Search

The geochemistry of Archean anorthosite-bearing layered intrusions has major implications for the thermal and chemical state of the Archean crust/mantle system, as originally posited by Bowen (1917) as “the anorthosite problem” and expanded on by Ashwal (1993). Debates have centred on the nature of the parental magmas, emplacement mechanisms and geodynamic settings of Archean anorthosites, many of which have megacrystic textures. In this review, we have compiled whole-rock major and trace element and Nd isotope geochemical data from Archean anorthosite-bearing layered intrusions worldwide to address the outstanding questions outlined above regarding the petrogenesis of anorthosites. Archean anorthosite-bearing layered intrusions were not significantly affected by hydrothermal alteration and were derived from depleted mantle sources and most (85%) were emplaced in oceanic settings. Some intrusions were intruded in continental settings or ocean-continent transition zones, reflecting the emergence of continents in the Paleoarchean into the Neoarchean. Based on their petrography and major and trace element geochemistry, Archean anorthosite-bearing layered intrusions mostly crystallised from hydrous Ca- and Al-rich tholeiitic magmas that fractionated from more primitive tholeiitic parental magmas. Archean layered intrusions formed by shallow- and deep-level fractional crystallisation of tholeiitic magmas and predominantly formed in back-arc suprasubduction zone and volcanic arc settings. Archean anorthosite-bearing layered intrusions started forming at ca. 3850 Ma, most of them representing relicts of dismembered Archean subduction-related ophiolites. Modern-style plate tectonic processes have operated at least since the earliest Archean and were the predominant contributor to Archean crustal growth.

We use a sample of 8,869 firm-years from the highly regulated Main Market (MAIN) and relatively unregulated Alternative Investment Market (AIM) in the United Kingdom to analyse the impact of financial restrictions on optimal cash holdings in the context of financial crises. Employing system generalised methods of moments, we find that AIM firms have a faster adjustment speed of cash as confirmed by precautionary and transaction motives over 2001-2017. However, AIM firms decrease (increase) their adjustment speed of cash more than MAIN firms during (after) the financial crises.

We use a sample of 8,869 firm-years from the highly regulated Main Market (MAIN) and relatively unregulated Alternative Investment Market (AIM) in the United Kingdom to analyse the impact of financial restrictions on optimal cash holdings in the context of financial crises. Employing system generalised methods of moments, we find that AIM firms have a faster adjustment speed of cash as confirmed by precautionary and transaction motives over 2001-2017. However, AIM firms decrease (increase) their adjustment speed of cash more than MAIN firms during (after) the financial crises.

Bulk-rock Lu-Hf and Sm-Nd isotope compositions, as well as major and trace element data are presented for metavolcanic rocks from the Mesoarchaean (ca. 3075 Ma) Ivisaartoq Supracrustal Belt in the Nuuk region of southern West Greenland. The epsilon Hf-t calculated at 3075 Ma range from +0.8 to +3.1 and the corresponding epsilon Nd-t values range from +0.7 to +3.6, which forms an array that is displaced off the mantle array for these two isotopic systems. Primitive mantle normalized trace element plots of the metabasalts display negative Nb- and Ti-anomalies in combination with the elevated Th abundances, which is consistent with a subduction zone affinity as proposed by previous studies of this metavolcanic belt. No significant correlations are observed between the isotope compositions and proxies of shallow crustal contamination in the Ivisaartoq rocks, despite clear evidence for inherited Eoarchaean zircon [Polat et al. (2009a) Chemical Geology 268, 248-271], which would have dominated the bulk-rock Hf-isotope budget. Furthermore, the measured samples are less radiogenic than the estimate for the depleted mantle composition at 3075 Ma. The lack of isotope and trace element correlation suggests incomplete equilibration between the crustal contaminant and the parental Ivisaartoq melts. We prefer a petrogenetic model with some combination of slab-derived metasomatism of the mantle source region for the Ivisaartoq magmas, which homogenized their trace element contents, in combination with the incorporation of granitoid residue with unradiogenic Hf-isotope composition at higher degrees of partial melting and finally the eruption of mechanically entrained Eoarchaean crust without significant chemical equilibration. The geochemical arc-affinity and lower than depleted mantle (DM) isotope compositions of these metavolcanic rocks support the notion that crustal recycling and plate tectonics has been operating on Earth since at least the Mesoarchaean Eon. (C) 2016 Elsevier

