•We interrogate a global dataset of Archaean TTG bulk rock compositions.•Many elements show maxima, minima and/or change points in the interval 3.3–3.0 Ga.•The data are consistent with a transition ...from stagnant- to mobile-lid tectonics.•An increasing data spread through time reflects lithospheric differentiation.
It is estimated that around three quarters of Earth's first generation continental crust had been produced by the end of the Archaean Eon, 2.5 billion years ago. This ancient continental crust is mostly composed of variably deformed and metamorphosed magmatic rocks of the tonalite–trondhjemite–granodiorite (TTG) suite that formed by partial melting of hydrated mafic rocks. However, the geodynamic regime under which TTG magmas formed is a matter of ongoing debate. Using a filtered global geochemical dataset of 563 samples with ages ranging from the Eoarchaean to Neoarchaean (4.0–2.5 Ga), we interrogate the bulk rock major oxide and trace element composition of TTGs to assess evidence for secular change. Despite a high degree of scatter in the data, the concentrations or ratios of several key major oxides and trace elements show statistically significant trends that indicate maxima, minima and/or transitions in the interval 3.3–3.0 Ga. Importantly, a change point analysis of K2O/Na2O, Sr/Y and LaN/YbN demonstrates a statistically significant (>99% confidence) change during this 300 Ma period. These shifts may be linked to a fundamental change in geodynamic regime around the peak in upper mantle temperatures from one dominated by non-uniformitarian, deformable stagnant lid processes to another dominated by the emergence of global mobile lid or plate tectonic processes by the end of the Archaean. A notable change is also evident at 2.8–2.7 Ga that coincides with a major jump in the rate of survival of metamorphic rocks with contrasting thermal gradients, which may relate to the emergence of more potassic continental arc magmas and an increased preservation potential during collisional orogenesis. In many cases, the chemical composition of TTGs shows an increasing spread through the Archaean, reflecting the irreversible differentiation of the lithosphere.
Adakite petrogenesis Castillo, Paterno R.
Lithos,
March 2012, 2012-03-00, Volume:
134-135
Journal Article
Peer reviewed
Adakite was originally proposed as a genetic term to define intermediate to high-silica, high Sr/Y and La/Yb volcanic and plutonic rocks derived from melting of the basaltic portion of oceanic crust ...subducted beneath volcanic arcs. It was also initially believed that adakite only occurs in convergent margins where young and, thus, still hot oceanic slabs are being subducted. Currently, adakite covers a range of arc rocks ranging from primary slab melt, to slab melt hybridized by peridotite, to melt derived from peridotite metasomatized by slab melt. Adakites can occur in arc settings where unusual tectonic conditions can lower the solidi of even older slabs and their source also includes subducted sediments. Results of adakite studies have generated controversies due to (1) the specific genetic definition of adakite but its reliance on trace element chemistry for its distinguishing characteristics, (2) curious association of adakite with alkalic rocks enriched in high field-strength elements and Cu-Au mineral deposits and (3) existence of adakitic rocks produced through other petrogenetic processes. Other studies have shown that adakitic rocks and a number of the previously reported adakites are produced through melting of the lower crust or ponded basaltic magma, high pressure crystal fractionation of basaltic magma and low pressure crystal fractionation of water-rich basaltic magma plus magma mixing processes in both arc or non-arc tectonic environments. Thus, although adakite investigations enrich our understanding of material recycling and magmatic processes along convergent margins, economic deposits and crustal evolutionary processes, the term adakite should be used with extreme caution.
In order to check the heterogeneity of the CI chondrites and determine the average composition of this group of meteorites, we analyzed a series of six large chips (weighing between 0.6 and 1.2g) of ...Orgueil prepared from five different stones. In addition, one sample from each of Ivuna and Alais was analyzed. Although the sizes of the chips used in this study were “large”, our results show evidence for minor chemical heterogeneity in Orgueil, particularly for alkali elements and U. After removal of one outlier sample, the spread of the results is considerably reduced. For most of the 46 elements analyzed in this study, the average composition calculated for Orgueil is in very good agreement with previous CI estimates. This average, obtained with a “large” mass of samples, is analytically homogeneous and is suitable for normalization purposes.
