Anion Composition of Apatite in the Au-Cu Epithermal Deposit of Palai-Islica (Almería, SE Spain) as an Indicator of Hydrothermal Alteration

The Palai-Islica deposit (Almería, SE Spain) is an Au-Cu epithermal deposit hosted in Neogene calc-alkaline andesites and dacites from the Cabo de Gata-Cartagena volcanic belt in the Betic Cordillera. Major element compositions of apatite from Palai-Islica orebody and related hydrothermally altered and unaltered volcanic rock from the region hosting the deposit were obtained to clarify the processes involved in their formation. Apatite in the host volcanic rocks is rich in chlorapatite and hydroxylapatite components (50–57% and 24–36%) and poor in fluorapatite components (12–21%), indicating assimilation processes of cortical Cl-rich material in the magmatic evolution. Apatite in the orebody sometimes has corrosion textures and is mostly fluorapatite (94–100%). Apatite from the hydrothermally altered host rock of the orebody systematically bears signs of corrosion and has variable and intermediate fluorapatite (19–100%), chlorapatite (1–50%), and hydroxylapatite (0–47%) components. The style of zonation and the composition are related to the proximity to the orebody. These features can be interpreted as the result of hydrothermal modification of high Cl, OH-rich volcanic apatites into F-rich apatites. The enrichment of F is related to the intensity of hydrothermal alteration and could therefore constitute a geochemical index of alteration and of mineralization processes.

New Perspectives on the Chronostratigraphy and Magmatic Evolution of the Auckland Volcanic Field, New Zealand

<p>Understanding the eruptive history of a volcanically active region is critical in assessing the hazard and risk posed by future eruptions. In regions where surface deposits are poorly preserved, and ambiguously sourced, tephrostratigraphy is a powerful tool to assess the characteristics of past eruptions. The city of Auckland, New Zealand’s largest urban centre and home to ca. 1.4 million people, is built on top of the active Auckland Volcanic Field (AVF). The AVF is an intraplate monogenetic basaltic volcanic field, with ca. 53 eruptive centres located in an area of ca. 360 km2. Little is known however, about the evolution of the field because the numerical and relative ages of the eruptions are only loosely constrained, and therefore the precise order of many eruptions is unknown. Here I apply tephrostratigraphic and geochemical techniques to investigate the chronology and magmatic evolution of the AVF eruptions. First, I present an improved methodology for in-situ analysis of lacustrine maar cores from the AVF by employing magnetic susceptibility and X-ray density scanning on intact cores. These techniques are coupled with geochemical microanalysis of the tephra-derived glass shards to reveal details of reworking within the cores. These details not only allow assessment of the deposit relationships within cores (e.g. primary vs. reworked horizons), but also to correlate tephra horizons between cores. Through the correlation of tephra units across cores from a variety of locations across the field, an improved regional tephrostratigraphic framework for the AVF deposits has been established. Following on from this, I detail the methods developed in this study to correlate tephra horizons within the maar cores back to their eruptive source. This technique uses geochemical fingerprinting to link the glass analyses from tephra samples to whole rock compositions. Such an approach has not been previously attempted due to the complications caused by fractional crystallisation, which affects concentrations of certain key elements in whole rock analyses. My method resolves these issues by using incompatible trace elements, which are preferentially retained in melt over crystals, and therefore retain comparable concentrations and concentration ratios between these two types of sample. Because of the primitive nature of the AVF magmas, their trace element signature is largely controlled by the involvement of several distinct mantle sources. This leads to significant variability between the volcanic centres that thus can be used for individually fingerprinting, and correlating tephra to whole rocks. Nevertheless, in some cases geochemistry cannot provide an unambiguous correlation, and a multifaceted approach is required to allow the correlation of the tephra horizons to source. The other criteria used to correlate tephra deposits to their source centre include, Ar-Ar ages of the centres, modelled and calculated ages of the tephra deposits, the scale of eruption, and the deposit locations and thicknesses. The results of this research outline the methodology for assessing occurrence and characteristics of basaltic tephra horizons within lacustrine maar cores, and the methodology for correlating these horizons to their eruptive source. In doing this the relative eruption order of the AVF is accurately determined for the first time. Temporal trends suggest acceleration of eruption repose periods to 21 ka followed by deceleration to present. Although no spatial evolution is observed, coupling of some centres is seen when spatial and temporal evolution are combined. The geochemical signature of the magmas appears to evolve in a cyclic manner with time, incorporating increasing amount of a shallow source. This evolution is seen both during a single eruption sequence and throughout the lifespan of the AVF. Finally, pre-eruptive processes are assessed as part of the study of the magmatic evolution of the AVF. The effects of contamination from the crust and lithosphere through which the magma ascends are evaluated using the Re-Os isotope system. The results show there are variable inputs from crustal sources, which have previously not been identified by traditional isotope systems (e.g. Pb-Sr-Nd isotopes). Two sources of contamination are identified based on their Os systematics relating to two terranes beneath the AVF: the metasedimentary crust and the Dun Mountain Ophiolite Belt. The identification of this process suggests there is interaction of ascending melt with the crust, contrary to what previous studies have concluded. This body of research has provided a detailed reconstruction of the chronostratigraphy and magmatic evolution of the AVF to aid accurate and detailed risk assessment of the threat posed by a future eruption from the Auckland Volcanic Field.</p>

New Perspectives on the Chronostratigraphy and Magmatic Evolution of the Auckland Volcanic Field, New Zealand