Bulk-rock Lu-Hf and Sm-Nd isotope compositions, as well as major and trace element data are presented for metavolcanic rocks from the Mesoarchaean (ca. 3075 Ma) Ivisaartoq Supracrustal Belt in the Nuuk region of southern West Greenland. The epsilon Hf-t calculated at 3075 Ma range from +0.8 to +3.1 and the corresponding epsilon Nd-t values range from +0.7 to +3.6, which forms an array that is displaced off the mantle array for these two isotopic systems. Primitive mantle normalized trace element plots of the metabasalts display negative Nb- and Ti-anomalies in combination with the elevated Th abundances, which is consistent with a subduction zone affinity as proposed by previous studies of this metavolcanic belt. No significant correlations are observed between the isotope compositions and proxies of shallow crustal contamination in the Ivisaartoq rocks, despite clear evidence for inherited Eoarchaean zircon [Polat et al. (2009a) Chemical Geology 268, 248-271], which would have dominated the bulk-rock Hf-isotope budget. Furthermore, the measured samples are less radiogenic than the estimate for the depleted mantle composition at 3075 Ma. The lack of isotope and trace element correlation suggests incomplete equilibration between the crustal contaminant and the parental Ivisaartoq melts. We prefer a petrogenetic model with some combination of slab-derived metasomatism of the mantle source region for the Ivisaartoq magmas, which homogenized their trace element contents, in combination with the incorporation of granitoid residue with unradiogenic Hf-isotope composition at higher degrees of partial melting and finally the eruption of mechanically entrained Eoarchaean crust without significant chemical equilibration. The geochemical arc-affinity and lower than depleted mantle (DM) isotope compositions of these metavolcanic rocks support the notion that crustal recycling and plate tectonics has been operating on Earth since at least the Mesoarchaean Eon. (C) 2016 Elsevier

Radiogenic isotope compositions of Hf and Nd are typically coupled in Phanerozoic and Proterozoic mafic rocks due to a similar behaviour of Lu-Hf and Sm-Nd during mantle melting. Eoarchean rocks, for instance those from southern West Greenland, exhibit an apparent decoupling of Hf and Nd isotope compositions. This apparent decoupling may either indicate metamorphic disturbance or, alternatively, mirror early differentiation processes in the silicate Earth. To evaluate the issue, we performed combined measurements of Hf-Nd isotope compositions together with major and trace element concentrations for well preserved >3720 to >3800 Ma old tholeiitic metabasalts and gabbros from the similar to 3700 Ma and similar to 3800 Ma old terranes of the Isua Supracrustal Belt, southern West Greenland. In contrast to younger mafic rocks, calculated initial epsilon Hf-epsilon Nd values of the Isua tholeiites show similar spreads and are both near chondritic to strongly depleted (-0.7 to +6.3 and -0.8 to +4.4, respectively), also in contrast to previously reported more depleted signatures in nearby boninite-like metabasalts of the Garbenschiefer unit. An evaluation of alteration effects based on preserved major and trace element arrays reveals pristine magmatic trends and therefore the measured isotope compositions indeed in most cases characterize contrasting Eoarchean mantle sources. In accord with this view, compositions of the Isua metabasalts yield Eoarchean regression ages in Sm-Nd and Lu-Hf isochron spaces, overlapping with emplacement ages inferred from crosscutting relationships with tonalites. Lutetium-Hf systematics of the Isua metabasalts studied here, yield clear isochron relationships. For both terranes, there is some scatter in Sm-Nd space, indicating early disturbance of the Sm-Nd system close in time to the extrusion ages, possibly by seafloor alteration. Trace element compositions of the metabasalts indicate an arc setting and a strong source overprint by melt-like

Radiogenic isotope compositions of Hf and Nd are typically coupled in Phanerozoic and Proterozoic mafic rocks due to a similar behaviour of Lu-Hf and Sm-Nd during mantle melting. Eoarchean rocks, for instance those from southern West Greenland, exhibit an apparent decoupling of Hf and Nd isotope compositions. This apparent decoupling may either indicate metamorphic disturbance or, alternatively, mirror early differentiation processes in the silicate Earth. To evaluate the issue, we performed combined measurements of Hf-Nd isotope compositions together with major and trace element concentrations for well preserved >3720 to >3800 Ma old tholeiitic metabasalts and gabbros from the similar to 3700 Ma and similar to 3800 Ma old terranes of the Isua Supracrustal Belt, southern West Greenland. In contrast to younger mafic rocks, calculated initial epsilon Hf-epsilon Nd values of the Isua tholeiites show similar spreads and are both near chondritic to strongly depleted (-0.7 to +6.3 and -0.8 to +4.4, respectively), also in contrast to previously reported more depleted signatures in nearby boninite-like metabasalts of the Garbenschiefer unit. An evaluation of alteration effects based on preserved major and trace element arrays reveals pristine magmatic trends and therefore the measured isotope compositions indeed in most cases characterize contrasting Eoarchean mantle sources. In accord with this view, compositions of the Isua metabasalts yield Eoarchean regression ages in Sm-Nd and Lu-Hf isochron spaces, overlapping with emplacement ages inferred from crosscutting relationships with tonalites. Lutetium-Hf systematics of the Isua metabasalts studied here, yield clear isochron relationships. For both terranes, there is some scatter in Sm-Nd space, indicating early disturbance of the Sm-Nd system close in time to the extrusion ages, possibly by seafloor alteration. Trace element compositions of the metabasalts indicate an arc setting and a strong source overprint by melt-like

Growing evidence from the accessible geological record reveals that crust mantle differentiation on Earth started as early as 4.4 Ga. In order to assess the extent of early Archean mantle depletion, we obtained Lu-176-Hf-176, Sm-147-Nd-143, and high field strength element (HFSE) concentration data for the least altered, well characterized boninite-like metabasalts and associated metasedimentary rocks from the Isua supracrustal belt (southern West Greenland). The metasediments exhibit initial epsilon Hf-(3720) values from -0.7 to +1.5 and initial epsilon Nd-(3720) values from +1.6 to +2.1. Initial epsilon Hf-(3720) values of the least altered boninite-like metabasalts span a range from +3.5 to +12.9 and initial epsilon Nd-(3220) values from -0.3 to +3.2. These initial Hf-isotope ratios display coherent trends with SiO2, Al2O3/TiO2 and other relatively immobile elements, indicating contamination via assimilation of enriched components, most likely sediments derived from the earliest crust in the region. This model is also consistent with previously reported initial gamma Os-(3720) values for some of the samples. In addition to the positive epsilon Hf-(3720) values, the least disturbed samples exhibit positive epsilon Nd-(3720) values and a co-variation of epsilon Hf-(3220) and epsilon Nd-(3220) values. Based on these observations, it is argued, that the most depleted samples with initial epsilon Hf-(3220) values of up to +12.9 and high Lu-176/Hf-177 of similar to 0.05 to similar to 0.09 tap a highly depleted mantle source with a long term depletion history in the garnet stability field. High precision high field strength element (HFSE) data obtained for the Isua samples confirm the contamination trend. Even the most primitive samples display negative Nb-Ta anomalies and elevated Nb/Ta, indicating a subduction zone setting and overprint of the depleted mantle sources by felsic melts generated by partial melting of eclogite. Collectively, the data for boninite-like meta

Growing evidence from the accessible geological record reveals that crust mantle differentiation on Earth started as early as 4.4 Ga. In order to assess the extent of early Archean mantle depletion, we obtained Lu-176-Hf-176, Sm-147-Nd-143, and high field strength element (HFSE) concentration data for the least altered, well characterized boninite-like metabasalts and associated metasedimentary rocks from the Isua supracrustal belt (southern West Greenland). The metasediments exhibit initial epsilon Hf-(3720) values from -0.7 to +1.5 and initial epsilon Nd-(3720) values from +1.6 to +2.1. Initial epsilon Hf-(3720) values of the least altered boninite-like metabasalts span a range from +3.5 to +12.9 and initial epsilon Nd-(3220) values from -0.3 to +3.2. These initial Hf-isotope ratios display coherent trends with SiO2, Al2O3/TiO2 and other relatively immobile elements, indicating contamination via assimilation of enriched components, most likely sediments derived from the earliest crust in the region. This model is also consistent with previously reported initial gamma Os-(3720) values for some of the samples. In addition to the positive epsilon Hf-(3720) values, the least disturbed samples exhibit positive epsilon Nd-(3720) values and a co-variation of epsilon Hf-(3220) and epsilon Nd-(3220) values. Based on these observations, it is argued, that the most depleted samples with initial epsilon Hf-(3220) values of up to +12.9 and high Lu-176/Hf-177 of similar to 0.05 to similar to 0.09 tap a highly depleted mantle source with a long term depletion history in the garnet stability field. High precision high field strength element (HFSE) data obtained for the Isua samples confirm the contamination trend. Even the most primitive samples display negative Nb-Ta anomalies and elevated Nb/Ta, indicating a subduction zone setting and overprint of the depleted mantle sources by felsic melts generated by partial melting of eclogite. Collectively, the data for boninite-like meta

We report trace element, samarium (Sm)-neodymium (Nd) and lead (Pb) isotopic data for individual micro-and mesobands of the Earth's oldest Banded Iron Formation (BIF) from the 3.7-3.8 Ga Isua Greenstone Belt (IGB, West Greenland) in an attempt to contribute to the characterization of the depositional environment and to the understanding of depositional mechanisms of these earliest chemical sediments. Rare earth element (REE)-yttrium (Y) patterns of the individual mesobands show features of modern seawater with diagnostic cerium (Ce/Ce), presodymium (Pr/Pr) and Y/holmium (Ho) anomalies. Very low high field strength elements (HFSE) concentrations indicate essentially detritus-free precipitation. Uranogenic Pb isotope data define a correlation line with a slope of 3691 ± 41 Ma, indicating that the uranium (U)-lead (Pb) system remained closed after the formation of this BIF. High 207Pb/204Pb relative to 206Pb/204Pb ratios compared to average mantle growth evolution models are a feature shared by BIF, penecontemporaneous basalts and clastic volcanogenic metasediments and are indicative of the ultimate high-µ (238U/204Pb) character of the source region, an essentially mafic Hadean protocrust. Sm-Nd isotopic relations on a layer-by-layer basis point to two REE sources controlling the back-arc basin depositional environment of the BIF, one being seafloor-vented hydrothermal fluids (eNd (3.7 Ga)

We report trace element, samarium (Sm)-neodymium (Nd) and lead (Pb) isotopic data for individual micro-and mesobands of the Earth's oldest Banded Iron Formation (BIF) from the 3.7-3.8 Ga Isua Greenstone Belt (IGB, West Greenland) in an attempt to contribute to the characterization of the depositional environment and to the understanding of depositional mechanisms of these earliest chemical sediments. Rare earth element (REE)-yttrium (Y) patterns of the individual mesobands show features of modern seawater with diagnostic cerium (Ce/Ce), presodymium (Pr/Pr) and Y/holmium (Ho) anomalies. Very low high field strength elements (HFSE) concentrations indicate essentially detritus-free precipitation. Uranogenic Pb isotope data define a correlation line with a slope of 3691 ± 41 Ma, indicating that the uranium (U)-lead (Pb) system remained closed after the formation of this BIF. High 207Pb/204Pb relative to 206Pb/204Pb ratios compared to average mantle growth evolution models are a feature shared by BIF, penecontemporaneous basalts and clastic volcanogenic metasediments and are indicative of the ultimate high-µ (238U/204Pb) character of the source region, an essentially mafic Hadean protocrust. Sm-Nd isotopic relations on a layer-by-layer basis point to two REE sources controlling the back-arc basin depositional environment of the BIF, one being seafloor-vented hydrothermal fluids (eNd (3.7 Ga)

We report trace element, samarium (Sm)-neodymium (Nd) and lead (Pb) isotopic data for individual micro-and mesobands of the Earth's oldest Banded Iron Formation (BIF) from the 3.7-3.8 Ga Isua Greenstone Belt (IGB, West Greenland) in an attempt to contribute to the characterization of the depositional environment and to the understanding of depositional mechanisms of these earliest chemical sediments. Rare earth element (REE)-yttrium (Y) patterns of the individual mesobands show features of modern seawater with diagnostic cerium (Ce/Ce), presodymium (Pr/Pr) and Y/holmium (Ho) anomalies. Very low high field strength elements (HFSE) concentrations indicate essentially detritus-free precipitation. Uranogenic Pb isotope data define a correlation line with a slope of 3691 ± 41 Ma, indicating that the uranium (U)-lead (Pb) system remained closed after the formation of this BIF. High 207Pb/204Pb relative to 206Pb/204Pb ratios compared to average mantle growth evolution models are a feature shared by BIF, penecontemporaneous basalts and clastic volcanogenic metasediments and are indicative of the ultimate high-µ (238U/204Pb) character of the source region, an essentially mafic Hadean protocrust. Sm-Nd isotopic relations on a layer-by-layer basis point to two REE sources controlling the back-arc basin depositional environment of the BIF, one being seafloor-vented hydrothermal fluids (eNd (3.7 Ga)