Finally, the Cu and Zn isotopic ratios are homogeneously distributed within the CI parent body with a spread of less than 100ppm per atomic mass unit (amu).
To expand the newly developed ARM glasses as reference materials for in situ microanalysis of isotope ratios and iron oxidation state by a variety of techniques such as SIMS, LA‐MC‐ICP‐MS and EPMA, ...we report Li‐B‐Si‐O‐Mg‐Sr‐Nd‐Hf‐Pb isotope data and Fe2+/ΣFe ratios for these glasses. The data were mainly obtained by TIMS, MC‐ICP‐MS, IR‐MS and wet‐chemistry colorimetric techniques. The quality of these data was cross‐checked by comparing different techniques or by comparing the results from different laboratories using the same technique. All three glasses appear to be homogeneous with respect to the investigated isotope ratios (except for B in ARM‐3) and Fe2+/ΣFe ratios at the scale of sampling volume and level of the analytical precision of each technique. The homogeneity of Li‐B‐O‐Nd‐Pb isotope ratios at the microscale (30–120 μm) was estimated using LA‐MC‐ICP‐MS and SIMS techniques. We also present new EPMA major element data obtained using three different instruments for the glasses. The determination of reference values for the major elements and their uncertainties at the 95% confidence level closely followed ISO guidelines and the Certification Protocol of the International Association of Geoanalysts. The ARM glasses may be particularly useful as reference materials for in situ isotope ratio analysis.
Key Points
The homogeneity of isotope ratios (Li‐B‐Si‐O‐Mg‐Sr‐Nd‐Hf‐Pb) and Fe2+/ΣFe in three andesite reference material glasses (ARM‐1 to ‐3) were estimated using in situ and bulk techniques.
New reference values for the major elements in all three glasses are provided.
Reference values for isotope ratios (Li‐B‐Si‐O‐Mg‐Sr‐Nd‐Hf‐Pb) and Fe2+/ΣFe values are provided for the three glasses.
Volcanic ashes in coal and coal-bearing sequences typically occur as persistent bands within coal seams (generally as tonsteins, but in a few cases as bentonites, K-bentonites, or as clay-free ...partings), as an intimate mixture with organic matter, as host rocks (such as roof and floor strata), or as thick layers in coal-bearing strata that are stratigraphically separated from coal seams, including those of thick, laterally persistent tuffs, and in the broader sense fragmental clay rocks and flint clays. Altered volcanic ashes have been found in numerous coals with rank ranging from lignite through various bituminous coals to anthracite, as well as in all the continents where coal beds are present.
The main primary minerals in volcanic ash that survive post-depositional alteration include high-temperature quartz, plagioclase, sanidine, zircon, apatite, monazite, micas, rutile, and anatase. Alteration of volcanic glass and less stable primary minerals may result in the formation of kaolinite, smectite, illite, mixed-layer I/S and, in some cases, chlorite and zeolites. In addition to mineralogical and petrographic characteristics, identification of the parent magma type is commonly based on relatively immobile elements rather than the total alkali-silica contents, which are often affected by post-depositional alteration. Four types of volcanic ashes have been identified in coal and coal-bearing sequences, namely felsic, mafic, intermediate, and alkali. Altered mafic volcanic ashes are characteristically enriched in Sc, V, Cr, Co, and Ni; have positive Eu anomalies; and are of a medium-REE enrichment type. Altered alkali volcanic ashes are unique in their significant high concentrations of rare metals such as Nb, Ta, Zr, Hf, REE, and Ga, and are characterized by distinct negative Eu anomalies. Compared with altered alkali volcanic ashes, felsic tonsteins have relatively lower REE concentrations and less pronounced negative Eu anomalies, but a greater fractionation between light and heavy REEs. The compositional variation of different types of altered volcanic ashes is attributed to the tectonic framework and geodynamic controls.
