title: A continuous 5300-yr Holocene cryptotephrostratigraphic record from northern New Zealand and implications for tephrochronology and volcanic-hazard assessment.

authors: Gehrels, Maria J.

subjects: crytotephra

abstract: A continuous  5280 calendar (cal.) yr long cryptotephrostratigraphic record of a peat core from northern New Zealand demonstrates that cryptotephra studies can enhance conventional tephra records by extending the known distribution of ash fall and enabling re-assessment of volcanic hazards. A systematic sampling strategy was used to locate peaks in glass-shard concentrations and to determine loci of individual geochemical populations, and a palynological method involving spiking samples with Lycopodium spores was adapted to facilitate accurate counting of glass-shard concentrations. Using glass shard major element compositions, and a core chronology based on eight AMS 14C ages and two visible macroscopic tephra layers, Taupo Tephra (Unit Y) (1688-1748 cal. BP) and Tuhua Tephra (6800-7230 cal. BP) (2cr-age ranges), four cryptotephras were correlated with known eruptions: Whakaipo (Unit V) (2743-2782 cal. BP), Stent (Unit Q) (4240-4510 cal. BP), and Unit K (4970-5290 cal. BP), erupted from Taupo Volcanic Centre, and Whakatane Tephra (5470-5600 cal. BP) erupted from Okataina Volcanic Centre. Mixed glass populations were found in the core, most likely an artefact of post-depositional remobilization of shards vertically (both up and down) in the peat or on its surface by wind, or a result of closely spaced eruption events, or a combination of these. A secondary glass population identified within the macroscopic Taupo Tephra was tentatively attributed to either an earlier phase within that eruption or to mixing with a slightly older Taupo-derived eruptive or (less likely) a currently unknown Okataina-derived eruptive. These results indicate that, in the absence of continuous cryptotephrostratigraphic analysis, a peak in shard concentrations may not in itself be indicative of the ‘true’ stratigraphic (ie, isochronous) level of a tephra layer. For cryptotephra studies of peat cores, we recommend (1) using a detailed sampling strategy for the analysis of distal tephra-derived glass to detect and account for any mixed populations and possible vertical spread of glass shards through the peat, and (2) analysing more shards from larger samples to help ‘capture’ sparsely represented cryptic andesitic tephra deposits.

text: A continuous 5300-yr Holocene cryptotephrostratigraphic record from northern New Zealand and implications for tephrochronology and volcanic-hazard assessment. A continuous  5280 calendar (cal.) yr long cryptotephrostratigraphic record of a peat core from northern New Zealand demonstrates that cryptotephra studies can enhance conventional tephra records by extending the known distribution of ash fall and enabling re-assessment of volcanic hazards. A systematic sampling strategy was used to locate peaks in glass-shard concentrations and to determine loci of individual geochemical populations, and a palynological method involving spiking samples with Lycopodium spores was adapted to facilitate accurate counting of glass-shard concentrations. Using glass shard major element compositions, and a core chronology based on eight AMS 14C ages and two visible macroscopic tephra layers, Taupo Tephra (Unit Y) (1688-1748 cal. BP) and Tuhua Tephra (6800-7230 cal. BP) (2cr-age ranges), four cryptotephras were correlated with known eruptions: Whakaipo (Unit V) (2743-2782 cal. BP), Stent (Unit Q) (4240-4510 cal. BP), and Unit K (4970-5290 cal. BP), erupted from Taupo Volcanic Centre, and Whakatane Tephra (5470-5600 cal. BP) erupted from Okataina Volcanic Centre. Mixed glass populations were found in the core, most likely an artefact of post-depositional remobilization of shards vertically (both up and down) in the peat or on its surface by wind, or a result of closely spaced eruption events, or a combination of these. A secondary glass population identified within the macroscopic Taupo Tephra was tentatively attributed to either an earlier phase within that eruption or to mixing with a slightly older Taupo-derived eruptive or (less likely) a currently unknown Okataina-derived eruptive. These results indicate that, in the absence of continuous cryptotephrostratigraphic analysis, a peak in shard concentrations may not in itself be indicative of the ‘true’ stratigraphic (ie, isochronous) level of a tephra layer. For cryptotephra studies of peat cores, we recommend (1) using a detailed sampling strategy for the analysis of distal tephra-derived glass to detect and account for any mixed populations and possible vertical spread of glass shards through the peat, and (2) analysing more shards from larger samples to help ‘capture’ sparsely represented cryptic andesitic tephra deposits.

