Individual Hornblende Grains Research Articles

Conventional and high-resolution heavy mineral analyses applied to flysch deposits: comparative provenance studies of the Ropianka (Upper Cretaceous–Paleocene) and Menilite (Oligocene) formations (Skole Nappe, Polish Carpathians)

Conventional and high-resolution analyses of heavy minerals from the gravity flow-deposited sandstones of the Campanian–Maastrichtian interval of the Ropianka (Upper Cretaceous–Paleocene) and Menilite (Oligocene) formations of the Polish Carpathians display similar compositions in terms of mineral species. Zircon, tourmaline, rutile, garnet, staurolite and kyanite belong to the main constituents in both formations. Apatite is common in the Ropianka Fm., while the Menilite Fm. almost lacks this mineral. Furthermore, individual hornblende grains were found in the Ropianka Fm., while andalusite is present only in the Menilite Fm. The Ropianka Fm. is relatively richer in zircon, tourmaline, garnet and apatite, while the Menilite Fm. contains more staurolite and kyanite. Zircon and tourmaline colour and morphological varieties are similar in both formations. The similarities of the heavy mineral assemblages studied suggest origin of these minerals from lithologically similar rocks. Negative correlations between the zircon + tourmaline + rutile (ZTR) values and the content of garnet and staurolite in the Ropianka Fm. may indicate, to a large extent, first-cycle delivery of garnet and staurolite to the formation. Negative, but low, correlation valid only for ZTR and garnet and positive correlations for ZTR and staurolite and kyanite in the Menilite Fm. suggest delivery of these minerals from sedimentary rocks or/and palimpsest sediments. The data obtained on mineral relationships and their morphology suggest mixed first-cycle and recycled provenance of the heavy minerals studied. Additionally, the first-cycle material input seems to be larger during the Ropianka Fm. sedimentation, while during the deposition of Menilite Fm. the contribution of material delivered from erosion of recycled sediments appears more prominent. The heavy mineral evidence suggests a change at the northern margin of the Skole Basin from an immature passive margin with a high relief during sedimentation of the Campanian–Maastrichtian part of the Ropianka Fm. to a mature passive margin with a low relief during sedimentation of the Menilite Fm.

Centennial‐ to millennial‐scale ice‐ocean interactions in the subpolar northeast Atlantic 18–41 kyr ago

In order to monitor the evolution of the British‐Irish Ice Sheet (BIIS) and its influence in surface ocean structure during marine isotopic stages (MIS) 2 and 3, we have analyzed the sediments recovered in core MD04‐2829CQ (Rosemary Bank, north Rockall Trough, northeast Atlantic) dated between ∼41 and ∼18 ka B.P. Ice‐rafted debris flux and composition,40Ar/39Ar ages of individual hornblende grains, multispecies planktonic stable isotope records, planktonic foraminifera assemblage data and faunal‐based sea surface temperatures (SSTs) demonstrate a close interaction between BIIS dynamics and surface ocean structure and water properties in this region. The core location lies beneath the North Atlantic Current (NAC) and is ideal for monitoring the shifts in the position of its associated oceanic fronts, as recorded by faunal changes. These data reveal a succession of BIIS‐sourced iceberg calving events related to low SST, usually synchronous with dramatic changes in the composition of the planktonic foraminifera assemblage and with variations in the stable isotope records of the taxaNeogloboquadrina pachyderma(sinistral coiling) andGlobigerina bulloides. The pacing of the calving events, from typically Dansgaard‐Oeschger millennial timescales during late MIS 3 to multicentennial cyclicity from ∼28 ka B.P., represents the build‐up of the BIIS and its growing instability toward Heinrich Event (HE) 2 and the Last Glacial Maximum. Our data confirm the strong coupling between BIIS instabilities and the temperature and salinity of surface waters in the adjacent northeast Atlantic and demonstrate the BIIS's ability to modify the NAC on its flow toward the Nordic Seas. In contrast, subsurface water masses were less affected except during the Greenland stadials that contain HEs, when most intense water column reorganizations occurred simultaneously with the deposition of cream‐colored carbonate sourced from the Laurentide Ice Sheet.

