Provenance, also known as geographic attribution, in geology refers to the origins or sources of particles within sediment and sedimentary rocks.
Metamorphic and igneous rocks are broken down via weathering and erosion into sediment as part of the rock cycle. These sediments are transported by wind, water, ice, or gravity, before being deposited in horizontal layers. As more sediment is deposited over time, earlier layers are covered and compacted. Finally, they are cemented to form a new rock.
Modern geological provenance research specifically refers to the application of compositional analyses to determine sedimental origins. This is often used in conjunction with the study of exhumation histories, forward-modeling of paleo-earth systems, and interpretation of drainage networks and their evolution. In combination, these help to characterize the "source to sink" journey of clastic sediments from the hinterland to a sedimentary basin. Sediments analyzed for provenance can provide tectonic, paleogeographic, and paleoclimatic histories.
Provenance studies are conducted to investigate scientific questions such as the growth history of the continental crust, the collision history of the Indian and Asian tectonic plates, Asian monsoon intensity, and Himalayan exhumation.
Background
Etymology Provenance () describes in detail the history of a certain object, with respect to its creation, ownership, custody, and location. The term is commonly used by art historians and archivists, who use it to authenticate a work, document, or other significant object.
History The study of sedimentary provenance involves several geological disciplines, including mineralogy, geochemistry, geochronology, sedimentology, and petrology. The development of provenance methods occurred alongside development of these mainstream geological disciplines.
The earliest provenance studies were based on paleocurrent and petrographic analysis (composition and texture of sandstone and conglomerate). In the 1970s, provenance studies expanded to include tectonic settings (i.e. magmatic arcs, collision orogens, and continental blocks) using sandstone composition. Similarly, bulk-rock geochemistry techniques were applied to interpret provenance, linking geochemical signatures to source rocks and tectonic settings.
In the 1980s, advancements in chemical and isotopic microanalysis methods continued. Inductively coupled plasma mass spectrometry (ICP-MS) and sensitive high-resolution ion microprobe (SHRIMP) enabled researchers to analyze single mineral grains.
Purpose The goal of sedimentary provenance studies is to reconstruct and interpret the history of sediment from parent rocks at a source area to detritus at a burial place, to investigate the characteristics of a source area by analyzing the composition and texture of sediments. Sedimentary provenance analysis can also be a powerful tool to track landscape evolution and changes in sediment dispersal pathways through time.
In Petrology of Sedimentary Rocks (1992), Boggs described the four main goals of provenance studies as follows:
"source(s) of the particles that make up the rocks erosion and transport mechanisms that moved the particles from source areas to depositional sites depositional setting and depositional processes responsible for sedimentation of the particles (the depositional environment), physical and chemical conditions of the burial environment and diagenetic changes that occur in siliciclastic sediment during burial and uplift"
Sediment path
Sources All exposed rocks are subjected to physical or chemical weathering. They are broken down into finer-grained sediments. Igneous, sedimentary, and metamorphic rocks can all serve as sources for detritus.
Transportation
Rocks are transported downstream from higher to lower elevations. 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 boulder size into sand or even clay. At the same time minerals within the sediment can also be changed chemically. Only minerals that are more resistant to chemical weathering can survive (e.g. ultrastable minerals such as zircon, tourmaline, and rutile). During transportation, minerals can be sorted by their density, and as a result, light minerals (such as quartz and mica) will be moved faster and further than heavy minerals (such as zircon and tourmaline).
Accumulation After a certain distance of transportation, detritus reaches a sedimentary basin and accumulates in one place. With the accumulation of sediments, they are buried to a deeper level and go through diagenesis, which turns separate sediments into sedimentary rocks (i.e. conglomerate, sandstone, mudrocks, limestone, etc.) and some metamorphic rocks (such as quartzite) which were derived from sedimentary rocks. After sediments are weathered and eroded from mountain belts, they can be carried by streams and deposited along rivers as river sands. Detritus can also be transported and deposited in foreland basins and offshore fans. The detrital record can be collected from all these places and can be used in provenance studies.
+Examples of detritus accumulation 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 is eroded from a source area, it is 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 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 °C. Zircon is resistant to physical abrasion and chemical weathering; so, zircon grains can survive multiple cycles of reworking. This means that if the zircon grain is reworked (re-eroded) from a foreland basin, and not from original mountain-belt source area, it will lose information of reworking (the detrital record will not indicate the foreland basin as a source area but will indicate the earlier mountain belt as the source). To avoid this problem, samples can be collected close to the mountain front, upstream from which there is no significant sediment storage.
Provenance methods Generally, provenance methods can be sorted into two categories: petrological methods and geochemical methods.
Petrological methods Examples of petrological methods include QFL ternary diagram, heavy mineral assemblages (apatite–tourmaline index, garnet zircon index), clay mineral assemblages and illite crystallinity, reworked fossils and palynomorphs, and stock magnetic properties.
Geochemical methods Examples of geochemical methods include zircon U-Pb dating (plus Hf isotope), zircon fission track, apatite fission track, bulk sediment Nd and Sr isotopes, garnet chemistry, pyroxene chemistry, and amphibole chemistry. See the summary table below for various types of provenance methods.
Plate tectonics This method has the ability to link sandstone composition to tectonic setting. This method is described in the 1979 Dickinson and Suczek paper.
Resolving provenance problems by dating detrital minerals
Geochronology and thermochronology have been applied to solve provenance and tectonic problems. Detrital minerals used in this method include zircon, monazite, white mica, and apatite. The ages dated from these minerals indicate the timing of crystallization and multiple tectono-thermal events. This method is based on the following considerations: "(1) the source areas are characterized by rocks with different tectonic histories recorded by distinctive crystallization and cooling ages; (2) the source rocks contain the selected mineral;" and (3) a detrital mineral such as 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 minerals ideal to record long histories of crystallization of tectonically complex source areas.
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 (source area) 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 (divided into 3 tectonic level, Tethyan Himalaya, Greater Himalaya and Lesser Himalaya), 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 (Paleoproterozoic) and about 16 grains yield age of ~2600 Myr (Archean). 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 (e.g. Lesser Himalaya sequence), then eroded and deposited at foreland basin.
The U–Pb geochronology of zircons was conducted by laser-ablated, multicollector, inductively-coupled plasma mass spectrometry (LA-MC-ICPMS).
Bulk sediment Nd and Sr
The properties of the samarium–neodymium dating (Sm–Nd) radioactive isotope system can provide age estimations of sedimentary source rocks; the system has been used in provenance studies. 143Nd is produced by α decay of 147Sm and has a half-life of 1.06×1011 years. A variation of 143Nd/144Nd is caused by the decay of 147Sm. Now the Sm/Nd ratio of Earth's mantle is higher than that of its crust and the 143Nd/144Nd ratio is also higher in the mantle than in the crust. The 143Nd/144Nd ratio is expressed in εNd notation (DePaolo and Wasserbur 1976).
,
where CHUR refers to the Chondritic Uniform Reservoir. So, ϵNd is a function of T (time).
The Nd isotope evolution in the mantle and crust is shown in the figure to the right. The bold line in the upper plot shows the evolution of the bulk earth or CHUR. The lower plot shows the evolution of bulk earth (CHUR) in the crust and mantle; 143Nd/144Nd is transformed to εNd. Normally, 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. In addition, Sr and Nd isotopes have been used to study both provenance and weathering intensity. Nd is mainly unaffected by weathering processes, but the 87Sr/86Sr value is more affected by chemical weathering.
Summary Provenance study methods are also listed in the table below: