Monthly Archives: October 2023

Geoscience, Minerals & Metals

Tracing Mantle-Oxygen Fugacity Changes Through the Great Oxidation Event: Insights from Apatite Inclusions in Brazilian Igneous Zircons

Leave a comment

Moreira, H., Storey, C., Bruand, E. et al. Sub-arc mantle fugacity shifted by sediment recycling across the Great Oxidation Event. Nat. Geosci. 16, 922–927 (2023). https://doi.org/10.1038/s41561-023-01258-4

Substantial accumulation of free oxygen in the atmosphere occurred between ~2.45 and 2.20 billion years ago , with permanent atmospheric oxygenation commencing between 2.3 and 2.2 Ga. This period is known as the Great Oxidation Event (GOE) and marks the most dramatic change in Earth’s surface chemistry and habitability. However, it remains unclear if these major atmospheric changes affected the amount of free or chemically available oxygen in the mantle and, consequently, the redox state of mantle-derived magmas. In the modern Earth, considerable amounts of surface-oxidized components infiltrate the mantle via slab fluids and subducted sediments, ultimately influencing the oxidation state of the mantle wedge and arc magmas.

Palaeoproterozoic magmatic transition.

a, Zircon U–Pb ages versus 176Hf/177Hf(t) ratios (expressed as ɛHf(t) values relative to chondrite at the time of crystallization t). Zircons from TTG magmas (n = 31) have significantly positive ɛHf(t), whereas zircons from the sanukitoid magmas (n = 33) are near the CHUR. A crustal evolution line links both suites of rocks to a DM melting event at ~2.5 Ga.
b, Zircon 18O/16O ratios (expressed as δ18O relative to Vienna Standard Mean Ocean Water) show that the basaltic crust was hydrothermally altered at high temperature (~4.5‰) before generating TTG magmas at 2.35 Ga and before remelting in the metasomatized mantle wedge at 2.13 Ga. The latter event generated sanukitoids that have zircons with heavier oxygen (~6.5‰). Individual error bars in a and b are shown at 2 standard errors.
c, Tectonic model for the generation of magmas in the Palaeoproterozic pre- and post-GOE peak. SCLM stands for subcontinental lithospheric mantle.

Mantle oxygen fugacity ( fO2) probably changed in the early Earth as a result of metallic Fe retention during core formation and further homogenization, but subsequent variations through time are debatable. The mantle fO2 is either described as largely unchanged or overall having a near-constant rate of increase through time.

Continue reading Tracing Mantle-Oxygen Fugacity Changes Through the Great Oxidation Event: Insights from Apatite Inclusions in Brazilian Igneous Zircons →
Miscellaneous

A Pioneer’s Recollections – Part 2

Leave a comment

Queensland’s Early Days
BY C. Duncan Laidley
Brisbane Courier (Qld. : 1864 – 1933), Saturday 27 October 1923, page 19

I find it most interesting to read the Brisbane Courier from 100 years ago on a Saturday morning. It brings into stark relief the modern world, that which remains much the same and that which has evolved beyond belief in such a short period of time. Here is a transcription of the recollections of Duncan Laidley who as a 9 year old arrived in Sydney, Australia in January 1842 after a 4-5 month voyage. It should be noted that the discussion of our Aboriginal brothers and sisters may not reflect modern sensitivities.

In this article Mr. Duncan, who is 90 years of age, tells of the Brisbane of 66 years ago, and of his experiences at Bald Hills, Ipswich, Mary borough, Laidley, and other places.

Carseldine’s General Store in Bald Hills in the late 1890’s. James Carseldine open the store in 1969 and his residence is to the right . Courtesy Kris Herron
Continue reading A Pioneer’s Recollections – Part 2 →
Minerals & Metals

THE 975 Oz CURTIS NUGGET – FOUND AT GYMPIE

Leave a comment

The Curtis Nugget, the largest in Queensland, was discovered in Gympie, Queensland in 1868 by Curtis and Brigg on a lease close to the initial claim of the discoverer of the Gympie Goldfield, James Nash.  The claim where the Curtis nugget was found had been pegged earlier by George Curtis and Charles Colin.  

Charles Colin

Charles Collin was the son of Jules  Andre Colin  de Souvigny, who with the  family emigrated to Brisbane, Australia in 1826, from Poitier, France. During the passage they shortened the family name to Colin likely to accommodate English sensitivities.  Gustav Colin, the lad to center right of the image below, was the Great Grandfather of this author.  Jules Colin with the aide of Government Land Grants purchased what is now the suburb of Kenmore in western Brisbane.  The Kenmore acreage was sold and the family moved to Bald Hills which was closer to the early Brisbance City. The funds from the sale of the nugget allowed for the return of several family members to France and for Charles Colin to establish a Christian Monastery and School in Sri Lanka. 

The family of Jules and Mathilde Colin de Souvigny – circa 1860. Charles Colin was likely the boy to the left.
Continue reading THE 975 Oz CURTIS NUGGET – FOUND AT GYMPIE →
Geoscience

Formation Environments of Advanced Argillic Minerals & The Exploration Implications

1 Comment

This post is a summary of the excellent recent paper on the above topic in Economic Geology: Jeffrey W. Hedenquist, Antonio Arribas; Exploration Implications of Multiple Formation Environments of Advanced Argillic Minerals. Economic Geology 2022;; 117 (3): 609–643. https://doi.org/10.5382/econgeo.4880

Introduction

Hydrothermal ore deposits are associated with alteration minerals, and one such alteration type is “advanced argillic,” found in relatively shallow geological environments where minerals like alunite, kaolinite, dickite, and pyrophyllite indicate the presence of reactive fluids. This term encompasses a range of minerals, including sericite, quartz, alunite, pyrite, tourmaline, topaz, and more. Some of these minerals are shared with other alteration types, like the kaolinite, dickite, and halloysite clays of the argillic term.