Provenance study of early and middle bronze age pottery from Kaman-Kalehöyük, Turkey, by heavy mineral analysis and geochemical analysis of individual hornblende grains

The provenance of Early Bronze Age and early Middle Bronze Age pottery produced between 2100 and 1970 B.C. and excavated from Kaman-Kalehöyük, Turkey was studied using mineralogical methods, including heavy mineral analysis and geochemical study of individual hornblende grains. The relative abundances of heavy minerals in the fabrics of 20 Early Bronze Age (EBA) and early Middle Bronze Age (MBA) pottery sherds, together with 27 local sediments collected within a radius of 25 km of the site were studied. The heavy mineral distributions in the Kaman region were statistically analyzed for consistencies in their occurrence. Amphibole was found to be the most abundant heavy mineral in most of the analyzed samples, followed by titanite and epidote. The amounts of other minerals such as garnet, clinopyroxene, and zircon are subordinate. Different proportions of heavy minerals in the fabrics allow categorization of the EBA and early MBA potteries into 5 groups. Comparative study of heavy mineral assemblages of local sediments and pottery suggests that half of the EBA and most of the MBA potteries were produced from sediment near the excavation site, whereas the other half of the EBA potteries were produced from sediment occurring at 25 km east or more east of the site. The geochemistry of individual amphibole grains was studied by electron-microprobe analysis. Most of the studied amphibole grains are calcic and aluminous, corresponding to hornblende. They can be further characterized into four types based on their chemical composition. The combination of heavy mineral identity and hornblende geochemistry provides diagnostic evidence for the origins of Kaman-Kalehöyük pottery. These and analogous heavy mineral techniques are effective archaeometric tools for determining pottery provenance.

The relationship of Heinrich events and their European precursors over the past 60 ka BP: a multi-proxy ice-rafted debris provenance study in the North East Atlantic

High resolution, multi-proxy records of ice-rafted debris (IRD) flux and provenance in the NE Atlantic detail the development, variability and decline of marine margins of the last glacial circum-North Atlantic ice sheets. Coupled lithological identification, Sr and Nd isotopic composition and 40Ar/ 39Ar ages of individual hornblende grains reduce ambiguity as to IRD potential source region, allowing clear differentiation between Laurentide (LIS), Icelandic and British (BIS) ice sheet sources (the Icelandic and BIS are collectively referred to as the NW European ice sheet, NWEIS). A step-wise increase in the flux of IRD to the core site at ∼26.5 ka BP documents BIS advance and glaciation of Ireland. Millennial-scale variability of the BIS at a ∼2 ka periodicity is inferred through clusters of pulsed IRD fluxes throughout the late glacial (26.5–10 ka BP). Combination of these European IRD events and the ∼7 ka periodicity of LIS instability is thought to account for quasi-synchronicity of the NWEIS and LIS IRD pulses at Heinrich event (H) 2 and H1, previously suggested to represent the possible involvement of the NWEIS in the initiation of H events. Furthermore, the lack of extensive NWEIS marine margin is inferred prior to H3 (31.5 ka BP), such that no ‘European precursor’ event is associated with either H5 or H4. This suggests that ‘precursor events’ were not directly implicated in the collapse of the LIS, and the persistent instabilities of the BIS that are clustered at a 2 ka periodicity are incompatible with the concept that both H events and their ‘precursors’ are independent responses to a common underlying trigger.

Contrasting conditions preceding MIS3 and MIS2 Heinrich events

This paper presents an integrated multi-tracer study performed on piston cores recovered in the glacial ice-rafted detritus belt, stretching from Newfoundland to the Irish margin across the North Atlantic (40–55°N), in order to compare in detail the internal structure of each Heinrich event (HE). These tracers are IRD counts (quartz, dolomite, volcanic grains), their Nd isotopic composition and Ar–Ar datings of individual hornblende grains. A focus on the detailed structure of HE confirms that all intervals of massive sediment flux, specifically Heinrich layers HL1-to-5 (HLs), were dominated by North American, Laurentide ice-sheet surges from Hudson Strait, that are evident as far east as the Bay of Biscay (European margin). The sequences of events leading up to the HLs, however, present significant dissimilarities. One important difference is that HL2 and HL1 were preceded by “precursor events” (increases in the number of lithic grains per gram from non-Laurentide sediment sources). Sediment debris derived from near-simultaneous iceberg releases originating from the European ice-sheet are only detectable close to the European margin. In contrast there are no comparable precursor events before HL5 and HL4. This observation implies that precursor events are unlikely to be mechanistically linked to the triggering of HEs. The similarity of the HLs, against contrasting background conditions, is a significant observation that should add constraints to their origin.

Ice-rafted detritus evidence from 40Ar/ 39Ar ages of individual hornblende grains for evolution of the eastern margin of the Laurentide ice sheet since 43 14C ky

During the last glacial interval, the North Atlantic ice sheets expanded and contracted in approximate synchronicity with orbitally forced global climate change. Variation in ice rafted detritus content in North Atlantic marine sediment cores record the waxing and waning of glaciers, as well as the abrupt temperature changes at millennial time scales. The background variations of ice rafting are punctuated by Heinrich layers, which appear to record the catastrophic collapse of the Laurentide ice sheet through the Hudson Strait. The objective of this paper is to document the evolution of glaciation on Laurentia during the last 43 14C kyr. We present a provenance study based on 40Ar/ 39Ar dates of individual hornblende grains from 57 samples taken at 2 cm spacing between 4 and 134 cm from core V23-14 (43.4°N, 45.25°W, 3177 m). Sedimentation rates outside of the Heinrich layers are very low in this core, but the Heinrich layers are easily identified. Laurentide glaciation did not extend into the ocean south of 55°N until about 26 14C kyr, and retreated to the coastline or beyond by 14 14C kyr. Documenting the history of this major ice sheet has significant implications for understanding ice rafting sources in more distal locations where mixing among different ice sheets is likely.