Volcanic ashes in coal may serve as chronostratigraphic markers to identify and correlate coal seams, and can also be used for radiometric age determination. The ashes may have provided terrigenous materials that served as substrates for peat development; and may have terminated peat accumulation if volcanic ash in large quantities fell into the peat swamp; they can also be used to indicate the geodynamic processes of coal formation; and perhaps explain mass extinction events. Admixed volcanic ash may lower the quality of the coal if not removed in the preparation plant. From a practical viewpoint, alkali volcanic ashes may be significantly enriched in rare earth elements, Y, Nb, Ta, Zr, Hf, and Ga, which have potential economic significance. This paper reviews the distribution, geochemical and mineralogical compositions, and the significance and applications of volcanic ashes in coal and coal-bearing sequences.
Fluids in the deep crust and upper mantle appear to have played roles in the long-term evolution of the subcratonic lithospheric mantle and the stabilities of the continents, in the geochemical ...cycles of the elements from subduction zones to Earth’s surface environment, and in the formation of diamonds. Much evidence of the chemistry of deep fluids has accumulated from studies of fluid inclusions in diamonds and xenoliths. But the origins of the fluids and their behavior are still unclear. In part, this is due to the lack of a comprehensive theoretical model of aqueous, high-pressure fluids. Traditional models have used a C-O-H-type of model, which contains no major rock-forming elements or aqueous ions or metal-complexes.
In the present study, we use experimentally measured solubility data for multicomponent K-free eclogite, K-free peridotite and K-bearing peridotite rocks at upper mantle conditions from the literature to construct aqueous speciation solubility models that enabled calibration of the thermodynamic properties of ions and metal-complex species involving the elements Na, K, Mg, Ca, Fe, Al, Si, and C in an extended Deep Earth Water (DEW) model. New equilibrium constants were retrieved for the aqueous bisilicate anion, a silica trimer, silicate complexes of Ca, Fe, and Al, a silicate complex of Mg and bicarbonate, and formate complexes of Fe and Ca. The aqueous speciation and solubility model also took account of decreases in the activity of water and aqueous activity coefficients of neutral dissolved gases and included consideration of H2CO30. Based on the temperature and pressure dependences of the equilibrium constants, and supporting data covering a wide range of conditions, we then developed aqueous equation of state characterizations of the ions and metal-complex species. Overall, the results form a basis for modeling fluid-rock interactions under upper mantle conditions consistent with experimental solubility measurements.
We have mapped the major-element composition of Mercury's surface from orbital MESSENGER X-Ray Spectrometer measurements. These maps constitute the first global-scale survey of the surface ...composition of a Solar System body conducted with the technique of planetary X-ray fluorescence. Full maps of Mg and Al, together with partial maps of S, Ca, and Fe, each relative to Si, reveal highly variable compositions (e.g., Mg/Si and Al/Si range over 0.1–0.8 and 0.1–0.4, respectively). The geochemical variations that we observe are consistent with those inferred from other MESSENGER geochemical remote sensing datasets, but they do not correlate well with units mapped previously from spectral reflectance or morphology. Location-dependent, rather than temporally evolving, partial melt sources were likely the major influence on the compositions of the magmas that produced different geochemical terranes. A large (>5×106 km2) region with the highest Mg/Si, Ca/Si, and S/Si ratios, as well as relatively thin crust, may be the site of an ancient and heavily degraded impact basin. The distinctive geochemical signature of this region could be the consequence of high-degree partial melting of a reservoir in a vertically heterogeneous mantle that was sampled primarily as a result of the impact event.