year: 2006-02

title: Alteration mineralogy and whole-rock geochemistry of the Tauhara Geothermal Field

authors: Mauriohooho, Kate

subjects: geothermal

abstract: The Tauhara Geothermal Field lies over a central zone of rhyolitic volcanism and caldera formation in the TVZ where extension allows magma to penetrate the crust and NE-SW oriented faults permit hot fluids to ascend and convect as individual hydrothermal systems.  The Jurassic greywacke basement hosts the heat source critical to geothermal systems, the Waiora Formation is the permeable reservoir hosting fluids and the Huka Falls Formation is the cap rock to the system. 304 drill cutting samples and six drill core samples were collected from three wells across the Tauhara field (TH9, TH10 and TH12) at 20 m intervals, and a variety of petrographic, mineralogical and geochemical techniques were applied to the samples.  New information on fluid flow pathways (feed zones), alteration types, alteration zones, hydrothermal conditions and mineral inferred temperatures were identified and interpreted from the data collected.

Using both alteration mineralogy data (from powder x-ray diffraction, short wave infrared or SWIR, and petrography) and geochemical data (from x-ray fluorescence and portable x-ray fluorescence), three main zones of alteration were identified across all wells:  A shallow zone of argillic to intermediate argillic alteration followed by a transition zone with increasing depth as propylitic alteration begins to dominate and an intense propylitic alteration zone at the deepest levels.

Mineral inferred temperatures are generally cooler than measured well temperatures in all wells except in TH10.  Here, mineral inferred temperatures agree with hotter measured well temperatures (> 280 °C) and strong potassic alteration indicates that TH10 is closest to the major upflow zone, making south Tauhara a prospective area for geothermal production.  In TH12, mineral inferred temperatures are inconsistent with measured well temperatures implying a recent increase of fluid temperature.  However petrographic evidence of an illite-calcite overprint suggests a cooler fluid at the bottom of well TH12 which is supported by petrography in this study showing pervasive illite alteration.  TH12 is perhaps more marginal to the upflow zone and the change in temperature regime may be related to blind faults at depth bringing the cooler fluid in.  An alteration halo based on geochemical data was identified in the vicinity of the dike in TH9 at ~1400 mRF by high alteration index values determined by pXRF data, which is useful because illite and chlorite detected by SWIR did not show any appreciable mineralogical alteration halo.

The pXRF technique on cuttings correlated well with lab XRF on powdered cutting samples crushed using a ring mill, showing excellent correlation (rs >0.8) for elements As, Ba, Ca, K, Nb, Rb, Sr, Y, and Zr but poorer correlation (rs <0.7) for others.  Samples analysed by XRF were classified as rhyolite/dacite or andesite/andesite-basaltic.  One sample was reclassified as andesite/andesite-basaltic rather than ignimbrite and this finding was corroborated by downhole pXRF geochemical data which show it belongs in the andesitic lithic crystal breccia unit. These results mean that downhole pXRF geochemical trends of immobile elements (Zr, Y, Nb) can be used to refine lithological boundaries, which cannot be identified from visual logging alone.  In addition, downhole immobile element trends are able to separate lithology packets and distinguish subunits or internal stratigraphic variations.  Conversely, variations in mobile element concentrations (Ba, K, Ca, As, Rb and Pb) indicated feed zones or permeable zones of fluid movement by depletions or elevations in element concentrations.  The (AlOH / H2O) ratio derived from SWIR data was plotted against depth downhole and changes in the ratio appear to coincide with some of the locations of feed zones (fluid pathways) in TH9 and TH12.

This study is the first to apply pXRF to geothermal drill cuttings and has proven that the data collected can be used to aid visual logging, and quantify alteration intensity.  Application of rapid geochemistry on existing and future wells will generate detailed geology models and enable correlation between wells and across fields.  This newly developed approach can be applied worldwide to any geothermal fields, provided that appropriate quality assurance techniques are used to ensure consistency of data.

text: Alteration mineralogy and whole-rock geochemistry of the Tauhara Geothermal Field The Tauhara Geothermal Field lies over a central zone of rhyolitic volcanism and caldera formation in the TVZ where extension allows magma to penetrate the crust and NE-SW oriented faults permit hot fluids to ascend and convect as individual hydrothermal systems.  The Jurassic greywacke basement hosts the heat source critical to geothermal systems, the Waiora Formation is the permeable reservoir hosting fluids and the Huka Falls Formation is the cap rock to the system. 304 drill cutting samples and six drill core samples were collected from three wells across the Tauhara field (TH9, TH10 and TH12) at 20 m intervals, and a variety of petrographic, mineralogical and geochemical techniques were applied to the samples.  New information on fluid flow pathways (feed zones), alteration types, alteration zones, hydrothermal conditions and mineral inferred temperatures were identified and interpreted from the data collected.

