Provenance (geology)
Provenance in geology, is the reconstruction of the origin of sediments. The Earth is a dynamic planet, and all rocks are subject to transition between the three main rock types: sedimentary, metamorphic, and igneous rocks. Rocks exposed to the surface are sooner or later broken down into sediments. Sediments are expected to be able to provide evidence of the erosional history of their parent source rocks. The purpose of provenance study is to restore the tectonic, paleo-geographic and paleo-climatic history.
In the modern geological lexicon, "sediment provenance" specifically refers to the application of compositional analyses to determine the origin of sediments. This is often used in conjunction with the study of exhumation histories, interpretation of drainage networks and their evolution, and forward-modelling of paleo-earth systems. In combination, these help to characterise the "source to sink" journey of clastic sediments from hinterland to sedimentary basin.
Introduction
, is the place of origin or earliest known history of something. In geology, the term provenance deals with the question where sediments originate from. The purpose of sedimentary provenance studies is to reconstruct and to interpret the history of sediment from parent rocks at a source area to detritus at a burial place. The ultimate goal of provenance studies is to investigate the characteristics of a source area by analyzing the composition and texture of sediments. The studies of provenance involve the following aspects: " the source of the particles that make up the rocks, the erosion and transport mechanisms that moved the particles from source areas to depositional sites, the depositional setting and depositional processes responsible for sedimentation of the particles, and the physical and chemical conditions of the burial environment and diagenetic changes that occur in siliciclastic sediment during burial and uplift". Provenance studies are conducted to investigate many scientific questions, for example, the growth history of continental crust, collision time of Indian and Asian plates, Asian monsoon intensity, and Himalayan exhumation Meanwhile, the provenance methods are widely used in the oil and gas industry. "Relations between provenance and basin are important for hydrocarbon exploration because sand frameworks of contrasting detrital compositions respond differently to diagenesis, and thus display different trends of porosity reduction with depth of burial."Transportation of detritus
Rocks are transported downstream from higher elevation to lower elevation. Source rocks and detritus are transported by gravity, water, wind or glacial movement. The transportation process breaks rocks into smaller particles by physical abrasion, from big boulder size into sand or even clay size. At the same time minerals within the sediment can also be changed chemically, only minerals that are more resistant to chemical weathering can survive. During the transportation, minerals can be sorted by their density, and as a result, light minerals like quartz and mica can be moved faster and further than heavy minerals.Accumulation of detritus
After a certain distance of transportation, detritus reaches a sedimentary basin and accumulates in one place. With the accumulation of sediments, sediments are buried to a deeper level and go through diagenesis, which turns separate sediments into sedimentary rocks and some metamorphic rocks which were derived from sedimentary rocks. After sediments are weathered and eroded from mountain belts, they can be carried by stream and deposited along rivers as river sands. Detritus can also be transported and deposited in foreland basins and at offshore fans. The detrital record can be collected from all these places and can be used in provenance studies.| Detritus Type | Depositional environment | Location | Coordinates | Reference |
| Loess sand | Loess | Loess Plateau | ||
| Detrital apatite | Continental margin | East Greenland Margin | ||
| Detrital zircon | Modern river | Red River | ||
| Heavy mineral | Accretionary complex | South-central Alaska | ||
| Detrital zircon | Ancient passive continental margin | Southern Lhasa terrane | ||
| Detrital zircon | Foreland basin | Nepal Himalayan foreland basin |
Reworking of detritus
After detritus are eroded from source area, they are transported and deposited in river, foreland basin or flood plain. Then the detritus can be eroded and transported again when flooding or other kinds of eroding events occur. This process is called as reworking of detritus. And this process could be problematic to provenance studies. For example, U-Pb zircon ages are generally considered to reflect the time of zircon crystallization at about 750° Celsius and zircon is resistant to physical abrasion and chemical weathering. So zircon grains can survive from multiple cycles of reworking. This means if the zircon grain is reworked from a foreland basin it will lose information of reworking. To avoid this problem, samples can be collected close to the mountain front, upstream from which there is no significant sediment storage.Development of provenance methods