Evidence from 40Ar/39Ar Ages of Individual Hornblende Grains for Varying Laurentide Sources of Iceberg Discharges 22,000 to 10,500 yr B.P.

The abundance and lithic content of ice rafted detritus in glacial North Atlantic sediment cores vary abruptly on millennial time scales that have been correlated to Dansgaard-Oeschger cycles in the Greenland ice cores. There is growing evidence that various ice sheet outlets contributed increased iceberg fluxes at multiple discrete intervals, and the relative timing of iceberg discharges from different sources is important for understanding interactions between oceans and ice sheets. We present a provenance study based on 40Ar/39Ar dates of individual hornblende grains from 20 samples taken at 600 to 700 yr spacing between 10,500 and 22,000 yr B.P., from Orphan Knoll core EW9303-GGC31. Heinrich layers are characterized by a dominant Paleoproterozoic hornblende provenance consistent with published studies. A change in provenance between Heinrich events H2 and H1 indicates contributions of iceberg calving from the Newfoundland and southern Labrador margins. Between H1 and the Younger Dryas interval, Paleoproterozoic ice rafted grains remained dominant. The dominance of Baffin Island (or Greenland?) sources to the ice rafted detritus is ascribed to the retreat of the southern Laurentide ice sheet at about the time of H1—a retreat that isolated Newfoundland and southern Labrador ice from the shelf-slope boundary.

40Ar/39Ar and Pb-Pb study of individual hornblende and feldspar grains from southeastern Baffin Island glacial sediments: implications for the provenance of the Heinrich layers

Lead isotope analysis of individual feldspar grains and 40Ar/39Ar analysis of individual hornblende grains from three samples taken from Baffin Island ice-proximal till deposits are consistent with a provenance recording a two-stage geologic history including formation of continental crust in the Late Archean and partial isotopic resetting and (or) reequilibration during major Paleoproterozoic regional metamorphism and magmatism. Significantly, although the populations are small, and the two-stage Archean-Paleoproterozoic history of their sources are similar, the Pb isotope compositions of samples from Baffin Island do not match the results from Heinrich layer H2 in core HU87-033-009 from northeast of the Hudson Strait in the Labrador Sea. Specifically, in H2 samples from HU87-033-009, the 207Pb/204Pb is high for a given value of 206Pb/204Pb. Additionally, the range in 206Pb/204Pb is greater in HU87-033-009 samples, and there are two distinct trajectories of 208Pb/204Pb. The steeper of the 208Pb/204Pb versus 206Pb/204Pb trends is not seen in the studied Baffin Island glacial sediment samples. Samples of feldspar grains from Heinrich layers H1, H2, H4, and H5 from core V28-82 in the eastern North Atlantic have a 207Pb/204Pb versus 206Pb/204Pb relation similar to that of Baffin Island till samples, but there is also a high 208Pb/204Pb that we have not captured in this suite of glacial sediment samples. Our results highlight the strength of a combined radiogenic isotope approach to characterizing the geological history of sediment sources and provide a first step in assessing the variability in isotope character in this region of dynamic glacial events during the late Quaternary Period.

Laser probe 40Ar [sbnd]39Ar measurements of loss profiles within individual hornblende grains from the Giants Range Granite, northern Minnesota, USA

A Nd-glass laser has been used as a “probe”, mapping natural argon diffusion in partially outgassed hornblende grains at a resolution of around 50 μm. The hornblendes are from the Giants Range Granite (2700 Ma), Minnesota, taken approximately 2 km from the contact with the Duluth Gabbro Complex (1100 Ma). The full range of ages can be discerned within individual hornblende grains using the laser extraction technique, but more importantly, diffusion profiles up to 500 μm in length have been detected. The profiles reflect large variations in “domain” size within single grains. Diffusive argon loss was controlled not only by lattice-dominated volume diffusion, but a network of fast diffusion paths (now defined by biotite inclusions) and also indirectly by mineral reactions associated with the contact metamorphism. Three natural argon loss profiles from subgrains of different sizes yield good fits to theoretical volume diffusion loss curves, and a mean time-integrated diffusion parameter ( D(t)dt) of (7.1 ± 0.9) × 10 −5 cm 2. This parameter constrains the possible heat loss models for the Duluth Gabbro.