Recognition and interpretation of advanced argillic minerals have increased during mineral exploration in recent decades, facilitated by SWIR spectrometry. This extensive mineralogical information provides insights into the formation environment and its relation to potential mineral deposits.

Understanding advanced argillic and related alteration types is essential for mineral exploration and assessment. The provided framework guides exploration efforts and the study of alteration mineralogy across a wide range of hydrothermal settings, from subaerial to submarine, and varying depths and temperatures.

Executive Summary

  • Advanced argillic minerals include alunite, anhydrite, aluminosilicates (kaolinite, halloysite, dickite, pyrophyllite, andalusite, zunyite, and topaz), and diaspore.  Advanced argillic minerals are key indicators of specific geological alteration environments and are commonly associated with hydrothermal systems and volcanic activity.
  • The formation of these minerals is closely tied to the pH levels, depths, and geochemical conditions of their respective environments. 
  • These minerals form in five distinct geologic environments of hydrolytic alteration, with pH ranging from 4 to less than 1, often at depths below 500 meters.
Continue reading Formation Environments of Advanced Argillic Minerals & The Exploration Implications →
Miscellaneous

A Pioneer’s Recollections – Part 1

Leave a comment

I find it most interesting to read the Brisbane Courier from 100 years ago on a Saturday morning. It brings into stark relief the modern world, that which remains much the same and that which has evolved beyond belief in such a short period of time. Here is a transcription of a recollections of Duncan Laidley who as a 9 year old arrived in Sydney, Australia in January 1842 after a 4-5 month voyage. Below is the text, followed by the scanned copy of the relevant pages and a summary in poetic form generated by AI.

SYDNEY’S EARLY DAYS.

BRISBANE IN 1857.

By C. DUNCAN LAIDLEY.

Born on December 8, 1833, near Brechin, in Forfarshire, Scotland, I am in the shadows of eventide, and ere the break of the new and perfect day I venture to put down some of the recollections of the past, that those who live in these times of ease and plenty may know something of what the pioneers endured in opening up this country. 

Bowerman, Henry Boucher, 1789-1840
This panorama landscape depicts the Moreton Bay Settlement in 1835. The viewpoint is from South Brisbane, on the site now occupied by the Queensland Cultural Centre. The Brisbane landscape and buildings of the period are depicted. Buildings depicted are the Windmill, with a fence in front and the treadmill building to the left; the row of buildings from left to right are the surgeon’s cottage and convict and military hospitals (three low set buildings in a row); the convict barracks, a multi-storey building with a walled yard; the military barracks, a multi-storey building with a low set guard house just visible to the left; the Engineer’s House, used during Bowerman’s time as offices for the commandant and commissariat staff; the kitchen for, and then the Parsonage building, which by 1835 was being used as quarters for commissariat staff; the Commissariat Stores buildings, with an arched wharf with a crane and sentry box to the front, a small boat house to the left of the wharf, and boat builders hut and storeroom to the right of the wharf; and the Commandant’s House, with a small kitchen/convicts’ quarters building to the left.
The building in the far right of the painting, shown behind a row of trees growing on the river bank, was the Government Gardeners house.
Continue reading A Pioneer’s Recollections – Part 1 →
Geoscience

A new Lithospheric and Tectonic Model for Earth

Leave a comment

Introduction

Global depth to the LAB and plate boundaries (after (Afonso, 2019) and (Hasterok, 2022)

The Earth’s lithosphere is its rigid outer shell, which rests on the more fluid asthenosphere. The thickness of the lithosphere varies, from a few kilometers at ocean spreading centers to 250-300 kilometers within continental cratons. There are two significant seismic boundaries in the crust and upper mantle: the Mohorovicic Discontinuity (Moho), indicating a change from felsic-to-mafic rocks to ultramafic peridotites, and the Lithosphere-Asthenosphere Boundary (LAB), which signifies a shift from a strong, plate-like layer to a weaker, convective asthenosphere over geological time. This transition occurs around the conductive-adiabatic geotherm intersection, where heat transfer shifts from conduction to convection.

The thickness of the continental lithosphere depends on its tectono-thermal age, increasing from 60-80 kilometers in active extensional regions to 100-160 kilometers in older terranes and up to 200-300 kilometers in ancient cratons. Some exceptions exist in cratons affected by more recent tectonic and magmatic events. The lithosphere appears as a seismic high-wavespeed layer, or “lid,” over a low-wavespeed zone or a gradual decrease in seismic wavespeed with depth. This boundary is referred to as the “8°-discontinuity” or the mid-lithosphere discontinuity (MLD). Different seismic methods may detect different depths for the LAB or MLD, depending on their sensitivity.

The cratonic LAB is subject to debate, with some proposing a broad thermal boundary zone and others suggesting a sharper transition influenced by factors such as chemical composition, melt content, or vertical anisotropy variation. The presence of an observable S-to-P (Sp) conversion in seismic data requires a thermal gradient of at least 20°C per kilometer. While such gradients are common beneath oceanic and non-cratonic areas, cratons typically exhibit much lower gradients. Multiple factors, including various scales of mantle convection, can contribute to localized high thermal gradients at the LAB.

Continue reading A new Lithospheric and Tectonic Model for Earth →