•MESSENGER X-ray Spectrometer data used to produce global Mg/Si and Al/Si maps.•Major-element variations define large-scale geochemical terranes on Mercury.•Chemical heterogeneities do not match units mapped from morphology and color.•A heterogeneous mantle likely gave rise to the surface compositional variations.
The combination of U–Pb, Lu–Hf and O isotopic analyses in global zircon databases has recently been used to constrain continental crustal growth and evolution. To identify crust-forming events, these ...studies rely on the assumption that new crust is formed from depleted mantle sources. In contrast, this work suggests that post-collisional mafic magmas and their derivatives represent a non-negligible contribution to crustal growth, despite having zircons with “crust-like” Hf–O isotopic characteristics. We address this paradox and its implications for crustal evolution on the basis of a case study from the Variscan French Massif Central (FMC). The late stages of continental collisions are systematically marked by the emplacement of peculiar mafic magmas, rich in both compatible (Fe, Mg, Ni, Cr) and incompatible elements (K2O, HFSE, LREE) and displaying crust-like trace element patterns. This dual signature is best explained by melting of phlogopite- (and/or amphibole-) bearing peridotite, formed by contamination of the mantle by limited amounts (10–20%) of crustal material during continental subduction shortly preceding collision. Mass balance constraints show that in melts derived from such a hybrid source, 62–85% of the bulk mass is provided by the mantle component, whereas incompatible trace elements are dominantly crustal in origin. Thereby, post-collisional mafic magmas represent significant additions to the crust, whilst their zircons have “crustal” isotope signatures (e.g. −2<εHft<−9 and +6.4<δO18<+10‰ in the FMC). Because post-collisional mafic magmas are (i) ubiquitous since the late Archean; (ii) the parental magmas of voluminous granitoid suites; and (iii) selectively preserved in the geological record, zircons crystallized from such magmas (and any material derived from their differentiation or reworking) bias the crustal growth record of global zircon Hf–O isotopic datasets towards ancient crust formation and, specifically, may lead to an under-estimation of crustal growth rates since the late Archean.
•Post-collisional mafic magmas (PCMM) are ubiquitous since the late Archean.•The source of PCMM is a mantle contaminated by 10 to 20% crustal materials.•Around three quarters of the PCMM bulk mass originates from the mantle.•Voluminous magmatic suites derive from PCMM.•Post-collisional magmatism contributes to crustal growth.
Three synthetic reference glasses were prepared by directly fusing and stirring 3.8 kg of high‐purity oxide powders to provide reference materials for microanalytical work. These glasses have ...andesitic major compositions and are doped with fifty‐four trace elements in nearly identical abundance (500, 50, 5 µg g−1) using oxide powders or element solutions, and are named ARM‐1, 2 and 3, respectively. We further document that sector‐field (SF) ICP‐MS (Element 2 or Element XR) is capable of sweeping seventy‐seven isotopes (from 7Li to 238U, a total of sixty‐eight elements) in 1 s and, thus, is able to quantify up to sixty‐eight elements by laser sampling. Micro‐ and bulk analyses indicate that the glasses are homogeneous with respect to major and trace elements. This paper provides preliminary data for the ARM glasses using a variety of analytical techniques (EPMA, XRF, ICP‐OES, ICP‐MS, LA‐Q‐ICP‐MS and LA‐SF‐ICP‐MS) performed in ten laboratories. Discrepancies in the data of V, Cr, Ni and Tl exist, mainly caused by analytical limitations. Preliminary reference and information values for fifty‐six elements were calculated with uncertainties 2 relative standard error (RSE) estimated in the range of 1–20%.
Key Points
Three synthetic andesite reference glasses (ARM‐1, ARM‐2 and ARM‐3) were prepared, and characterised by a variety of analytical techniques.
The homogeneity of the ARM glasses was evaluated by multiple EPMA and LA‐ICP‐MS spot analyses on randomly selected glass splits.
Preliminary reference values of the ARM glasses are provided.
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