The study of sedimentary provenance involves several geological disciplines, including mineralogy, geochemistry, geochronology, sedimentology, igneous and metamorphic petrology. The development of provenance methods are heavily dependent on the development of these mainstream geological disciplines. The earliest provenance studies were primarily based on paleocurrent analysis and petrographic analysis. Since the 1970s, provenance studies shifted to interpret tectonic settings using sandstone composition. Similarly, bulk rock geochemistry techniques are applied to interpret provenance linking geochemical signatures to source rocks and tectonic settings. Later, with the development of chemical and isotopic micro-analysis methods and geochronological techniques, provenance researches shifted to analyze single mineral grains. The following table has examples of where provenance study samples are collected.Provenance methods
Generally, provenance methods can be sorted into two categories, which are petrological methods and geochemical methods. Examples of petrological methods include QFL ternary diagram, heavy mineral assemblages, clay mineral assemblages and illite crystallinity, reworked fossils and palynomorphs, and stock magnetic properties. Examples of geochemical methods include zircon U-Pb dating, zircon fission track, apatite fission track, bulk sediment Nd and Sr isotopes, garnet chemistry, pyroxene chemistry, amphibole chemistry and so on. There is a more detailed list below with references to various types of provenance methods.| Method | Case studies | Strength | ||
| Zircon U–Pb dating | Determine detrital zircon age of crystallization | |||
| Zircon U–Pb plus Hf isotopes | εHf > 0, Granite melts formed by the melting of young crust recently formed from depleted mantle generates zircons with radiogenic initial isotopic compositions similar to that of their mantle source; εHf < 0, Felsic melts derived from melting of reworked, old continental crust generates zircons with unradiogenic initial Hf isotope ratios. | |||
| Apatite fission track | Thermochronological age. | |||
| Zircon fission track | Thermochronological age, crystallization age, lag time | |||
| Zircon He and U–Pb double dating | "This method gives both the high temperature U–Pb crystallization and low temperature /He exhumation ages for the same zircon." | |||
| Bulk sediment Nd and Sr | Nd model age , ultimate protolith or source area | |||
| Bulk sediment Pb isotopes | Complicated Pb isotopes systematics makes it powerful tool to exam a source rock's geologic history especially in ancient heritage. | |||
| Heavy mineral assemblages | Heavy mineral assemblage of sedimentary rock is a function of the source rock type. For example,kyanite and sillimanite assemblage-rich indicates high-grade metamorphic source rocks | |||
| Garnet geochemistry | N/A | |||
| Ar–Ar mica dating | Indicate time of mica cooling through Ar-Ar closure temperature due to exhumation. | |||
| Nd isotopes in apatite | Nd model age , ultimate protolith or source area. | |||
| Pyroxene chemistry | Variable chemistry composition Ca-Mg-Fe indicative of source magma and source rock. | |||
| Amphibole chemistry | Major and trace element analyses of amphibole grains are used to provenance studies. | |||
| Pb isotopes in K-feldspar | N/A | |||
| Clay mineralogy | Original abundance of clay minerals in source determines the assemblege distribution in detrital record. The weathering and change of chemical composition also affect distribution. | |||
| Monazite U–Pb dating | Determine detrital monozite mineral age of crystallization. | |||
| Heavy mineral stability during diagenesis | N/A | N/A | ||
| Bulk sediment trace element chemistry | More sensitive indicators of geological processes than major elements | |||
| Rutile U-Pb | N/A | Determine detrital rutile mineral age of crystallization | ||
| U–Pb detrital titanite | Determine detrital titanite age of crystallization | |||
| Zircon REE and Th/U | Zircon grain derived from different types of granite can be discriminated by their REE ratios. | |||
| Reworked fossils and palynomorphs | Use reworked fossil and Palynomorphs | Use ESR intensity to correlate detrital record with source rock. | ||
| Rock magnetic properties | Substitute or supplement geochemical provenance data, using magnetic susceptibility, hysteresis loops, theromagnetic curves and iron-oxide mineral petrography to correlate sediment with source area. |
Examples of provenance methods
Sandstone composition and plate tectonics
This method is widely used in provenance studies and it has the ability to link sandstone composition to tectonic setting. This method is described in the Dickinson and Suczek 1979 paper. Detrital framework modes of sandstone suites from different kinds of basins are a function of provenance types governed by plate tectonics. Quartzose sands from continental cratons are widespread within interior basins, platform successions, miogeoclinal wedges, and opening ocean basins. Arkosic sands from uplifted basement blocks are present locally in rift troughs and in wrench basins related to transform ruptures. Volcaniclastic lithic sand and more complex volcano-plutonic sands derived from magmatic arcs are present in trenches, forearc basins and marginal seas. Recycled orogenic sands, rich in quartz or chert plus other lithic fragments and derived from subduction complexes, collision orogens, and foreland uplifts, are present in closing ocean basins. Triangular diagrams showing framework proportions of quartz, the two feldspars, polycrystalline quartzose lithics, and unstable lithics of volcanic and sedimentary parentage successfully distinguish the key provenance types."Resolving provenance problems by dating detrital minerals
