 47 mya and 27 mya indicate that by this time considerable erosion of the lava pile would have resulted in exhumation of deeper layers and much lower burial temperatures. The fluid inclusions in calcite 3 and apophyllite indicate crystallization at temperatures between 144 deg C and 244 deg C. Some earlier work by Shrikantappa and Mookherjee on fluid inclusions in apophyllite from Savda indicate even higher temperatures reaching 280 deg C. Such boiling conditions at a shallow burial level implies the formation of a hydrothermal system. The presence of powellite, a calcium molybdate, indicates oxidizing fluids. Such a system must have formed repeatedly at widely separated time intervals. The study does not put forth an explanation of the geological events that could have triggered the formation of these high temperature fluid circulation systems.

In summary, Stage 1 and Stage 2 involves a locally formed circulation system. Elements were scavenged from adjacent regions of the lava flow and incorporated into growing secondary minerals. Stage 3 involves a larger circulation system. The concentration of vanadium in basalts is low, in the range of few tens of ppm (parts per million). In Apophyllite, the concentration of vanadium is on the order of 3000 ppm.  This suggests that fluids attained this element concentration by circulating and reacting with a large volume of basalts over a widespread area.

The researchers have stressed that their proposed explanation applies to the specific mineral sequence observed at Savda quarries near Jalgaon and the lava flows near Nasik and is not to be taken as a general explanation for secondary mineralization covering the entire Deccan basalts. That would require much more extensive sampling from different regions and stratigraphic levels. The observations from other studies and the mineral sequence I have personally observed in the Pune area though does suggest that Stage 1 and Stage 2 sequences at least are common everywhere, although the specific combinations of zeolites may vary. Early near surface reaction of hot lava with groundwater and then progressive burial with mineralizing fluids getting contributions from both meteoric water and magmatic residual fluids would have been a common trajectory of fluid rock interaction across the volcanic province.

Such a timing of mineralization, contemporaneous with volcanism and continuing after burial, has been noted from lava provinces in Iceland and Iran too. The Stage 3 event though is much younger and would depend on later geologic triggers that may differ from place to place. In eastern Iceland for example, a late stage of mineralization has been linked to heat provided by the intrusions of dikes ( sheet like bodies of magma injected along fractures). The geochronology of apophyllites in the Deccan Volcanic Province needs to be validated by more such work.

Some questions do remain.

First, nearly 20 to 40 million years after Deccan volcanism ended, what could be the source of heat for the initiation of the late stage fluid circulation systems that precipitated calcite 3 and apophyllite?

And secondly, the lack of carbon and oxygen isotope analysis of calcite 3 prevents us from identifying the source of precipitating fluids. That is a lacunae that future studies must aim at filling.

References:

Srikantappa, C.; Mookherjee, A. Water, Aqueous, H2O-CO2 and Gaseous Inclusions in Cavity Minerals in the Basaltic Lava flows around Pune, India: Evidence for Boiling. In Proceedings of the Second Meeting of the Asian Current Research on Fluid Inclusions (ACROFI–2), Kharagpur, West Bengal, India, 12–14 November 2008; p. 176.

 

Konkan Road Trip Photos: Murud Dabhol Tural

Last week beginning Monday July 15th, I took a four day road trip to Konkan, India west coastal plains. We went first to the small village of Murud and then drove south via Dabhol to Tural highlands.

The phrase 'coastal plains' is something of a misnomer since between the high Western Ghats and the Arabian Sea there are hill ranges with altitudes reaching 50 m to 200 m ASL. Tural is a community living on one of these ranges. We stayed there in the family home of a friend.

The map below shows a portion of the Konkan region through which we traveled.

The region had come alive due to the monsoons, although that week we caught a small break in the rains. It did rain heavily in short bursts, but there were enough interludes to go for long walks and enjoy the sun too.

Some pictures of landscapes that we came across.