Using both alteration mineralogy data (from powder x-ray diffraction, short wave infrared or SWIR, and petrography) and geochemical data (from x-ray fluorescence and portable x-ray fluorescence), three main zones of alteration were identified across all wells:  A shallow zone of argillic to intermediate argillic alteration followed by a transition zone with increasing depth as propylitic alteration begins to dominate and an intense propylitic alteration zone at the deepest levels.

Mineral inferred temperatures are generally cooler than measured well temperatures in all wells except in TH10.  Here, mineral inferred temperatures agree with hotter measured well temperatures (> 280 °C) and strong potassic alteration indicates that TH10 is closest to the major upflow zone, making south Tauhara a prospective area for geothermal production.  In TH12, mineral inferred temperatures are inconsistent with measured well temperatures implying a recent increase of fluid temperature.  However petrographic evidence of an illite-calcite overprint suggests a cooler fluid at the bottom of well TH12 which is supported by petrography in this study showing pervasive illite alteration.  TH12 is perhaps more marginal to the upflow zone and the change in temperature regime may be related to blind faults at depth bringing the cooler fluid in.  An alteration halo based on geochemical data was identified in the vicinity of the dike in TH9 at ~1400 mRF by high alteration index values determined by pXRF data, which is useful because illite and chlorite detected by SWIR did not show any appreciable mineralogical alteration halo.

The pXRF technique on cuttings correlated well with lab XRF on powdered cutting samples crushed using a ring mill, showing excellent correlation (rs >0.8) for elements As, Ba, Ca, K, Nb, Rb, Sr, Y, and Zr but poorer correlation (rs <0.7) for others.  Samples analysed by XRF were classified as rhyolite/dacite or andesite/andesite-basaltic.  One sample was reclassified as andesite/andesite-basaltic rather than ignimbrite and this finding was corroborated by downhole pXRF geochemical data which show it belongs in the andesitic lithic crystal breccia unit. These results mean that downhole pXRF geochemical trends of immobile elements (Zr, Y, Nb) can be used to refine lithological boundaries, which cannot be identified from visual logging alone.  In addition, downhole immobile element trends are able to separate lithology packets and distinguish subunits or internal stratigraphic variations.  Conversely, variations in mobile element concentrations (Ba, K, Ca, As, Rb and Pb) indicated feed zones or permeable zones of fluid movement by depletions or elevations in element concentrations.  The (AlOH / H2O) ratio derived from SWIR data was plotted against depth downhole and changes in the ratio appear to coincide with some of the locations of feed zones (fluid pathways) in TH9 and TH12.

This study is the first to apply pXRF to geothermal drill cuttings and has proven that the data collected can be used to aid visual logging, and quantify alteration intensity.  Application of rapid geochemistry on existing and future wells will generate detailed geology models and enable correlation between wells and across fields.  This newly developed approach can be applied worldwide to any geothermal fields, provided that appropriate quality assurance techniques are used to ensure consistency of data.

year: 2015

title: Reducing our prison population: Past failures and new approaches

authors: Tauri, Juan Marcellus

abstract: Earlier this year the Minister of Justice, Andrew Little announced the latest in a long line of reviews, taxpayer-funded summits and inter-agency, ‘wholeof-government’ projects stretching back to the late-1980s, all aimed in some form or other, at making the criminal justice system more ‘safe’ and ‘effective’. Officially launched at a summit held in Porirua in August 2018, the aim of the latest review is to reduce New Zealand’s prison muster by 30% over the next 15 years. And unsurprisingly, given our significant over-representation in prisons, a specific focus of the review is on identifying ways to significantly reduce Māori over-representation in the prison population. This commentary represents a modest offering in response to the current government’s stated aim of making the justice system safer and more effective, and to reduce the prison muster, and it is based on two core arguments, namely that successive governments and the public service have failed to reduce offending and imprisonment and that successful solutions require community empowerment and government/public service accountability.

text: Reducing our prison population: Past failures and new approaches Earlier this year the Minister of Justice, Andrew Little announced the latest in a long line of reviews, taxpayer-funded summits and inter-agency, ‘wholeof-government’ projects stretching back to the late-1980s, all aimed in some form or other, at making the criminal justice system more ‘safe’ and ‘effective’. Officially launched at a summit held in Porirua in August 2018, the aim of the latest review is to reduce New Zealand’s prison muster by 30% over the next 15 years. And unsurprisingly, given our significant over-representation in prisons, a specific focus of the review is on identifying ways to significantly reduce Māori over-representation in the prison population. This commentary represents a modest offering in response to the current government’s stated aim of making the justice system safer and more effective, and to reduce the prison muster, and it is based on two core arguments, namely that successive governments and the public service have failed to reduce offending and imprisonment and that successful solutions require community empowerment and government/public service accountability.

year: 2019