<p>Understanding the eruptive history of a volcanically active region is critical in assessing the hazard and risk posed by future eruptions. In regions where surface deposits are poorly preserved, and ambiguously sourced, tephrostratigraphy is a powerful tool to assess the characteristics of past eruptions. The city of Auckland, New Zealand’s largest urban centre and home to ca. 1.4 million people, is built on top of the active Auckland Volcanic Field (AVF). The AVF is an intraplate monogenetic basaltic volcanic field, with ca. 53 eruptive centres located in an area of ca. 360 km2. Little is known however, about the evolution of the field because the numerical and relative ages of the eruptions are only loosely constrained, and therefore the precise order of many eruptions is unknown. Here I apply tephrostratigraphic and geochemical techniques to investigate the chronology and magmatic evolution of the AVF eruptions. First, I present an improved methodology for in-situ analysis of lacustrine maar cores from the AVF by employing magnetic susceptibility and X-ray density scanning on intact cores. These techniques are coupled with geochemical microanalysis of the tephra-derived glass shards to reveal details of reworking within the cores. These details not only allow assessment of the deposit relationships within cores (e.g. primary vs. reworked horizons), but also to correlate tephra horizons between cores. Through the correlation of tephra units across cores from a variety of locations across the field, an improved regional tephrostratigraphic framework for the AVF deposits has been established. Following on from this, I detail the methods developed in this study to correlate tephra horizons within the maar cores back to their eruptive source. This technique uses geochemical fingerprinting to link the glass analyses from tephra samples to whole rock compositions. Such an approach has not been previously attempted due to the complications caused by fractional crystallisation, which affects concentrations of certain key elements in whole rock analyses. My method resolves these issues by using incompatible trace elements, which are preferentially retained in melt over crystals, and therefore retain comparable concentrations and concentration ratios between these two types of sample. Because of the primitive nature of the AVF magmas, their trace element signature is largely controlled by the involvement of several distinct mantle sources. This leads to significant variability between the volcanic centres that thus can be used for individually fingerprinting, and correlating tephra to whole rocks. Nevertheless, in some cases geochemistry cannot provide an unambiguous correlation, and a multifaceted approach is required to allow the correlation of the tephra horizons to source. The other criteria used to correlate tephra deposits to their source centre include, Ar-Ar ages of the centres, modelled and calculated ages of the tephra deposits, the scale of eruption, and the deposit locations and thicknesses. The results of this research outline the methodology for assessing occurrence and characteristics of basaltic tephra horizons within lacustrine maar cores, and the methodology for correlating these horizons to their eruptive source. In doing this the relative eruption order of the AVF is accurately determined for the first time. Temporal trends suggest acceleration of eruption repose periods to 21 ka followed by deceleration to present. Although no spatial evolution is observed, coupling of some centres is seen when spatial and temporal evolution are combined. The geochemical signature of the magmas appears to evolve in a cyclic manner with time, incorporating increasing amount of a shallow source. This evolution is seen both during a single eruption sequence and throughout the lifespan of the AVF. Finally, pre-eruptive processes are assessed as part of the study of the magmatic evolution of the AVF. The effects of contamination from the crust and lithosphere through which the magma ascends are evaluated using the Re-Os isotope system. The results show there are variable inputs from crustal sources, which have previously not been identified by traditional isotope systems (e.g. Pb-Sr-Nd isotopes). Two sources of contamination are identified based on their Os systematics relating to two terranes beneath the AVF: the metasedimentary crust and the Dun Mountain Ophiolite Belt. The identification of this process suggests there is interaction of ascending melt with the crust, contrary to what previous studies have concluded. This body of research has provided a detailed reconstruction of the chronostratigraphy and magmatic evolution of the AVF to aid accurate and detailed risk assessment of the threat posed by a future eruption from the Auckland Volcanic Field.</p>

Magmatic thickening of crust in non–plate tectonic settings initiated the subaerial rise of Earth’s first continents 3.3 to 3.2 billion years ago

When and how Earth's earliest continents—the cratons—first emerged above the oceans (i.e., emersion) remain uncertain. Here, we analyze a craton-wide record of Paleo-to-Mesoarchean granitoid magmatism and terrestrial to shallow-marine sedimentation preserved in the Singhbhum Craton (India) and combine the results with isostatic modeling to examine the timing and mechanism of one of the earliest episodes of large-scale continental emersion on Earth. Detrital zircon U-Pb(-Hf) data constrain the timing of terrestrial to shallow-marine sedimentation on the Singhbhum Craton, which resolves the timing of craton-wide emersion. Time-integrated petrogenetic modeling of the granitoids quantifies the progressive changes in the cratonic crustal thickness and composition and the pressure–temperature conditions of granitoid magmatism, which elucidates the underlying mechanism and tectonic setting of emersion. The results show that the entire Singhbhum Craton became subaerial ∼3.3 to 3.2 billion years ago (Ga) due to progressive crustal maturation and thickening driven by voluminous granitoid magmatism within a plateau-like setting. A similar sedimentary–magmatic evolution also accompanied the early (>3 Ga) emersion of other cratons (e.g., Kaapvaal Craton). Therefore, we propose that the emersion of Earth’s earliest continents began during the late Paleoarchean to early Mesoarchean and was driven by the isostatic rise of their magmatically thickened (∼50 km thick), buoyant, silica-rich crust. The inferred plateau-like tectonic settings suggest that subduction collision–driven compressional orogenesis was not essential in driving continental emersion, at least before the Neoarchean. We further surmise that this early emersion of cratons could be responsible for the transient and localized episodes of atmospheric–oceanic oxygenation (O2-whiffs) and glaciation on Archean Earth.

Supplemental Material: Magmatic evolution and architecture of an arc-related, rhyolitic caldera complex: The late Pleistocene to Holocene Cerro Blanco volcanic complex, southern Puna, Argentina

Methodology, calculations, and data analysis.