1) The coast near Murud. After a brutal summer, the feel of cool winds and sounds of monsoon waves crashing on the shore was very refreshing.

2) Lonely stretch of a shimmering beach near Murud.

3) Loading our car on to the ferry at Dabhol.

4) Colourful fishing boats at Dabhol jetty.

5) Continental erosion writ in mud! River Vashishti meets the Arabian Sea.

6) Rice fields in a quiet community in Tural highlands.

7) Tural highlands is capped by a flat surface.

8) This plateau cap is made up of iron rich laterite. It formed during late Miocene times (~10 million  years ago) by prolonged chemical weathering of the underlying basalt rock and pediment (layer of weathered rock debris) . The picture shows the hard laterite surface, which would have been a low lying peneplain in late Miocene times.

9) Subsequent to lateritization, the western margin (Konkan coastal region) underwent some uplift, resulting in the formation of a plateau or 'table land' as it is commonly called. As the land rose, invigorated streams cut into the laterite surface forming deeply entrenched channels.  The picture below shows a close up of the laterite plateau dissected by a dendritic stream network (blue arrows).

10) The evolution of the Konkan coastal region from a low lying undulating surface undergoing lateritization, to an uplifted and dissected plateau is depicted in the schematic below.

Source: Evolution of Laterite in Goa: Mike Widdowson  2009

11) The laterite is a commonly used building material in this region. Small quarries pockmark these highlands. The picture shows large bricks of laterite. The plateau cap is hard laterite that can't be cut into regular brick shaped pieces. Below this crust though is a softer iron rich soil. This semi indurated material is cut into brick shapes and left to dry. It hardens upon dehydration into a usable stone.

12) We took long walks in cool lush forest patches.

13) Deep in the forest we visited my friend's family temple, a hidden jewel with a spring fed bath. These temples act like a social glue, bringing families and communities together on religious and other occasions.

14) On the way back via Kumbharli Ghat we caught sight of the majestic Western Ghat Escarpment.
 

until next time! 

Groundwater Must Be The Focus Of India National Water Policy

India's Water Management Crisis

A piercingly clear essay by Himanshu Thakkar on why India must realign its water resources priorities from big dams and river linking projects to protecting, managing, and regulating ground water.

Just take a look at the numbers:

"Most of the water that India uses today comes from over 30 million wells and tubewells. Irrigation is India’s biggest user of water and over two thirds of irrigated area gets water from groundwater. 85% of rural domestic supply, over 55% of Urban and Industrial water supply comes from groundwater. The graph of % of water in each sub sector coming from groundwater has been going up for at least four decades. In fact, some estimates show that over 90% of additional water India used in last four decades have come from groundwater. It sounds like an immitigable blessing. That’s not how blessings work, unfortunately.

Central Ground Water Board’s data shows that in about 70% of areas, groundwater is depleting and at many places it has exhausted or is on verge of exhaustion. The quality is deteriorating. Warnings have been available for decades now, but the government has done little to address the emerging crisis.

In fact, India’s water resources establishment, lead by the Big dam ideologues at Central Water Commission have ensured that the government do not even acknowledge that groundwater is India’s water lifeline"....

Scary.

Some States have taken initiatives to manage ground water. Maharashtra recently passed the Maharashtra Ground Water Act which provides a framework for management and regulation of ground water. How much diligent enforcement of the rules actually takes place remains to be seen.

Additional Reading:

The Maharashtra Groundwater (Development and Management) Act 2009 - Shashank Deshpande, Deputy Director GSDA.

A Decade Of The Maharashtra Ground Water Legislation: Analysis Of The Implementation Process - Sanjiv Phansalkar and Vivek Kher.

Papers: Indus Civilization- Resilience, Fragility And Rural Complexity

Diversity, variability, adaptation and ‘fragility’in the Indus Civilization- Cameron A. Petrie

We are inheritors of a rural civilisation’: rural complexity and the ceramic economy in the Indus Civilisation in northwest India - Danika Parikh and Cameron A. Petrie

These two recently published papers are worth reading.