and thermochronology are more and more applied to solve provenance and tectonic problems. Detrital minerals used in this method include zircons, monazites, white micas and apatites. The age dated from these minerals indicate timing of crystallization and multiple tectono-thermal events. This method is base on the following considerations: " the source areas are characterized by rocks with different tectonic histories recorded by distinctive crystallization and cooling ages; the source rocks contain the selected mineral;" Detrital mineral like zircon is ultra-stable which means it is capable of surviving multiple phases of physical and chemical weathering, erosion and deposition. This property make these detrital mineral ideal to record long history of crystallization of tectonically complex source area.The figure to the right is an example of U–Pb relative age probability diagram. The upper plot shows foreland basin detrital zircon age distribution. The lower plot shows hinterland zircon age distribution. In the plots, n is the number of analyzed zircon grains. So for foreland basin Amile formation, 74 grains are analyzed. For source area, 962, 409 and 666 grains are analyzed respectively. To correlate hinterland and foreland data, let's see the source area record first, Tethyan sequence have age peak at ~500 Myr, 1000 Myr and 2600 Myr, Greater Himalaya has age peaks at ~1200 Myr and 2500 Myr, and Lesser Himalaya sequence has age peaks at ~1800 Ma and 2600 Ma. By simply comparing the foreland basin record with source area record, we cam see that Amile formation resemble age distribution of Lesser Himalaya. It has about 20 grains with age ~1800 Myr and about 16 grains yield age of ~2600 Myr. Then we can interpret that sediments of Amile formation are mainly derived from the Lesser Himalaya, and rocks yield ago of Paleoproterozoic and Archean are from the Indian craton. So the story is: Indian plate collide with Tibet, rocks of Indian craton deformed and involved into Himalayan thrust belt, then eroded and deposited at foreland basin.
U–Pb geochronology of zircons was conducted by laser ablation multicollector inductively coupled plasma mass spectrometry.
Bulk sediment Nd and Sr
Depend on properties of Sm–Nd radioactive isotope system can provide age estimation of sedimentary source rocks. It has been used in provenance studies. 143Nd is produced by α decay of 147Sm and has a half life of 1.06×1011 years. Variation of 143Nd/144Nd is caused by decay of 147Sm. Now Sm/Nd rato of the mantle is higher than that of the crust and 143Nd/144Nd ratio is also higher than in the mantle than in the crust. 143Nd/144Nd ratio is expressed in εNd notation.. CHUR refer to Chondritic Uniform Reservoir. So ϵNd is a function of T. Nd isotope evolution in mantle and crust in shown in the figure to the right. The upper plot, bold line shows the evolution of the bulk earth or CHUR. The lower plot shows evolution of bulk earth crust and mantle, 143Nd/144Nd is transformed to εNd. Normally, the most rocks have εNd values in the range of -20 to +10. Calculated εNd value of rocks can be correlated to source rocks to perform provenance studies.What's more, Sr and Nd isotopes have been used to study both provenance and weathering intensity. Nd is mainly unaffected by weathering process but 87Sr/86Sr value is more affected by chemical weathering.Lab data acquisition and instruments
To pick suitable lab data acquisition to sediment provenance, grain size should be taken into consideration. For conglomerates and boulders, as original mineral paragenesisis preserved, almost all analytical methods can be used to study the provenance. For finer grained sediments, as they always lose paragenetic information, only a limited range of analytical methods can be used.Lab data acquisition approaches for provenance study fall into the following three categories: analyzing bulk composition to extract petrographic, mineralogical and chemical information. analyzing specific groups of minerals such as heavy minerals and analyzing single mineral grains about morphological, chemical and isotopic properties.
For bulk composition analysis, samples are crushed, powdered and disintegrated or melted. Then measurement of major and trace and rare-earth elements are conducted by using instruments like atomic absorption spectroscopy, X-ray fluorescence, neutron activation analysis etc.
Sand-sized sediments are able to be analyzed by single-grain methods. Single-grain methods can be divided into the following three groups: Microscopic-morphological techniques, which are used to observe shape, color and internal structures in minerals. For example, scanning electron microscope and cathodoluminescence detector. Single grain geochemical techniques, which are used to acquire chemical composition and variations within minerals. For example, laser-ablation inductively coupled plasma mass spectrometry. Radiometric dating of single grain mineral, which can determine the geochronological and thermochronological properties of minerals. For example, U/Pb SHRIMP dating and 40Ar/39Ar laser-probe dating.
For more information about instruments, see
*
*
Problems and limitations of provenance studies