The first one reviews settlement patterns, water availability, agricultural strategies and craft production in urban and rural Indus settlements. It draws inferences on the type of power structures and hierarchies that may have prevailed within cities and villages and between different regions. And there is the perennial question on the link between climate change, water stress and the decline of urban sphere of the Indus civilization. There were different response from the urban and rural spheres to environmental stress, with the more flexible and adaptable rural lifeways showing more resilience and sustainability.

"Petrie et al. (2017; Petrie 2017) have suggested that the weakening of the ISM around c. 2200–2100 bc meant that the climate in the subsequent period became ‘unpredictably unpredictable’. By this we meant that before and during the Indus urban phase, populations were familiar with ‘predictable unpredictable’ conditions and their farming strategies were tailored to make use of water supplied by combinations of rainfall, inundation, small-scale irrigation and/or lifted water (cf. Miller 2006). Populations in specific areas across the Indus zone might have been able to survive one, two, or even more years of drought, either through reliance on their own resources, or through support from other regions. However, when this range was exceeded, such as when populations were faced with protracted periods of drought, the local and medium-to-long range provisioning and support networks may not have been able to sustain the status quo. I have suggested that in such a situation, farmers may have had to engage in constant risk mitigation, thereby reducing opportunities to produce surpluses, and in such situations it is possible that living in large groups (i.e. urban centres) was not an option".

The Indus cultural sphere lasted a long time after its cities declined. In the graphic below the upper left and right panels show distribution of settlements during the urban phase with modern winter (left) and summer (right) rainfall contours overlain. The bottom panel shows the post urban settlement patterns. There are denser habitations nearer the Himalaya front in the post urban phase. This shift from Rajasthan, Cholistan and Haryana eastwards and closer to the Himalaya foothills followed more reliable monsoons in that region. Gujarat on the other hand wasn't depopulated as much suggesting regional differences in monsoon strength and varied water harvesting strategies. However, the urban center of Dholavira and nearby settlements were abandoned.

Source: Cameron Petrie

Even the decline of the cities was not a sudden event. Indus societies did not collapse due to any one catastrophic environmental change such as one big river changing course or a very rapid decline in monsoon. Urbanization was at a peak between 2600 B.C and 1900 B.C. But at Mohenjodaro for example, signs of abandonment and depopulation begin by 2200 B.C. On the other, Harappa continued to be occupied throughout the urban phase and well into the late Harappan Phase, although analysis of skeletons do suggest increasing physical stress.

The second paper by Danika Parikh and Cameron Petrie concentrates on bringing out the complexities and variation in rural lifeways and economies. Ceramic products from four Indus age villages in Haryana are analyzed and described and some interesting inferences drawn on urban rural (in)dependence and the socio-economic role of villages in the larger Indus sphere.

"The regional rural ceramic economy innorthwest India was clearly complex and shows a considerable degree of variation. Rural communities produced some ceramic forms similar to Classic Harappan forms, and others that were quite different, and they used some decorativemotifs that were common and others that we had previously not seen. This pattern of similar ceramic forms but different techniques and decoration is particularly interesting, given what we understand of how pottery production is learned. Pottery forming is often learned through ‘vertical transmission’, inter-generationally; shape and decorative motifs are more easily imitated and are often transmitted horizontally, or peer-to-peer (Knappett 2011, 106–107; see also Gosselain 2000). The use of different techniques to produce the same forms suggests that Classic Harappan and Haryana Harappan ceramicmaterial was not produced in the same workshops, and that these potters are unlikely to have been members of the same communities of practice".

The rural populations were not only engaged in agro-pastoralism. Villages had varied occupations such as functioning as workshops for specialized craft production and as factory sites making goods for larger towns and cities.

Open Access.

Groundwater Worries: Saving Pune's Hill Slopes

 SCRAP HCMTR - MAKE PUNE A MODEL FOR SUSTAINABLE DEVELOPMENT

I wrote a short note on my Facebook page on Pune groundwater and its growing importance in response to a citizen's petition to save natural recharge areas that occur on several of Pune's hill slopes. There is a proposal for two roads (the petition mentions only one) to cut across these hills, which will result in the paving over of the recharge surface. In the picture below, the roads will be built at two levels across the slopes behind the green and yellow colored building.

 Forested Slopes of Law College Hill

Besides the threat to groundwater, there are other objections to the road, including its outdated route  and its preference for private vehicles over public transport.

Pune nature lovers and others too... do consider signing the petition and help save part of our precious remaining natural heritage.


Cross Posted from Facebook:

A couple of days ago it rained about 75 mm in central parts of Pune. If you consider a 1 sq.km area around your neighborhood, about 75 million litres of water came crashing down in a few hours. If just 2 percent of that infiltrated into the ground, about 1.5 million liters were added to our ground water resource through a 1 sq. km surface area.

Surprised that rock can hold this much water? Those who grew up swimming in the Tilak Tank of old won't be. It was fed by a natural spring. At any one time the pool held about 2.5 million liters of water. The Tilak Tank hole in the ground would have held much more, but excess water was being drained into a nullah. More recently, the Suvarnarekha building on Prabhat Road was demolished, and the builder excavated a hole for a basement. It soon filled up with ground water. It held about 5.5 million liters of water. As the builder started pumping out the water, more kept rushing in. Over a one and half year period up to 100 million liters of water was likely pumped out. All this water held under a few hundred square meter area!

Pune receives about 650 mm of rain annually. That means 260 billion liters of water falls yearly over a 400 sq.km area. How much of that is infiltrating into the ground.. 5 percent, 10 percent, 20 percent? No one knows for sure and the amount will be highly variable across a surface. But we do have an idea how much is being taken out. A recent estimate by ACWADAM, one of the leading experts on local hydrogeology, puts our annual extraction of groundwater to be 3-4 TMC, which amounts to 80-100 billion liters of water. This suggests that we are extracting more than the natural recharge, since ground water levels are beginning to dip at many places.

The Pune dam cluster collectively store 826 billion liters of water out of which 315 billion liters are allocated annually to Pune. Our increasing use of ground water at 80-100 billion liters annually, underscores the critical role ground water is beginning to play in our lives.

It is imperative that we redress the growing imbalance between extraction and natural recharge. We can do this by individual action of leaving ground uncovered around our homes, and also by protecting larger swaths of recharge areas where infiltration rates are particularly high. The Vetal - Hanuman Tekdi slopes have been identified by ACWADAM as an important natural recharge zone. They must be protected from being covered over by concrete by the proposed HCMTR and Balbharati roads.

Please sign this petition for saving Pune's tekdi slopes from being destroyed! Say no to the HCMTR and Balharati Roads.

SCRAP HCMTR - MAKE PUNE A MODEL FOR SUSTAINABLE DEVELOPMENT

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A small addition to the above post. An earlier study by Raymond Duraiswamy and colleagues published in 2009 had identified the Hanuman (Law College) Hill slopes as potential recharge zones. The map below shows recharge potential of parts of Pune as identified using hydrogeologic criteria. The red rectangle (my addition) roughly outlines the hill slopes under threat of being paved over by roads. The study highlights an area inside the red rectangle (Table 20: Balbharati Building) as an ideal site for enhancing recharge and also points out old quarries in nearby places which could be repurposed to store runoff water for recharging the underlying aquifers. Unfortunately, some of these quarries are now being encroached by slums and also being used as dumping grounds for construction debris. 

Source: Raymond Duraiswamy, Vrishali Dumale and Usha Shetty 2009 - Geospatial mapping of potential recharge zones in parts of Pune city.

ACWADAM will be soon releasing a detailed study of the ground water situation of Pune along with maps of aquifers. I will post that information when it is made available.